PRDS (Pressure Reducing & Desuperheating Station)

PRDS (Pressure Reducing & Desuperheating Station)


We have the knowledge and expertise to enable you to select the right pressure and temperature controlsolution for your specific need. Control valves and Desuperheaters are available with various options to suit a client's requirement.

Recommendations for efficient working of PRDS

  • Minimum straight length at outlet should be 4 mtrs.
  • 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 1.5 mtrs.
  • 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 feed water).
  • Instrument quality air is required.

Minimum water pressure requirement for Combined PRDS:

  • Top entry: Pw = [(P1+P2)/2 ] + 7 BAR
  • Bottom entry through stem : Pw = P2 + 7
  • Bottom entry through nozzle : Pw = [P1 / 2 ] + 7 BAR

Notes

  1. Minimum controllable temperature is Saturation Temperature + 7°C.
  2. The above are based on a specific set of parameters. These guidelines may change.

Features

  • All stations are engineered and factory assembled.
  • Most of the PRDS stations are supplied with ‘Combined PRDS’ valves which give the advantage of pressure reduction and temperature control in a single unit.
  • If a Combined PRDS unit is not suitable; we offer separate PRV and DSH unit.
  • Even with Combined PRDS valve, separate redundant Desuperheaters can be included in case of bypass operation when main valve is under maintenance.
  • The Water flow control valve and Waterline are also offered with the steam line, which ensures correct design and one to one matching of pressure and temperature control stations.
  • It is designed in line with good steam engineering practices using our proprietary software.
  • Forbes Marshall manufactures most of the components of pressure reducing and Desuperheating stations in-house which enables quality control as per our standards.
  • The entire assembly is hydro tested.
  • Various options and makes are available for isolation valve and bypass valves.
  • We ensure availability of steam to the process of the right quantity, quality, pressure and temperature.
  • For low sizes we maintain same pressure class at the inlet and outlet which allows high robustness and strength for the station. This ensures reliability of station even in case of any malfunction or failure.
  • Stations are available in various sizes from 25mm up to 1500mm or more. Pressure class available from ANSI 150# up to 2500# in A106 Gr. B / A335 P11 / A335 P22 materials.

Pressure Reducing Valve

Pressure control valve which, is the ‘Heart of the Station’ is designed, engineered and manufactured in house which ensures high quality.

Strainer

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. Helps to isolate the main pressure control valve in case of maintenance activities.

Bypass Valve

This can be supplied 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.

Desuperheaters Fixed or Variable Nozzle Desuperheaters are provided for accurate temperature control. These are designed, engineered and manufactured in house which ensures high quality.

Safety Valve

A good quality safety valve is essential in the event of a sudden steam pressure increase. Forbes Marshall safety valves are the most reliable and proven safety valves.

Gauges

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.

Instrumentation

High accuracy instrumentation for pressure and temperature control like transmitters, controllers, DCS system are offered as per client’s requirement with options of various makes and models.

Notes:

  • Special materials are offered as per client’s requirement.
  • Same pipe material is maintained throughout station as per design temperatures.
  • Entire assembly is provided with IBR Certificate.
  • Option of Self Regulating valve is also available.
  • We recommend minimum 4mtrs. Straight length from the point of water injection.
  • We recommend minimum 10 mtrs. Distance of temperature sensor from the point of water injection.
  • Interconnecting pipe between water and steam line, supporting pads, supporting structures, any other item which is not specified in line diagram is not a part of standard package but can be supplied upon request.
  • Pneumatic testing can be offered for the entire assembly on request.
  • Multilayer redundancy for critical applications is also available.

For nearly three centuries steam has been used to transport energy in the form of heat from the place of its origin- the boiler- to its usage point a steam engine, a turbine, a heat exchanger or any other process. For this reason steam is generated in boilers at high pressure and temperature and subsequently expanded in equipment to obtain thermal and mechanical energy. In condensers this reduced steam is converted back in to liquid phase and by means of boiler feed water pump this condensate is pressurized again and transported to the boiler. The water/steam cycle is closed.

Hence, Pressure and Temperature reduction is inevitable. The reasons for pressure and temperature reduction can be summarized as:

  • Steam boilers are usually designed to work at high pressures. Working it at lower pressures can result in carryover of water.
  • Steam at high pressure has a lower specific volume which means that a greater weight can be carried by a pipe of given size. Forbes Marshall recommends distribution of steam at high pressure and reduction at point of usage. This reduces capital costs or piping / insulation and also reduces distribution losses.
  • Steam pressure may be reduced to save energy. Steam at lower pressures has higher latent heat. Reduced pressure of steam also leads to reduced heat loss and lower flash steam formation from open vents.
  • Since the pressure and temperature of steam are related, controlling the pressure enables us to control the temperature in the heating process.
  • Pressure must be reduced so that they are within the within the rated safety limits.
  • In plants where steam usage takes place at many different pressures, pressure reduction allows generation of steam at a single high pressure and subsequent reduction to the desired pressure at the point of usage.
  • Temperature control is necessary to control superheat temperatures.
  • Temperature control is necessary to protect piping systems and components from excessive temperatures and/ or to provide process steam at required temperatures.