Normally open and close valve .Why there are two types.Based on normal seating position ,pneumatically operated spring return valves are two types ,
Normally close or Air to open
Normally open or Air to close
We all know this .But have you ever thought why we need these two types .????
We can position them at our desired point by electric or pneumatic signal .Then why there is two types???
For example take a situation .A normally open control valve is used for a specific purpose .Say for boiler feed water control .Why should we didn’t use normally close control valve .We are able put the valve at any desired position ,wheather it is normally closed or normally open. we can control them as our desire
Then why there normally open valve ?
What happens if we use normally close valve there instead of normally open? Any criteria for their selection for a specific process???
The fact is that the selection is not random for normally open or normally close valve .One important consideration while selecting the type is safety .A failsafe design is preferred especially in process environment .This selection of normally open or normally close valve also based on failsafe strategy .
if a normally close valve fails because of some reason .Like electric signal failure, pneumatic signal failure etc .Then the valve will attain it’s normal position due to its spring tension ,which is close .It is considered as it’s failsafe position .Which means under failure conditions the valve will close . similarly a normally open valve opens at such conditions .
So if a flow of a fluid stream should be blocked under failure conditions then what will you choose???
Yes..You should choose a normally close valve in that flow line . Because the stream is considered to be safe if it is close under failure conditions .For example fuel lines ,gas lines etc .
And if we need passage of fluid under such conditions you should choose a normally open valve .
Ex: valves in cooling water lines ,feed water valve for boiler etc .Here flow of fluid is ensured under failure condition of the valve . ,
In the above discussed example . Suppose we are using a normally close valve instead of normally open valve for the boiler feed water .Water is supplied to the shell side through this valve .The water in the boiler shell is converted to steam by the heating medium supplied through the tubes .And suppose air to the control valve fails .Or electric signal to the control valve fails .what will happen ????
The control valve will attain close position due to spring tension .This is because valve is normally close valve .Its failure position is close .So control valve closes .So the feed water flow interrupted .This is the danger situation .The lack of water in the boiler will cause the damage to its water tubes because of the heating medium..This situation is not desired .So instead of using normally close valve,if we use normally open valve there the system will be more safer .