A pressure reduction and metering station takes gas from a high-pressure line, lowers it to the pressure the consumer can use, and measures the quantity delivered at the same point. Known as an RMS (regulating and metering station) or PRMS (pressure reducing and metering station), it is designed around two questions: in what order should regulation and metering sit along the gas path, and how is the downstream system protected from overpressure if a regulator fails?
This page covers the layout of the combined station, run redundancy and the overpressure protection chain. Detailed regulator selection and heater types are dealt with on their own pages.
Where RMS stations are used
Combined regulating and metering stations serve points that are fed from a transmission or high-pressure distribution line and need an outlet pressure of their own:
· Industrial zones and distribution areas: feeding the local network at a suitable pressure and measuring total consumption at the entry point.
· Large industrial consumers: stable pressure for furnaces, boilers and process burners, with metering at the plant inlet.
· Power plants: pressure reduction ahead of gas turbines or engines and fuel consumption monitoring; the turbine fuel specification is assessed separately.
City gate stations also reduce pressure and meter gas, but as the interface between transmission and distribution they carry extra duties such as odorization and continuity of network supply. An RMS, by contrast, serves a specific consumer or delivery point.
Typical station layout
1. Inlet: isolation valve and inlet pressure measurement.
2. Filtration: protects regulator pilots, protective devices and the meter from contaminants.
3. Preheating: installed ahead of regulation when cooling across the pressure drop would exceed acceptable limits.
4. Regulator runs: each run carries a slam-shut valve, a monitor regulator and an active (working) regulator.
5. Meter run: meter, pressure and temperature transmitters, and a volume corrector or flow computer.
6. Outlet: relief valve, outlet pressure measurement and outlet isolation valve.
The protection chain: active, monitor, slam-shut and relief
The guiding principle of a pressure reduction station is that no single component failure should cause a dangerous pressure rise downstream. The regulator run is therefore built as a layered chain of protection:
· Active regulator: holds the outlet pressure at set point during normal operation.
· Monitor regulator: installed in series with the active regulator and set slightly higher; if the active regulator fails open, the monitor takes over pressure control and supply continues.
· Slam-shut valve: normally open, it shuts off flow automatically and completely when the monitored pressure exceeds its set value. It is commonly reset by hand, so the run cannot restart before the cause has been investigated.
· Relief valve: vents limited pressure rises, such as seat leakage from a closed regulator or thermal expansion, to a safe location; whether full-capacity relief is required depends on the risk assessment and applicable regulations.
The steps between these set points are worked back from the maximum allowable operating pressure of the downstream system. Because a regulator is not always designed to act as a safety device, overpressure protection is treated as a separate function. European standards set out distinct requirements for the station, for gas pressure regulators and for safety shut-off devices.
Building in run redundancy
Where supply must not be interrupted, regulation is split into two or more parallel runs. The most common arrangement is two identical runs, one working and one on standby. The standby regulators are set to a slightly lower outlet pressure; if the working run's slam-shut trips and outlet pressure falls, the standby run opens and takes over on its own. Where one run cannot carry the full capacity, arrangements such as two working runs plus one standby are used.
Redundancy of regulator runs and of meter runs are separate decisions: the first protects continuity of supply, the second keeps measurement going during maintenance and verification. At a billing point, a standby meter run or an explicit rule for any unmetered bypass is needed.
Metering upstream or downstream of regulation?
Where the meter run sits affects both the size of the station and the quality of measurement:
Location | Potential benefits | Points to watch |
Upstream of regulation (high-pressure side) | Actual volume is smaller, so a smaller meter may be sufficient; measurement takes place away from regulator-induced flow disturbance. | Meter run and transmitters must suit the inlet pressure class; inlet pressure swings have to be considered when setting the measuring range. |
Downstream of regulation (low-pressure side) | Pressure is stable, and metering equipment can be selected for a lower pressure class. | Actual volume grows for the same quantity of gas, so the meter may need to be larger; adequate distance or mitigation is needed for regulator-induced disturbance and for noise that can affect some meter types. |
At billing points the metering location is usually fixed by the contractual delivery point, and the technical choice is made within that framework.
