Pressure relief and sequence valves

Pressure relief and sequence valves

 an example of a pressure relief (safety) valve. In Figure 2.84a the valve is being used in a hydraulic circuit. Pressure is controlled, by opening the exhaust port to the reservoir tank against an opposing force such as a spring. In Figure 2.84b, the valve is being used in a pneumatic circuit so it exhausts to the atmosphere.

Figure 2.84c and d shows the same valves except that this time the relief pressure is variable, as indicated by the arrow drawn across the spring. If the relief valve setting is used to control the normal system pressure as well as acting as an emergency safety valve, the adjustment mechanism for the valve must be designed so that the maximum safe working pressure for the circuit cannot be exceeded.

Figure 2.84e,f shows the same valves with the addition of pilot control. This time the pressure at the inlet port is not only limited by the spring but also by the pressure of the pilot circuit superimposed on the spring. The spring offers a minimum pressure setting and this can be increased by increasing the pilot circuit pressure up to some predetermined safe maximum. Sometimes the spring is omitted and only pilot pressure is used to control the valve.

Sequence valves are closely related to relief valves in both design and function, and are represented by very similar symbols. They permit the hydraulic fluid to flow into a subcircuit, instead of back to the reservoir, when the main circuit pressure reaches the setting of the sequence valve. You can see that Figure 2.85 is very similar to a pressure relief valve (PRV) except that, when it opens, the fluid is directed to the next circuit in the sequence instead of being exhausted to the reservoir tank or allowed to escape to the atmosphere.

Flow control valves

Flow control valves, as their name implies, are used in systems to control the rate of flow of fluid from one part of the system to another. The simplest valve is merely a fixed restrictor. For operational reasons this type of flow control valve is inefficient, so the restriction is made variable as shown in Figure 2.86a. This is a throttling valve. The full symbol is shown in Figure 2.86b. In this example the valve setting is being adjusted mechanically. The valve rod ends in a roller follower in contact with a cam plate.

inlet pressure to the valve does not affect the flow rate from the valve. Under these circumstances we use a pressure compensated flow control valve (PCFCV). The symbol for this type of valve is shown in Figure 2.87. This symbol suggests that the valve is a combination of a variable restrictor and a pilot operated relief valve. The enclosing box is drawn using a long-chain line. This signifies that the components making up the valve are assembled as a single unit.