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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 begins to open when a certain static differential pressure (upstream pressure minus the downstream pressure acting on the valve) across the valve has been reached. 

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

When the opening force outbalances the closing force, the valve mechanism opens (opening pressure). The necessary opening pressure depends on the spring tension or the mass of the counterweight, the position of installation, and the size of the check valve.

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

Once the opening pressure is reached and it exceeds the closing force, flow can begin. 

When the pressure upstream of the check valve drops, and the closing force once again outbalances the opening force, the valve begins to close. 

This process will continue until the flow is large enough such that the increased dynamic differential pressure greatly exceeds the closing force therefore keeping the valve open.

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

Depending on the flow rate, the check valve opens either completely or only partially. During partial opening, for example, at a reduced flow rate, clatter may occur. 

This is produced by the impact of the valve disc, cone, or flap repeatedly closing on the check valve seat. In some instances, instead of generating a clatter, a high-frequency humming tone is heard. 

An incessant noise is a sure sign that too large of a check valve was selected; the valve was not chosen optimally to match the requirements of the application. 

The possibility of utilizing a smaller, nominally sized check valve is not feasible if the ensuing pressure drop at full load is not acceptable for the application.

Non-return check valve in situ

At times, check valve noise can be only heard during start-up, shutdown, or during partial load conditions. 

In such cases, the valve is temporarily oversized only during certain operating phases. It may be possible to solve the noise problem by reducing the closing force by selecting a check valve with a weaker spring or a reduced counterweight option. 

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