When preheating comes into play
Gas cools as its pressure is reduced (the Joule-Thomson effect). For small pressure drops the cooling may not matter; across large drops, the outlet temperature can fall below what the regulators, valves and downstream equipment can accept. The heating requirement follows from the outlet temperature calculated at the lowest inlet temperature, the largest pressure drop and the actual gas composition. If a heater is needed, it goes ahead of the regulator runs.
Design criteria
Parameter | Why it matters in design | Effect on selection |
Minimum and maximum inlet pressure | Defines the regulator's control range and pressure ratio | Regulator type and size, run pressure class |
Outlet set point and allowable deviation | Defines the stability the consumer needs | Regulator accuracy class, monitor and protection set-point steps |
Maximum allowable operating pressure downstream | Sets the upper limit of the protection chain | Slam-shut and relief settings, number of protection layers |
Flow range and expected growth | Run capacity and meter range are based on it | Number of runs, redundancy arrangement, meter type and size |
Gas temperature and composition | Determine how much the gas cools across the pressure drop | Need for and duty of a heater |
Purpose of measurement | Consumption monitoring and billing impose different requirements | Meter run redundancy, bypass rule, record keeping |
Noise and gas velocity limits | Large pressure drops can generate noise and vibration | Regulator selection, outlet pipe size, need for silencing |
Monitoring and remote access
Inlet and outlet pressures, gas temperature, filter differential pressure, slam-shut position and metering data are usually gathered in a local panel and passed to the operator's SCADA or DCS. Immediate alarms for events such as a slam-shut trip or a standby run taking over are essential for a fast response at unmanned stations.
What TLY Enerji contributes
On RMS and PRMS projects, TLY Enerji provides engineering support for reviewing process data, selecting measurement technology and supplying measurement and control equipment. Depending on project scope, installation of transmitters and flow meters, PLC or SCADA integration, testing, commissioning, supervision and maintenance support can also be included. The scope of supply for regulating and protection equipment is defined for each project against the technical specification.
Data needed for a technical review
· Role of the station and type of consumer served (area supply, plant inlet, power plant)
· Minimum and maximum inlet pressure, required outlet pressure and acceptable deviation
· Maximum allowable operating pressure of the downstream system
· Minimum and maximum instantaneous flow, seasonal variation and expected growth
· Inlet gas temperature range and composition data
· Expected redundancy for regulator runs and meter runs
· Whether the measurement is used for billing, and how the contract defines the delivery point
· Preference for an enclosed or open-air installation, and remote monitoring requirements
Related pages
· Gas pressure regulating skids: Detailed selection of direct-acting and pilot-operated regulators and protection devices.
· Gas heating, regulating and metering skids: Heater types and temperature control for large pressure drops.
· City gate stations: Odorization and staged regulation at the transmission-to-distribution interface.
· Natural gas metering stations: Meter run architecture and volume conversion.
· Gas filtration and metering skids: Filtration that protects regulators and meters.
Frequently asked questions
Is there a difference between an RMS and a PRMS?
In practice the two abbreviations usually describe the same type of station: RMS stands for regulating and metering station, PRMS for pressure reducing and metering station. Some specifications use PRMS to stress the pressure reduction duty. What matters is not the label but the functions defined in the specification: pressure steps, redundancy arrangement, metering purpose and heating needs.
Why is a slam-shut valve needed if there is already a monitor regulator?
The monitor regulator is there to keep supply going when the active regulator fails; the slam-shut valve protects the downstream system by stopping flow when pressure control is lost altogether. They address different failure scenarios. Since regulators are not always designed as safety devices, overpressure protection is normally provided by an independent shut-off element; the mandatory layers are set by the project specification and regulations.
How does the standby regulator run take over?
In the usual arrangement the standby regulators are set slightly below the outlet pressure of the working run. While the working run operates normally, outlet pressure stays above that value and the standby run remains closed. If the working run's slam-shut trips and outlet pressure drops, the standby regulators open and pick up the supply automatically. Reporting the event to the control room is important so the fault can be investigated.