Modular hydraulic pressure relief valve and hydraulic cartridge pressure relief valve are available in almost all hydraulic systems, so it is important to have a full and deep understanding of the characteristics of the relief valve used.

Hydraulic Relief Valve Function and Application
Hydraulic relief valve consists of valve housing A, valve poppet B and spring C and other basic components, at least there are one inlet (P) port and one outlet (T) port. The main role of hydraulic pressure relief valve is essentially pressure limiting: to limited the working pressure is by pressure relief.cartridge-relief-valve-structure-and-symbol
The load determines the pressure, the hydraulic relief valve only limits the pressure of the hydraulic system, but the pressure relief valve itself cannot generate pressure.

Basic Common Features Of Various Types Of Hydraulic Pressure Relief Valves:
1) When the inlet pressure reaches the pressure presetting point, the pressure relief valve will open and relief the pressure then.
2) The spring chamber is generally connected to the outlet port and the back pressure is equal to the outlet pressure. Therefore, in addition to the type of unloading outside, the pressure at outlet port is directly increased by 1:1 proportion. If you want the opening pressure to be completely independent of the outlet pressure, you should consider using a sequence valve.

Basic Functions Of Hydraulic Relief Valves:
A. As a safety valve
used to limit the maximum pressure of hydraulic system, it plays the following role:cartridge-relief-valve-for-pump-and-motor-protection
1. Avoid the high pressure damages to the hydraulic system, components and piping
2. Avoid hydraulic power source such as electric motors, diesel engines, or gasoline engines stops while operation due to heavy loads
3. Avoid excessive force/torque generated by the hydraulic cylinder or hydraulic motor, which may damage connected or pushed components; Avoid hydraulic cylinder or hydraulic motor with large inertia load, due to excessive inertial force during acceleration braking or torque, resulting in damage.
   In these operating conditions, the pressure relief valve is normally closed.

B. As pressure control valve, hydraulic pressurepressure relief valves maintain the pressure in whole hydraulic system or partial system within a specific level or range, for example:cartridge-relief-valve-for-hydraulic-throttle
1. Relief flow for the inlet and outlet throttling speed control circuit, which can be referred to as the constant pressure valve at this time
2. Generate the back pressure on the return oil to improve motion stability, which is often referred to as back pressure valve.
3. Unloading the pressure in double pump circuit or accumulator circuit, which is often referred to as pressure unloading valve.
In these conditions, the hydraulic pressure relief valve is normally open.

There are many types of hydraulic pressure relief valves, which can be classified from many different angles.
Classification according to valve poppet structure:
1) Ball type:
 The ball type relief valve is simple in structure and low in cost, but it is only suitable for small flow.
2) Poppet type: The poppet hydraulic pressurerelief valve can pass through a large flow, with less leakage, quick response and longer life, it is most widely used.
3) Spool type: The spool type relief valve can flow through a larger volume but with a smaller pressure range regulation.

Classification According To The Type Of Action
1) Direct acting pressure relief :
 The direct-acting type has fast response and small overshoot and is suitable as a safety valve to reduce the impact, but the pressure regulation deviation is large, that is, the control pressure fluctuates greatly with the flow rate.
2) Pilot operated pressure relief: The pressure regulation deviation of pilot type is small, control pressure accuracy is high, and it is used in occasions where pressure control is more accurate needed, but the response is slower
3) Soft pressure relief type : Soft pressure relief type will be able to relief the pressure before the pressure reaches to presetting value and surges in the inlet port.cartridge-relief-valve-symbols
The pressure relief valve in the constant rate pump with the open central spool directional control valve, or the (constant pressure) variable pump with the closed neutral directional control valve circuit generally requires a low leakage, fast response, anti-pollution, vibration reduction, can be considered using direct-acting type.
In the hydraulic constant rate pump throttle circuit, continuous flow supply and high-precision pressure control are generally required. When small amounts of internal leakage have little effect, pilot type pressure relief valves may be considered.

Classification According to the location of the application in the loop
1) Main hydraulic pressure relief valve, installed in the same direction of directional valve, the side of the hydraulic pump
2) Secondary hydraulic pressurerelief valve mounted after the directional valve, the side of driving device

Classification according to function

cartridge-relief-valve-symbols-as-per-various-function1) Normal type
2) Pressure-safe type. After opening, it will not be closed until the inlet pressure drops to zero. Not suitable for circuits that require load holding.
3) With reverse check valve type. The general relief valve does not pass in the reverse direction.
4)  Bi-directional type. In the hydraulic motor drive circuit, the pressure relief valve, or a two-way relief valve, should generally be installed on both sides to prevent excessive pressure on one side due to external loads during neutral conditions.
5) External control type. The opening pressure can be changed by an additional control type such as a hydraulic control, air control, a solenoid switch, or an electrical proportional control.
* There is also a so-called thermal protection relief valve. Actually, it is a small direct flow relief valve that is used as a safety valve. It starts to relief the pressure when the temperature rises causing thermal expansion of the closed liquid to protect the components (mainly hydraulic cylinders) from high pressure damage.
From the point of oil connection, ordinary types are two-way. The three-port and four-port ports are generally available for external control.

