What is a second-stage pressure regulator in a gas network?

Some pipelines distribute gas in two stages. The Portuguese ACSS specification ET 03/2006 (revised 2024), for example, sets oxygen and nitrous oxide at 800 kPa (±100) in the primary network, then 480 kPa (+100) for oxygen and 420 kPa (+100) for nitrous oxide in the secondary networks; medical air goes from 900 kPa (±100) to 450 kPa (±100). ACSS also allows single-stage systems, for which it recommends ISO 7396-1 and ISO 10524-2. ISO 10524-2 is the standard for manifold and line pressure regulators in medical gas pipeline systems.

How it is installed

• ACSS asks for each department to be fed by at least two second-stage reduction sets in bypass, installed where they can be seen, and notes that the two sets should not work at the same time.
• HTM 02-01 Part A (NHS England, 2026 edition) calls the assembly a pressure-reducing station (PRS). Where gas is distributed above the nominal terminal unit pressure, the maximum working pressure should not exceed 1100 kPa and a PRS should be installed.
• The PRS should be duplex, not simplex, so that the flow is not interrupted by maintenance or a component failure. Each side has a pressure regulator, an analogue gauge, a safety relief valve, a test terminal unit, a non-return valve and a pressure transducer between two line valves, with lead and lag regulators set no more than 50 kPa apart.
• It should be installed in an area with good ventilation, not in a ceiling space, with the pressure-relief valves piped away individually to a safe location.

Alarms

HTM 02-01 asks for an alarm at the PRS, repeated on the plant alarm system as its own column, with a digital readout of both regulator outputs and high- and low-pressure conditions; a station fault is raised when both sets of regulators have a pressure fault. ACSS asks for the alarm systems of each department's pressure reducers to allow connection to the building automation system.

Ultra Controlo supplies second-stage pressure reducers, including versions with an integrated area valve, and an alarm relay for second-stage zone monitoring with high- and low-pressure alarms.