Hydraulic Elevator Systems
Domain 5 covers direct-acting and roped hydraulic elevators. Questions are usually about the valve assembly, pressure and area arithmetic, and what each safety device protects.
Layout and valves
A hydraulic power unit has a tank, pump, motor, valve assembly and muffler. In-ground cylinders sit in a casing and are the classic environmental concern; holeless units use above-ground cylinders beside the rails; roped (2:1) units halve plunger stroke and double car speed for the same oil flow. The valve assembly typically includes a check valve (holds the car and closes when the pump stops), a down (lowering) valve, a pressure relief valve, a manual lowering valve, and a pipe-rupture valve that limits oil loss if a line fails. The relief valve is set above full-load pressure and limited by code to roughly 150% of full-load pressure.
Arithmetic you will need
Pressure = load ÷ plunger area. Speed comes from flow: one gallon is 231 in³, so plunger speed in in/min = (GPM × 231) ÷ area in in². Doubling pump flow doubles speed; doubling plunger area halves it for the same flow.
Worked example
A 6 in diameter plunger has area π × 3² = 28.27 in². A 30 GPM pump gives 30 × 231 = 6,930 in³/min, and 6,930 ÷ 28.27 = 245 in/min ≈ 20.4 ft/min. To run 100 ft/min (1,200 in/min) at that same plunger area, flow must be 1,200 × 28.27 ÷ 231 ≈ 147 GPM - a fast plausibility check on a proposed unit.
Traps
Using diameter instead of radius in the area formula; confusing the relief valve with the check valve; assuming a holeless unit has no cylinder or plunger inspection obligations; lowering a loaded car manually without control.