Differential Pressure Flow Characteristics Test For hydraulic pressureRelief Valve
(1) Test circuit
See the figure according to 1S003:1988 and GB/105-1987
1. Hydraulic power source. It is required that the flow rate be smoothly adjustable throughout the test range, which is difficult to achieve with a single variable pump or a fixed pump with a speed control valve. Especially in the initial stage, because the required flow rate is very small, it is often below 0,1 L/min. Using multiple speed control valves in parallel, or bypass throttle valve can take in to consideration.
2. The pressure relief valve in the system is used for safety protection only. The pressure set value must be higher than the test range, but it does not exceed the allowable pressure of the valve under test.
5. Pressure sensor. 5a measures the inlet pressure. 5b measures the outlet pressure. If the outlet pipe is very short, thick, the pressure loss is negligibly small even can be omitted. Or only by a low pressure gauge is used for monitoring.
6. Tested valve
7. Flow sensor. Gear flow sensors may be considered here because they have a much larger measurement range than turbine flow sensors.
8. X-Y recorder, or digital oscilloscope, or computer data acquisition record display system.

Test Process Of Differential Pressure Flow Characteristics
Preparation stage:

Connect the X-Y recorder with flow qv7 as the X axis, differential pressure P5a-P5b or pressure P5a as the Y axis. Allow the oil temperature to reach the predetermined value.

Test procedure:
1. Turn on the hydraulic pressure source
2. The tested valve 6 is adjusted to the minimum value of the given pressure adjustment range.
3 start recording. Slowly increase the output flow of the hydraulic source from zero to the maximum test flow, then slowly decrease it to zero and stop recording. 4. The tested valve 6 is adjusted to the maximum value of the given pressure adjustment range and the steps are repeated step 3.
5. Set a few more values between the maximum and minimum values of the pressure adjustment range and repeat step 3.
*Keep the oil temperature relatively constant throughout the test. The test curve thus obtained is the valve’s characteristics under the operating conditions of the valve, which is typical test curve.

Transient Response Characteristics Test Of hydraulic pressurePressure Relief Valves
Test circuit: According to ISO03:1988 and GB/T8m5-1987:cartridge-relief-valve-Transient-Response-Characteristics-Test

  1. Hydraulic power source. It is best not to use a variable pump to avoid the dynamic response characteristics of the variable mechanism affecting the test results.
  2. The system pressure relief valve is for safety protection only. The pressure presetting value must be significantly higher than the test range. Pressure relief should not occur during the test, otherwise it will reduce the system pressure rise gradient.
  3. Bypass valve. Where 3a is used for pressure loading, it must be a quick-response valve, otherwise it will not be able to obtain a sufficient gradient of system pressure rise. 3b is used to control the pressure in the pilot chamber of an externally controlled relief valve. This part of the circuit must be modified accordingly with the type of valve being used.
  4. Thermometer.
  5. Pressure sensor. Response characteristics better than 5000Hz
  6. The valve being tested.
  7. Flow meter. If the flow is known, it can be removed or moved to the return line of the tested valve in order to reduce the volume of the pipe.
  8. Throttle valve. Set initial pressure.
  9. Zero flow display. It is used to monitor whether the flow bypasses the test. It can be a measuring cup or an ordinary container.
  10. Quick recorder. If digital, sampling time is shorter than 0.2ms.

Testing Process:

1. Connect the pressure sensor output P5 to the quick recorder.
2. Start the hydraulic power source. Allow the oil temperature to reach the predetermined value.
3. The tested valve 6 under is adjusted to a certain opening pressure.
4. Bypass valve 3a open. Using the throttle valve 8 to set the initial pressure, it should approach but not exceed the opening pressure of the valve 6 under test.
Test Procedure:
1. At the beginning of the test, the flow from the hydraulic power source 1, is directly drawn through the bypass valve 3a and the inlet pressure P5 of the valve under test is lower than the opening pressure and is closed by the tested valve 6 .

2. Quickly close the bypass valve 3a, P5 quickly rises, open the tested valve 6. After a while, P5 has stabilized. By recording the transient changes in pressure P5, the transient behavior of the tested valve will be able to be seen.

3. If the pressure in the pilot chamber of the tested valve is externally controlled, the bypass valve 3b can be quickly connected to make the measured 6 unloaded. P5 quickly decreased, which can measure the transient characteristics of the tested valve when unloading the pressure