There are two installation types of hydraulic check valves modular hydraulic check valves and cartridge check valves.
Hydraulic check valves also known as one way hydraulic check valve or isolated check valve, allow only fluid flows in one direction and stops the fluid flow from the other direction.
Hydraulic check valve can be divided into two major categories: normal check valve and pilot operated hydraulic check valve by the functional point of view check valve are two-port valves, pilot-operated check valve is a three-port valve, the pressure of the additional control port can control the on or off of the hydraulic oil in the main chamber.

Hydraulic check valve is generally composed of a check valve housing, a spring and a valve poppet, and a non-spring check valve is rarely used. According to the recommendations of ISO 1219-1:2006, the symbol of the function of the hydraulic check valve should be as shown in the figure:
In the graphic symbol of the general one-way valve, the spring is often abbreviated.cartridge-check-valve-symbol
When the pressure at port ① is higher than the pressure at port ② plus spring pressure, the valve poppet is pushed open and the oil flow passage is opened, then the oil flows positively through the hydraulic check valve (①->②).
When the pressure at port ② is higher than the pressure at port ①, the valve poppet is pressed against the valve seat by spring force and hydraulic pressure, and then the flow is cut off.

Hydraulic CCheck Valves Are Used In Many Hydraulic Systems:
1) Hydraulic check valves are used to protect the heat exchanger and avoid overpressure when the heat exchanger is blocked. At the same time, the heat exchanger can also be bypassed: only a part of the liquid passes through the heat exchanger. The flow rate depends on the opening pressure of the hydraulic check valve and is less affected by the total flow.cartridge-check-valve-application
2) The hydraulic check valve is used to maintain the pressure in the accumulator. Port ① is the filling port. If it is not filled, the check valve can close this port. In this way, the accumulator pressure will not be lost.
3) The hydraulic check valve is used to make the hydraulic motor variable in both directions at different speeds. If port ② is filled with oil, all pressure oil will flow to the motor due to the function of the check valve closing. If port ① is filled with oil, part of the pressure oil is bypassed through the two-way regulator flow valve and check valve. The speed of the hydraulic motor in this direction will be lower.
4) The check valves with higher opening pressure are also used as low pressure relief valves. There is no essential difference between the structure and function of the two.

Hydraulic check valve can be divided into one-way screw-in type and plug-in type according to the different installation methods.
The plug-in type (Like Logic Valve) is a hydraulic check valve installed inside the cavity of integrated valve block. It is not exposed outside the valve block, this plug-in valve completely enters the inside of the valve block. The screw-in type uses its own thread or other components, such as pipe joints and O-ring rings to installed in the valve block. General depth is as per the valve and bloc calculated before design. According to the different valve poppet, hydraulic check valve can be divided into ball valve poppet type, spool poppet type and spool type.

From the structural point of view, there are flow from the end face, but also from the side flow. This can sometimes simplify the design of hydraulic manifold chamber and reduce the pressure drop.
In addition, there are some additional features of hydraulic check valve, such as with reverse relief valve type. If the hydraulic check valve installed in the hydraulic cylinder that can be used to support the foot of a mobile machine can hold pressure but not over-pressure.
In general, an excellent hydraulic check valve should have small forward flow resistance, rapid reverse closure, reliable sealing, and long life.

Hydraulic Check Valve Flow Differential Pressure Characteristics And Testing
In the way of forward flow, because the pressure drop caused by the spring force and the friction force between the valve housing and the valve poppet needs to be overcome, the through-flow pressure drop cannot be less than the opening pressure.cartridge-check-valve-pressure-curve
In order to obtain smaller flow losses, the spring should be as long and soft as possible. In this way, after opening the check valve, the flow increases, the opening increases, and the spring is compressed, the spring force does not rise too quickly.
The flow differential pressure curve for a hydraulic check valve with opening pressures of 0.34 bar and 2.10bar, respectively.
Test circuit diagram as per ISO6403 the recommended test circuit:

  1. Cartridge-Check-Valve-Flow-Differential-Pressure-Characteristics-And-Testing
  1. Hydraulic power source. The maximum output flow is at least greater than the nominal flow of the test check valve being tested. It is required that its flow should be smoothly adjustable throughout the test range
  2. A relief valve, as a pressure loading valve
  3. Shut off valve (not necessary)
  4. Testing Valve
  5. A pressure gauge or pressure sensor
    5a. Measure the pressure at inlet port
    5b. measures the pressure at the outlet port. If the outlet pipe is very short and thick and the pressure loss is negligibly small, it may not be necessary to monitor it, to install a low-range pressure gauge only
  1. A thermometer
  2. A flow sensor
  3. X-Y recorder

Hydraulic Check Valve Testing Preparation
Connect the X-Y recorder: Flow qv7 is input as the X-axis, pressure difference p5a-p5b or pressure p5a as Y-axis. Allow the oil temperature to reach the predetermined value. Usually choose 32 oil, 40 °C. Adjust the relief valve, release to maximum loose condition, and pressure gauge should show no pressure at the moment.

