A hydraulic pilot operated check valve is a type of valve that allows fluid flow in one direction and blocks it in the opposite direction, unless a pilot pressure is applied. It is commonly used in hydraulic systems to lock an actuator in position, prevent backflow, and protect the pump from pressure spikes.
Basic Principle
A hydraulic pilot operated check valve consists of a poppet, a spring, and a pilot piston. The poppet is normally held closed by the spring force, preventing fluid flow from the outlet port to the inlet port. When the pressure at the inlet port is higher than the pressure at the outlet port, the poppet is pushed open by the fluid force, allowing free flow from the inlet port to the outlet port. This is the normal check valve function.
However, when the pressure at the outlet port is higher than the pressure at the inlet port, the poppet remains closed by the spring force, blocking the fluid flow from the outlet port to the inlet port. This creates a locked position for the actuator connected to the outlet port. To unlock the actuator and allow reverse flow, a pilot pressure must be applied to the pilot port of the valve. The pilot pressure acts on the pilot piston, which pushes the poppet open against the spring force, allowing fluid flow from the outlet port to the inlet port. This is the pilot operated function.
The following table summarizes the flow conditions for a hydraulic pilot operated check valve:
| Inlet Pressure | Outlet Pressure | Pilot Pressure | Flow Direction |
|---|---|---|---|
| Higher | Lower | Any | Inlet to Outlet |
| Lower | Higher | Zero | Blocked |
| Lower | Higher | Higher | Outlet to Inlet |
Applications
Hydraulic pilot operated check valves are widely used in hydraulic systems that require an actuator to be locked in position, such as clamping, braking, holding, and lifting devices. They can also prevent backflow of fluid from the actuator to the pump, which could cause contamination, loss of prime, or cavitation. Moreover, they can protect the pump from pressure spikes that may occur when the actuator is stopped abruptly by a directional control valve.
Some examples of hydraulic systems that use pilot operated check valves are:
- Forklifts: Pilot operated check valves are used to lock the forks in position when lifting or lowering loads, and to prevent the forks from dropping in case of a hose rupture or power failure.
- Cranes: Pilot operated check valves are used to lock the boom and the hook in position when lifting or lowering loads, and to prevent the boom or the hook from falling in case of a hose rupture or power failure.
- Presses: Pilot operated check valves are used to lock the ram in position when applying pressure, and to prevent the ram from retracting in case of a hose rupture or power failure.
Advantages and Disadvantages
Hydraulic pilot operated check valves have some advantages and disadvantages compared to other types of valves, such as:
- Advantages:
- They can lock an actuator in position without consuming power or generating heat.
- They can prevent backflow of fluid and protect the pump from pressure spikes.
- They can provide smooth and controlled motion of the actuator when piloted.
- Disadvantages:
- They require a pilot pressure to allow reverse flow, which may not be available in some situations.
- They may cause pressure intensification in the actuator, which could damage the seals or the cylinder.
- They may introduce some leakage or hysteresis in the system, which could affect the accuracy or repeatability of the actuator.