LPG regulation and choosing the right test

Four pressures, four meanings

Container pressure, regulator outlet pressure, appliance working pressure and tightness-test pressure are different things. Common fixed domestic supply reference values:

• propane: about 37 mbar
• butane: about 28 to 30 mbar
• natural gas: about 21 mbar at the domestic meter outlet

Low-pressure LPG pipework is sized for no more than 2.5 mbar loss from the regulator to each appliance inlet, compared with 1 mbar from the meter for natural gas. These values do not define every LPG arrangement: some caravan regulators are approved for propane and butane at 30 mbar. Take the required fuel and working pressure from the actual appliance data and instructions, then verify it under the prescribed conditions.

What the regulator does

Where liquid and vapour coexist at equilibrium, container pressure follows the fuel’s vapour pressure at that temperature. Withdrawal cools the contents, and pressure is not a simple measure of remaining liquid. Regulation reduces the supply pressure to the required downstream range. With two stages, the first can supply an intermediate pressure to the final regulator. Both stages must suit their inlet range and demand; a correctly rated single-stage arrangement can also be suitable. Regulation does not add odorant or measure consumption.

Choose a regulator or staged arrangement approved for:

1. the approved fuel or fuels, with connections suitable for the actual cylinders
2. the inlet-pressure range
3. the required outlet pressure
4. the total demand of the appliances

A natural-gas meter governor is not interchangeable, even if the connection fits. A wrong or undersized regulator gives dangerous over-pressure or starved, unstable burners. Protect regulators from damage, water and blockage.

Protection if the regulator fails

An automatic changeover supplying fixed pipework from four or more refillable LPG cylinders needs protection against excessive downstream pressure if its regulator malfunctions. The changeover keeps the gas flowing; it does not provide that pressure protection.

HSE recommends suitable overpressure protection for all refillable-cylinder installations feeding fixed pipework, including smaller groups. Identify the protective arrangement and check it suits the installation; a normal outlet pressure does not show the failure protection works.

Where the second stage sits

On a typical two-stage bulk supply, the first regulator reduces vessel pressure to an intermediate supply. The final regulator reduces that again for the downstream installation. The service pipe between the tank and building carries whichever pressure reaches it from the tank-side equipment.

• First stage at the tank, final stage at the building: the cross-site pipe is between stages and is commonly at medium pressure.
• Both stages at the tank: the final reduction happens before the cross-site pipe, which then carries the low-pressure supply.

The diagram shows those two arrangements. Identify the installed regulators and confirm their specifications and operating pressures; counting the visible fittings does not tell you the pressure. In the small bulk guidance, low pressure is below 75 mbar, while medium pressure runs from 75 mbar to 2 bar gauge. A comparable leak can release a greater quantity at the higher pressure, so the boundary matters when assessing risk and selecting a procedure.

Check working pressure under load

A no-flow value can look satisfactory even if supply pressure will sag when burners run. A low or high settled value also needs investigation; do not dismiss it simply because no gas is flowing. Use a manometer at the appliance inlet test point while the model runs at its prescribed test condition, including simultaneous demand where the applicable method calls for it. Compare the working result with the model’s limit. An excessive drop calls for checks of regulator capacity and condition, hose or pipe restriction, and cylinder vapour delivery. Pressure under flow and pressure loss in an isolated tightness test answer different questions.

If a converted boiler’s pressure is outside the manufacturer’s permitted range, keep it out of use while the discrepancy is resolved. Verify the test point, instrument, load and model specification before tracing the supply, regulation, pipework and conversion setup. The reading does not tell you which part has failed. Heating the cylinder, drilling injectors or substituting a natural-gas target is not a remedy.

Choose the standard for the installation

UP/1B Edition 4 includes caravans and leisure accommodation vehicles when its scope conditions are met. Small LPG installations in boats and other vessels use PD 54823 or BS EN ISO 10239, as applicable. LPG service pipework has a separate route under Liquid Gas UK Code of Practice 22. A similar regulator pressure does not make these installations interchangeable for testing.

Tightness testing LPG

After work that may disturb a gas connection, follow the applicable tightness procedure over the affected section before returning it to service. A result from before the work does not cover a newly remade joint.

1. Select the procedure for the fuel, pressure stages, volume and boundary. Include the section affected by the work; a pass applies to that tested boundary, not an untested branch. The UP/1B Edition 4 scope includes an installation-volume limit of 0.035 m³, so a 0.040 m³ section is outside it whatever its pressure. LPG service pipework is outside it too.
2. Carry out the let-by check: the isolating valve must hold shut.
3. Allow the prescribed stabilisation and maintain suitable conditions throughout the test. Outdoor sun, wind or shade can still change gas temperature after the initial wait. Cooling can resemble a leak and warming can mask one; resolve invalid conditions rather than inventing a correction.
4. Under a valid, isolated and stable test, pipework has no permissible loss. A perceptible fall requires making safe, investigation, rectification and a satisfactory retest.
5. Assess an eligible existing-appliance loss only after the pipework is proved tight, using LPG criteria, never natural-gas figures. Where appliances cannot reasonably be isolated, the connected test must show no perceptible movement.
6. Check joints with approved compatible leak detection fluid, never a flame, and follow its application and residue instructions. A high-pressure cylinder connection can lie outside the downstream test section and require a separate check.
7. Reseal every test point and prove it gas-tight.
8. Before handover, confirm tightness, the correct working pressure under load and that any changeover works. Purge where required: under Edition 4, PV = 1.5 × IV.