Check Valves - The Most Overlooked Component in a ...

Check valves are the simplest form of hydraulic devices in that they permit free flow in one direction and block flow in the opposite direction. Check valves may be used as a directional or pressure control in a hydraulic system. In the illustration, oil is flowing in from the left side port, through the check valve and out the right side port. If the pressure equalizes or is higher in the right side port, the check valve will close and block flow in the opposite direction.

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The spring rating varies based on how it is used in the system. One of the most common locations for a check valve is immediately downstream of the hydraulic pump. When used in this application, the spring pressure rating is usually 1 -5 PSI. In this case, the valve is used as a directional control in that it allows oil flow from the pump to the system but blocks flow in the reverse direction. This is commonly called a pump isolation check valve and serves four purposes in the system:

1. The check valve will block pressure spikes back to the pump. Oil flows from the pump to the system a speed of 15 - 30 feet per second in systems depending on the pressure in the system. When a directional is de-energized to block flow, the oil is rapidly deadheaded. The pressure in the line can rapidly increase 2 to 3 times the pressure in the line. The check valve should then close and block pressure spikes to the pump. A plywood plant recently changed 4 pumps due to cracking of the pumps' housings. This occurred over a weeks time on their Debarker hydraulics. When they ran out of pumps they FINALLY took the check valve out and found that the piston and spring were no longer in the valve! This $150 check valve costs the company $15,000 in the costs of the pumps and another $50,000 in machine downtime. That was one expensive check valve! The truth is that if one mechanic had looked at the schematic, known why the check valve was in the system, the replacement of the pumps and subsequent expense would have been avoided.
2. When a system is shutdown, it is important to maintain oil in the lines. In many cases the pump is mounted below the level of the system valves, cylinders and motors. The check valve downstream of the pump will prevent the lines from draining once the electric motor is turned off. If the oil in the lines drain through the pump and into the reservoir, a vacuum will occur. Air will be pulled in through the O'rings and seals of the valves and actuators and into the lines. This can create issues when re-starting the system as the air will need to be bled out.
3. Some systems have a hydraulic accumulator installed downstream of the pump and check valve. When the system is turned off there is pressurized fluid inside the accumulator. The check valve will block flow from the accumulator preventing the reverse rotation of the pump. You can observe the pump shaft or electric motor fan to verify that the check valve is good. Note: All systems using an accumulator should have a method of bleeding the hydraulic pressure down to 0 PSI when the system is turned off.
4. On many systems one pump is used as a back up or spare pump. Each pump will have a check valve at the pump outlet port. The check valve will block flow from the online pump to the offline pump, preventing reverse rotation.

Check valves are many times used in the case drain lines of variable displacement pumps. This is particularly common when multiple pumps are on one skid and the case drain lines connect to a single header. I was called in to a papermill a few years ago that kept losing one of the two pumps on their Chemi-Washer drives. The shaft seal of one one pump kept blowing out. Matter of fact, they ran out of spares and had to air freight their last pump to the factory in New York. The timeline was so critical due to downtime costs, that the pump was still warm when they received it back from the factory. Just prior to installing the pump, we removed the check valve in the case drain line and found it stuck in the closed position. This prevented the oil in the pump case from draining which resulted in blowing out of the seal.

In many cases a check valve is used as a pressure control. A common application is using it as a relief valve to protect a heat exchanger. In this case, the spring rating is usually 65-100 PSI. I was teaching a class at sawmill a few years ago when the students were doing their hands on exercises on the Edger. Although there was a check valve shown on the schematic to protect the air cooler, the lines to the check valve were plugged off. I asked one of the mechanics about it and he said the check was taken off years ago AND that they had changed the cooler last week because of ruptured tubes!

When troubleshooting hydraulics, most everyone is looking for something large (pump, valve, cylinder) to be the problem. Every component in the hydraulic system has a function. Be sure you know the purpose of the check valves in your system when troubleshooting!

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