Steam and Hydronic Line Controls

The pipelines of steam and hot-water heating systems contain a number of different devices or controls designed to maintain and control the circulation of the steam or water through the steam and hot-water lines. These steam — and hot-waterline controls include such devices as pumps, manifolds, valves, flow switches, steam traps, and expansion tanks. Some of these devices, such as pumps, manifolds, and valves, are designed to directly control the rate and direction of flow of the steam or water. Others indirectly influence the flow rate.

Most of the line-control devices commonly used in steam and hot-water heating systems are covered in this chapter. However, with the exception of certain specialized applications, valves are described more fully in Chapter 9 (V alves and Valve Installation).

Steam and Hydronic System Pumps

Heating and circulating pumps are used to maintain the desired flow rate of steam or hot water in a heating system.

The two types of pumps used in steam heating systems are (1) condensate pumps and (2) vacuum heating pumps. The condensate pump is used in a gravity steam heating system to return the con­densation to low — or medium-pressure boilers. Vacuum heating pumps are used in either return-line or variable-vacuum heating systems to return condensation to the boiler and to produce a vacuum in the system by removing air and vapor along with the condensation.

Circulators are used in hot-water (hydronic) heating systems to maintain a continuous flow of the water in the system. They are smaller in size than either a condensate or a vacuum pump and are usually of the motor-driven centrifugal type.

Condensate Pumps

The design of a conventional gravity-flow steam heating system is such that the heat-emitting units cannot be placed at a level lower than the water level in the boiler. The movement of the condensa­tion depends on gravity and, therefore, must move from a higher to

A lower level until it reaches the boiler (at the lowest level). This is illustrated by the one-pipe steam heating system in Figure 10-1. Each heating unit has a single pipe through which it receives the hot steam and returns the condensate in the opposite direction. The dependence on gravity to return the condensate to the boiler places certain limitations on the design of the heating system unless mechanical means are used to compensate for the lack of gravity. A condensate pump serves this purpose.

Many different types of condensate pumps have been developed for use in steam heating systems. Screw, rotary, turbine, reciprocating, and centrifugal pumps are some of the types used for this purpose. One of the most common uses of condensate pumps in low-pressure steam heating systems is the motor-driven centrifugal pump equipped with receiver (tank) and float-control automatic switch.

In operation, condensation enters the receiver and fills the tank. A float connected to an automatic switch rises with the water until the tank is almost full. At that point, the float closes the switch and starts the pump motor. The water is pumped from the receiver and the float drops, causing the switch to open and shut off the pump motor.

Steam and Hydronic Line Controls

The centrifugal pump shown in Figure 10-2 is used to pump con­densation from a lower level return line to one at a higher level, or against a higher pressure. These units are also used to pump con­densation from a flash tank to a boiler (see Figure 10-3) and for other special applications.

As shown in Figure 10-4, the basic components of this pump consist of an impeller (A), with an inlet at its center rotating on a shaft (D). The condensation enters the inlet orifice and flows radi­ally through vanes to the outer periphery (F) of the impeller; it has approximately the same velocity as the periphery. The head of pres­sure developed by the pump is the result of the velocity imparted to the condensation by the rotating impeller.

When the condensation leaves the outer periphery of the impeller, it flows around the volute casing (B) and through the dis­charge orifice (E) of the pump. A wear ring (C) is provided to pre­vent bypassing of the condensation.

Condensate pumps are available in either single or duplex units. The latter are used in installations where it is necessary to have a pump available for use at all times. A duplex unit is actually two condensation pumps fitted with a mechanical alternator. Both pumps feed into the same receiver. If one pump malfunctions, the other starts automatically and continues to provide uninterrupted pumping service for the system.

Steam and Hydronic Line Controls

./water

Oos

подпись: oosLEVEL

BOILER

CONDENSATE

ZJ______________________ c

WATER LEVEL

FLASH TANK

:=»-

7

11/ INCH PIPE LENGTH =

-—50 FEET

Steam and Hydronic Line Controls

VENT

 

-b.

 

Steam and Hydronic Line Controls

T

 

VERTICAL DISTANCE FROM BOILER LEVEL TO PUMP DISCHARGE = 10 FEET

 

—k.

 

PUMP VENT

 

I

подпись: i—»ff—p-

Steam and Hydronic Line Controls

PUMP RECEIVER ‘

4

подпись: 4F

DRAIN,1

DRAIN

подпись: drainFigure 10-3 Pumping condensation from flash tank to boiler. (Courtesy Spirax Sarco Co.)

Steam and Hydronic Line Controls 501 DISCHARGE B

Steam and Hydronic Line Controls

Figure 10-4 Basic components of a centrifugal condensate pump.

(Courtesy Spirax Sarco Co.)

Vertical condensation pumps are available for use in installations where the space for the pump is limited, where the returns run below the floor, or where it is undesirable to place a horizontal pump in a sunken area. Vertical condensation pumps are also available in both single and duplex units.

The use of a condensate pump in a two-pipe steam heating sys­tem is shown in Figure 10-5. Note the arrangement of gate and check valves on the discharge side of the pump. This type of system is commonly referred to as a condensate-return steam heating sys­tem. Using a condensate pump to return the condensation provides greater design flexibility for the system. The major disadvantage is that larger steam traps and piping must be used than in vacuum heating systems.

Condensate pumps can also be used as mechanical lifts in vac­uum steam heating systems (see Figure 10-6). By connecting the vent outlet of the condensation pump to a return line above the level of the vacuum heating pump, the same vacuum return con­dition is maintained in the piping below the water level as in the rest of the system. In this arrangement, the only purpose of the condensate pump is to lift the condensation from the lower level to the higher one without reducing the capacity of the vacuum heating pump.

Steam and Hydronic Line Controls

Figure 10-5 Condensate pump in a two-pipe steam heating system.

(Courtesy ITT Hoffman Specialty)

Vacuum Pumps

Vacuum heating pumps are used to maintain the vacuum in mechanical vacuum heating systems by removing air, vapor, and condensation from the lines. Many vacuum pumps are available in both single and duplex units, and are designed to automatically adjust themselves to the varying conditions of the system. The duplex units have an advantage over the single pumps, because they provide automatic standby service. If one of the pumps in a duplex unit should happen to malfunction, the other one cuts in and picks up the load.

A vacuum pump is operated either by steam or electricity. Steam — driven vacuum pumps are sometimes used in high-pressure steam systems, but these pumps have been generally replaced by auto­matic motor-driven return-line pumps. Examples of vacuum heat­ing pumps are shown in Figures 10-7 and 10-8.

In operation, the pump is started before the steam enters the sys­tem. When the pump removes the air from the lines, steam quickly fills the radiators of the system. The radiators remain full of steam, because the air is automatically removed as fast as it accumulates. By quickly exhausting the air and condensation from the system, the vacuum pump causes the steam to circulate more rapidly, resulting in faster warm-up time and quieter operation.

Steam and Hydronic Line ControlsVALVE

BOILER WATER LEVEL

1

UNIT

HEATER

подпись: 1
unit
heater

VACUUM VENT

подпись: vacuum ventVACUUM HEATING PUMP

CONDENSATE

DISCHARGE

 

DRIP

TRAP

 

CONDENSATION PUMP AS A MECHANICAL LIFT

 

503

подпись: 503Figure 10-6 A condensate pump used as a mechanical lift. (Courtesy Nash Engineering Co.)

Steam and Hydronic Line Controls

Figure 10-7 Return-line vacuum heating pump and receiver.

(Courtesy Nash Engineering Co.)

The condensation of steam in the lines creates the vacuum, and the pump maintains it by continuing to pump air from the system. The vacuum maintained by the pump is only a partial one, because it is not possible with this device to extract all the air. Each stroke of the pump piston or plunger removes only a fraction of the air, depending on the percentage of clearance in the pump cylinder, the resistance of valves, and other factors; hence, an infinite number

Steam and Hydronic Line Controls

Figure 10-8 Duplex return-line vacuum and boiler feed pump for a vacuum steam heating system. (Coun. esy Chicago Pump Co.)

Of strokes would theoretically be necessary to obtain a perfect vacuum, not considering line and pump resistance.

Vacuum pumps designed to remove only air from a system are referred to as dry pumps. Those that remove both air and conden­sation are called wet pumps. When a wet pump is used, the con­densation is pumped back to the boiler. In operation, the air, being heavier than steam, passes off through thermostatic retainer valves to the pump. When the steam reaches the retainer valves, they close automatically to prevent the steam passing into the dry return line to the pump and breaking the vacuum. The air from the pump is passed into a receiver, where it is discharged through an air vent. The condensation is pumped back to the boiler generally by means of a centrifugal pump.

In most vacuum systems, the pump is controlled by a vacuum regulator and a float control. The vacuum regulator cuts in when the vacuum drops to a preset level and cuts out when the vacuum reaches its highest point. The float control operates independently from the vacuum regulator, starting the pump when condensation reaches a certain level in the receiver.

