Wednesday, September 5, 2012

Check Valve - Backflow Preventer

What is a Check Valve or Non-return valve (NRV)?

Check valves or non-return valves are self actuated valves and designed to prevent the reversal of flow in a piping system. In other words we can say that Check valves are basically backflow preventers.Valve Check continues to work even if plant loses it power,air supply or hydraulic power source. Even these flow preventers or one way valves do not need operator,s assistance to function.

How Does  a Check valve Work?

Check valves are activated by the flowing material in the pipeline.This flowing material may be gas,water ,air ,oil or steam in pipeline. The pressure of the fluid passing through the system opens the check valve, while any reversal of flow will close the valve.Opening of valve is basically lifting or swinging of check valve disc thus allowing the flow in one direction.Closure in check valve, for backflow prevention, is accomplished by the weight of the check mechanism, by back pressure, by a spring, or by a combination of these means. 

Check valves

Why we use Check Valves or Non-Return Valves or one way valves?

Sometimes reverse flow may be detrimental for equipment or process so it needs to be uni-directional for operational safety. This operational safety is accomplished by valve check. Valves check are self actuated and self operating devices. 
  • Check Valves  are used to protect the equipment that can be affected by reverse flow, such as flow meters, strainers and flow control valves.
  • Check Valves  are used to check the pressure surges associated with hydraulic forces, for example, water hammer
  • Check Valves  are used to prevent the flooding that can take place if flow reversed.
  • Check Valves  are used to prevent the reverse flow on system shutdown.
  • Check Valves  are used to to prevent the flow under gravity.
  • Check Valves  are used for relief of vacuum conditions.

  Basic Types of Check Valves

It is very important for HVAC mechanical engineer to know about various valves being used in mechanical industry.Various check valve types have been developed to cater to the diversified requirements in piping systems in various applications.These various types of check valves offer many advantages and disadvantages .One type of check valve ideal for one application may be misfit for another application.Plant engineers have to look into many factors to decide which type of valve would be they using in that particular application.
check valve types   
Following are the main check valve types used in various applications.
  1. Swing Check Valve
  2. Tilting Disk Check Valves
  3. Lift Check Valves
  4. Piston Check Valves
  5. Butterfly Check Valves
  6. Stop Check Valves
  7. Other Check valve types (Ball check valve, diaphragm check valve etc)
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