Treat a refrigeration circuit as a pressure vessel with a compressor attached: every pressure rating, test pressure and relief setting must come from a recognised code or from an existing certified component, never from a rule of thumb, and the relief device must open below the vessel’s rated maximum and be impossible for the operator to adjust, isolate or block. A fluid’s vapour pressure climbs steeply with temperature, so the hottest day or a nearby fire, not the normal running pressure, sets the demand on the vessel.
Part of Refrigeration
How a charged refrigeration circuit can burst, the order in which its pressures must be kept, and the requirements on its relief device and its testing.
The Circuit Holds Energy, And The Weakest Part Sets Its Rating
The circuit is a closed shell full of vapour and liquid. If a part gives way, the fluid expands and the part flies. Pressure Vessel Theory explains why the energy stored is the danger, not the pressure itself, and why hot or liquefied contents store the most. Hoop and Longitudinal Stress gives the wall stress as P × r ÷ t for a thin cylinder. Two things follow from the formula without needing any figure: stress rises in direct proportion to pressure, and a thinner wall or a larger radius raises it. Whatever that formula gives is a floor for understanding a vessel, never a thickness to build, as that article stresses.
The rating of the whole circuit is the rating of its weakest part. Gauges, valves, hoses, glass level gauges, gaskets, the compressor casing, thin-walled coils and threaded joints all have their own limits, and the low side of a machine is often not rated for what the high side sees.
Where The Pressure Comes From
- Heat. Vapour pressure rises steeply with temperature. Ammonia gives its order of magnitude. At standstill the whole circuit settles at the vapour pressure of its warmest liquid-bearing part, so a hot day with the machine off loads the low side too (recalled behaviour, flagged).
- A blocked outlet with the heat still on. A closed condenser line or a clogged restriction on a fired generator keeps the pressure rising.
- Liquid trapped between closed valves. Warming expands it against rigid walls; see System Construction.
- Fire. A vessel in a fire gets far more heat than the machine ever put in.
- Weakening of the wall. Corrosion thins it, often under insulation or at the low point where moisture collects, and cycling fatigues welds. Failure Modes describes the same set for an autoclave.
The Pressure Ladder, In Order, With No Numbers
The corpus pressure-vessel articles give the ordering any properly specified vessel keeps. Every figure belongs to the vessel and its code, not to this page.
- Operating pressure: what the process runs at in normal service.
- Relief setting: above operating pressure so the device does not chatter, and at or below the vessel’s maximum allowable working pressure.
- Maximum allowable working pressure: the rating assigned by the designer from the wall, welds and material.
- Test pressure: what the vessel is proved to with water before use, as the governing code specifies.
- Burst pressure: where the wall actually fails, which nothing in normal practice approaches.
The corpus does not agree on how far above operating pressure a relief device should be set. Pressure Relief Valve sets out three incompatible rules from other corpus articles and resolves it by giving only the ordering and capping the setting at the vessel’s rated maximum. This page does the same and gives no setting. Do not confuse that article with Pressure Relief Valves, which concerns hydraulic circuits.
What You Need
- A rating for every part, from a nameplate, a code or a maker’s data. A part without a known rating is a part that cannot be used in the pressure circuit.
- A relief device on the high side, of a type and capacity matched to the heat source, with a fixed setting that the operator cannot change and no valve between it and the vessel; see Pressure Relief Valve for the requirements, written for steam but identical in principle.
- A discharge path to a place where the released fluid harms nobody. A relief that opens releases toxic vapour for an ammonia plant; it is the lesser harm compared with a burst, so place the outlet accordingly. Piping the discharge into water is a recalled practice for ammonia, because the vapour dissolves, and it is flagged for checking.
- A pressure gauge rated above the highest pressure the circuit can reach.
- A water test and a vessel log, as in Vessel Construction.
Method: Before First Heat
Step 1: List every pressure part and its rating. If any part is unknown, replace it with one that is known.
Step 2: Check the order. Operating pressure below the relief setting, relief setting at or below the lowest rating in the circuit.
Step 3: Prove the circuit with water, at the code’s test pressure for the weakest vessel, then leak-test with soap.
Step 4: Fit and check the relief device. It lifts freely, closes again, discharges where intended, and cannot be adjusted without breaking a seal.
Step 5: Decide what happens if the heat cannot be removed. The heat source must be one that can be cut at once.
Worked Example: The Relief Must Pass What The Fire Can Make
If the outlet is blocked and the fire keeps burning, the heat input boils fluid at a steady rate and the relief must pass it as vapour. The same reasoning appears in Pressure Relief Valve for steam. Here, assume a 2 kW heat input to an ammonia generator (an assumption for the arithmetic) and a latent heat of ammonia of about 1,370 kJ/kg (a recalled value, flagged).
- Vapour rate: 2 kJ/s ÷ 1,370 kJ/kg = 0.00146 kg/s.
- Per hour: 0.00146 × 3,600 = about 5.3 kg/h.
So a relief device on that generator has to pass at least about 5.3 kg of ammonia vapour an hour at its set pressure. That is a minimum under ideal flow. The device’s capacity at its set pressure is a rated figure that comes from its maker or a test, not from this arithmetic. A fire under the vessel makes the heat input larger than the intended 2 kW, so the capacity must cover the worst case the heat source can produce.
Common Mistakes
- A shut-off valve between the vessel and its relief device. An isolating valve lets someone close it off.
- Adding weight or tightening the spring to stop a simmering valve. Find out why it lifts.
- Treating the gauge as the safety device. It only shows the pressure.
- A cast-iron part in a pressure circuit. Cast iron shatters instead of bending, as the corpus boiler article says.
- Testing with air first. Stored air energy is far greater than water’s.
Safety
The relief device is the last thing between a blocked line and a burst vessel
Never tie down, weight, plug, throttle, isolate or reset a relief device. Never test it by raising the pressure of an untested vessel. If it lifts in service, cut the heat and find out why before the next run. This page deliberately gives no working pressure, test pressure, relief setting or wall thickness: they come from a code or from a certified component, and a circuit built to a guess is a bomb.
What’s Next
- Learn next: Safety — the toxic and flammable side of the same plant
- Combine with: Failure Modes — the shared ways a pressure shell fails