Two typical installations in which vacuum heating pumps are used are illustrated in Figures 10-9 and 10-10. In the vacuum air-line heating system, shown in Figure 10-10, thermostatic-type air-line valves are used instead of radiator air vents. The primary purpose of the vacuum heating pump is to expel air from the system.

The vacuum return-line system is very similar to a condensation — return steam heating system, except that a vacuum pump is used to provide a low vacuum in the pipes and to return the condensation to the boiler. Because of the vacuum condition, smaller steam traps and piping can be used.

An accumulator tank must be installed in a vacuum pump steam heating system if the returns are below the inlet connection of the vacuum pump receiver. As shown in Figure 10-10, the condensate flows by gravity from the baseboard heating units to the accumula­tor tank, where it is lifted to the vacuum pump receiver.

Circulators (Water-Circulating Pumps)

Hydronic heating systems use small compact pumps to provide the motive force to circulate the water in the pipes. They are usually referred to as circulators or water-circulating pumps. The circulator is used to move the water from the boiler to the heat-emitting units and back again. It is not used for lifting, as is the case with vacuum and condensate pumps, but simply for circulating the water through a closed loop.

Steam and Hydronic Line Controls

SUPPLY VALVE

THERMOSTATIC TRAP

STEAM SUPPLY MAIN

VACUUM

PUMP

Y

STRAINER

F &TTRAP

GATE VALVE CHECK VALVE — VACUUM PUMP

F & T TRAP Y STRAINER

Figure 10-9 Vacuum pump in a two-pipe steam heating system.

(Courtesy ITT Hoffman Specialty)

Circulators were first introduced in the 1930s to augment water circulation in the traditional hot-water space-heating systems. Prior to their introduction, the hot-water systems relied on the density difference between cold and hot water to provide the motive force for water circulation. These systems were called gravity hot-water heating systems, and the pumps were added to boost circulation. These early pumps (sometimes called three-piece circulators or booster pumps) are still with us today, although as more technically advanced models. All modern hydronic heating systems are closed — loop installations that use one of several different types of pumps to circulate the water.

Three-Piece Booster Pumps

The three-piece booster pump illustrated in Figures 10-11 and 10-12 is an example of the circulators used in small-to medium-size residen­tial and light commercial hydronic heating systems since the 1930s.

Steam and Hydronic Line Controls

STEAM SUPPLY MAIN

SUPPLY VALVE

THERMOSTATIC TRAP

F & T TRAP

CHECK VALVE——

VACUUM PUMP

LIFT

FITTING ACCUMULATOR TANK

Figure 10-10 Vacuum pump with accumulator tank in two-pipe steam

Heating system. (Courtesy ITT Hoffman Specialty)

A typical three-piece booster pump consists of the following three sections: (1) the pump body (also called the volute, body assembly, or waterway), (2) the coupling assembly, and (3) the motor assembly (also called the shaft-and-motor assembly).

The three-piece booster pump contains hermetically sealed sleeve bearings, a carbon/ceramic seal, and a coupler that uses springs in tension to provide quiet operation. The motor of a three-piece booster pump can be serviced by removing it from the pump body. Consequently, there is no need to drain the system or disconnect the pump from the piping for servicing.

The three-piece booster pump has an inline volute, which means the inlet and discharge ports are located along the same centerline. It has a strong starting torque, which enables it to free a stuck impeller without any difficulty.

Note

The volute is just another term for the pump body. It contains the motor bracket, the impeller, the volute gasket, the inlet and discharge ports, and the pump mounting flanges. The shape of the volute will determine how the circulator is connected to the piping.

Steam and Hydronic Line Controls

Figure 10-11 Bell & Gossett Series PL three-piece booster pump.

(Courtesy ITT Bell & Gossett)

A three-piece booster pump requires periodic inspection. The mechanical seal will sometimes need replacement. After removing the old seal, clean the shaft and sleeve before installing a new one. The pump manufacturer will provide step-by-step instructions for servicing the pump mode.

This pump also requires periodic lubrication. A wool wicking is used to draw the lubricating oil into the bearing assembly (see Figure 10-13). Check the pump manufacturer’s operating and maintenance instructions for the recommended lubricating sched­ule. As shown in Figure 10-13, the three-piece booster pump must always be installed with the oil ports facing upward and with the motor, motor shaft, and bearing assembly in a horizontal position.

Caution

Never plug or cover the weep hole, or you will trap the excess oil in the pump body. Any dirt or sediments in the oil may damage the bearings and shorten their service life.

Steam and Hydronic Line Controls

Figure 10-12 Cross sectional view of a Bell & Gossett Series PL three-piece booster pump. (Courtesy ITT Bell & Gossett)

Warning

Use only the lubricant specified by the pump manufacturer. An SAE 20 (nondetergent) or I0W-30-weight oil can be substituted if the pump manufacturer’s recommended oil is unknown.

The three-piece booster pump shown in Figures 10-11 and 10-12 can be installed to discharge in any direction (e. g., up, down, horizontally, etc.), but the motor shaft must always be in the hor­izontal position, the arrow on the pump body must always point in the direction of flow, and the conduit box must be positioned on top of the motor housing.

Wet-Rotor Circulators

The wet-rotor circulator is also used in small — to medium-size resi­dential and light-commercial hydronic systems (see Figure 10-14). This is a small, close-coupled pump with an integral 40125-watt motor. It is a sealed pump that does not require lubrication. The wet-rotor pump combines the motor, the shaft, and the impeller in a single assembly housed in a chamber filled with system fluid. In other words, it is both cooled and lubricated by the system fluid, hence the name wet-rotor pump. These circulators do not have a mechanical seal, as is the case with booster pumps. Consequently, seal replacement is not a problem.

Wet-rotor circulators are small, require no maintenance, and provide a long service life before they have to be replaced. A princi­pal disadvantage using the wet-rotor pump is that it cannot be ser­viced or repaired while connected to the piping; the system must be

Steam and Hydronic Line Controls

Figure 10-13 Bell & Gossett Series 100 booster pump.

(Courtesy ITT Bell & Gossett)

Drained before it can be removed. They are sometimes called throw-away pumps, because it is less expensive to replace them than it is to repair them.

Note

The problem of having to drain the system in order to remove the pump can be avoided if shutoff (check) valves are installed in the return line on either side of the unit.

Inline Centrifugal Circulators

An inline centrifugal circulator is another type of pump commonly used in hydronic heating systems of small — to medium-size residen­tial and light-commercial structures. The Bell & Gossett Series 90 inline centrifugal pump shown in Figures 10-15 and 10-16 is an example of this type of pump. It is a close-coupled, low-maintenance unit that can be mounted inline both vertically and horizontally. Because the motor can be disconnected from the pump body for servicing, there is no need to remove the pump body from the piping circuit for servicing.

End Suction Pumps

End suction pumps are used in the hydronic systems of hotels, stores, theaters, and other large structures. The Bell & Gossett Series VSC pump shown in Figure 10-17 is a double suction unit
characterized by a vertically split casing perpendicular to the pump shaft. It is used in hydronic heating and cooling systems, and for condenser water, cooling towers, refrigeration, and general service. These pumps have volutes that differ in design from the smaller cir­culators used in small to medium hydronic systems, and they are commonly installed at a 90 “angle to the system piping with a lateral offset between the discharge and inlet ports.

Circulator Selection

A number of different factors must be considered when selecting circulators for both large and small heating systems. The selection factors include:

• Amount of water to be handled

• Temperature of the water to be handled

• Head against which the pump must operate

Steam and Hydronic Line Controls

Figure 10-15 Bell & Gossett Series 90 inline centrifugal pump.

(Courtesy ITT Bell & Gossett)

• Working head of the system

• Pump suction head

Data necessary for selecting a suitable water-circulating pump are supplied by pump manufacturers. Always check the manufac­turer’s specifications for the maximum working pressure and the maximum operating temperature of the pump before deciding on which model to use. These operational limits can be found in the manufacturer’s specification sheet, in the installation manual, or on the pump nameplate. The maximum working pressure and maxi­mum operating temperature of the system must not exceed those of the pump. If they do, it can lead to possible property damage, serious injury, or even death.

Steam and Hydronic Line Controls

Steam and Hydronic Line Controls

DRAIN

PLUG

Steam and Hydronic Line Controls

(Counesy ITT Bell & Gossett)

The temperature of the water handled by the pump will deter­mine the type of pump packing selected. The working head of the system is the sum of the static head and the friction. The pump will have either a positive or negative suction head.

Pump Head and Pressure Drop

Two terms you will encounter when dealing with circulators are pump head and pressure drop. The term pressure drop refers to the friction created when the fluid (water) flows against the inside sur­face of the piping and passes through valves, fittings, or other system components. This friction will slow down the flow rate of the system fluid (water). The circulator must be powerful enough to overcome this friction and produce a steady, uniform flow rate. The term pump head (or pressure head) refers to the force developed by the circula­tor to overcome pressure drop.

