Pressure Reducing and Desuperheating Station (PRDS)

When there is requirement of lower pressure steam, than what is generated, by various equipments / processes, PRDS are used so as to fulfill the requirement. PRDS is used for steam conditioning by reducing the pressure & temperature of steam. It is a combination of control valve for pressure reduction and an atomising / spraying nozzle for desuperheating. The outlet steam should be 5 to 7 degrees higher than saturation temperature or else there may be danger of supplying wet steam.

Types of PRDS

Type- 1 : Split PRDS

In this type of PRDS, desuperheating is done external to the pressure reduction. Cooling water is sprayed downstream of pressure control valve through a separate desuperheater unit after the pressure reduction of steam is completely done. In this type of PRDS, pressure reducing station can have a manual bypass valve for pressure reduction with a common desuperheater unit.

Type-2 : Combined PRDS

This is an integral type PRDS which means the pressure control action and water spray take place in the same unit. This type of control valve consists of an inlet & outlet process connection and a nozzle for the entry of the spraying water. The cooling water enters through the valve & sprays water in the low pressure zone. If any bypass is required in this type of PRDS then the bypass valve has to be the same control valve. Manual bypass will not do as it will again require a separate Desuperheater for the bypass line. 

Spray Water

As the spray water is mixed with the steam to desuperheat the steam, the spray water quality should be same as that of steam. Hence, either Boiler feed water or Steam condensate shall be used as spray water. Spray water is tapped either from Boiler Feed Pump or Condensate Extraction pump based on the pressure & temperature requirments. For high pressure steam, boiler feed water is used whereas for low pressure steam, condensate is used.

Spray water Pressure requirement (This data needs to be confirmed from PRDS Vendor. However for preliminary calculations / specifications, following can be used.)

For Type-1

Spray water pressure = Steam pressure + 5-7 bar

For Type-2

Top Entry   : Pw = [(Pi + Po)/2] + 7 Bar

Bottom Entry through stem  : Pw = Po + 7 Bar

Bottom Entry through nozzle: Pw = (Pi / 2) + 7 Bar

PRDS Calculations

Inlet Steam Flow rate = Mi , Outlet Steam Flow rate = Mo

Spray water Flow rate = Mw

Inlet Steam Enthalpy = Hi , Outlet Steam Enthalpy = Ho

Spray water Enthalpy = Hw

Enthalpies are to be found from Steam table based on the Inlet & outlet operating parameters

Mi + Mw = Mo ………………………………. Eqn. 1

Mi x Hi + Mw x Hw = Mo x Ho ………….. Eqn. 2

Using the above two equations, any two unknown parameters can be evaluated. Either inlet or outlet steam flow rate is always known. The spray water flow rate requirement can be calculated through these equations. An example below for better understanding.

Inlet Steam Conditions;

Pressure = 42 kg/cm2 (g), Temp = 380°C

Hi = 755.74 Kcal/Kg

Outlet Steam Conditions;

Pressure = 4 kg/cm2 (g), Temp = 156°C (Atleast 5° above saturation temp.)

Mo = 30 TPH, Ho = 658.68 Kcal/kg

Spray water Conditions;

Temp. = 60°C, Hw = 60 Kcal/kg

Steam inlet flow rate & Spray water flow rate needs to be calculated.

Mi + Mw = Mo

Mi + Mw = 30 ……………………………………… Eqn. 1

Mi x Hi + Mw x Hw = Mo x Ho

Mi x 755.74 + Mw x 60 = 30 x 658.68 ………. Eqn. 2

Solving both the equations,

Mi = 25.8 TPH & Mw = 4.2 TPH

Layout Requirements

1. When there is space constraint, Type-2 PRDS is a better option as it consumes lesser space than Type-1. 

2. If PRDS is a continuous process then Type-1 is cost effective as it can have a manual bypass valve.

3. Minimum straight length required at the outlet of PRDS is 4-5 mtrs. It varies from manufacturer to manufacturer.

4. Minimum distance of temperature sensor from the point of water injection shall be 12 m.