Vibration limits

For tests in a manufacturers works

The acceptable vibration limits for complete and assembled fans in accordance with ISO 14694 are given in Table 15.6. These are r. m.s. velocity values filtered to the fan rotational fre­quency and to be taken at the design duty.

The vibration levels give in Table 15.7 are guidelines for accept­able operation and are for filter-out measurements taken on the bearing housings. Newly commissioned fans should be at or below the start-up level increasing with time, as wear and tear take place, until it reaches the “alarm” level. Remedial action should then take place.

Vibration testing for product development and quality assessment

Just as measurement of displacements will give most weight to low frequencies, so acceleration measurements will weight the level towards the higher frequency components. Velocity mea­surements are intermediate and most fans have a reasonably flat velocity spectrum.

Fans produced for higher pressures and flowrates — greater speeds and stresses — may be required for more critical appli­cations. With direct drive units, especially at 2-pole speeds, high frequency vibrations will be generated by the bearings and also by the many electromagnetic forces.

Nevertheless, a quick method of vibration testing for production purposes is considered essential. It may be that for initial ac­ceptance/rejection, acceleration decibel readings in the usual frequency octave bands can be a quality tool. The method of mounting the accelerometer to the measuring point is of para­mount importance in obtaining accurate and repeatable results.

Bad mounting can drastically reduce the frequency range of the accelerometer. Whilst a threaded stud onto a flat machined sur­face is an ideal fixing, this is very seldom possible. An interme­diate holding block for adhesive fixing may therefore be used, this being stuck in position with Araldite® or Loctite®. The de­sign of such a block is shown in Figure 15.9. It will be seen that the tapped holes for the accelerometer are in three planes. Thus it is possible to obtain, readings in the horizontal, vertical and axial directions.

Accelerometer positions may be standardised as shown in Fig­ure 15.10. As the absolute readings may be very low, it is es­sential for the fan to be soft-mounted and an “A” frame assem-

Fan application category	R. m.s. velocity mm/s
Rigidly mounted	Flexibly mounted
BV-1	9.0	11.2
BV-2	3.5	5.6
BV-3	2.8	3.5
BV-4	1.8	2.8
BV-5	1.4	1.8

Table 15.6 Vibration limits for the manufacturer’s works tests

For tests on site

5 Grooves 8 x 90°

The in-situ vibration level of any fan is not the sole responsibility of the manufacturer. Apart from the design and balance quality, it also depends on installation factors such as the mass and stiffness of foundations for supporting structures.

Application Category	Rigidly mounted r. m.s velocity mm/s	Flexibly mounted r. m.s velocity mm/s
Start-up	Alarm	Shut-down	Start-up	Alarm	Shut-down
BV-1	10	10.6	*	11.2	14.0	*
BV-2	5,6	9.0		9.0	14.0	»
BV-3	4.5	7.1	9.0	6.3	11.8	12.5
BV-4	2.8	4.5	7.1	4.5	7.1	11.2
BV-5	1.8	4.0	5.6	2.8	5.6	7.1
* To be determined from historical data

Support bracket ®2 Fan case — Inlet flange

8 а 8 8 10 10 10 10 10

I i 6 i I —

U4=M

2- IN N» NYLON RQPЈS REQUIRED ^t’fcCiA «124" BETWEEN CRS OF THIMBUS FOR SUNDtNQi FANS

_ … No NYLON REQUiSEO 9′:& ИКTWКEN CRS ОHIM8ECS FOR FANS

Figure 15.11 Assembly for vibration testing of axial and centrifugal fans

DT

(g1 Duct flange — Motor end ®2 Duct flange — Motor end

Figure 15.10 Accelerometer position for sling testing

Bly with rubber cords and nylon slings in accordance with ISO 14695 should be used as shown in Figure 15.11 and Figure

15.12. Whilst not absolute in its accuracy, it would enable con­sistent readings to be taken and comparability to be established. The low natural frequency of the ropes ensures that the fan is completely isolated from any outside influences.

The first 20 fans of a given type should be tested and readings taken at the prescribed accelerometer positions. All these fans have to be assessed as satisfactory according to the normal subjective inspection then current. In this case the acceptance level AdB in each octave band may be set at 85% pass i. e. the acceptance level is set at the fourth highest reading obtained for all units in all directions. It must be appreciated that these levels will be unique to a particular design of fan at a particular speed. Those for some typical small machines are shown in Table 15.8.

Readings must be taken in all three directions and be within the acceptance level set down. No differentiation is made between horizontal, vertical or axial measurements. Such acceptance levels are constantly under review. Each fan should be logged and trends noted. The intention should be to gradually reduce the acceptance levels.

Recent improvements in balancing procedures (compound balancing of fan impeller and motor rotor/shaft to quality grade G 1) has indicated that levels in the appropriate band — 63 Hz for

Figure 15.12 Vibration test on 34" axial flow fan

2-pole speeds, < Hz for all others, can be significantly reduced. For large Category 2 and 3 fans about 1250 mm diameter, it is

Fan size mm and type	Speed R/min	Power KW	AdB re 10 5 m/s2 in each Octave band Hz
<45	63	125	250	500	1k	2k	4k	>5.6k
180 Axial	3500	0.3	79	86	90	102	102	117	108	107	104
800 Axiat	1180	22.5	82	91	98	109	113	111	110	108	98
315 Centrifugal	3500	1.5	86	97	102	105	109	111	109	111	112
900 Centrifugal	1180	10.0	91	95	100	98	98	104	103	96	91

Table 15.8 Typical vibration acceptance standards for small Category 3 or marine fans

Impractical to suspend the unit due to the weight and physical dimensions involved.

Furthermore, it is desirable to obtain as much information as possible, with a view to determining the source of all vibrations. Such fans therefore should be bolted down on a rigid founda­tion block. Complete discrete frequency analyses of displace­ment, velocity and acceleration should be taken at each fan bearing together with motor bearings where applicable. It may also be necessary to take readings at other particular points of interest e. g. shaft seals, fan feet etc.

To enable objective assessments to be finalized and for accep­tance standards to be set, a manufacturer will need to make routine tests. The combinations of fan size, speed, blade form, duty, specific width etc., lead to many permutations. Repeat­able tests will take a long time. Nevertheless the velocity stan­dards set in Table 15.5 can be followed and fans must meet these before despatch.

Posted in Fans Ventilation A Practical Guide