The circulators used in small residential hot-water heating sys­tems are installed directly in the return line. These are commonly single-suction pumps in which both the motor and the pump share a common shaft. The major objection to installing a pump directly in the return line is that the pipe connections may have to be broken to remove the pump for service and repair. However, some pumps such as the Bell & Gossett Series 90 model shown in Figures 10-15 and 10-16 can be serviced without removing the entire unit. If the circulator must be completely removed from the return line, a good idea is to install shutoff (check) valves above and below the unit so that the entire system does not have to be drained.

The operating head of circulators used in smaller hot-water heat­ing systems is limited, and it is often general practice to size the pipelines of the system after selecting a pump capable of meeting the requirements of the system.

Pump manufacturers recommend that cast-iron pumps be selected for circulating water in a hydronic space heating system. Bronze pumps are recommended for pumping domestic (potable) hot water.

Circulator Installation

Always follow the pump manufacturer’s installation instructions. These instructions will accompany the pump or can be obtained from the pump manufacturer (often by going online and download­ing the manual from their web site).

Note

Most circulator manufacturers recommend using their cast-iron models for circulating water in a hydronic space heating system and their bronze models for pumping domestic (potable) hot water.

Warning

Never install a circulating pump with operational limits (maximum working pressure and maximum operating temperature) less than those of the hydronic system. Make certain the electrical rating of the circulator is appropriate for the installation.

In residential and light-commercial hydronic heating systems, the circulator should be mounted with its inlet port close to the point at which the expansion tank connects to the return line, which normally is the point of no pressure change in the system. Placing the inlet side of the circulator close to the expansion tank is important, because the latter controls the pressure of the water in the system. The circulator only circulates the water. It does not cre­ate pressure. The circulator should also be mounted in the return line as close as possible to the boiler. Install it with the flow arrow on its body facing the boiler. This will ensure that its discharge port also faces the boiler. Never mount a circulator in the highest part of the system.

The circulators used in residential and light-commercial hydronic systems should be installed with their motor shafts in a horizontal position. Installing the circulator with the motor shaft in a vertical position places unwanted weight on the bushings, rotor, and impeller.

When installing a circulator, it is also a good idea to install shut — off valves in the line above and below it. Doing so will allow removal of the circulator without having to first drain the system.

The header piping should be strong enough to hold the weight of the circulator. This can be a problem if it is installed in copper pipe or tubing, because copper has neither the strength nor the rigidity of steel. To prevent sagging from the weight of the circulator, sup­port the header pipe or tubing with metal strap hangers or brackets, or use steel pipe or tubing for the header and connect the copper lines to either end of it.

Circulator Operation

A hydronic system circulator is always filled with system fluid (water). As soon as the pump moves water out of its discharge port, it is immediately replaced by an equal volume of water entering through its inlet port. Remember, these are closed systems that are always completely filled with water. If the system is not filled with water, either there is a leak or the operator has failed to check the water level before startup.

Warning

A circulator pump should never be run without any water in it. Doing so will damage the pump. Always check to be sure it is filled with water before startup.

Water enters the inlet port of the circulator and flows directly to the impeller located in the volute (see Figure 10-18). The impeller is a rotating wheel that creates the centrifugal force required to move the water through the piping. The pump drive shaft enters the back of the impeller and exits the front through an opening called an eye. The centrifugal force of the rotating impeller accelerates the veloc­ity of the water, forcing it away from the eye and around the inside of the pump body. The water is directed toward a discharge port that is much smaller than the pump inlet port. Squeezing the water through this smaller discharge port converts the water velocity to

Steam and Hydronic Line Controls

Figure 10-18. Circulator impeller and volute. (Courtesy ITT Bell & Gossett)

Pressure. This is the pressure head (or pump head) developed by the pump to overcome the friction (pressure drop) created by the water flowing through the piping, valves, and other components of a hydronic heating system.

Circulator Troubleshooting, Service, and Maintenance

Hermetically sealed, self-lubricating pumps should never be oiled or lubricated. It is not only unnecessary, but could also damage the pump. Very little maintenance is required for these pumps.

Sometimes the failure of a hot-water (hydronic) heating system to produce heat can be traced to a malfunctioning circulator (pump). Before attempting to repair or replace the unit, check the electric power to the pump from the system controls. The problem may be due to an electrical failure, instead of the mechanical failure of the pump itself. Always first check for a blown fuse or a tripped circuit breaker.

If repairs are required, manufacturers commonly provide parts lists for their pump models so that the unit can be serviced on site. For the circulator shown in Figure 10-16, it is possible to replace the shaft sleeve, seals, gaskets, and impeller (all parts subject to wear) without removing the pump from the piping.

Note

The pH value of the water is an important consideration when operating circulators. For optimum operation, the water should have a pH ranging from 7 to 9. The water pH value can change during the service life of the pump. These changes occur as a result of a change in water quality or chemical additives. If the pH value of the water falls outside the 7-9 range, it can cause circulator seal failure or corrosion of system components.

Important Safety Tips

To avoid possible injury or even death from electrical shock, always shut off the electrical power and disconnect the pump before attempting to service or repair it.

To avoid scalding burns, always allow the system water to cool to room temperature before attempting to remove a pump for servicing.

Steam Traps

A steam trap is an automatic valve that opens to expel air and con­densation from steam lines and closes to prevent the flow of steam. The functions of a steam trap are:

• Remove (vent) air from the system so that steam can enter. (Air in the pipes will block the flow of steam into the radiators.)

Prevent steam from leaving the system until all of its latent heat is removed.

Remove (drain) the condensate from the system after the latent heat has been removed. (Draining the condensate from the system prevents corrosion and water hammer.)

All steam traps operate on the fundamental principle that the pressure within the trap at the time of discharge will be slightly in excess of the pressure against which the trap must discharge. This includes the friction head, the velocity head, and the static head on the discharge side of the trap. The steam trap cannot operate unless the excess pressure of discharge is greater than the total back pressure.

Each steam trap used in steam heating is designed for a specific range of applications that its operating characteristics best suit. Although there is no universal steam trap per se, the many different types can be grouped into the following three classes on the basis of their operating characteristics:

• Separating traps

• Return traps

• Air traps

Separating traps are designed to release condensation but close against steam. They are float-operated, thermostatically operated, or float-and-thermostatically operated. Thermostatic traps are designed to release air and condensation but close against them.

Return traps may be operated to receive condensation under a vacuum and return it to atmosphere or a higher pressure. Air traps are generally operated by a float.

Steam Trap Information

The 80-page Steam Traps & Repair Parts catalog from State Supply Company, Inc. of St. Paul, Minnesota is filled with information necessary for troubleshooting and repairing steam traps. It contains repair part drawings, capacity tables, and product ordering guides from all the major steam-trap manufacturers (Dunham-Bush/MEPCO, Monash, Illinois, Marsh, Spirax-Sarco, Hoffman Specialty, Trane, Armstrong, Sterling, and Warren-Webster). A copy can be obtained by writing or calling State Supply:

The State Supply Company, Inc.

597 East Seventh Street St. Paul, MN 55101-2477 800-772-2099 Info@statesupply. com Www. statesupply. com

Sizing Steam Traps

Selecting the correct size steam trap for a system is an important factor in its operational efficiency. For example, an oversized trap will operate less efficiently than a correctly sized one, and will tend to create abnormal back pressure. Moreover, the installation cost will be higher and the operational life expectancy will be reduced.

Manufacturers of steam traps provide information in the form of capacity ratings and related data to make these selections easier. Data should be based on hot condensation under actual operating conditions rather than cold-water ratings. If possible, always try to determine the basis for a manufacturer’s ratings. Table 10-1 and the sizing example were provided by Sarco Company, Inc., a manufac­turer of steam traps.

Steam Trap Maintenance

Isolate the steam-trap assembly from the return and supply lines and allow the pressure to normalize (return to atmosphere) before starting any maintenance. The steam trap can be disassembled for maintenance as soon as it has cooled down.

Note

Many steam trap manufacturers produce repair kits for current

Models and even for traps no longer in production

Automatic Heat-Up

Select a steam trap with a pressure rating equal to or greater than the pressure in the steam supply main but with a capacity based on the estimated pressure at the trap inlet. The pressure at the inlet of the steam trap can be considerably less than the pressure in the steam supply main.

If the steam trap is connected into a common piping return sys­tem, it may have to operate against a certain amount of static pres­sure. This static (back) pressure can cause a reduction in the operating capacity of the steam trap. Table 10-2 illustrates the effect of back pressure on steam trap capacity.

The safety factor for a steam trap is the ratio between its maxi­mum discharge capacity and the condensation load it is expected to handle. The actual safety factor to use for any particular applica­tion will depend upon the accuracy of the estimated condensation load, the accuracy of the estimated pressure conditions at trap inlet and outlet, and the operational characteristics of the trap.

