Operation Principle of Relief Valve

Relief valve is used to set the maximum pressure in a hydraulic system. There are many designs and varieties of relief valve, but they can be denoted by direct-acting and two-stage relief valve.

The function of a relief valve is to set the maximum pressure in a hydraulic system. Although there are many designs and varieties, they can all be denoted by the general symbol (a) in Figure 2. It is a normally closed valve which partially opens permitting flow to the tank port when the pressure at the inlet port overcomes the spring force. Symbol (a) does however more accurately represent a direct acting valve. If there is no arrow through the spring the valve is pre-set, i.e. non-adjustable. A two-stage relief valve may be shown as Figure 2(b) as this more closely illustrates its operation which will be described later.

Figure 1. Pressure-control valve.

When the pressure control valve shown in Figure 1 is closed the inlet and outlet ports are isolated by the valve spool. An adequate hydraulic seal is obtained-owing to the minute clearance between the spool and its housing. This seal becomes less efficient as the working pressure increases. Many simple direct acting relief valves employ either a conical poppet or a ball to seal against a mating valve seat. Being a contact type seal this is more effective at high pressure. In the poppet type relief valve (Figure 3) pressure at port P acts on the exposed surface of the poppet to apply a force which is resisted by the spring force. When the pressure at port P has risen sufficiently to overcome the spring force, the poppet is lifted off its seat permitting fluid to flow to the tank port T, relieving the pressure in the system.

Figure 2. Relief valve symbols. (a) General or direct-acting. (b) Two-stage.

Relief valves of the ball or poppet type have a rapid response to pressure surges, typically 25 ms, but the pressure flow characteristic is not constant. The poppet or ball tends to hammer on the seat giving rise to relief valve whine; seat damage can occur with resultant leakage and they are best suited for infrequent duty. A variation is the guided poppet type relief valve which has the advantage of a direct poppet valve but is more suitable for continuous duty.

Figure 3. Poppet type direct acting relief valve.

The guided piston relief valve (Figure 4) is of much quieter operation but is best suited for low-pressure applications (up to 100 bar) under constant flow conditions. The response time is still fast although slightly slower than the direct poppet-type relief valve. In common with the preceding direct-acting relief valves, it has a high pressure over-ride characteristic. The pressure override is the difference between the cracking pressure or opening pressure and the pressure drop across the valve when it is passing the maximum rated flow at the same valve setting.

Figure 4. Guided piston relief valve.

The differential piston/poppet type relief valve (Figure 5) is suitable for pressures up to 350 bar. The pressure acts on the differential areas between the poppet and the seat. When the valve operates, a large flow area opens for a relatively small poppet movement. This results in a low-pressure over-ride, but the reset pressure may be appreciably lower than the opening pressure.

Figure 5. Differential poppet relief valve : force to overcome sping = pressure x (a-b).

The pilot-operated relief valve .(Figure 6) is a two-stage valve which gives good regulation of pressure over a wide range of flow. It consists of a main spool controlled by a small built-in direct-acting relief valve. Pressure is sensed at the pilot relief valve via a small hole or jet in the spool or through the housing. When the control valve is closed the main spool is in hydraulic balance; it is however held onto its seat by a light spring. Any increase in pressure sufficient to open the control valve throws the main spool out of balance owing to the pressure drop across the jet, and the spool lifts against the spring relieving the major flow from the pressure to tank port. The small amount of flow which passes through the control section is also returned to the tank port (i.e. it is internally drained). Alternatively the control section may have an external drain connection to avoid the effect of back pressures in the tank line.

Figure 6. Pilot-operated relief valve.

A separate pilot or vent port V which is plugged for normal operation is fitted so that the valve can be remotely operated. This port is on the control side of the main spool and connection to the tank causes the main spool to imbalance at a very low pressure. This venting feature is a useful method of unloading a pump or circuit. Alternatively, the main valve can be remotely controlled by connecting another relief valve to the vent port V. This will regulate the pressure from its minimum up to the limit set by the main valve pilot section. Both these features are demonstrated in Figure 7 in which a three-position solenoid actuated directional-control valve enables the relief valve to be remotely operated by an electrical signal to give three different pressure settings, one of which is nominally zero, the relief valve then being vented. With solenoid a energized, internal pressure control is achieved; with solenoid b energized, remote pressure control is achieved; and with both solenoids a and b de-energized, the valve is vented. The directional cofitrol valve may be integral with the relief valve or a separate valve connected to the vent port.

