What an overspeed valve does on a hydraulic elevator
Hydraulic elevators and traction elevators keep the car under control in different ways. On the hydraulic side, one device does a specific and important job: the overspeed valve, also called a pipe rupture valve.
How a hydraulic elevator moves
Instead of ropes and a counterweight, a hydraulic elevator pushes oil under pressure to raise the car and releases it in a controlled way to lower the car. That controlled release is normally handled by the control valve, which meters the oil so the ride is smooth and the car stops level with the floor.
Where the risk is
The concern is a supply line failure. If a line breaks and oil escapes faster than the system intends, the downward speed of the car can climb past normal. That is the exact situation the overspeed valve is built for.
What the overspeed valve does
The valve watches the rate of oil flow. If that flow spikes past a set point, whether from a ruptured line or an abnormally high flow rate, the valve closes and stops the car. It is adjusted in the field to a specific tripping point, and it is built to the ASME A17.1 / CSA B44 standard.
What it does not do
This is the part worth being clear about. The overspeed valve protects against a line or flow failure. It does not stop the car from overspeeding due to a leaking underground cylinder, and it is not a substitute for cylinder replacement or the elevator's other safety devices. Those are separate problems with separate solutions.
If your elevator is older
Many older hydraulic elevators were installed without an overspeed valve. In most cases one can be added. If you are not sure whether your car has one, it is a reasonable thing to ask on your next service visit.
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