When working on refrigeration systems, it’s very important to understand the terminology involved around the pressure rating of equipment especially when it comes to component selection. We must make sure that all components have been selected with a Maximum Allowable Pressure (PS) equal to or higher than what has been calculated for the system.

There is always some confusion in terminology used in pressure rating of components e.g. valves, pipe, flanges, vessels. We see many different terminologies in common usage e.g.

  • Design Pressure
  • Maximum Allowable Working Pressure
  • Maximum Operating Pressure

Let’s take a look at the terminology in more detail:

1) Design Pressure (DP)

Design Pressure – the pressure used in the design of a component to determine its required wall thickness and physical characteristics.

The design pressure should be equal to or less than the Maximum Allowable Pressure (PS) but can be anywhere from 10% to 25% higher than the MOP.

2) Maximum Allowable Pressure (PS)

Also referred to as the Maximum Allowable Working Pressure (MAWP), it is the maximum pressure at which a system or its component is designed to operate safely. With refrigeration systems, we often refer to this as the Maximum Allowable Pressure (PS).

The ‘PS’ is shown on the vessel nameplate and may also be the same as the design pressure.

PS can be > or = the Design Pressure

3) Maximum Operating Pressure (MOP)

Maximum Operating Pressure (MOP) is the maximum pressure a component or system is normally operated and is usually 10-20% below the PS.

How to determine the Maximum Allowable Pressure (PS)

Firstly, determine which column to use based on the design ambient temperature for the region you are working in. For example, let’s say we are working on an air cooled freezer system located within the United Kingdom, so we would select the <32°C ambient column. If the system has an air-cooled condenser, then use a saturation temperature of 55°C from the table below. On the comparator (or gauges) find the refrigerant that is in the system and then find 55°C on that scale. Follow the line until it reaches the pressure scale. This would be the Maximum Allowable Pressure (PS) for the high side of the system. The low side would be tested separately at either 27°C.

Refrigerant Type: R407F

High Side of system: R407F @ 55°C = 26.16Bar (PS)

Low Side of system: R407F @ 27°C = 11.07Bar (PS)


Example for Maximum Allowable Pressure (PS):

Determine appropriate test pressures to BS EN378 standards.


After an installation or repair has been completed three specific pressure tests must be carried out, prior to commissioning the system i.e. 2 x positive pressure (Strength & Tightness) & 1 x negative vacuum rise tests. Before any testing takes place, establish the correct strength test pressure and remember that only Oxygen Free Nitrogen or other inert gas can be used.


How to conduct a strength test to BS EN378 standards


A strength test is considered a dangerous test (especially for R410a and other high pressure refrigerants) and all risks must be eliminated prior to starting work.

‘The pressure in the system shall be built up gradually and monitored by remote calibrated gauge or other indicator located in a safe place.’

‘During the strength test all personnel shall be evacuated from the area of risk and precautions shall be taken to minimize risk to property.

‘The full strength test pressure shall be held for at least the minimum time sufficient for the pressure to equalize throughout the system. The test shall be deemed satisfactory provided that no visible permanent distortion of any component or part of the system results from this test.’

The strength test is often carried out only on the area that has been repaired. If the system has isolation valves then these can be closed to prevent the high pressure from going into the low side of the system.

Note: The high and low sides should be isolated where possible as there is a risk that the high side pressure exceeds the maximum allowable pressure rating for the evaporator.

It is not recommended that service manifold gauges are used for strength test purposes, especially those that have a manifold sight glass. There have been cases, where the sight glass has blown out of the manifold and injured the service engineer. Most refrigerant wholesalers now supply purpose built nitrogen test rigs, designed specifically for this purpose.

Remember to isolate the section of pipe being tested.

Calculation of strength test = 1.1 ᵡ PS


HIGH SIDE (Air cooled condenser)

55⁰C on R407F = 26.16 Bar

26.16 Bar × 1.1 = 28.78 Bar on the high side

LOW SIDE (Heat exchanger exposed to indoor ambient)

27⁰C on R407F = 11.07 Bar

11.072 Bar × 1.1 = 12.18 Bar

High Side Strength Test Pressure = 28.78 Bar

Low Side Strength Test Pressure = 12.18 Bar

REMEMBER: The high side test pressure could exceed the rating of the pressure relief valves (PRV’s). If this is the case, then they should be removed and plugged for the strength test procedure.


After the strength test is completed to BSEN378:2016 standards, the tightness test can be applied. To calculate the Tightness Test Pressures, multiply the Maximum Allowable Pressure by a factor of Tightness Test Pressure = 1.0 x Maximum Allowable Pressure.

 Note: Under no circumstances will a trace gas of refrigerant be added to the nitrogen pressure as this will be classed as deliberate venting to atmosphere.

Once the correct tightness test pressure has been achieved, isolate the nitrogen cylinder and check for any pressure drop on the gauge which would provide an immediate indication of a leak. If no pressure drop occurs, go over all joints and leak points using a bubble solution. If there are no signs of any leaks, isolate all manifold valves and remove the nitrogen bottle from the system. Ensure there is a separate gauge fitted to monitor the system pressure.

