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In our last set of article, we discussed about combined pressure reducing and desuperheating. In this article, we will discuss about some useful & important features of Combined Pressure Reducing & Desuperheating (PRDS)

What is the advantage of combining Pressure Reduction & Desuperheating (PRDS) ?

A Combined Pressure Reducing & Desuperheating (PRDS) Station will control pressure & temperature together. The desuperheater portion will be useful for the safe handling of very high drops and the control of high velocities associated with steam power generation. The best possible use would be with back-pressure turbines supplying exhaust steam to process. In case the turbine be shuts down for any reason, the steam rejected will be bypassed through the Combined PRDS which serve to reduce it to the required process conditions.

Some of the main advantages can be as below:

  • The water flow control valve and water line would be used with the steam line, which ensures correct design and one to one matching of pressure and temperature control stations
  • One can ensure availability of steam to the process with right quantity, quality, pressure and temperature
  • This is much compact unit compared with separate Pressure Reducing Valve & a Desuperheater
  • All stations can be engineered and factory assembled, thus reducing installation & commissioning time at the site
  • No water hammer
  • No water carryover problems
  • There will be immediate response to flow changes
  • It can be designed in various types like water entry from top or bottom as per water quantity and pressure available at site
  • Water is injected at the 'Vena Contracta' point which is the most turbulent zone causing complete atomization
  • High turndown ratio possible
  • Improved rate of heat transfer
  • Easy maintenance

Recommendations for efficient working of PRDS Valves

  • Minimum distance of Temperature Sensor from the point of water injection should be 10 to 12 mtrs
  • Minimum distance of Pressure Sensor from PRDS Valve should be 1.5mtrs
  • It is recommended to install a strainer of 0.8 mm mesh before water control valve
  • Spray water should be very clean (equivalent to boiler feedwater)
  • Instrument quality air is required
  • Minimum straight length at outlet should be 4 mtrs

Typical Applications

  • Turbine Bypass
  • Condenser Dump
  • Main Steam Line
  • Turbine Extraction
  • Auxiliary PRDSH
  • Deaerator Pegging
  • Ejector and Gland Sealing

Typical components of a Combined PRDS

Pressure Reducing Valve The pressure control valve is the ‘Heart of the Station’. It is good to have it designed, engineered and manufactured by the supplier, so that quality can be assured


This ensures clean steam going into station and avoids burrs, foreign particles entering in main valve.

Isolation Valves

Zero leakage Gate / Piston valves prevent inline leakage. These also help to isolate the main pressure control valve in case of maintenance activities.

Bypass Valve

This can be either with manual or actuated operation. This is a Globe type valve and ensures reduced steam pressure and uninterrupted supply in case the main pressure control valve is under maintenance.


The Fixed or Variable Nozzle Desuperheaters are provided for accurate temperature control. It is good to have these designed, engineered and manufactured by the supplier, so that quality can be assured.

Safety Valve

A good quality safety valve is essential in the event of a sudden steam pressure increase. One needs to ensure that the safety valves are from a reliable supplier with reliable and proven designs


Good quality pressure and temperature gauges are essential at inlet and outlet of station. These ensure correct parameters at the inlet and measure the result of the pressure reducing station at the outlet.


Highly accurate instrumentation for pressure and temperature control like transmitters, controllers, DCS system need to be considered based on the process requirement.

In our next article, we will discuss about a new topic, the Velocity Monitoring of Flue Gases & why it is necessary.