Relief Valve Symbol

Definition: The Relief Valve symbol represents the normally closed pressure-control valve that limits maximum system pressure by diverting flow to tank when pressure exceeds its spring setting, drawn per ISO 1219-1 as a single square (envelope) with an offset flow arrow held closed by a spring on one side and piloted open by a dashed pressure-sensing line from the Inlet, discharging to the Outlet/Tank port.

Also known as: pressure relief valve, PRV, safety valve, main relief, system relief valve, pilot-operated relief valve, direct-acting relief valve.

What the Relief Valve symbol means

The Relief Valve symbol denotes the safety ceiling of a hydraulic circuit. Drawn as one square because it is an infinitely-positionable (modulating) valve rather than a discrete-position directional valve, its internal arrow is shown offset from the port connections — normally closed — with a spring holding it shut and a dashed pilot line tapping the Inlet pressure onto the opposite side of the element. The reading is mechanical and literal: inlet pressure pushes against the spring; when force from pressure exceeds the spring setting, the arrow shifts into alignment and flow passes Inlet → Outlet/Tank.

In a circuit with a fixed-displacement pump, the relief valve is not optional: the pump delivers flow regardless of resistance, so the relief valve teed off the pressure line immediately downstream of the pump is what prevents a blocked circuit from bursting. It defines maximum force at every actuator (F = p_relief × area) and, importantly, it is a heat generator — every litre dumped across it converts the full pressure drop into heat, which is why systems that idle against the relief for long periods either overheat or get redesigned with unloading valves or pressure-compensated pumps. Distinguish it from its sibling symbols: a pressure-reducing valve is drawn normally open with its pilot sensing downstream, and a sequence valve looks like a relief but discharges into a second circuit rather than tank.

How to identify the Relief Valve symbol

Look for a single square with an arrow that does not line up with the ports (normally closed), a spring symbol (zigzag) on one side of the square, and a dashed pilot line running from the Inlet port to the side opposite the spring. An arrow struck diagonally through the spring marks an adjustable setting — which almost all relief valves have. The Outlet routes to the tank symbol, usually directly and drawn short. A pilot-operated (balanced-piston) relief valve is often drawn as the same simplified symbol, or in detailed form as a small pilot relief stacked with a main stage, sometimes with an external vent port that enables remote control or unloading.

The classic confusion pair is relief vs reducing valve: the relief valve is normally closed and senses upstream (protecting the supply side by dumping excess); the reducing valve is normally open and senses downstream (limiting pressure delivered to a branch). Spring position and pilot-line origin on the symbol tell you which you have. Since fluid power drawings worldwide follow ISO 1219-1 (legacy ANSI Y32.10 used the same forms), there is no separate American symbol to learn.

Function in a circuit

In a direct-acting relief valve, inlet pressure acts on a poppet or spool held by the spring; at the cracking pressure the poppet begins to lift and a small flow passes to tank, and as pressure rises further the valve opens fully until it passes the pump's whole output at the full-flow pressure. The difference between cracking and full-flow pressure (pressure override) can be substantial in direct-acting valves — the spring gets stiffer as it compresses — so a valve that cracks at 180 bar may need 210 bar to pass full flow.

Pilot-operated relief valves fix that: a small pilot poppet controls the pressure behind a balanced main piston, so the main stage opens with only a few bar of override, giving flat, accurate pressure control at high flows. Their vent port offers a bonus — venting the pilot chamber to tank through a small solenoid valve drops system pressure to near zero (unloading), a standard energy-saving trick drawn as a solenoid valve hanging off the relief's vent line. Relief valves are set typically 10–20% above maximum working pressure, verified with a gauge while deadheading the circuit at reduced flow.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Cracking-pressure settings are adjustable across ranges such as 5–50, 50–210, and 100–350 bar (up to 630 bar in high-pressure cartridges); the setting convention is 10–20% above maximum working pressure. Direct-acting valves suit flows to roughly 60–120 L/min with pressure override of 10–30%; pilot-operated valves handle 40–2,000+ L/min with override of just 2–5 bar. Response times: direct-acting ~2–10 ms (good for shock spikes), pilot-operated ~20–50 ms. Heat generated when relieving = Δp(bar) × Q(L/min) ÷ 600 in kW — a 210 bar valve passing 40 L/min dissipates 14 kW into the oil.