The application for which a steam trap is to be used is also an important factor in its selection. For example, a float and thermostatic trap is recommended for use as a steam-line drip trap at pressures ranging from 16 psig to 125 psig. For the same application at pres­sures of 126 psig or above, an inverted bucket trap is suggested. Unusual operating conditions may also influence the choice of a steam trap for a particular application. A careful reading of the selection guide and related literature provided by the manufacturer will greatly reduce the possibility of error in choosing a suitable steam trap.

Table 10-1 Sizing Traps for Steam MainsCondensation Load in P ounds per Hour per 1000 Feet of Insulated Steam Main*Ambient Temperature 70 FInsulation 80% Efficient

Steam

Pressure

(PSIG)

2"

2’A"

3"

4"

5"

Main Size 6" 8"

10"

12”

14"

16"

18"

20"

24"

0°F*

Correction

Factor

10

6

7

9

11

13

16

20

24

29

32

36

39

44

53

1.58

30

8

9

11

14

17

20

26

32

38

42

48

51

57

68

1.50

60

10

12

14

18

24

27

33

41

49

54

62

67

74

89

1.45

100

12

15

18

22

28

33

41

51

61

67

77

83

93

111

1.41

125

13

16

20

24

30

36

45

56

66

73

84

90

101

121

1.39

175

16

19

23

26

33

38

53

66

78

86

98

107

119

142

1.38

250

18

22

27

34

42

50

62

77

92

101

116

126

140

168

1.36

300

20

25

30

37

46

54

68

85

101

111

126

138

154

184

1.35

400

23

28

34

43

53

63

80

99

118

130

148

162

180

216

1.33

500

27

33

39

49

61

73

91

114

135

148

170

185

206

246

1.32

600

30

37

44

55

68

82

103

128

152

167

191

208

232

277

1.31

* Chart loads represent losses due to radiation and convection for standard steam.

П For outdoor temperature of 0°F, multiply load value in table for each main size by correction factor corresponding to steam pressure. (Courtesy Spirax Sarco Co.)

IIS

подпись: iis

Table 10-2 Effect of Back Pressure on Steam Trap Capacity (Percentage Reduction in Capacity)

%

Back Pressure

Inlet Pressure PSIG

5

25

100

200

25

6

3

0

0

50

20

12

6

5

75

38

30

25

23

(Courtesy Spirax Sarco Co.)

Installing Steam Traps

The installation of steam traps requires the following modifications in the piping:

1. Install a long vertical drip and a strainer between the trap and the apparatus it drains. The vertical drip should be as long as the installation design will permit. Exception: Thermostatic traps in radiators, convectors, and pipe coils are attached directly to the unit without a strainer.

2. A gate valve should be installed on each side of a trap, along with a valved bypass around the traps if continuous service is required. This permits removal of the trap for servicing, repair, or replacement without interrupting service.

3. A check valve and gate valve should be installed on the dis­charge side of a trap used to discharge condensation against back pressure or to a main located above the trap (as for lift service).

Always carefully follow the steam trap manufacturer’s instructions for installing a steam trap. If these instructions are not available, call the factory or an authorized representative for information before attempting to install the trap. The following are offered as guidelines for installing steam traps:

1. All work must be performed by qualified personnel trained in the correct installation of the trap.

2. Installation work must comply with all local codes and ordinances.

3. Allow the boiler to cool down to approximately 80F and the pressure to drop to 0 before attempting to do any work on the trap.

4. Wear heat-resistant gloves to prevent serious burns when opening and shutting steam valves.

5. Cap off the gate valves if they are not connected to a drain or not in use for test or pressure-relief purposes to prevent prop­erty damage, serious injury, or death.

6. Connect a temporary pipe between the steam pipe opening and a drain to prevent injury from steam pipe blow-down. In lieu of installing a temporary pipe, stand at least 100 feet from the pipe opening.

7. Open supply valves slowly after installing the trap.

Note

Check the trap seat rating on the nameplate before installing it. The rating must be equal to or greater than the maximum pressure differential across the trap.

Float Traps

A float trap (see Figure 10-19) is operated by the rise and fall of a float connected to a discharge valve. The change of condensation level in the trap determines the level of the float. When the trap is empty, the float is at its lowest position and the discharge valve is closed. As the condensation level in the trap rises, the float also rises and gradually opens the valve. The pressure of the steam then pushes the condensation out of the valve. Because the opening of the valve is proportional to the flow of condensation through the trap, the discharge of condensation from the trap is generally con­tinuous. On some float traps, a gauge glass is used to indicate the height of the condensation in the trap chamber.

One of the principal disadvantages of a float trap is the tendency of the valve to malfunction. Valve malfunctions can result from the

Figure 10-19 Float trap.

AIR

Steam and Hydronic Line Controls

Sticking of moving parts or excess steam leakage due to unequal expansion of the valve and seat.

Float traps are designed for steam pressures ranging from vac­uum conditions to 200 psig and are used to drain condensation from heating systems, steam headers, steam separators, laundry equipment, and other steam process equipment. When used in heat­ing systems, a float trap should be equipped with a thermostatic air vent (see Float and Thermostatic T raps ’in this chapter).

Thermostatic Traps

The operation of a thermostatic trap (see Figure 10-20) is based on the expansion or contraction of an element under the influence of heat or cold.

Thermostatic traps are of the following two types: (1) those in which the discharge valve is operated by the relative expansion of metals and (2) those in which the action of the liquid is utilized for this purpose. The latter is probably the most commonly used ther­mostatic trap found in modern steam heating systems. Thermostatic traps of large capacity for draining blast coils or very large radiators are called blast traps.

Modern thermostatic traps consist of thin, corrugated-metal bel­lows or discs enclosing a hollow chamber that is filled with a liquid or partially filled with a volatile liquid. When steam comes in con­tact with the expansive element, the liquid expands or becomes a gas and thereby creates a certain amount of pressure. The element expands as a result of this pressure and closes the valve against the escape of the steam.

Steam and Hydronic Line Controls

OUTLET OUTLET

DISC TYPE

Figure 10-20 Thermostatic trap.

BELLOWS TYPE

Balanced-Pressure Thermostatic Steam Traps

As shown in Figure 10-21, the principal parts of a balanced-pressure thermostatic trap consist of a flexible bellows, a valve head, and a valve seat. The bellows is partially filled with a volatile fluid and hermetically sealed. The fluid sealed in the bellows has a pressuretemperature relationship that closely parallels, but is approximately 10F below, that of steam. When the condensation surrounding the bellows reaches approximately 10F below satu­rated steam pressure, the fluid inside the bellows begins to build up pressure. When the temperature of the condensation approaches that of steam, the pressure inside the bellows exceeds the external pressure. This pressure imbalance causes the bellows to expand, driv­ing the valve head to its seat and closing the trap (see Figure 10-22). When the condensation surrounding the bellows cools, the vaporized fluid condenses and reduces the internal pressure. The reduction of internal pressure causes the bellows to contract, opening the trap for discharge.

A balanced-pressure thermostatic steam trap is vulnerable to water hammer and corrosive elements in the condensation. The lat­ter problem can be handled by fitting the trap with anticorrosive internal components. On the plus side, this type of trap has rela­tively large capacity and high air-venting capability. It is completely self-adjusting within its pressure range.

Additional information about balanced pressure steam traps can be found in Chapter 2 (Radiators, Convectors, and Unit Heaters)’ in Volume 3.

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Maintenance

These valves are factory sealed and no repair parts are available. If defective, they must be replaced. The valves must be completely iso­lated from both the supply and return lines before removal for replacement.

Float and Thermostatic Traps

A float and thermostatic trap (see Figure 10-23) has both a thermo­static element to release air and a float element to release the con­densation. As such, it combines features of both the float trap and the thermostatic trap.

These traps are recommended for installations in which the vol­ume of condensation is too large for an ordinary thermostatic trap to handle. Float and thermostatic traps are also used in low-pressure steam heating systems to drain the bottom and end of steam risers (see Figures 10-24 and 10-25). Other applications include the draining of condensation from unit heaters, preheat and reheat coils in air conditioning systems, steam-to-water heat exchangers, blast coils, and similar types of process equipment.

Note

Check the trap-seat pressure before installation to make sure its rating is equal to or greater than the steam supply of the boiler.

Steam and Hydronic Line Controls

INLET

BALL FLOAT

OUTLET

THERMOSTATIC AIR VENT

VALVE HEAD

VALVE SEAT

Figure 10-23 Float and thermostatic steam trap.

(Courtesy Spirax Sarco Co.)

STEAM

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Figure 10-25 Draining bottom of low-pressure steam riser.

(Courtesy Spirax Sarco Co.)

If the trap is operating properly, it will immediately and continu­ously discharge condensation, air, and noncondensable gases from the system that enter the inlet orifice of the trap.

