13.3: The Role of a Relief Valve in System Protection

In nearly all hydraulic systems, a relief valve is included as a safety mechanism to prevent dangerous pressure buildup. Excessive pressure can occur if fluid flow is restricted or blocked within the system.

How High Pressure Can Develop

Pressure spikes commonly occur when:

• A directional control valve is in a blocked-center position, preventing fluid from moving freely.
• A cylinder reaches full extension or retraction, also known as deadheading, when it stops fluid movement.
• An actuator is stalled by an excessive load, restricting the pump’s output flow.

In these situations, a relief valve ensures system protection by opening when pressure reaches its preset limit, diverting excess flow to the reservoir. Without this safeguard, components such as pumps, hoses, seals, and actuators could be damaged due to over-pressuring.

By functioning as both a pressure regulator and a safety feature, the relief valve ensures that hydraulic systems operate efficiently and reliably, even under fluctuating load conditions.

The Role of a Relief Valve in a Hydraulic Circuit

In a hydraulic system, pressure must be carefully managed to prevent damage to system components. One of the key safety devices responsible for this function is the relief valve, which ensures that pressure never exceeds a predetermined limit.

Imagine a hydraulic circuit where a fixed displacement pump continuously supplies oil to the system. This type of pump delivers a constant volume of fluid, regardless of demand. If the flow is suddenly blocked, such as when a directional control valve (DCV) is in a center position, or a cylinder reaches the end of its stroke, the pressurized oil has nowhere to go. Without a relief valve, the pressure in the system would rise uncontrollably, potentially leading to burst lines, damaged components, or even motor failure.

The relief valve prevents this dangerous situation by acting as a pressure-sensitive gateway. Under normal conditions, it remains closed, allowing oil to flow through the system as intended. However, when pressure at the valve’s inlet reaches its preset limit, the valve opens, providing an escape path for excess oil. This diverted flow is sent directly back to the reservoir, preventing further pressure buildup.

The relief valve’s behavior can be observed by monitoring flow and pressure changes in the system. When the relief valve remains closed, a flowmeter downstream of the relief valve would read zero, indicating that all oil is moving through the working circuit. The moment the valve opens, the flow through the relief port jumps to full pump capacity, confirming that excess pressure is being safely redirected.

Relief valves can be either internally or externally drained. Most are internally drained, meaning no external drain line is required. However, some designs feature an external drain, which must be connected to the return manifold to function properly.

In essence, the relief valve serves as the hydraulic system’s pressure safeguard, automatically regulating flow when necessary to protect components from excessive force. By opening precisely when needed, it ensures both operational efficiency and system longevity.

Connecting and Testing a Relief Valve to Limit System Pressure

In hydraulic systems using a fixed displacement pump, oil is constantly being pushed into the system whenever the pump is running. If that flow has nowhere to go —if it’s "deadheaded" —pressure will climb rapidly. Without a safeguard, this pressure can become dangerously high, potentially bursting hoses or causing the electric motor to stall.

That’s where a relief valve comes in. A relief valve is a safety device designed to limit maximum system pressure. It does this by opening at a preset pressure level and diverting excess fluid back to the reservoir.

Figure \(\PageIndex{1}\): Schematic diagrams of a double-acting cylinder in extension and retraction.

Imagine a situation where a directional control valve (DCV) operates a double-acting cylinder. Cycling the cylinder (extending and retracting), the relief valve behaves at various stages. Those stages would include closed at pressures lower than the previous set and open completely when pressures exceed the set point. These stages of the relief valve protect the system during blocked or high-pressure conditions.

Observing the Relief Valve in Action

Relief valves come in two drainage types:

• Internally drained, where excess oil is routed back through the valve body into the tank line.
• Externally drained, which requires a separate line to carry excess fluid back to the reservoir or return manifold.

If no external drain line is shown in a schematic, the valve is assumed to be internally drained. The hydraulic stand that we work with uses an externally drained relief valve, which must have a drain line connected to the return manifold —this prevents pressure from building up behind the valve spool and interfering with operation.

What is Happening Physically

Let’s walk through what’s occurring when the actuator is blocked:

• The pump continues to deliver flow, but the flow path is restricted.
• This causes pressure to climb quickly, which is the set pressure on the relief valve.
• Once the pressure at the inlet of the relief valve reaches the set pressure, the valve pops open, relieving excess pressure and allowing oil to flow safely back to the reservoir.

This process protects both the pump and the hydraulic components from damage. In a live system, the momentary opening of a relief valve may not even be noticeable—unless you’re watching the flowmeter or pressure gauge. But it's a crucial part of keeping the system safe and functional.