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Swing Check Valve - Why & Where ? : Week 1/13/2014

In our last article, we discussed about some issues related to Textile industry. The article was about how important it is to maintain right pH value in the process to ensure the right dyeing quality. In this article, we will talk about Power Industry. This one is about Quick Closing Non Return Valve (QCNRV)

Swing Check Valve : Why & Where?

In power plants, conventionally Swing Check type non return valves have been used to allow flow in one direction and in the event of reversal of flow due to any reason, the disc Swings back in the passage and prevents the medium going back and flooding the source.

While are ideal on Compressible mediums like Steam , on applications where Incompressible mediums are involved conventional Swing Check valve have been found to pose some problems.

In other words operation of Swing Check Valves have been found to be quite unstable in systems with Pulsating flow. Sudden closing of the valve due to a reversal of medium can lead to slamming of the Disc onto the seat as well as lead to damage to the pump casing due to water hammer.

Here is an example : Imagine a pump in a pumping station being switched off. The flow will then begin to lose its speed, it then stops and then there is reversal or back-flow. The check Valve then closes. The flow reversal can occur in an extremely short time, anywhere between 1/100 to 1/10 seconds. If the check valve does not respond quickly, we may experience a closure that will occur at a high negative speed backflow condition.

This can result in :

  • Violent contact of the valve plug against its seating,
  • Water Hammer with a steep wave front.
  • The shock and overpressure caused by water hammer can result in cracks and breakdown of the valve´s components with serious consequences for the pipeline & other critical equipment

 

Why QCNRV?

A QCNRV (Quick Closing Non Return Valve) does a similar function in a power plant except that the valve is additionally provided with an Pneumatic or Hydraulic Actuator and other accessories to reduce the closing time. Assisted by a quick closing device like a Power Cylinder these valves perform the function of preventing reversal of flow and also provide a quick and positive shut off, thus limiting the damage to the source equipment in event of a trip. Since this the movement of the disc is controlled by a pneumatic or a hydraulic cylinder the disc closes without slamming.

This Non-slam function also increases the life of the seat / disc. In applications like on Feed water line QCNRV have found to have helped reduce damaging effect due to water hammer. Thus the role of QCNRV is crucial in a Power plant and it therefore needs to be reliable and dependable. Some of the typical application areas of a QCNRV Valves are listed below:-

  • Discharge lines of Boiler Feed Water Pump
  • Main Steam Extraction lines
  • Cold Reheat Line preventing the Reheat steam from entering Turbine Exhaust line in event of turbine trip
  • For preventing backflow of steam from feed water hearers, auxiliary turbines and other equipment to the main turbine after sudden load reductions or turbine trip
  • In Refinery and Hydrocarbon units these valves are effectively used on Compressor Discharge lines and discharge lines of Ethylene Compressor, Wet Gas Compressor as well as Air Compressors.
  • Turbine back pressure.

Who can benefit?

The applications are enumerable. However, some of the main types of industries who can benefit from QCNRV’s include

  • Turbine suppliers
  • Power plants
  • Cogen plants
  • Refineries & Petrochemical plants
  • In our next article, we will talk about yet another new idea.