Safety Devices, Controls and Protective Equipment
Safety devices, controls and protective equipment - this domain tests the layered defence around a refrigeration plant: pressure relief, automatic cutouts, machinery-room safeguards, and the order in which each device should act.
Relief protection
Every pressure vessel and every isolatable section of high-side piping needs relief protection sized to CSA B52. The device is set at or below the vessel's maximum allowable working pressure, has no stop valve that could isolate it, and discharges to a safe place - outdoors, above the roofline, for ammonia. Where a valve is used to change reliefs in service it must be a three-way type with each relief sized for full capacity.
Protective controls
- High-pressure cutout, manual reset, set below the relief setting.
- Low-pressure cutout to stop the machine below its design suction pressure.
- Oil pressure failure switch, sensing oil-pump pressure against crankcase pressure with a time delay (commonly 30–120 s).
- High discharge temperature, motor overload and overcurrent protection.
- Liquid level, high-level and pump-down controls; chiller flow and freeze protection.
- Ammonia detection, machinery-room ventilation, emergency stop and eyewash/shower.
Worked example
An ammonia high side has a design pressure and relief setting of 1,724 kPa (250 psig). Compressor capacity starts unloading at 1,450 kPa, the high-pressure cutout trips at 1,550 kPa on manual reset, and the relief valve is set at 1,724 kPa. The order is deliberate: unloading handles normal swings, the cutout shuts the machine down before the relief lifts, and the relief is the last line of defence. Raising the cutout to stop nuisance trips turns the relief valve into the operating limit.
Traps
Never field-adjust a relief valve or plug a leaking one. Manual-reset cutouts must not be reset until the cause is found. A low-pressure cutout is protection, not temperature control. Valving off a transmitter to "test" a switch may prove nothing about the switch itself. Relief discharge piped indoors, or reduced in size, is a direct code violation.