We were discussing “Hydraulic flow control valves” in our previous post, where we have seen the basic concepts and
application concepts of flow control valves in hydraulic units. We have also
seen the classifications of flow control valves in our previous post, where we
have seen the “Non pressure compensated flow control valve”.
Today we will concentrate here the pressure
compensated flow control valves with the help of this post.
We were discussing in our previous post
that non-pressure compensated flow control valves could not be used in an
application where working load varies because pressure at the outlet of valve
i.e. outlet pressure will also vary if working load varies and therefore
pressure drop or pressure difference across the valve will also vary.
As we know that flow of hydraulic oil
flowing through an orifice depends on the pressure difference across the
orifice, hence flow of hydraulic oil flowing through the orifice or valve will
also vary due to variation in pressure difference across the valve.
Speed of hydraulic actuator will vary
due to variation in flow rate of hydraulic oil entering or leaving the actuator
because speed of actuator depends on the flow rate of hydraulic oil leaving or
entering the actuator. Therefore non-pressure compensated flow control valves
could not be used with hydraulic actuators that must actuate with constant
speed.
For such applications where we need hydraulic
actuators to move with constant speed, regardless of pressure, pressure
compensated flow control valves will be used.
Now let us consider one condition where
load increases and therefore outlet pressure increased to 90 bar from 70 bar due
to increment in working load as shown in figure. Hence, compensator spool will
be moved towards left to maintain the constant pressure difference across the
needle valve.
Pressure compensated flow control valve will
have also a needle valve section like in other flow control valve, but there
will be one spring loaded compensator spool also as shown in figure. This
spring loaded compensator spool will reduce the size of orifice when pressure
drop across the valve increases. If pressure drop across the valve decreases,
in that case this spring loaded compensator spool will increase the size of
orifice.
In short, pressure compensated flow
control valve will provide the constant flow to the actuator by changing the
size of orifice in proportion with change in the system pressure. In order to
provide the constant flow to a hydraulic actuator, spring loaded compensator
will maintain the constant pressure difference across the valve.
In case of pressure compensated flow
control valve, let us see how spring loaded compensator maintains the constant
pressure drop across the valve. Upstream pressure or inlet pressure of needle
valve will be indicated here with PI and it will be measured at the
left hand of spring loaded compensator spool. Outlet pressure or downstream
pressure of valve will be indicated by PO and will be measured at
the right hand side of the spring loaded compensator spool. Let the spring pressure
which will act over the compensated spool towards left side is 8 bar for easily
understanding the concept.
In order to secure the balancing of
spring loaded compensator spool, following equation must be fulfil.
PI-
PO = P Spring
But spring pressure will be constant and
therefore spring loaded compensator spool will maintain the pressure drop across
the valve constant.
PI-
PO = Constant
Now let us consider one condition where
load increases, what will happen? Outlet pressure of needle valve also
increases i.e. PO increases due to increment in the load. As PO
increases, therefore spring loaded spool will be unbalanced and will be moved
towards left and hence inlet pressure at valve will be increased due to
movement of spool towards left. Spool will come again in balance position, once
pressure difference across the valve becomes equivalent with spring pressure.
Therefore, spring loaded compensator
spool will be moved in order to maintain the pressure difference across the needle
valve constant. Once pressure difference across the valve will be constant,
flow rate of hydraulic oil will also be constant regardless of system pressure.
Therefore, pressure compensated flow control valve has solved the problem caused by non-pressure compensated flow
control valve by changing the orifice size in relation of change in the system
pressure.
Following figure displayed here
represents the graphic symbol of pressure compensated flow control valve.
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