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How do non-return (check) valves work?

How do non-return (check) valves work?

A check valve provided with a spring or counterweight commences opening when certain static pressure is differential (pressure upstream minus pressure down-stream of the valve) has been established. 

This produces an opening force (pressure differential x crosssectional area of valve), acting against the closing force. 

When the opening force outbalances the closing force, the valve element is opened (opening pressure). The necessary opening pressure depends on the spring tension or the weight of the counterweight, the position of installation and the size of the check valve.

On start-up of your plant, static pressure will first build up between the pressure generator and the check valve. 

Once the opening pressure has been reached and exceeds the closing force, the volume flow begins. 

The pressure upstream of the check valve drops, so that the closing force once again outbalances the opening force, causing the valve to close. 

This process will repeat itself until the volume flow is large enough so that, because of the additional dynamic pressure differential, the closing force is no longer great enough to close the valve.

Non-return (check) valves in steam and condensate systems

Depending on the volume flow, the check valve opens either completely or only partly. During partial opening, i.e. at a reduced volume flow, clatter may occur. 

This is produced by the impact of the valve disc, cone or flap onto the seat. It is possible that, instead of clatter, a high-frequency humming tone is heard. 

An incessant noise is a sure sign of check valves sized too large; the valve was not chosen optimally to match the requirements of the plant. 

The possibility of using a valve with a smaller nominal size is no longer given if the pressure drop at full load would become too great.

Non-return check valve in situ

Sometimes, the noise can only be heard during start-up or shut-down, or only at partial load. 

Then the valve is oversized only for these operating phases, and it might be possible to solve the problem by reducing the closing force, for example by using a weaker spring or a reduced counterweight. 

It may even be possible to use a valve without any spring at all – in vertical pipelines with upward flow.