Where the Relief Valve symbol is used

• Immediately downstream of every fixed-displacement pump as the main system relief on hydraulic power units
• Mobile-equipment valve banks, as port (work-line) reliefs protecting individual cylinder and motor circuits from load-induced spikes
• Crossover relief pairs across hydraulic motor ports, absorbing shock when a directional valve closes on a spinning load
• Press safety circuits, capping maximum tonnage regardless of control-system faults
• Pilot-operated main reliefs with solenoid vent valves, providing pump unloading during machine idle
• Hydrostatic transmission charge circuits and cooling loops, where low-pressure reliefs maintain charge pressure

Example

In a power-unit schematic drawn to ISO 1219-1, the Relief Valve symbol's Inlet pin tees off the pump's pressure line and its Outlet/Tank pin drops straight to the reservoir; the dashed pilot line from the Inlet works against the adjustable spring, set to 230 bar on a system whose working pressure is 200 bar. When the press cylinder bottoms out, the fixed pump's 32 L/min has nowhere to go, pressure rises to 230 bar, the valve cracks and passes the full flow to tank — dissipating about 12 kW as heat, which is why the machine's idle logic vents the relief to unload the pump between cycles.

Key facts

• ISO 1219-1 draws the relief valve as a single envelope with an offset (normally closed) arrow, a spring on one side, and a dashed pilot line sensing the Inlet on the other — pressure literally fights the spring on the page.
• Every fixed-displacement pump circuit requires a relief valve: positive-displacement flow with a blocked path otherwise raises pressure until something fails.
• Cracking pressure is where the valve first passes flow; full-flow pressure is higher by the pressure override — large (10–30%) in direct-acting valves, small (2–5 bar) in pilot-operated ones.
• Standard setting practice: 10–20% above maximum working pressure, verified by deadheading the circuit against a gauge.
• Relieving converts the full pressure drop to heat: kW = Δp(bar) × Q(L/min) ÷ 600 — sustained relieving overheats systems and signals a design or setting problem.
• Pilot-operated reliefs have a vent port: venting it to tank through a small solenoid valve unloads the pump to near-zero pressure — the standard idle-energy-saving circuit.
• Relief vs reducing valve: relief is normally closed and senses upstream; reducing is normally open and senses downstream — the spring and pilot-line placement on the symbol distinguishes them.
• A relief valve is a modulating (infinite-position) valve, hence one envelope square — directional valves get one square per discrete position.

Frequently asked questions

How do I read the relief valve symbol?

Read it as a force balance. The square is the valve; the arrow inside is offset from the ports, meaning no flow path exists at rest (normally closed). The zigzag spring on one side holds it closed at the set pressure; the dashed line from the Inlet applies system pressure to the other side. When inlet pressure beats the spring, imagine the arrow sliding into line with the ports: flow passes Inlet to Outlet/Tank. The diagonal arrow through the spring means the setting is adjustable.

What is the difference between cracking pressure and full-flow pressure?

Cracking pressure is where the valve just begins to open and pass a trickle. As flow through the valve increases, the spring compresses further and pressure rises above the cracking point; full-flow (or full relief) pressure is where the valve passes the pump's entire output. The difference is the pressure override — 10–30% for direct-acting valves, only 2–5 bar for pilot-operated designs, which is why high-flow systems use the latter.

What should a hydraulic relief valve be set to?

Conventionally 10–20% above the maximum working pressure the circuit needs — high enough not to interfere with normal operation or waste energy through seepage near the setting, low enough to protect the weakest-rated component. Set it by deadheading the actuator (or blocking the line) at reduced pump flow while watching a gauge, then locking the adjuster. Never set it above the lowest component rating in the circuit.

What is the difference between a relief valve and a pressure-reducing valve?

Opposite jobs, mirror-image symbols. A relief valve is normally closed, senses its own inlet (upstream), and dumps excess flow to tank to cap system pressure. A pressure-reducing valve is normally open, senses its outlet (downstream), and throttles shut to limit the pressure delivered to one branch — e.g. a clamping circuit that must not exceed 50 bar in a 210 bar system. On the symbol, check whether the pilot line comes from upstream (relief) or downstream (reducing) and whether the arrow is offset (closed) or aligned (open) at rest.

Why does my hydraulic system overheat when idling?

Very likely the pump is dumping its whole flow across the relief valve while nothing moves — every litre relieved converts the full pressure drop to heat (kW = bar × L/min ÷ 600). Fixes visible on the schematic: a vented pilot-operated relief with a solenoid unloading valve, a tandem-centre directional valve that routes pump flow to tank in neutral, or replacing the fixed pump with a pressure-compensated variable pump. A relief valve that relieves continuously is a symptom, not a fault of the valve.

Is a hydraulic relief valve the same as a safety valve on a boiler or air receiver?

Same principle — spring-opposed, pressure-opening — but different standards worlds. Hydraulic relief valves are ISO 1219-1 schematic components engineered to ISO 5781, set and adjusted by the machine builder. Boiler and pressure-vessel safety valves fall under ASME Section VIII / API 520/526 (or PED/EN 764 in Europe), are certified, sealed, and legally mandated devices. Do not substitute one for the other, and do not read hydraulic-schematic conventions into piping P&IDs.

Related symbols

• Check Valve symbol
• Flow Control Valve symbol
• Pressure Regulator symbol
• Pressure Switch symbol
• Pump (Motor-driven) symbol
• Solenoid Valve symbol

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