Pressure Reducing Valves and Downstream Safety Valves
Steam is distributed at high pressure and used at low pressure. A pressure reducing valve between the two holds a steady lower pressure for the user whatever happens upstream. Laid out properly — with a separator, strainer, isolation, a bypass and a safety valve — a PRV station is reliable for years.
You'll learn
- Explain why steam pressure is reduced before use
- Describe direct-acting and pilot-operated PRVs
- Lay out a complete PRV station
- Understand downstream overpressure protection
Before this: Latent heat, Enthalpy, Isolation and control valves
Why reduce pressure
- The process needs a particular temperature, and for saturated steam that means a particular pressure.
- Lower-pressure steam carries more latent heat per kilogram: 2,135 kJ/kg at 3 kg/cm²(g) against 2,002 at 10, about 7% more.
- The user’s equipment may be rated for a lower pressure than the main.
- Lower pressure means less flash steam from the user’s condensate and less loss from leaks.
So the main runs at high pressure, where small pipes carry a lot of steam, and a PRV near each user, or each group of users, drops it to what the process needs.
How PRVs work, and the station around one
- Direct-acting: a spring holds the valve open; downstream pressure acting on a diaphragm closes it. Simple and cheap, but the controlled pressure droops as flow rises.
- Pilot-operated: a small pilot valve senses downstream pressure and drives the main valve. Holds pressure far more accurately across a wide flow range.
- Upstream isolation valve and a separator with its trap, to remove water droplets.
- Strainer, to keep scale off the PRV seat.
- Pressure gauge upstream and downstream.
- The PRV, sized for the flow, usually smaller than the line.
- Downstream pipe expanding to suit the larger volume of lower-pressure steam.
- A safety valve on the low-pressure side.
- Downstream isolation, and a manual bypass that is normally shut.
Enthalpy is unchanged through the valve, so dry saturated steam at 10 kg/cm²(g), reduced to 3 kg/cm²(g), leaves at about 162 °C — 19 °C of superheat above its 143.2 °C saturation temperature. Slightly wet steam is dried by the same effect. The superheat is soon lost in the downstream pipe.
Protecting the low-pressure side
A PRV can fail open: dirt holds the seat off, a diaphragm ruptures, or someone opens the bypass. Then the full upstream pressure arrives on equipment that may only be rated for the downstream pressure. A safety valve downstream of the PRV, set at or below the rating of the weakest downstream item and sized to pass the full flow the PRV or its bypass could pass when wide open, is what stops that from becoming a burst vessel.
- For a large pressure ratio, two PRVs in series often control better and quieter than one.
- Sensing lines for pilot PRVs are taken from a straight, undisturbed length downstream, not right at the valve outlet.
- Noise and erosion rise with pressure drop; low-noise trims and expanded outlet pipe help.
Key takeaways
- Distribute at high pressure; reduce near the user for temperature, latent heat and equipment rating.
- Pilot-operated PRVs hold pressure accurately; direct-acting ones droop with flow.
- Station: isolation, separator and trap, strainer, gauges, PRV, safety valve, isolation, bypass.
- Steam reduced from 10 to 3 kg/cm²(g) leaves slightly superheated (about 162 °C).