Hydraulic Check Valve Testing Test Procedure
1. Turn on the hydraulic power source (The electric motor drives the hydraulic motor to establish the pressure)
2. Start recording.
Adjusting the pressure relief valve 2 slowly turn the regulation know, increasing the pressure p5a slowly and the flow qv7 increases to the maximum flow. Then slowly open the pressure relief valve 2 to reduce the pressure to zero.
Stop recording.

Cracking Pressure And Testing
Cracking pressure refers to the pressure in the inlet cavity when the valve poppet is just opened. It is generally between 0 and 2.5 MPa.
The opening pressure of hydraulic check valve is slightly higher than the spring preload pressure.
Check valves are generally spring installed. The spring preload generally exceeds the weight of the spool at least. To ensure that, regardless of the direction of one-way installation, the spool can be reset.
Sometimes harder springs are used to ensure that the spool or poppet will be able to close quickly, or to achieve better compact performance, to ensure a certain opening pressure.
But the opening pressure can theoretically be calculated based on spring preload and effective area, accurate is not so easy actually.

Proposed ISO6403 Check Valve Testing Circuit
1. A hydraulic power source. As long as the flow is larger than the opening flow.
2. A relief valve, for establishing pressure. (Because the opening pressure of cartridge check valve is mostly low, in the 0~0.2Mpa range, if there is no pressure valve that can work in this range, use the throttle valve then)
3. A shut-off valve (Not necessary)
4. Testing Valve
5. A pressure gauge
6. A thermometer
7. A flow meter. Actually, when the valve is turned on, the flow rate is very small, the flow it cannot be accurately measured with an ordinary flow meter. If there is no appropriate micro flow meter, just consider using a measuring cup and stopwatch instead.

ISO6403 Test Process
A. The relief valve (throttle valve) 2 is fully released. Power on the hydraulic pump 1, at this time, there should be no flow at the outlet of the tested valve
B. Slowly close the relief valve (throttle valve) until there is a small flow at the outlet of the tested valve, about 5 drops/sec.
C. At this point, the pressure gauge shows that it is the opening pressure of the valve.

Internal Leakage Testing Of Hydraulic Check Valve
Internal leakage means the amount of leakage through the sealing surface between the valve poppet and the valve seat when the hydraulic check valve is in the working condition of reverse sealing (Reverse Flow).
For hydraulic check valve, the internal leakage should generally be as small as possible, preferably without leakage.
The standard for internal leakage is mL/min. However, because of the small amount, therefore, often expressed as drops/min. and 1 mL of mineral oil is about 16-20 drops.
Internal Leakage Direct Measurement

1) Test loop
The picture shows the hydraulic check valve test loop recommended by ISO6403.

In the picture shows:

  1. A hydraulic power source. As long as the flow rate is significantly larger than the maximum amount of leakage
  2. A pressure relief valve, for establishing pressure
  3. A shut-off valve, not necessary.
  4. The valve being tested.
  5. A pressure gauge.
  6. A thermometer.
  7. A flow meter. However, as the leakage flow is usually very small, it cannot be accurately measured with an ordinary flowmeter. If there is no appropriate micro flow meter, you can consider using a measuring cup and stopwatch instead of the flow meter.

2) Test process

  1. The relief valve 2 is fully loosened before testing, then turn on the hydraulic pump 1 to adjust the relief valve to the testing pressure.
  2. After the leak oil has stabilized, use a measuring cup and stopwatch to measure the amount of leakage in a certain.

Compressed Air Testing Measurement

An excellent hydraulic check valve, should be no leak in a few minutes, often too slow to use the cup. Therefore, there are also compressed air inspections, which can be used as factory inspection. Not suitable for formal testing.

1) Test circuit

  1. The air source.
  2. A switching valve, manually or electrically controlled available
  3. An air pressure gauge for monitoring purposes.
  4. Testing valve