# Why do medical gas pipelines need pressure relief valves?

**Why do medical gas pipelines need pressure relief valves downstream of the line regulators, and where does HTM 02-01 place them?**

Under HTM 02-01 (2026), the NHS England guidance, pipeline distribution systems should have a pressure-relief device downstream of the line pressure regulator, and cylinder manifolds a self-closing safety valve after each regulator with at least that regulator's flow capacity; the HTM speaks of relief valves fitted to protect downstream systems.

HTM 02-01 (2026 edition), the NHS England guidance for medical gas pipeline systems, calls these devices pressure safety valves or pressure-relief valves. Its cylinder management chapter, on equipment connected to cylinders, speaks of relief valves fitted to protect downstream systems, and its dental air section of valves protecting distribution pipework. It states a discharge capacity for the valves on manifolds (at least the flow of the upstream regulator) and on air plant (the compressor throughput or the system demand flow, depending on position). Part B explains why they are not tested by raising line pressure: doing so could make anaesthetic equipment fail, and a valve that does not reseat could release a considerable amount of gas.

## Where they are fitted

- Pipeline distribution: a pressure-relief device downstream of the line pressure regulator, connected through a three-way cock so that it can be exchanged for a certificated replacement (Part B, 11.120).
- Cylinder manifolds: self-closing valves downstream of the pressure regulators and upstream of the isolation valves, with a flow capacity at least equal to that of the regulator before them (Part A, 5.38).
- Medical air plant: on the delivery pipe of each compressor, able to discharge that compressor's total throughput; on each air receiver and dryer tower, able to discharge the throughput of all the compressors (not needed on dryer columns already protected by the receiver valve and downstream equipment); and immediately downstream of each pressure regulator, able to discharge the system demand flow before any isolating valve (Part A, 7.54).
- Pressure-reducing stations, for example from surgical to medical air: each side of the duplex station has its own regulator, gauge, safety relief valve, test terminal unit and non-return valve (Part A, 3.74).

On air plant, safety valves should conform to BS EN ISO 4126-1 and be of the closed-bonnet type. For dental air pipework, HTM 02-01 gives a lifting pressure 10% above nominal line pressure for valves protecting distribution pipework.

## Where the gas goes

On manifolds, each safety valve should have its own discharge pipe, at least one size larger than its inlet, vented outside the building where the discharge will not create a fire hazard or injure anyone. The outlet should end at least 3 m from any doorway, window or natural vent that can be opened and at least 5 m from mechanical ventilation or air intakes, be turned downwards against dirt and moisture, and have warning signs. On air plant the valves should discharge through pipework to a safe location away from the plant, and pressure-reducing station valves should be piped away individually so that the station can be worked on safely.

## Testing and replacement

HTM 02-01 does not recommend lifting safety valves. Valves should be individually tested and certificated, should not be tested on site, and should be replaced within five years of their stamped test date under a planned procedure; Part B presents this five-yearly replacement as an alternative to pressure testing, which would need shutdowns. During manifold servicing the valve exhaust is soap-tested, and a passing valve is replaced with a sealed, certificated unit. The asset list should record each regulator and safety relief valve with its location, pressure rating, serial number, installation date and five-yearly replacement due date.

### Can a medical gas safety valve be tested by raising the line pressure?

Not under HTM 02-01: Part B says it is not appropriate to test by raising the line regulator setting or unseating the valve, because anaesthetic equipment could fail; the valve should be replaced every five years instead.

### Where does a manifold safety valve discharge?

Through its own pipe, at least one size larger than the inlet, to outside the building, at least 3 m from openable doors, windows or vents and 5 m from ventilation intakes (HTM 02-01).

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems, Part A (2026), paras 3.71, 3.72, 3.74, 3.77, 5.38 to 5.46, 7.54, 7.55, 15.170 and 15.225](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [NHS England, HTM 02-01 Medical gas pipeline systems, Part B (2026), paras 4.32, 6.80, 9.175, 11.118 to 11.120 and 11.170](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-b-operational-management-aug-26.pdf)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/why-do-medical-gas-pipelines-need-pressure-relief-valves/