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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