Check Valve, Hydraulic, Description
The function of a check valve is to prevent flow in one direction and allow flow in the other direction. Check valves commonly use a poppet and light spring to control flow as shown in the figure below. If P1A1 > P2A2 + spring force + friction, then flow occurs in the direction of the arrows. If P1A1 < P2A2 + spring force + friction, then the poppet would be pushed to the left, against the stop, prohibiting flow in the reverse direction.
Figure 1 Simplified Check Valve Schematic
The most common method for designing a check valve is illustrated in the Figure 1. Different manufacturers may utilize other design approaches. For example, another type of check valve is a ball that pushes against a spring. Operation is similar to the check valve shown in Figure 1 except a ball replaces the piston.
Check valves are used in hydraulic systems anytime flow in a selected direction is not desirable or may create a problem. Check valves are not used in bidirectional flow lines, such as to and from actuators. Some examples where check valves are used are
Return lines - to prevent a leak upstream of the reservoir from draining the reservoir
Return lines – to prevent a pressure spike from migrating back up a return line to a component (this is especially important where actuators have mechanical locks and a pressure spike in the return line could cause the lock to disengage)
Ground Service lines – to ensure flow only flows in the proper direction when servicing and also to prevent a ground service line leak from draining the system
Pressure lines – to ensure no reverse flow through a hydraulic motor
Low Pressure (Feed) Line to a Pump – to prevent fluid in the pump from flowing back to the reservoir without going through a filter
Charged Accumulator Lines - check valves can be used to trap pressure in a given volume and maintain the charge pressure for a specified time interval. An example of this application would be using a pump to charge an emergency brake accumulator through a check valve. In this case, when the pump is turned off pressure is maintained downstream of the check valve and is available for emergency braking (see Figure 2).
Figure 2 Charged Brake Accumulator using Check Valve
Power Control Units (PCUs) – to prevent backflow out of a PCU’s supply pressure port should the supply pressure to that PCU fail. This is usually required to ensure hydraulic fluid is maintained in the PCU in order to prevent lost of the actuator stiffness. This is generally required on PCU’s that drive flight control surfaces where flutter is of concern. The inlet check valve is generally used with a return line pressure compensator that will maintain a low pressure (50-150 psi) in the PCU internal porting for a given amount of time following the removal of the inlet supply pressure to the PCU.
Parallel Pump Installations – to prevent outlet flow from one pump at a slightly higher pressure from flowing to the other pump (see Figure 3). In this type of pump installation, one of the two check valves is designed to open at a slightly higher pressure than the other. For example, say the left pump is powered by the aircraft’s engine and the right pump is power by an electric motor. In this type of application the engine driven pump generally has a larger flow capacity over that of the electrical driven pump. The electrical driven pump is intended as a backup for the engine driven pump. In this case the check valve on the electrical driven pump could be set to open (crack) about 50 psi more than the engine driven pump. This will ensure the engine driven pump supplies the system leakage flow during periods of no hydraulic flow demands and will also help reduce electrical motor noise variability.
Figure 3 Use of Check Valves in a Parallel Pump Arrangement
When considering the use of a check valve, the following factors should be evaluated:
Pressure Rating – make sure the valve is rated appropriately for the system pressure
Regulation Range – what is the minimum and maximum delta p required to go into and out of the checked flow position
Pressure Drop Across the Valve – this will affect design pressure available to downstream components and thus, the sizing of those components
Temperature Rating – valve should be rated for fluid temperatures and applicable environmental temperatures
Valve Materials – valve should be sufficient to pass proof and burst testing, not be susceptible to corrosion and other environmental considerations, operate properly under temperature extremes
Seals/Clearances – affects overall reliability of the valve. Some valves may not use seals and will maintain tight clearances between piston and housing to minimize leakage through the valve. The design characteristics can be affected by environmental conditions and aging/wear over time.
Leakage – what is the leakage through the valve in the checked position under all environmental conditions?
Failure Modes – the dominant failure modes consist of the piston jamming in either the open or the closed position. Clogging is also a possibility.
Chattering – valve should be evaluated for potential to exhibit chattering or limit cycle behavior under certain upstream or downstream conditions. This will be a function of the natural frequency of the servo and the damping.