Marine hydraulicsLesson 10 of 20
Check valves and pilot-operated check valves: how a load is held
A check valve lets oil through one way and blocks it the other. A pilot-operated check adds a small piston that can push it open on command, so a cylinder holds its load until you deliberately lower it.
6 minutes · For signed-up learners, free
A check valve is the simplest valve in hydraulics: a ball or poppet, a seat and a light spring. Oil pushing one way lifts it and flows. Oil pushing the other way presses it harder onto its seat and stops. Add a small pilot piston that can push the poppet open on purpose and you have the pilot-operated check, the valve that holds hatch covers, outriggers, davit arms and press rams where they are. In the animation the valve sits in the line under a loaded cylinder: raise, hold, then lower, and watch the pilot piston do its job.
In simple terms
Raising. The pump pushes oil towards the cylinder. The oil lifts the poppet off its seat against a light spring and goes straight through. This part is an ordinary check valve: it needs only a couple of bar to open.
Holding. When the lever goes to Hold, the oil under the piston wants to flow back out. That pressure pushes the poppet onto its seat, and the heavier the load, the tighter it seals. Because it is metal on metal, a clean poppet holds with practically no leakage, which a spool valve can’t do.
Lowering. Now the pump feeds the top of the cylinder. That same pressure also travels down a thin pilot line to a small piston behind the poppet. The piston has a bigger area than the poppet’s seat (four times bigger in the default setting), so it only needs a quarter of the load pressure to push a pin into the poppet and open it. The oil under the piston can then flow back to tank and the load comes down.
The two faults are the two problems we find most. A particle trapped on the seat stops the poppet closing, so the load sinks slowly. A blocked or crushed pilot line means the pilot piston never gets pressure, so the load won’t come down at all.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Raise, hold and lower a load through a pilot-operated check, and change the pilot ratio.
- Break it: dirt under the seat (the load drifts) and a blocked pilot line (it won't come down).
Continue this lesson free
The rest of this lesson, the interactive Pilot-operated check valve holding a cylinder and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.
Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.
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This lesson is also published by our sister site hydraulicsuae.com.
