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Combined Pressure Reduction and Desuperheating (PRDS) : Week 10/23/2012

In our last set of article, we discussed about advantages of Variable Nozzle Desuperheater (VND) concept & it’s applications. We also discussed about some good engineering practices for efficient desuperheating. In this article, we will discuss about combined pressure reducing and desuperheating :

Why Pressure and Temperature Reduction?

To understand the need for Combined Pressure Reducing & Desuperheating (PRDS), one needs to first understand why pressure & temperature reduction is necessary.

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 equipments to obtain thermal and mechanical energy. In condensers this reduced steam is converted back into 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. Various 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. It is recommended that distribution of steam happens at high pressure and reduction happens 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 need 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.

These reasons give an idea of how important it is to manage pressure and/or temperature conditions at various stages at various processes / applications.

Why ‘Combined’ Pressure and Temperature Reduction?

Conventional method for pressure reducing and desuperheating of steam calls for pressure reducing valve followed with desuperheater. However, it is much more beneficial to reduce pressure and temperature of steam in a single unit called 'Combined PRDS' valve. During the process of pressure reduction in the valve, water is being injected simultaneously into a highly turbulent zone called the 'Vena Contracta' zone. This causes instantaneous evaporation of water for complete desuperheating.

In our next article, we will discuss more about Combined Pressure Reducing & Desuperheating (PRDS) and also about some useful & important features of it.