Relief valve & pressure limiting
The load decides the pressure — and a blocked line could send it sky-high. The relief valve is the system's pressure ceiling: the one part that keeps everyone safe.
Source: Rabie, Fluid Power Engineering, Ch. 5.
Before you start
What you need first
- Hydraulic power, \(N = pQ\) — pressure times flow is a power (Topic 4).
- The basic circuit & ISO symbols — pump, lines, tank and how a valve taps a line (Topic 5).
- The pump makes flow; the load makes the pressure (Topic 19).
What you'll be able to do
- Explain why every hydraulic system must limit its pressure.
- Read the relief-valve symbol and say where it sits in the circuit.
- Tell a direct from a pilot-operated relief valve, and use the terms cracking and override pressure.
- Work out the power wasted when oil dumps over the valve, \(N = pQ\).
Start here · the one big idea
Why we must limit the pressure
Remember the rule from the pumps theme: the pump pushes a steady flow, and the load sets the pressure. So what happens if the load suddenly becomes a wall — a cylinder reaches the end of its stroke, or a valve shuts and blocks the line?
The flow has nowhere to go, but the pump keeps pushing. The pressure shoots up — in principle without limit — until something gives: a hose bursts, a seal blows, or the pump itself breaks.
The relief (safety) valve
The relief valve sets the maximum pressure the system is allowed to reach. It is normally closed: most of the time it does nothing. If the pressure tries to climb above its set value, it opens and dumps the excess oil straight back to the tank — capping the pressure.
Read its symbol as a box carrying three clues:
- an arrow — the flow path the oil takes when it opens;
- a spring — what holds it shut and sets the pressure;
- a dashed pilot line — it senses the inlet pressure.
Where it sits in the circuit
The relief valve branches off the pressure line, right after the pump, with its other port going to tank. Its dashed pilot line senses the same pressure line it is protecting.
While the system pressure is below the setting, the branch is shut and all the pump flow goes to the work. Only when the pressure reaches the ceiling does the valve crack open and bleed the surplus to tank.
The simple kind
Direct-operated: a spring against the pressure
Inside the simplest relief valve is a poppet held onto its seat by a spring. The inlet pressure pushes the poppet one way; the spring pushes back.
- Pressure low → the spring wins → valve closed.
- Pressure high → the oil wins → poppet lifts → oil escapes to tank.
Turn the adjusting screw to squeeze the spring more (higher setting) or less (lower setting).
Two pressures to name
The cracking pressure \(P_r\) is the pressure at which the valve just starts to open — the setting. To pass more flow the poppet must lift further, which needs a little more pressure. That extra rise above the setting is the override pressure.
A good valve keeps the override small, so the pressure stays close to the setting even at full flow. A direct valve would need a very stiff spring to do that at high flow — hard to build, which is exactly why the pilot type exists (next).
The high-flow kind
Pilot-operated: almost flat pressure at high flow
A pilot-operated relief valve is a two-stage design. A small pilot valve sets the pressure; it then controls a big main valve that handles the large flow.
- Below the setting: both stages closed, the main valve held shut.
- At the setting: the tiny pilot cracks open first.
- That trickle of pilot flow creates a pressure drop that lets the main valve open wide for the big flow.
Because the pilot stage passes only a trickle, its override is tiny — so the whole valve holds an almost constant pressure, even at full flow. That is why the pilot type is the go-to relief valve for large flows.
The price of dumping: wasted power
Every litre the relief valve sends to tank was pressurised by the pump — and that energy is thrown away as heat. The power lost is just the hydraulic power of the dumped stream (Topic 4):
| Symbol | Meaning | SI unit |
|---|---|---|
| \(N\) | power wasted as heat at the valve | W |
| \(p\) | pressure the oil is dumped at (the relief setting) | Pa |
| \(Q\) | flow forced over the valve to tank | m³/s |
✏️ Try it yourself
- Is a relief valve normally open or normally closed, and where does the oil go when it opens?
- In one line each: what is the cracking pressure and what is the override pressure?
- A relief valve holds \(p = 250~\text{bar}\) and dumps \(Q = 15~\text{L/min}\) to tank. Find the wasted power.
Common mistakes to avoid
| Mistake | Fix |
|---|---|
| "The relief valve makes the pressure" | It limits the pressure. The load makes it; the valve only caps the maximum. |
| Thinking it is open most of the time | It is normally closed — it only opens to protect the system. |
| Confusing cracking with override | Cracking = the setting (just starts to open); override = the extra rise above it at full flow. |
| Leaving \(p\) in bar / \(Q\) in L/min in \(N=pQ\) | Convert first: \(1~\text{bar}=10^{5}~\text{Pa}\), \(Q[\text{m}^3/\text{s}]=Q[\text{L/min}]/60{,}000\). |
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| The job | Sets the system's maximum pressure (a safety ceiling) |
| State at rest | Normally closed; opens to dump surplus to tank |
| Two pressures | Cracking = the setting; override = extra rise at full flow |
| Direct vs pilot | Direct = simple, more override; pilot = flat pressure at high flow, can unload the pump |
| The cost | Dumped flow wastes power \(N=pQ\) as heat |