The second type of valve is a pressure-reducing valve. This type of valve (which is normally open) is used to maintain reduced pressures in specified locations of hydraulic systems. It is actuated by downstream pressure and tends to close as this pressure reaches the valve setting. Schematic diagram of pressure reducing valve is shown in Fig. 1.6, symbolic representation is shown in Fig. 1.7 and three-dimensional view is shown in Fig. 1.8.
A pressure-reducing valve uses a spring-loaded spool to control the downstream pressure. If the downstream pressure is below the valve setting, the fluid flows freely from the inlet to the outlet. Note that there is an internal passageway from the outlet which transmits outlet pressure to the spool end opposite the spring. When the outlet (downstream) pressure increases to the valve setting, the spool moves to the right to partially block the outlet port. Just enough flow is passed to the outlet to maintain its preset pressure level. If the valve closes completely, leakage past the spool causes downstream pressure to build up above the valve setting. This is prevented from occurring because a continuous bleed to the tank is permitted via a separate drain line to the tank.
Reverse free flow through the valve is only possible if the pressure exceeds the valve setting.The valve then closes, thus making reverse flow impossible. Therefore, pressure-reducing valves are often equipped with a check valve for reverse free flow.
External forces acting onto a linear actuator increase the pressure between the pressure-reducing valve and the actuator. In some systems, it is therefore desirable to relieve excess fluid from the secondary system to the tank in order to maintain a constant downstream pressure, regardless of such external forces.
A reducing valve is normally open. It reads the downstream pressure. It has an externaldrain.This is represented by a line connected from the valve drain port to the tank. The symbol shows that the spring cavity has a drain to the tank.
Figure 1.9 shows an application for a pressure-reducing valve. Here two cylinders are connected in parallel. The circuit is designed to operate at a maximum pressure p1, which is determined by the relief valve setting. This is the maximum pressure at which cylinder 1 operates. By the function of the machine, cylinder2 is limited to pressurep2 (p2<p1).This can be accomplished by placing a pressure-reducing valve in the circuit in the location shown in Fig. 1.9.If the pressure in the cylinder2 circuit rises above p2, the pressure-reducing valve closes partially to create a pressure drop across the valve. The valve then maintains the pressure drop so that the outlet pressure is not allowed to rise above p2 setting.
The disadvantage of this method is that the pressure drop across the reducing valve represents the lost energy that is being converted into heat. If the pressure setting of the reducing valve is set very low relative to the pressure in the rest of the system, the pressure drop is very high, resulting in excessive heating of the fluid. When the hydraulic oil becomes too hot, its viscosity reduces causing increased component wear.
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