No Heat or Uneven Heat

Float and thermostatic traps are used in two-pipe steam heating sys­tems to pass air and condensate into the return piping while simultane­ously preventing the steam from moving past the radiators and ends of the steam mains. Failure of these traps results in the following problems:

Trap fails in the open position causing the steam to pass into the return lines. As a result, the pressure in both the return and supply piping is the same and, without a pressure differential between the two, the steam cannot move to the radiators.

Trap fails in the closed position, blocking the entry of air and steam into the radiators.

The condensation is handled by the ball float, which is con­nected by a level assembly to the main valve head. Condensation entering through the trap inlet causes the ball float to rise, moving the level assembly and opening the valve for discharge.

Air and noncondensable gases are discharged through the thermo­static air vent. The thermostatic element is also designed to prevent the flow of steam around the float valve.

Float and thermostatic traps operate under pressures ranging from vacuum to a maximum pressure of 200 psig; however, the great majority of them are designed for 40 psig or less.

These traps have limited resistance to water hammer. They are also vulnerable to corrosive elements in the condensation unless fitted with anticorrosive internal components.

Thermodynamic Steam Traps

A thermodynamic steam trap (see Figures 10-26 and 10-27) con­tains only one moving part, a hardened stainless-steel disc that functions as a valve. Because of its construction simplicity, this is an

Steam and Hydronic Line Controls Steam and Hydronic Line Controls

1 •* ?

подпись: 1 •* ?

DISC

REPLACEABLE VALVE SEAT

подпись: disc
replaceable valve seat

TRAP BODY

подпись: trap body Steam and Hydronic Line ControlsQ

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Figure 10-27 Thermodynamic steam trap. (Courtesy Spirax Sarco Co.)

Extremely rugged trap and is especially well suited for service on medium — and high-pressure steam lines operating under pressures up to 600 psig. The minimum operating pressure for some makes of these traps is as low as 3.5 psig.

Thermodynamic steam traps are small, unaffected by water hammer, and can be mounted in any position. The operating princi­ples of a thermodynamic steam trap are illustrated in Figure 10-28.

Bucket Traps

Bucket traps are either of the upright or inverted design and are used in both low- and high-pressure steam heating systems. Both types of bucket traps are designed to respond to the difference in density between steam and condensation. The construction of a bucket trap is such that it has good resistance to water hammer. On the other hand, most bucket traps, unless modified, have limited air-venting capabilities. Bucket traps also have a tendency to lose their waterseal and blow steam continuously during sudden pressure changes.

In an upright bucket trap (see Figure 10-29), the condensation enters the trap and fills the space between the bucket and the walls of the trap. This causes the bucket to float and forces the valve against its seat, the valve and its stem usually being fastened to the bucket. When the water rises above the edges of the bucket, it floats into it and causes it to sink, thereby withdrawing the valve from its seat. This permits the steam pressure acting on the surface of the water in the bucket to force the water to a discharge opening. When the bucket is emptied, it rises and closes the valve and another cycle begins. The discharge from this type of trap is intermittent.

DISC HELD UP BY PRESSURE AT INLET

FLASH STEAM CREATES PRESSURE HERE

OUTLET

HOLES

DISC
MOVES
DOWN

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JET CREATES
LOW PRESSURE

HERE

 

ONLY OUTLET PRESSURE ABOVE DISC

 

■SM wm

 

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DISCHARGE

подпись: discharge

CONTROL PRESSURE ON LARGE AREA

подпись: control pressure on large area

INLET ON SMALL AREA

подпись: inlet on small area Steam and Hydronic Line Controls

DISC

RISES

подпись: disc
rises
Steam and Hydronic Line Controls

CONDENSATE OR MIXTURE OF AIR AND CONDENSATE

Pressure of condensate or air lifts the disc off its seats. Flow is across the underside of the disc to the three outlet holes. Discharge continues until the flashing condensate ap­proaches steam temperature.

CONDENSATE REACHING TEMPERATURE OF SATURATES STEAM

A high-velocity jet of flash steam reduces pressure under the disc and at the same time, by recom­pression, builds up pressure in the control chamber above the disc.

This drives the disk to the seats ensuring tight closure without

Steam loss. STEAM

CONDENSES

HERE

HEAT TRANSFER FROM HERE MAINTAINS PRESSURE IN CONTROL CHAMBER

Steam pressure in the control chamber, acting over the total disc area, holds the disc closed against inlet pressure acting over the smaller inlet seat area.

CONDENSATE ACCUMLATION REDUCES HEAT TRANSFER TO CONTROL CHAMBER

As soon as condensate collects, even at steam temperature, it reduces heat transferred to the control chamber Pressure in thechamber decreases as steam trapped there condenses. The disc is lifted by inlet pressure and condensate is discharged.

Figure 10-28 Operating principles of a thermodynamic steam trap.

(Courtesy Spirax Sarco Co.)

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In the inverted bucket trap (see Figure 10-30), steam floats the inverted submerged bucket and closes the valve. Water entering the trap fills the bucket, which sinks, and through compound leverage opens the valve, and the trap discharges.

An inverted bucket trap with its seat open to vent air or drain condensate is shown in Figure 10-31. During start-up, air is vented into the return line through a bleed hole located at the top of the bucket. The condensate enters the trap, moves around the bucket, and drains from the open trap seat.

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Figure 10-31 Inverted bucket trap seat open to vent air or drain

Condensate. (Courtesy ITT Hoffman Specialty)

As steam flows into the trap, it collects at the top of the bucket. When enough steam has collected there, its buoyancy causes the bucket to rise and close the trap seat (see Figure 10-32). The closed trap seat blocks the exit of the steam until more condensate enters the bucket trap. The cycle repeats itself as long as the boiler is producing heat. Some inverted bucket traps have a thermal vent at the top of the bucket, which produces faster venting during start-up.

Typical installations for bucket traps are shown in Figure 10-33. Note the following installation recommendations:

• Provide enough room around the trap to allow easy access for service and maintenance.

• Locate the trap as close as possible to and below the equip­ment being drained.

• Install the trap in a straight run of horizontal pipe that is slightly pitched to allow condensate to flow down into the trap inlet.

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Figure 10-32 Inverted bucket trap with seat closed to retain steam.

(Courtesy ITT Hoffman Specialty)

Flash Traps

A flash trap (see Figure 10-34) is used to drain condensation from steam lines; steam, water, and oil heaters; unit heaters; and other equipment in which the pressure differential between the steam supply and condensation return is 5 psig or more.

The operation of a flash trap depends upon the property of con­densation at a high pressure and temperature to flash into steam at a lower pressure. The condensation flows freely through the trap due to the pressure difference between the inlet and outlet orifices. The free flow of the condensation is interrupted by the introduction of steam into the inlet chamber, where it mixes with the remaining condensation. The steam heats the condensation and causes it to flash, thereby temporarily halting its flow through the orifice and allowing it to accumulate in the trap.

Except for an adjustable orifice used for adjusting the pressure differential, a flash trap contains no other moving parts. Flash traps operate intermittently. They are generally available for pressures ranging from vacuum to 450 psig.

Impulse Traps

An impulse trap (see Figure 10-35) operates with a moving valve actu­ated by a control cylinder. When the trap is handling condensation,

EQUIPMENT DRAIN POINT INLET PIPE

PIPE

SHUT-OFF VALVE UNION

/ N

BUCKET

TRAP

/outlet pipe

STATIC

HEAD

I I i

TT|NG ^

Y STRAINER I’

DIRT GATE VALVE

PIPE ELBOW

/-

TEE FITTING

SHUT-OFF

VALVE

X

POCKET

GATE VALVE

FOR BLOW DOWN TO DRAIN (A) Trap draining to open drain.

WN

М-У

^ EQUIPMENT DRAIN POINT
INLET PIPE

PIPE

SHUT-OFF VALVE UNION

T /

" R-

TEE FITTING BUCKET

STATIC

HEAD

TRAP

SHUT-OFF VALVE TO

JL

J TO RETURN LINE

PIPE ELBOW

EQUIPMENT DRAIN POINT

STATIC

HEAD

TEE FITTING BUCKET TRAP

/-

TEE FITTING

DIRT

POCKET

CHECKSHUT-OFF VALVE VALVE

RETURN LINE STATIC HEAD AGAINST DISCHARGE ’/2 PER/FT, (0.035 BAR PER.3 M) I

PRESSURIZED RETURN LINE

GATE VALVE

GATE VALVE FOR BLOW DOWN

FOR TEST &

PRESSURE TO DRAIN—^ RELIEF

(C)Trap draining to overhead return line

Or pressurized return line.

Y STRAINER

DIRT

POCKET

Goart bvawvdown, goart t -©

FOR BLOW DOW^ PRESSURE GRAVITY RETURN TO

N—TO DRAIN RELIEF VENTED RECEIVER

(B) Trap draining to gravity return line.

