PRESSURE COMPENSATED FLOW CONTROL VALVE

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. Following figure displayed here represents the pressure compensated flow control valve.

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 P_I 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 P_O 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.

P_I- P_O = P _Spring

But spring pressure will be constant and therefore spring loaded compensator spool will maintain the pressure drop across the valve constant.

P_I- P_O = Constant

Now let us consider one condition where load increases, what will happen? Outlet pressure of needle valve also increases i.e. P_O increases due to increment in the load. As P_O 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.

We will discuss Gate valve working principle in our next post.

Image courtesy: Google

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