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Achieving Efficient De-superheating : Week 10/1/2012

In our last set of article, we discussed about Variable Nozzle Desuperheater (VND) & how desuperheating can be done effectively. In this article, we will discuss about advantages of VND concept & it’s applications. We’ll also discuss about some good engineering practices for efficient desuperheating.

Advantages of using Variable Nozzle Desuperheater (VND)

  • The Variable Nozzle Desuperheater (VND) is advantageous in many ways. These can be listed as below:
  • It can be custom designed for any size & the installation is pretty simple
  • As a VND ensures fine conical spray that evaporates almost instantaneously, it avoids pipe impingement
  • There is no need for a separate coolant valve.
  • More turndown ratios of possible in case of VND.
  • Because of number of available nozzles, large desuperheating range if possible

Applications :

Some of the important application areas of VND would include:

  • Turbine Bypass
  • Turbine Exhaust
  • Turbine Bleed Steam
  • Auxillary Steam
  • Process Steam
  • Attemperator
  • Pressure Reducing Valve Outlet Steam
  • Boiler Superheaters
  • Boiler Re-heaters

All the above areas would need fine control on steam temperature & a VND can just do that for the user.

Good engineering practices for efficient Desuperheating

There are some good engineering practices one should follow to ensure efficient Desuperheating. Many times, these practices are not followed & the users don’t get the desired results. These are the suggested Engineering practices:

  • Minimum inlet velocity should be 15 meters / sec for FND and 7 meters / sec for VND
  • Minimum straight length downstream beyond the Desuperheater should be 4 meters
  • Optimum distance between water injection point to the temperature sensor mounted downstream should be 10 meters
  • It is necessary to keep the steam flow rates (for max. and min conditions) as practical as possible. Else the turndown ratio increases unnecessarily which necessitates the selection and use of a combination of nozzles in the same spray cylinder
  • Installation of a strainer with a mesh size of approximately 100 ì in the water supply line to prevent the nozzles from clogging is essential

System Comparison :

Conventional FND (Fixed Nozzle Desuperheater)

Conventional injection water systems consist of:

  • Fixed size spray nozzle
  • Control valve
  • Steam pipe section

The water injection quantity is regulated by the control valve. As a consequence of this, the downstream water pressure P2 varies as a function of the valve plug position. At reduced capacity, the control valve starts to throttle, reducing P2. Hence the available water to steam pressure difference results in larger droplet size and poor atomisation. The water evaporation rate slows down and temperature control becomes troublesome.

This typical system problem becomes compounded as nozzles and valves are usually sized for the design capacity but normally operate significantly below these design conditions. This oversizing results in a partially open control valve even at normal operating conditions. With reducing load, downstream water pressure P2 decays rapidly resulting in larger droplet size. Conventional systems therefore will work satisfactorily only at relatively steady load

The VND (Variable Nozzle Desuperheater)

The built in desuperheater valve mechanism regulates the amount of injection water by varying the number of injection nozzles. This enables the water pressure to remain constant, independent of the number of injection nozzles in operation. This results in an excellent and near uniform spray quality over the entire operating range. Control of nozzle opening is achieved by the positioning of a piston which is operated directly by an actuator mounted onto the valve. Through this simple design there is no separate water control valve necessary.

In our next article, we will discuss about Combined Pressure Reducing & Desuperheating (PRDS)