Steam and Hydronic Line Controls
Steam and Hydronic Line Controls

TEE FITTING

 

Steam and Hydronic Line Controls
Steam and Hydronic Line Controls

Steam and Hydronic Line Controls Steam and Hydronic Line Controls Steam and Hydronic Line Controls

ADJUSTABLE

Steam and Hydronic Line Controls

GAUGE GLASS ■

AND BLOW DOWN RE-EVAPORATING VALVE OPENING CHAMBER

подпись: gauge glass ■
and blow down re-evaporating valve opening chamber

DRAIN

PLUG

подпись: drain
plug

OUTLET

подпись: outletORIFICE

The pressure required to lift the valve is greater than the reduced pressure in the control cylinder, and consequently the valve opens, allowing a free discharge of condensation. As the remaining con­densation approaches steam temperature, flashing results, flow through the valve orifice is choked, and the pressure builds up in the control chamber, closing the valve.

Tilting Traps

The operation of a tilting trap (see Figure 10-36) is intermittent in nature. With this type of trap, condensation enters a bowl and rises until its weight overbalances that of a counterweight, and the bowl sinks to the bottom. As the bowl sinks, a valve is opened, thus admitting live steam pressure on the surface of the water, and the trap then discharges. After the water is discharged, the counter­weight sinks and raises the bowl, which in turn closes the valve, and the cycle begins again.

Steam and Hydronic Line Controls

^OU

4» INLET

Figure 10-35 Impulse trap.

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Lifting Traps

A lifting trap (see Figure 10-37) is an adaptation of the upright bucket trap and is available for pressures ranging from vacuum to 150 psig.

Condensation in the chamber of the trap accumulates until it reaches a level high enough to cause the steam valve in the high — pressure inlet to open. Steam then enters the auxiliary high-pressure inlet on the top of the trap at a pressure higher than the trap inlet pressure. This high-pressure steam forces the condensation to a point above the trap and against a back pressure higher than that which is possible with normal steam pressure. As the condensation is pushed out of the trap, the float or bucket descends to the bottom and causes the valve in the high-pressure inlet to close, shutting off the steam supply. Condensation then begins to refill the float chamber, and the cycle is repeated.

Boiler Return Traps

A boiler return trap (or alternating receiver) is a device used in some vapor-steam heating systems to return condensation to the boiler under varying pressure conditions of operation up to the working limit of the boiler. A vapor-steam heating system in which a boiler return trap is used is sometimes referred to as a return-trap system. A typical installation in which a boiler return trap is used is shown in Figure 10-38.

Steam and Hydronic Line Controls

BLOW OFF

подпись: blow off

Figure 10-37 Lifting trap.

подпись: figure 10-37 lifting trap.

INLET

подпись: inletAs shown in Figure 10-39, a boiler return trap consists of a chamber containing a float, which is linked to two valves. These valves control the openings to two connections on the top of the trap. One of these connections (the steam inlet) is connected to the steam header and direct boiler pressure. The other connection is vented to the atmosphere.

Condensation returning from the radiators is unable to enter the boiler by ordinary gravity flow, because the higher pressure of the boiler keeps the check valve closed. As a result, the condensation is forced to back up in the vertical pipe connected to the boiler return trap. As the condensation rises, it fills the bottom of the trap and lifts the float. At a certain level, the float causes the air valve to close and the steam valve to open, allowing steam at boiler pressure to enter the top of the trap. This steam at boiler pressure plus the gravity head (a boiler return trap must be located at least 6 inches above the water level in the boiler) is sufficient to force the conden­sation back down the pipe, through the check valve, and into the boiler.

539

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Figure 10-38 Boiler piping of a return-trap steam heating system. (Courtesy Dunham-Bush, Inc.)

 

подпись: 539

Steam and Hydronic Line Controls

Steam and Hydronic Line Controls

Figure 10-39 Boiler return trap. (Courtesy Spirax Sarco Co.)

Expansion Tanks

Expansion tanks (also sometimes called compression tanks) are installed in hydronic (hot-water) space heating systems to limit increases in pressure to the allowable working pressure of the equipment and to maintain minimum operating pressures.

When the temperatures rise during the operation of the system, the water volume also increases and builds up pressure. The pres­sure in the system is relieved to a certain extent by the storage of the excess water volume in the expansion tank. When temperatures drop, there is a corresponding drop in water volume and the water returns to the system.

Maximum pressure at the boiler is maintained by an ASME pressure-relief valve. Minimum pressure in the system is generally maintained by either an automatic or manual water-fill valve.

Closed steel expansion tanks and diaphragm tanks are used to contain the expanding volume of heated water in residential and light-commercial hydronic heating systems. Some typical installa­tions using ITT Bell & Gossett expansion tanks are illustrated in Figures 10-40 through 10-42.

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Figure 10-40 Typical installation with an inline Airtrol air separator.

(Courtesy ITT Bell & Gossett)

Closed Steel Expansion Tanks

The closed steel expansion tank has no moving parts (see Figure 10-43). It is normally two-thirds filled with water and one-third with air. As heated water expands and its excess volume enters the tank, it compresses the air at the top of the tank. The compression of the air in the tank results in an increase of system pressure, which is indicated on the boiler pressure gauge.

EXPANSION TANK

Steam and Hydronic Line Controls

TO SYSTEM

USE THE SHORTEST NIPPLE POSSIBLE

3/4" OR LARGER

PRESSURE REDUCING VALVE

COLD WATER

SAFETY SUPPLY

RELIEF VALVE

PUSH TUBE ALL THE WAY DOWN

%

FLO-CONTROL

Ss

F EXPANSION 1 TANK

CO

G

VALVE

Vn

AIRTROL I,

TANK FITTING

PITCH UP TO TANK

Steam and Hydronic Line Controls Steam and Hydronic Line Controls

RELIEF VALVE DRAIN PIPE

 

AIRTROL J BOILER FITTING ‘/

 

Figure 10-42 Typical installations on top outlet boilers.

(Courtesy ITT Bell & Gossett)

Note

If the expansion tank is properly sized, the pressure increase should be not more than about a pound before the system high-limit temperature is reached.

When the system water cools down, its volume contracts, and the air in the tank expands back to its original volume, causing sys­tem pressure to fall. To sum it all up, the rise and fall of system pressure is created by the expansion and contraction of the air in the expansion tank.

Steam and Hydronic Line Controls

One problem encountered with a closed steel expansion tank directly connected into the system is that the system water can absorb the air and send it to the radiators and convectors by gravity circula­tion. Installing a gravity-flow check valve on the expansion tank will prevent gravity circulation.

Diaphragm Expansion Tanks

The air in a diaphragm-type expansion tank is separated from the water by a flexible rubber membrane (see Figure 10-44). These tanks are smaller than the closed steel tanks and come from the manufacturer precharged with compressed air. When the tank arrives at the site, the diaphragm is fully expanded against its inside surfaces. When the tank is installed and connected to the system piping, water enters the other side of the tank chamber and presses down on the diaphragm.

As a rule, diaphragm tank manufacturers will precharge their tanks to 12 psi, which is sufficient to match the water-fill pres­sure requirements of the typical house or small commercial building.

Sizing Expansion Tanks

Many problems are caused by using an expansion tank of inade­quate size. Table 10-3 lists recommended sizes for expansion tanks in both open and closed tank systems.

BOOSTER

Steam and Hydronic Line Controls

Nominal Capacity—Gallons

Open System

Square Feet of Radiation

10

300

15

500

20

700

26

950

Closed System

Nominal Capacity—Gallons

Square Feet of Radiation

18

350

21

450

24

650

30

900

35

900

35

1100

Troubleshooting Expansion Tanks

An undersized expansion tank or one that is completely filled up with water will cause the boiler pressure to increase when the water heats. Because the expansion tank is too small or too filled with water to absorb the excess pressure, the relief valve will begin to drip. The dripping relief valve is only symptomatic of the real prob­lem, and replacing the valve will in no way solve it.

There is not much you can do about an undersized expansion tank except replace it. As a rule-of-thumb, expansion tanks should be sized at 1 gal. for every 23 ft2 of radiation, or 1 gal. for every 3500 Btu of radiation installed on the job. In Table 10-3, the allowance is slightly higher.

If the problem is a completely filled tank, it should be partially drained so that there is enough space to permit future expansion under pressure. The first step in draining an expansion tank is to open the drain valve. The water will gush out at first in a heavy flow and then tend to gurgle out because a vacuum is building up inside the tank. Inserting a tube into the drain valve opening will admit air and break the vacuum, and the water will return to its normal rate of flow. After a sufficient amount of water has been removed, the drain valve can be closed.

Air Eliminators

Sometimes air pockets will form in the pipelines of steam or hot — water heating and cooling systems and retard circulation. One method of eliminating these air pockets is to install one or more air eliminators at suitable locations in the pipeline.

An air eliminator (or air vent) is a device designed to permit automatic venting of air. These automatic venting devices are avail­able in a number of sizes, shapes, and designs. Not only are air eliminators used for venting convectors, baseboard radiators, and other heat-emitting devices; they are also frequently used for this purpose on overhead mains and circulating lines.