Figure 7. Solenoid-controlled relief valve

1. Dual-relief valves

Frequently, relief valves are required in pairs to relieve the pressure on either side of an actuator. These usually take the form of a sandwich block which can be built into a valve stack as shown symbolically in Figure 8. The pressure in the service lines (A and B) may be relieved directly to the tank line T (port relief) or to the opposite service line (cross-line relief).

Another application of the dual-relief valve is in hydrostatic transmission when it is usually referred to at a cross-line relief valve.

Figure 8. Dual valves. (a) Port relief. (b) Cross-line relief.

2. Relief volve selection and pressure setting

Considerable care should be taken in choosing the correct type of relief valve for 3 particular application and in selecting the pressure at which il should just open or crack Most direct acting valves have high pressure override characteristics which make them unsuitable for systems with widely varying flows. The reset pressure (that is the pressure at which an open valve closes) must also be considered. This may be as low as 50% of the opening pressure, owing to the flow forces and the design and construction of the valve Response time may be the most important criterion in a specific application.

In general, two stage valves give good pressure regulation over a wide range of flow with low pressure override and close tolerance between the opening (or cracking) am resetting pressures. Direct acting valves have rapid response limes. Poppet types are the most tolerant to fluid contamination and also tend to have less internal leakage than spool valves, which makes them suited to high pressure working.

A frequently used rule .of thumb is for the main relief valve in a circuit to be set a 10-20% above the maximum required working pressure, taking into account the type of valve, its position relative to the actuator and the pressure losses in the system. Where there is  more than one pressure valve in a circuit or when used in conjunction with pressure compensated pumps, the controls must not be set at pressures which are too dos together as interaction or hunting may result. It is usual to set secondary relief valves such as port or cross-line reliefs at a pressure higher than the main relief valve.

3. Unloader valves

A relief valve can be unloaded in two ways: by pressure release, i.e. venting, or by a pilot pressure.

3.1. PRESSURE RELEASE (VENTING)

It was seen that the two-stage relief valve in Figure 6 could be unloaded by connecting the vent port V to tank (Figure 7). Venting causes the main spool to be unbalanced and open at a very low pressure dumping the pump flow from P to T. The main flow may be quite large but the flow through the vent port will be very small.

3.2. PILOT PRESSURE

The valve in Figure 1 will function as a direct-acting unloader when subject to a remote pilot pressure. As long as the force resulting from the pilot pressure is greater than the force set by the control spring, the relief valve will open fully, allowing the main flow to go to tank at low pressure.

3.3. DIFFERENCE BETWEEN VENTING AND PILOT-PRESSURE UNLOADING

In Figure 9(a), opening the vent port V releases pressure and causes the main spool to open. This is independent of the setting of the control spring. In Figure 9(b), the pressure signal at X from a remote source pilots the valve open against the spring setting.

Figure 9. Relief valve unloading. (a) By venting. (b) By pressure signal.

In a two-stage unloader valve pilot pressure from a remote source causes a piston to unseat the control poppet of the relief valve. The main spool is imbalanced and opens, dumping the main pump flow from P to T at a very low pressure. Although it is the high pressure on the pilot-port piston which causes the valve to unload, the action of pushing the control poppet off its seat vents the main spool. Generally, the valve will still respond to pressure at port P and function as a normal relief valve. A typical application is in a twin pump circuit (sometimes called a 'high-low’ or 'hi-to’ circuit). This is frequently employed on presses where both pumps supply fluid to move the tooling with just the small pump carrying out the pressing operation. Considerable savings in input power can be achieved. These circuits employ a check valve to isolate the high and low pressure halves of the circuit and sometimes the check valve is incorporated into the unloader valve (Figure 10). Another established application is with an accumulator and Figure 11 depicts the valve from Figure 10 in such a circuit.

Figure 10. Two-stage unloader valve with integral check valve.

It can be seen that whereas the direct-acting valve (Figure 1) can only be opened by a pressure at the pilot port, the two-stage valve (Figure 10) may still function as a normal relief valve responding to an internal pressure. However, since the pilot piston has a slightly larger area than the control-valve poppet, the external pilot pressure needed to open the valve is less than the direct pressure setting of the relief-valve spring.

A particular type of direct-acting unloader valve (not illustrated) specifically used in dual-pump systems unloads the secondary pump when the main pump circuit reaches a predetermined, non-adjustable pressure (say, 20 bar) below the relief-valve setting.

Figure 11.Accumulator circuit using the valve illustrated in Figure 10.