Every hydraulic system must have a relief valve that opens at a pressure below the lowest rating of any part and passes the whole pump flow back to the reservoir. The setting comes from the weakest component’s own marking, never from what the pump can make. While it is open every watt of pump power becomes heat, 1,668 W for 5 L/min at 20 MPa, and oil trapped between closed valves gains roughly 1 MPa for each kelvin it warms, so a closed section needs its own relief.
Part of Hydraulic Systems
A relief valve is a spring-loaded poppet or ball that opens when pressure reaches its setting and dumps oil to the reservoir, so pressure can never climb past what the system can hold.
A Spring Holds the Seat Shut
Following the carrier, a relief valve is a spring-loaded ball or poppet; when pressure exceeds the setting it opens and dumps fluid to the reservoir, and the spring preload sets the pressure. The spring force is set pressure times the seat area, so a smaller seat needs a lighter spring.
Worked example at a 20 MPa setting.
- Seat diameter 6 mm: area = π / 4 x 0.006² = 2.83 x 10⁻⁵ m²; spring force = 20,000,000 x 2.83 x 10⁻⁵ = 565 N
- Seat diameter 3 mm: area = 7.07 x 10⁻⁶ m²; spring force = 141 N
A hand-wound spring for 565 N is large; a small seat makes a lighter spring and a more compact valve. But the seat must pass enough flow when it opens, and a valve that lifts only a fraction of a millimetre may need more pressure than its setting to pass the whole flow. Use a larger seat or a pilot-operated design where the flow is big. The corpus gives no relief-flow design rule; test the valve by running the pump against it with a gauge and checking that pressure stays near the setting.
The Setting Is Anchored to the Weakest Part
The carrier says to set the valve at 80% of the weakest component’s rating. That rule works only if you know the rating: it must come from each part’s own marking or a real engineering reference, and this corpus cannot supply it for an unmarked part. As an illustration only, if the weakest part is marked for 25 MPa, 80% is 20 MPa. A part with no known rating is taken as having none, and the relief setting is then decided by a proof test on that part, not by the pump.
The carrier’s press example sets its relief at 25 MPa while its cylinder wall rule is stated for up to 20 MPa (Hydraulic Cylinders); read against that rule, the 25 MPa figure sits above what the wall was checked for, so check the actual parts before using it.
Setting and Testing
Step 1: Fit a gauge of known accuracy on the pump side of the valve.
Step 2: Dead-head the pump (block the outlet) and raise the pressure slowly. With a hand pump this is safe because it stores little energy; with a powered pump stand clear and keep the run short.
Step 3: Adjust the spring until the valve opens at the chosen pressure, then lock the adjusting screw so it cannot be changed.
Step 4: Check that it reseats and that the pressure falls cleanly; a valve that chatters or does not reseat has a worn seat or a weak spring.
Step 5: Route its outlet to the reservoir, not to the open air.
The Valve Is a Heater
While the relief valve is open, all the pump flow passes at the set pressure, and the energy goes into the oil as heat: power = pressure x flow. For 5 L/min (8.33 x 10⁻⁵ m³/s) at 20 MPa:
- Heat = 20,000,000 x 8.33 x 10⁻⁵ = 1,668 W
That warms the oil in a small reservoir quickly, so size the reservoir and avoid holding the cylinder against its stop with the pump running.
Trapped Oil Needs Its Own Relief
Oil that warms in a rigidly closed section, between two closed valves or inside a cylinder with blocked ports, cannot expand. Using a thermal expansion coefficient of about 7 x 10⁻⁴ per kelvin and a bulk modulus of 1.5 GPa (both recalled from tables, not from this corpus), the pressure rises by 7 x 10⁻⁴ x 1.5 x 10⁹ = 1.05 x 10⁶ Pa, about 1 MPa for each kelvin. A 20 K rise in a closed line takes it up by about 21 MPa. That is why a jack left under load in the sun, or a blocked line next to a forge, can burst. Give any closed section a small relief valve.
Common Mistakes
- Plugging or tying down the valve to get more force. The corpus’s boiler article records the toll this took with steam.
- Setting from the pump’s output. The pump can make more than the parts can bear.
- A valve too small for the pump flow, so pressure climbs above the setting before the whole flow is relieved.
- Mounting a valve after another valve. Nothing should sit between the pump and the relief valve.
Safety
If the relief valve fails, the weakest part bursts
A relief valve that is stuck, plugged, set too high or fitted behind a closed valve gives no protection, and the weakest line, hose or fitting bursts. A burst or pinholed line injects oil through skin; never search for a leak with a hand, use card or wood. Do not fit a relief valve that has not been tested at its setting, do not turn the adjusting screw to get more force, and never use a hose of unknown rating anywhere in the circuit.
What’s Next
- Learn next: Flow Control Valves — setting the speed without losing the protection
- Combine with: Hydraulic Press — where the relief setting decides the force