Three types of air eliminators (air vents) used in steam or hot — water heating and cooling systems are:

• Float-type air vents Thermostatic air vents

• Combination float and thermostatic air vents

A float-type air vent (see Figure 10-45) consists of a chamber (body) containing a float attached to a discharge valve by a lever assembly. The float-controlled discharge valve vents air through the large orifice at the top. The float action prevents the escape of any fluid, because the float closes the valve tightly when it rises. When the float drops, the lever assembly pulls the valve from its seat, and the unit discharges air.

Steam and Hydronic Line Controls

Float-type air vents are available for hot-water heating and cool­ing systems to 300 psi and low-pressure steam heating systems to 15 psi. A float-type air vent used in a steam heating system should be equipped with a check valve, which prevents air return under vacuum.

Steam cannot be maintained at its saturated temperature when air is present in the system. As shown in Table 10-4, the tempera­ture of the steam decreases as the percentage of air increases. A thermostatic air vent is specifically designed for removing air from a steam system. The one shown in Figure 10-46 consists of a valve head attached to a bellows, operating in conjunction with a ther­mostatic element. Its operating principle resembles that of a ther­mostatic steam trap. When air is present, the temperature of the steam drops. The temperature drop is sensed by the thermostatic element, which causes the valve in the vent to open and discharge the air. When the air has been discharged, the temperature of the steam rises, and the valve closes tightly.

Table 10-4 Effects of Air on Temperature of a Steam and Air Mixture

Mixture Pressure (Psig)

Pure Steam

5% Air

10% Air

15% Air

2

219°

21

6

O

21

3

O

210°

5

227°

225°

222°

21

4C

O

10

239°

237°

233°

230°

20

259°

256°

252°

249°

(Courtesy Spirax Sarco Co.)

In some special applications, it is necessary to use an air elimina­tor that will close when the vent body contains steam or water and opens when it contains air or gases. Combination float and thermo­static air vents have been designed for this purpose.

A combination float and thermostatic air vent (see Figure 10-47) consists of a vent body or chamber containing a float attached to a valve assembly. The float rests on a thermostatic element that responds to the temperature of the steam. The operation of this ele­ment is similar to the one used in a thermostatic steam trap. When the vent body is filled with air or gas, the float is at its lowest point, causing the thermostatic bellows to contract. Because the float is at a low point in the vent body, the head is moved off the valve seat and the vent discharges the air or gas. The head moves up and
closes the valve when either water or steam enters the vent body. The entry of water into the vent body forces the float upward and eventually closes off the valve. The entry of steam, on the other hand, causes the thermostatic bellows to expand and force the float upward, closing the valve.

Pipeline Valves and Controls

FLOAT

подпись: float

THERMOSTATIC

ELEMENT

подпись: thermostatic
element

SEAT

подпись: seat Steam and Hydronic Line Controls

CHECK VALVE HEAD —

подпись: check valve head -

Figure 10-47 Combination float and thermostatic air vent.

подпись: figure 10-47 combination float and thermostatic air vent.Pipeline valves and controls are used to regulate the temperature, pressure, or flow rate of the steam or water in the lines. Some valves (e. g., check valves) deal with only one of these functions; other valves are designed to handle more than one function.

Details about the design and construction of valves and the methods used for servicing, repairing, and installing them are found in Chapter 9 (V alves and Valve Installation)’. This chapter is lim­ited to a description of seven of the more common valves and con­trol devices used in steam and hydronic pipelines. They are:

Temperature regulators

• Electric control valves Water-tempering valves

• Hot-water heating control Flow control valves

• Electric zone valve

Balancing valves, valve adaptors, and filters

Temperature Regulators

Temperature regulators are used for many heating and cooling applications, including small-flow instantaneous heaters or coolers (shell-and-tube or shell-and-coil heat exchangers), small storage or tank heaters, and similar installations.

The Spirax Sarco Type 25T temperature regulator shown in Figure 10-48 is a diaphragm-operated valve used for regulating temperature in a variety of different process applications.

Before start-up, the main valve is normally in a closed position and the pilot valve is held open by spring force. The steam enters the orifice inlet, passes through the pilot valve and into the diaphragm chamber, and then out the control orifice. Control pres­sure builds up in the diaphragm chamber when the flow through the pilot valve exceeds the flow through the control orifice. This build-up in pressure opens the main valve.

The bulb of the temperature regulator is immersed in the medium being heated. At a predetermined temperature setting, the liquid in the bulb expands through the capillary tubing into the bel­lows and throttles the pilot valve. The main valve will deliver the required steam flow as long as the control pressure is maintained in the diaphragm chamber. The main valve closes when heat is no longer required.

Electric Control Valves (Regulators)

An electric control valve (regulator) is designed to provide remote electric on-off control in heating systems and steam process appli­cations (see Figure 10-49).

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Figure 10-49 Diagram of an electric on-off regulator.

(Courtesy Spirax Sarco Co.)

ELECTRICAL SIGNAL

DIAPHRAGM

подпись: electrical signal
 
diaphragm

The solenoid pilot at the top of the valve is connected to a room thermostat, automatic time clock, or some similar device from which it can receive an electrical signal. When the solenoid pilot is electrically energized, the pilot valve opens, and pressure builds up in the diaphragm chamber. As a result, control pressure is applied to the bottom of the main valve diaphragm, and the main valve is opened. The pilot valve closes when the solenoid pilot is de-energized, and control pressure is relieved through the bleed orifice. Steam pressure acting in conjunction with the force of the main valve-return spring combine to close the main valve.

Water-Tempering Valves

Water-tempering valves are used in hot-water space heating systems where it is necessary to supply a domestic hot-water supply at tem­peratures considerably lower than those of the water in the supply mains. The water-tempering valve automatically mixes hot and cold water to a desired temperature, thus preventing scalding at the fixtures. These valves are designed for use with hot-water space heating boilers equipped with tankless heaters, boiler coils, or high — temperature water heaters.

The A. W. Cash Type TMA-2 valve illustrated in Figure 10-50 is a thermostatic water-tempering valve that is shipped from the fac­tory preset to operate at 140F. These valves can also be field — adjusted to change the temperature of the mixed water leaving the valve by loosening the adjustment nut and turning the adjustment screw either clockwise (for colder water) or counterclockwise (for hotter water).

If turning the adjustment screw does not produce the desired mixed-water temperature, carefully touch the hot-water inlet on the valve to make sure hot water is being delivered. If you are certain hot water is getting into the valve and a temperature adjustment still fails to produce the desired results, the problem most likely lies inside the valve. Problems with these valves can usually be caused by one of the following:

• Binding of bonnet to push-rod

• Sticking of push-rod to O-ring Binding of piston

Sticking of body O-ring

Before attempting to service or repair these valves, close off the hot, cold, and mixed water connections. Water must not be allowed to enter the valve when the bonnet has been removed.

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Figure 10-50 Diagram of a model TMA-2 water-tempering valve.

(CourtesyA. W. Cash Valve Mfg. Corp.)

Access to the internal parts of the hot-water tempering valve illustrated in Figure 10-50 is gained by unscrewing the bonnet (i. e., turning it counterclockwise) and removing it. If the bonnet is bind­ing to the push-rod, pull the push-rod out of the bonnet and wipe it off with a crocus cloth. Do the same with the inside of the bonnet. If the push-rod O-ring is sticking, it should be removed and replaced. Reassembly is in reverse order; place the pushrod O-ring, reinsert the push-rod, and then screw the bonnet back on.

A binding piston should be removed, cleaned (with a crocus cloth), and lubricated. Access to the piston is also gained by unscrewing the bonnet. A body O-ring that is sticking should be removed and replaced. Access to the body O-ring is gained by unscrewing and removing the bonnet (leaving the push-rod in the bonnet). Push the piston and piston spring up and out through the top of the tempering valve. Lift out the post thermostat assembly. When reassembling, be sure to lubricate both the body O-ring and piston.

The design of the Watts No. N170 Series water-tempering valve differs from the one described above in that the discharge or mixed — water orifice is located in the bonnet (see Figures 10-51 and 10-52). The water temperature can be changed by loosening the locknut and turning the adjustment screw. Each full turn of the adjustment screw is equal to approximately a 10F change in temperature.

A typical installation in which a Watts No. N170 valve is used is shown in Figure 10-53. Tempered water at 140F can be delivered to the system. The thermostat in the valve makes trapping unnecessary except in extreme cases.

A two-temperature recirculating hot-water supply system is shown in Figure 10-54. In this system, a water-tempering valve and recirculating line are used to maintain approximate fixture water temperatures of 140F in the mains at all times. A relatively small

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ADJUSTING SCREW

COLD

HOT

HIGH-TEMPERATURE RESISTING RUBBER DISC

Figure 10-52 Components of a water-tempering valve.

(Courtesy Watts Regulator Co.)

BRONZE BODY

Capacity recirculator is used in the recirculating return piping, because very little hot water is required to maintain the low tem­perature in the mains. Long runs of recirculating piping should be insulated to reduce the heat loss from the piping.

