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PRESSURE REDUCING VALVE WORKING PRINCIPLE AND ITS INTERNAL CONSTRUCTION

We have seen working principle of pressure relief valve and its internal construction, we have also discussed the working principle of pilot operated pressure relief valve and its internal construction. Now we will look here the type of valve i.e. Pressure reducing valve. We will study here the working principle and internal construction of pressure reducing valve.

Pressure reducing valve will be used in hydraulic unit for limiting the pressure at specified location in hydraulic system or we can also say that pressure reducing valve limits the pressure value in a branch of hydraulic circuit lower than the required pressure value in main circuit of hydraulic unit. Pressure reducing valve will be normally open.

If we look the internal construction of pressure reducing valve, we will observe that there is one spring loaded spool and this spool will control the outlet pressure of valve. Inlet port of valve will be connected with main circuit and outlet port of valve will be connected with branch circuit of hydraulic unit. There will be one adjusting screw and pressure setting will be done in pressure reducing valve with the help of this adjusting screw.
Pressure reducing valve

We must see here that there will be one internal passage E from the outlet port and fluid will be transmitted to the spool end through this internal passage and therefore fluid will exert the pressure force over the spool against the spring. Valve will be in open condition, if the exerted pressure force by fluid is lower than the spring thrust force.

If downstream pressure, which is also termed as outlet pressure, is below the setting of valve then in that situation fluid will flow from inlet port C to outlet port D of the valve.

When pressure value at outlet port increases to the setting pressure of valve, spool will move in upward direction and hence will block the outlet port partially and it will increase the valve resistance to flow and therefore there will be pressure drop across the valve in order to maintain the desired outlet pressure.

Pressure reducing valve will be indicated by the symbol, as displayed here, in hydraulic circuit of a hydraulic system. Pressure reducing valve will read the outlet pressure. Pressure reducing valve will have one external drain line and this drain line will be connected with the tank.
Pressure reducing valve symbol

Let us see one hydraulic circuit, where we will understand the operation and importance of pressure reducing valve.
We can see here the hydraulic circuit of one hydraulic system. There are two hydraulic cylinders here. One is work cylinder which will be operated at full system pressure, other one is clamp cylinder which is used for clamping the part or workpiece. We need to clamp the workpiece for further engineering activities and we want that there should not be any clamping mark or dent over the surface of the workpiece.

Second cylinder i.e. clamp cylinder must be operated at lower pressure than the main system pressure as if clamp cylinder is also operated on main system pressure then it will crush or damage the workpice.

Let the system main pressure is P1 and clamp pressure is P2, as per our requirement P2 must be lower than the P1 i.e. P1>P2.

Our requirement could be secured easily by using one pressure reducing valve at the feed line of clamp cylinder as shown in hydraulic circuit diagram.

If pressure in clamp cylinder increases above P2, spool of valve will move in upward direction to block the outlet port partially and therefore there will be pressure drop across the valve in order to maintain the desired outlet pressure.

We must understand here that the above mentioned pressure drop will be the responsible of heating of fluid because loss of pressure energy will be converted in to heat energy. If pressure reducing valve setting is quite low than the main system pressure then there will be large pressure drop and hence there will be excessive generation of heat.

Image Courtesy: Google


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