To meet British Standards BSEN378:2016, this test pressure must be held for a minimum of 1hour. Larger systems may require a tightness test of approximately 24/48 hours.

The final step in pressure testing is to complete all the documentation and have it signed by the client as a witness.

Calculation of tightness test = 1.0 ᵡ PS


HIGH SIDE (Air cooled condenser)

55⁰C on R407F = 26.16 Bar

26.16 Bar × 1.0 = 26.16 Bar on the high side

LOW SIDE (Heat exchanger exposed to indoor ambient)

27⁰C on R407F = 11.07 Bar

11.072 Bar × 1.0 = 11.07 Bar

High Side Tightness Test Pressure = 26.16

Low Side Tightness Test Pressure = 11.07 Bar

Pressure Testing to BS EN 378 standards

  • Maximum Allowable Pressure (PS)           =          Refer to BSEN378:2016
  • Pressure Relief Valve setpoint                  =          1.0 × PS
  • Strength test pressure                               =          1.1 × PS
  • Tightness test pressure                             =          1.0 × PS
  • Tightness test with PRV fitted                   =          0.9 × PS
  • Pressure safety switch operates               =          0.9 × PS
  • Maximum operating pressure (MOP)        =          0.8 × PS

Pressure Relief Valves (PRV’s)

Pressure relief valves are selected and sized to prevent the system from being over-pressurized. The setpoint at which the valve operates will be equal to or less than the maximum allowable pressure (PS).

The Pressure Relief Valve setpoint should be equal to the design pressure + 10% but should never be higher than the maximum allowable pressure (PS).

There are a number of different types of relief devices

  • Fusible plugs (limited to vessels < 84L)
  • Rupture disk (generally used for low pressure systems & for installation downstream of a relief valve to monitor relief valve integrity).
  • Pressure relief valve

For vessels greater than 283L, most codes recommend a three way valve with two relief valves. This is just plain common sense in any event & system designers should always provide three way valves + two relief valves. If only one relief valve is provided there is very high potential to lose the entire refrigerant charge in the event of relief valve leakage or lifting due a system overpressure situation arising. This scenario would disable the plant until such time as the valve is inspected, repaired as necessary & recalibrated & the system recharged.

With two relief valves + valve manifold, the system can still operate by closing off the faulty or lifted valve & opening up the port to the standby valve. The faulty or lifted valve can then be inspected, repaired & recalibrated without interrupting plant operation.

Once a relief valve has popped, it should not be allowed to remain in operation until it is checked & calibrated.

Under no circumstances must any shut off valve be placed between the system or vessel protected by a relief valve & the relief valve itself. The danger of doing so is that the valve may accidentally be left in the closed position, which removes the protection provided by the relief valve & leaves the system totally unprotected against overpressure.

Relief valve discharge piping

Due to the asphyxiate, toxic or flammable properties of many refrigerants & for safety of plant operators, relief valves should discharge outside of the equipment space to atmosphere.

When sizing the discharge pipework for the pressure relief valve, care must be taken to ensure sufficient diameter to prevent the build up of back pressure in the pipework. This back pressure can have a subsequent effect on the effective discharge relief setpoint of the valve.

Pressure Testing Procedure – Use of Nitrogen

Ensure the Nitrogen Cylinder is secured so that it will not fall over or be knocked by other workers.

  • Ensure that the regulator valve is fully wound out (anti clockwise).
  • Fit the regulator to the cylinder.
  • Isolated the section of pipe to be tested. Remember do not put the high side pressure in the low side of the system.
  • Fit a separate nitrogen gauge to the system and ensure there are no isolated sections within the part of the system to be pressure tested. Do not use the engineers manifold set for pressure testing as there have been cases where the sight glass has blown out causing harm to the operator.
  • Fit the common hose to the nitrogen cylinder.
  • Open the valves.
  • Open the nitrogen cylinder valve.
  • Slowly wind the nitrogen regulator in (clockwise) to pressurise the system.
  • Pressurise the system in stages of no more than 5 bar at a time.
  • Only pressurise the relevant sections of the system to maximum allowable pressure.
  • Listen for audible pressure loss at every pressure increment increase.
  • If a leak is identified, the nitrogen should be vented, the leak repaired and the leak test procedure repeated.
  • When the maximum allowable pressure has been reached, isolate the nitrogen supply and close the nitrogen cylinder valve.
  • Wind the nitrogen regulator valve fully out (anti clockwise).
  • Test each joint with leak detection spray or soapy water. If leaks are found, they must not be repaired with the system pressurised.
  • Slowly vent the nitrogen. Repair any leaks found and then repeat the test procedure using OFN.
  • When no leaks are found, note the pressure shown on the high pressure gauge.
  • Maintain the system at the maximum allowable pressure for the duration of the test. (Minimum of 1 hour)
  • When it is established that the system is safe and leak tight the OFN can be vented and the system can be evacuated and dehydrated ready for the refrigerant.

It is good practice to have the client witness the test and sign a pressure test certificate confirming they have witnessed the system being tested to BSEN378:2008 standards.