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Data Sheet: Reducing Pressure the Right Way
Copyright Steamline ®, 1st August 2005

Pressure Reducing Stations, or PRV stations as they are sometimes called, are an integral part of any steam system. They are used to reduce the pressure (and correspondingly, the temperature) of steam. Steam is usually generated at high pressures at a central location (the boiler house) and transferred to usage points through the steam system. At the various points of steam usage, the pressure is dropped according to the requirement of each process.

Why can’t steam be generated at low pressure?
Most modern boilers operate at relatively high pressure. A typical process boiler operates at 10.5 to 17.5 kg/cm2g (150 – 250 psig). The operating pressure of a boiler determines various parameters like water holding capacity, steam space, water level etc. Such an arrangement has the added advantage that smaller diameter distribution pipes can be used, as the specific volume of steam occupied by steam is less at higher pressures. So it is usual practice to generate steam at high pressures at a central location, distribute the high pressure steam in small diameter pipes, and reduce the seam pressure as required at the usage points. The temperature of saturated steam varies with the pressure, so this also provides a simple method of controlling temperature of steam going to a process.

Economic benefits of Pressure Reduction
If steam at a higher pressure (and temperature) than required is used in a process, the heat will be sufficient for the process. So why reduce steam pressure? There is a powerful economic argument in favour of reducing the pressure: latent heat. The work done by steam in a process is primarily latent heat, which is far greater then sensible heat i.e. steam imparts heat to the process by condensing into water, thus giving off its latent heat.

A typical PRS
The figure below shows the general arrangement of a typical PRS. The Moisture Separator at the inlet ensures that any water slugs (condensate) travelling with the steam does not cause damage to the rest of the PRS, and provides dry steam to the process. The inlet valve can be used to shut-off steam supply to the PRS. The strainer removes dirt, welding slag or any other particles that could damage the seat and plug of the PRV.

As PRV sizing depends on the flow capacity (kv) required, it may not be of the same size as the inlet line. In fact, often the PRV size will be one size smaller then the inlet line. Also, as the pressure drops, the specific volume of steam increases, so the outlet size will be a size or two more than the inlet. Eccentric reducers and expanders are used to decrease and increase line sizes within the PRS. Concentric reducers should not be used.
A balancing line from the outlet is returned to the PRV actuator, to act as a feedback to balance pressure to the set pressure. For best results, this balancing line should be taken from a point 0.7 – 1m downstream of the PRV to ensure that the turbulence caused by the pressure reduction is overcome. A condensing pot is used in the balancing line to cool the steam so that the life of the diaphragm in the PRV is enhanced, as it does not come in direct contact with high temperature steam.
The outlet valve shuts off supply to the process, while the bypass line and valve provide an alternative flow path in case the PRV needs to be repaired or maintenance work is required. A suitably sized safety valve is required to ensure that the outlet steam pressure does not exceed the set-point, even if the PRV fails. This would normally be set to blow at 10% above the PRV set pressure, and should have the capacity to discharge the maximum flow of the PRS.
Inlet and outlet pressure gauges are used to check generation pressure, configure the set pressure and check PRV operation. These need to be provided with pig tail siphons to cool the steam, and isolating valves for removal / isolation of the gauge.

PRV Types
A PRV or Pressure Reducing Valve is the heart of the PRS. While a number of manufacturers implement the reduction of pressure in different ways, PRV’s are classified (by most modern engineering text references) into these three types:

Direct Acting
The simplest of PRVs, the direct acting type operates with either a flat diaphragm or convoluted bellows. Since it is self-contained, it does not need an external sensing line downstream to operate. It is the smallest and most economical of the three types and designed for low to moderate flows. PRV sizes are usually limited to 50 NB. Accuracy of direct acting PRVs is typically ± 15 – 20 % of the downstream set point. Typical examples of this type are the Spirax “BRV2”, Darling Muesco “model D-215”, Leader’s “type 24” or Hawa’s “OB” series PRV.

Internally Piloted Piston-Operated
This type of PRV incorporates two valves—a pilot and main valve—in one unit. The pilot valve has a design similar to the direct acting valve. The discharge from the pilot valve acts on top of a piston, which opens the main valve. This design makes use of inlet pressure in opening a larger main valve than could otherwise be opened directly. PRV sizes are usually limited to 80 NB. As a result, there is a greater capacity per line size and greater accuracy (± 5 – 10 %) than with the direct acting valve. As with direct acting valves the pressure is sensed internally eliminating the need for an external sensing line. Typical PRV’s of this type are the Spirax “DP17” or “DP143”, the Darling Muesco “model 1001” and the Mazda “Mark One”.

Externally Piloted
In this type, diaphragms replace the piston operator of the internally piloted design. This increased diaphragm area can open a larger main valve, allowing a greater capacity per line size than the internally piloted valve. PRV sizes up to 150 NB can be offered using this design. In addition, the diaphragms are more sensitive to pressure changes, and that means accuracy of ± 1 – 3 %. This greater accuracy is due to the location, external of the valve, of the sensing line where there is less turbulence. This valve also offers the flexibility to use different types of pilot valves (i.e., pressure, temperature, air loaded, solenoid or combinations). A typical externally piloted PRV is the Samson “41-23” series.

PRV Quality
As seen above, a variety of types and makes of PRV’s are available in the market. How does one make a comparison in terms of performance and quality? The parameters discussed below offer a guideline when comparing one PRV with another.

Materials of construction
As the Indian Boiler Regulations (IBR) allows the use of Cast Iron as a body material for valves used in steam lines up to 13 kg/cm2, most manufacturers use CI due to lower cost. However, if a PRS is specified by a consultant (e.g. Dalal / H&G etc.), the specification will almost always be for a PRV with a Cast Steel body. The reason for this is that CS is a far superior and durable material, and can also be used for higher pressures. Most PRV’s use SS valve plugs and seats. However, the diaphragm material can vary – usually, soft materials (such as EPDM) will have a longer life compared to steel. The table below shows a comparison of some common PRV’s.

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