WORKING PRINCIPLE OF A CHECK VALVE AND ITS TYPES IN HYDRAULIC SYSTEM

We have seen just an overview of pressure relief valve, pressure reducing valve and also pressure sequence valve in our previous post. Today we will discuss here basic principle of check valves and its various types with the help of this post.

Check valves

Check valves are widely used in hydraulic units. Such type of valve will permit the flow in one direction and will not permit the flow from other side and therefore check valves also termed as non return valves because such valves will not permit the return flow.

Check valves could be fixed independently in a line in hydraulic unit or such valves could also be fixed as integral component of pressure reducing valve, pressure sequence valve. Flowing fluid pressure force will open the check valve while the reverse flow pressure or spring force will close the check valve.

Check valve: Various types of check valves

Let us see here the various types of non-return valves i.e. check valves.

Standard Check valves

On the basis of relative position of ports, standard check valves are further classified in two types. First one is inline check valves and second one is right angle check valves. Working phenomena of inline check valves and right angle check valves are same. These valves will have valve seat, spring and ball or poppet as shown in below figure.

Hydraulic symbol of standard or conventional check valve

Flowing fluid entering to the check valve via its inlet side will exert the pressure force against the spring force and once flowing fluid pressure force overcomes the spring force, valve poppet or ball will leave the valve seat to permit the flow of fluid i.e. fluid will flow from port 1 to port 2.

Flowing fluid entering to the check valve via its outlet side will support or help to spring for holding the ball or poppet against the valve seat in order to avoid the flow of fluid in reverse direction i.e. port 2 to port 1.

Standard check valves that we have seen above are available in market with various cracking pressure. Cracking pressure is basically defined as the upstream pressure at which check valve will be operated.

Restriction Check valves

As name indicates, restriction check valve will permit the free flow of fluid in one direction but will restrict or reduce the fluid flow in other direction and this characteristic of restriction check valve make it unique from conventional check valves. In my opinion, such valves are used in meter-in and meter-out hydraulic circuit. If i am wrong, please correct me.

Hydraulic symbol of restriction check valve

Pilot operated check valve

Pilot operated check valve is also termed as PO check valve in industries. PO check valves will also work as other conventional check valve i.e. they will permit the free flow in one direction and will prevent the fluid flow in reverse direction. But in case of PO check valve, fluid flow in reverse direction will take place after applying adequate pilot pressure by a pilot line.

We may see in figure the condition A. PO check valve is working here as normal conventional check valve. Flowing fluid entering to the check valve via its inlet side will exert the pressure force against the spring force and once flowing fluid pressure force overcomes the spring force, valve poppet will leave the valve seat to permit the flow of fluid. There will not be any requirement of pilot pressure at this time.

Let us see the condition B, flowing fluid entering to the check valve via its outlet side will support or help to spring for holding the poppet against the valve seat in order to avoid the flow of fluid in reverse direction. There will not be any pilot pressure at this time also.

Now let us consider the case of C, pilot pressure is applied at pilot port for overcoming the spring force and other net force acting at other port, poppet will leave the valve seat as shown in figure due to the action of pilot pressure acting at pilot port and hence fluid flow in reverse direction will be start i.e. port 2 to port 1.

We will discuss the basic concept and working principle of pressure relief valve in our next post.

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