We were discussing " Hydraulic flow control valves" in our previous post, where we have seen the basic concept and application concepts of flow control valves in hydraulic units.

Today we will concentrate here the classifications of flow control valves with the help of this post. There are basically two types of hydraulic flow control valves.
1. Non pressure compensated flow control valve
2. Pressure compensated flow control valve

### Let us see here first the non pressure compensated flow control valve

Non pressure compensated flow control valves will be usually used where system pressure is relatively constant.  Flow rate of hydraulic oil flowing through an orifice will be dependent over the following two factors as mentioned below.
1. Opening of the orifice
2. Pressure difference across the valve
Flow rate of hydraulic oil flowing through an orifice i.e. Q will be directionally proportional to pressure difference across the valve. Flow rate i.e. Q will also be directionally proportional with size of opening of orifice.

Let we have considered that opening of the orifice is same, in that case flow rate of hydraulic oil flowing through an orifice will be dependent over the pressure difference or pressure drops across the valve. In the same way, if we have taken the pressure difference across the orifice is same then flow rate of hydraulic oil flowing through an orifice will be dependent over the opening of the orifice.

Let us see one example to understand the above statement. Let we have one application of hydraulic unit, where one hydraulic cylinder is pushing 1500 lb load and flow rate of hydraulic oil flowing through the orifice for a particular opening of the orifice is 5 GPM and pressure difference across the valve is 500 PSI.

Inlet pressure at the valve will be the pressure provided by the pump and it will be remain constant, hence pressure difference across the valve will be changed if pressure at the outlet of the valve changes. If pressure increases at the outlet of the valve, pressure difference across the valve will be reduced.

Let cylinder is pushing now higher load as compared to earlier in our above example, let now cylinder is pushing 2500 lb load. As load is increased at actuator i.e. cylinder, therefore pressure at the outlet of the valve will be increased and pressure difference across the valve will be reduced. Let now the pressure difference across the valve is 300 Psi. Now if we will determine the flow rate of hydraulic oil flowing through the orifice, it will be reduced and will be lower than the earlier i.e. it will be less than the 5GPM and speed of the cylinder will be reduced due to reduction in flow rate of hydraulic oil.

Therefore, we can say that the basic operation concept behind non pressure compensated flow control valve is that the flow rate of hydraulic oil through the orifice will be constant if pressure difference across the orifice is also constant.

This type of valve is termed as non pressure compensated flow control valve or also termed as throttle valve.
Non pressure compensated flow control valve

We will come to know with the above example that if load varies, speed of the cylinder piston could not be determined accurately with non pressure compensated flow control valve.

Non pressure compensated flow control valve is the simplest form of flow control valve. Such valve will have one adjustment screw with needle type of end. Flow rate of hydraulic oil will be changed by changing the orifice opening and Orifice opening will be changed by turning the adjustment screw.

There is one more form of non pressure compensated flow control valve, where there will be one check valve associated with this valve in order to control the flow rate of hydraulic oil in one direction only. With the provision of check valve, there will not be any restriction in fluid flow in opposite direction i.e. check valve permits here the free flow of fluid in opposite direction.

We will discuss the second type of flow control valve i.e. Pressure compensated flow control valve in our next post.

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