Tempering valves cannot compensate for rapid pressure fluctua­tions in the system. Where such water pressure fluctuations are expected to occur, a pressure equalizing valve should be installed.

The Watts 70A Series tempering valve, shown in Figure 10-55, is designed for small domestic water-supply systems and tankless

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Figure 10-53 Basic hot-water supply system using model N170

Tempering valve. (Courtesy Watts Regulator Co.)

Heater installations. Piping connections for these applications are shown in Figure 10-56. A balancing valve should be installed below the tempering valve in the cold-water line to compensate for the pressure drop through the heater.

This valve is available with both threaded and sweat connections. It is also available in both high — (120160F) and low — (100130F) temperature models. Temperature changes are made by turning the dial-type adjustment cap on the valve (see Figure 10-57).

The Spirax Sarco Type MB water blender (see Figure 10-58) has a 55F adjustment range for supplying tempered water to a system. It is a three-way double-ported balancing valve, essentially resem­bling the Watts and A. W. Cash valves in construction, except for an extended bonnet containing spirals. This type of construction allows a certain degree of pressure fluctuation between the hot — and cold-water inlets without disturbing the control of the tempered water.

Hot-Water Heating Control

A hot-water heating control consists of an outdoor liquid expansion­type bulb connected by a capillary system to a double-ported three-way automatic mixing valve (see Figure 10-59). It is designed to blend the hot water from the boiler with the cooler return water

THERMOMETER

подпись: thermometer

THERMOMETER

подпись: thermometerIt I!

555

WATTS AGA/ASME TEMPERATURE & PRESSURE RELIEF VALVE

 

*

ДQUASTAT

 

RECIRCULATOR

 

HAND VALVE

 

RECIRCULATING RETURN LINE

 

подпись: 555

Figure 10-54 Two-temperature hot-water supply system. (Courtesy Watts Regulator Co.)

 

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Figure 10-55 Model N70A series water-tempering valve.

(Courtesy Watts Regulator Co.)

In inverse proportion to the outside water and deliver the blended water to the circulating system.

In operation, the outdoor bulb reacts to changes in temperature and creates pressure. This pressure is transferred through a capil­lary system to the indoor bulb and then to the mixing valve, which is positioned to increase or decrease the amount of hot water from the boiler. Temperature-range adjustments can be made by turning an adjustment on top of the valve. A typical installation in which a hot-water heating control is used is shown in Figure 10-60.

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VACUUM RELIEF VALVE

COLD

8" TO 12"

RELIEF VALVE

DRAIN

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Figure 10-57 Dial-type adjustment cap. (Courtesy Watts Regulator Co.)

Flow Control Valve

A flow control valve is used (1) to prevent gravity circulation in a hydronic heating system when the circulator is not operating and (2) to permit the summerwinter operation of an indirect water heater. As shown in Figure 10-61, a flow control valve can be installed in either a vertical or horizontal position. Either location will require that it be installed with the arrow on the valve body facing the direction of flow.

When the circulator is operating, water passes through the flow control valve. When the circulator stops operating, the flow control valve remains closed and blocks the gravity flow of the water. A knob on top of the valve allows it to be manually opened to drain the system or to bypass it if there is a loss of electricity and only par­tial heating is possible. The knob is turned clockwise for the normal position and counterclockwise for the manual-bypass position.

Warning

Do not allow the valve to remain in the manual position when normal system operation resumes. Doing so will result in uncon­trolled heat.

Flow control valves do not require service or maintenance. A defective flow control valve should be replaced.

COLD WATER

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Figure 10-59 Hot-water heating control. (Courtesy Spirax Sarco Co.)

Electric Zone Valve

Some hydronic heating systems use electrically operated valves to control the flow of water into each zone. In these zoned hydronic systems, the valves operate in conjunction with a single circulator. The construction details of a typical electric zone valve are illus­trated in Figure 10-62.

When the room thermostat calls for heat, an electrical current is sent to the valve operator. The current flows through a normally closed switch and around a coil called a heat motor. The heat cre­ated in the heat motor causes a piston to move out and push against a spring-loaded lever that normally holds the valve closed. This action opens the zone valve. At the same time, the piston extends a bit further and trips an end switch that sends a signal through a relay back to the circulator, turning it on and sending water into the zone. The piston extends a bit further and reverses the sequence. This in-and-out movement of the valve piston will continue for as long as the room thermostat calls for heat.

560

подпись: 560

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Figure 10-60 Hot-water heating control used to control heating-system water in accordance with outside

Temperature. (Courtesy Spirax Sarco Co.)

 

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(A) Installed in vertical pipe

Figure 10-61

(B) Installed in horizontal pipe.

Flow control valve. (Courtesy ITT Bell & Gossett)

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T

PLUG

HERE

 

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Balancing Valves, Valve Adapters, and Filters

Hot-water heating and cooling systems often require additional balancing not foreseen in the preliminary planning. An effective method of balancing a heating or cooling system is to install bal­ancing valves, valve adapter units, or balancing fittings at suitable locations in the pipelines in order to regulate water flow through the radiators, convectors, baseboard panels, radiant coils, return mains, and branches.

A balancing valve is a control device that functions as a combi­nation balancing, indicating, and shutoff valve. This valve is used

REPLACEABLE HEAT HOTOR

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To balance a hot-water heating or cooling system and at the same time indicate the percent of flow through the valve. An example of one of these balancing valves is shown in Figure 10-63.

These valves are available in many body patterns and connection types, the selection depending on the requirements of the installa­tion. Among the body patterns available are angle, angle union, globe, and globe union. The different connections include screwed, sweat, male, or female unions.

The balancing valve shown in Figure 10-63 is fitted with a balanc­ing yoke that fits over the bonnet. The stem has a stop washer that shoulders on the balancing yoke. The yoke is rotated until the indica­tor on the calibrated dial points to the percent of flow required at a particular setting. At this point, it is locked in place with an Allen setscrew. The valve can then be opened until the stop washer contacts the top of the yoke, thereby permitting correct percentage of flow. For service work, the valve can be shut and, when opened, will never open beyond its set point. In addition, the valve is equipped with an O-ring that seals off against the bonnet to form a positive back seat, and the valve can be repacked under full line pressure.

Balanced fittings, such as the ones shown in Figure 10-64, are used for balancing branch or circuit resistance of radiators, convectors,

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Figure 10-63 Balancing valves. (Courtesy Spirax Sarco Co.)

Steam and Hydronic Line ControlsHeating or cooling coils, unit heaters, and other heat-transfer equipment employing hot or chilled water. Some are available with an integral manual air vent. They can also be obtained with a thread connection or with a sweat connection for nominal copper Figure 10-64 Balancing tube. When a balancing fitting is valve fittings. used, a stop valve must be installed (Courtesy Spirax Sarco Co.) with it to allow shutdown for neces­sary service.

Valve adapters are devices used to convert copper, bronze, cast brass, or cast-iron tees to balancing valves. These adapter devices can be threaded into cast-iron tees, or soldered or sweat-fitted into copper, bronze, or brass tees. They can also be inserted in a side outlet or run of tee of the same size to complete either a straightway or angle balancing valve. Because there is no inside reduction of

Pipe diameter, there is no water restriction except for the balancing required. Balancing is accomplished by using a screwdriver on the adjustment screw at the top of the adapter device.

Manifolds

A manifold (sometimes called a zone manifold) is a device used to connect multiple tubing lines to a single supply or return line in a hydronic radiant floor heating system (see Figure 10-65). Each heating system has at least two manifolds: a supply manifold and a return manifold. A supply manifold receives water from the boiler through a single supply pipe and then distributes it through a num­ber of different tubing lines to the rooms and spaces in the struc­ture. A return manifold provides the opposite function. It receives the return water from each of the rooms and spaces through as many tubing lines and sends it back to the boiler by a single return

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Pipe. A supply manifold and a return manifold are sometimes referred to jointly as a manifold station. Manifolds are described in greater detail in Chapter 1 (Radiant Heating)’ in V olume 2.

Pipeline Strainers

Rust, dirt, metal chips, scale, and other impurities are commonly found in both new and old pipelines. Unless these impurities are captured and removed from the pipes, they can damage valves, traps, and other equipment. Protection against these potentially damaging impurities is provided by installing a strainer or scrape strainer ahead of each mechanical device in the pipeline. Strainers are constructed with a screen socket placed at an angle to the nor­mal direction of flow. The impurities are captured by the screen.

Two typical examples of pipeline strainers are shown in Figures 10-66 and 10-67. These devices are constructed from bronze, semisteel, and steel, and are available in a variety of sizes. The screen socket is tapered and positioned to collect the impurities sus­pended in the steam or hot water. Both standard and specially designed screens are available from manufacturers.

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Posted in Audel HVAC Fundamentals Volume 2 Heating System Components, Gas and Oil Burners, and Automatic Controls


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