In this lesson we will identify basic features of pressure control valves and explain common characteristics of spring-loaded poppets.
Pressure controls are needed whenever there is the possibility of a pressure runaway, or when a second, lower pressure must be maintained in a part of the circuit. Pressure controls are also used to help control motion where gravity or other over-running forces are at work.
All pressure control valves are either normally closed or normally open.
Most types of pressure control valves are of the normally closed design. They will not pass fluid through them until pressure builds up to a certain level. Relief valves, sequence valves, unloading valves and counterbalance valves are all examples of normally closed, pressure valves.
Common to all of these valves is the main poppet or spool, and the main spring.
In many cases a pressure valve will have two poppets and two springs. The smaller of the two springs will be much stiffer and harder to compress. The small poppet and spring will be found in a section of the valve called the pilot section.
These valves with the two poppets and two springs are generally considered to be more precise.
In it's very simplest form, some pressure valves such a relief valve can be seen as just a spring loaded check valve.
In fact, on some schematics a basic relief valve has been shown, incorrectly with a check valve symbol.
While a pressure valve is not meant to be used as a check valve and vice versa, there are some similarities. They both have a poppet and a spring.
The difference is that relief valves limit the system pressure and check valves prevent flow in one direction.
Before looking at specific valve types, lets review some terms common to all pressure control valves.
Cracking Pressure is the system pressure value at which a valve has begun to pass flow. This is not be confused with the true, maximum pressure setting of the valve. Cracking pressure is always a lower pressure than the maximum pressure setting.
Pressure Override is the difference in pressure values between cracking pressure and full-flow, setting pressure.
Setting Pressure is the maximum system pressure that will occur under normal operating circumstances, and with all of the pump’s flow (full flow) passing through the valve.
Key Concept: Because they are operated by a spring, pressure control valves open over range of pressure, beginning at their cracking pressure and only opening fully once the system reaches the setting pressure.