Refrigeration systems contain pressurized fluid and there are safety standards that must be adhered to and there are certain legal requirements. Under the European Pressure Equipment Directive (PED) and the UK Pressure Equipment Regulations the main duties are placed on the user/owner of the system. They are a clear and practical means of legislating for safe practices in refrigeration. Responsible contractors and users will have been using such safe procedures well before the introduction of these regulations. In addition to the regulations themselves the HSE has published ‘Safety of pressure systems — Approved code of Practice’ which is a clear and helpful. The regulations apply to vapour compression refrigeration systems incorporating compressor drive motors, including standby compressor motors, having a total installed power exceeding 25 kW.
Factory-built equipment will be constructed to the relevant standards and will be pressure-tested for safety and leaks prior to shipment. In cases of doubt, a test certificate should be requested for all such items. Under the PED, vessels, including compressors, are categorized, depending on the refrigerant and volume. Those falling into certain categories will be CE marked and for smaller ones, not categorized, a statement of sound engineering practice can be obtained from the manufacturer.
It is necessary to hold a Safe Handling of Refrigerants Certificate to work with refrigerants. This can be obtained through short training courses. Maintenance engineers must keep themselves updated on safety procedures and training requirements.
Site-erected pipework, once complete, must be pressure tested for safety and leak tightness. The Institute of Refrigeration’s Code of Practice specifies that systems are pressure tested at 1.3 times the maximum working pressure. As an example, for R404A this is:
• 24.8 X 1.3 = 32.2 bar gauge (467 psig) on the high side
• 14.1 X 1.3 = 18.3 bar gauge (265 psig) on the low side
This assumes a maximum condensing temperature of 55°C and a maximum ambient temperature of 32°C, as specified in safety standard EN378.
Factory-built components and pressure vessels which have already undergone test should not be retested unless they form part of the circuit which cannot be isolated, when the test pressure must not exceed the original figure. Site hydraulic testing is considered unnecessary, owing to the extreme difficulty of removing the test fluid afterwards. However, it must always be appreciated that site testing with gases is a potentially dangerous process, and must be governed by considerations of safety. In particular, personnel should be evacuated from the area and test personnel themselves be protected from the blast which would occur if a pressure vessel exploded.
Systems should be pressure tested with dry (oxygen-free) nitrogen (OFN) or high-purity nitrogen. Nitrogen is used from standard cylinders, supplied at about 200 bar, and a proper reducing valve must always be employed to get the test pressure required. A separate gauge is used to check the test pressure, since that on the reducing valve will be affected by the gas flow.
If the high side is being tested, the low side should be vented to the atmosphere, in case there is any leakage between them that could bring excessive pressure onto the low side. It may be necessary to remove relief valves. Other valves within the circuit will have to be open or closed as necessary to obtain the test pressure. Servo-operated valves will not open on a ‘dead’ circuit, and must be opened mechanically.
The test pressure should be maintained for at least 15 min. If the pressure has not significantly reduced in that period the nitrogen is slowly vented until the pressure in the system has reduced to the pressure test (leak test) pressure.
Reference should be made to the codes of practice and guidance notes published by the Institute of Refrigeration (See Bibliography).
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