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Pressure Relief Valves: Function, Types, and Application in Hydraulic Systems

Pressure relief valves protect hydraulic circuits, pumps, and actuators by precisely limiting system pressure. They open when the set pressure is reached, diverting excess fluid in a controlled manner back to the tank or to a low-pressure side. For industrial applications in manufacturing technology and mechanical engineering, reliability, repeatability, and material resistance are crucial.

Basic Principle and Key Parameters

The basic principle is simple: a spring-loaded piston or diaphragm keeps the valve closed until the hydraulic force overcomes the spring preload. Important key parameters include set pressure, flow rate (Q), pressure drop (Δp) in the open position, repeatability of the set point, and leakage rates. The temperature range, viscosity range of the hydraulic oil, and the permissible operating pressure must be considered during selection.

Designs and Operating Principles

Direct-acting pressure relief valves open mechanically when the set point is reached and are suitable for small to medium flow rates and fast response times. Pilot-operated valves use a small pilot bore and an auxiliary valve to control the main valve with minimal force; this allows for high flow rates and precise pressure regulation. Combined valves integrate pressure limiting with other functions such as pressure holding or check valves. Materials such as free-cutting steel, stainless steel (e.g., 1.4301 / AISI 304 or 1.4404 / AISI 316L), and high-strength brass alloys are used depending on the medium, corrosion requirements, and temperature. Housings and internal parts use hardened surfaces to increase wear resistance in abrasive media. Seals are usually made of NBR (nitrile rubber) for standard oils, FKM (Viton) for higher temperatures and media resistance, or PTFE for aggressive fluids and low friction. Bearing and sealing losses, as well as aging resistance, vary according to the choice of material.

Connections, Mounting Position, and Dimensions

Common connection types include BSPP/BSPT, ISO 228/7, SAE flange, and metric threads. For high pressures, screw connections according to DIN EN or SAE standards are often used. The mounting position can influence switching behavior; however, many valves are designed for any position. Overall length, installation dimensions, and weight are relevant for integration into existing power unit components. For high-pressure applications, robust mounting surfaces and torque-specified screw connections are important.

Regulation and Adjustability

There are fixed and adjustable versions. Adjustable valves allow continuous adjustment of the trigger pressure, often via an adjustment screw with a lock nut or via a scale with an actuator for remote adjustment. Electronically controllable and pneumatically piloted variants offer remote monitoring and regulation, suitable for automated manufacturing processes. Documentation of hysteresis, reset travel, and locking torque is important to ensure reproducible settings.

Maintenance, Testing, and Service Life

Maintenance includes regular visual inspection, functional testing under defined conditions, replacement of seals, and cleaning of screens/pilot bores. Test intervals depend on operating cycles, filtration degree, and load profile. Measurement protocols for pressure testing and leakage measurement are part of the CE-relevant machine documentation. Service life is determined by load cycles, contamination, temperature changes, and corrosive environments. Spare part availability, easy disassembly, and modular design reduce downtime.

Industrial Applications

Pressure relief valves are used in hydraulic presses, injection molding machines, machine tools, conveying technology, and test benches. They serve as primary protection for pumps, as safety valves for cylinder circuits, and for pressure limiting in proportional valves and servo hydraulics. In mobile working machines, they secure axle and drive systems. For energy-efficient systems, valves with low leakage and low pressure drop values are preferred.

Practical Examples

Example 1 – Hydraulic power unit of a CNC press: A pilot-operated pressure relief valve with an adjustable setting range of 50–350 bar is installed in the pressure circuit to relieve the pump outlet in case of overload and return fluid to the tank. The pilot line is protected by a bellows filter, and the seals are made of FKM due to elevated oil temperatures. Test protocols document the trigger value after assembly. Example 2 – Machine tools with fine pressure regulation: For a hydraulic clamping system, an electro-piloted valve is used, which regulates the pressure in 1-bar increments via a PLC. The connection is made via an SAE flange connection; the setting is digital, and fault conditions are fed into the status monitoring. A fallback mechanism prevents overshooting in case of electronic failure. Example 3 – Mobile machine technology in harsh environments: In a hydrostatic circuit of a construction machine, a direct-acting valve with a stainless steel core and PTFE seals is used to ensure resistance to dirt and moisture. Fastening is done with reinforced screws and a locking plate; regular visual inspections and seal changes are part of the maintenance intervals.

Selection Criteria and Checklist

* Determine set pressure, maximum flow rate, permissible operating pressure, connection type, temperature range, and material requirements. When selecting, consider the filtration (recommendation: ≤25 µm) and the viscosity of the hydraulic oil used. Check the compatibility of the sealing materials with additives and additive changes. For safety-relevant applications, choose valves with defined and documented certification and test protocols.

Integration and System Design

Plan pressure relief valves close to the source of pressure build-up (e.g., pump outlet) for optimal response time. Ensure short pilot lines, clean bores, and accessible adjustment points. Use silencers in the return line if cavitation and noise are problematic. For parallel pump control, individual relief valves should be provided for each pump strand to avoid backflow and pressure surges.

Standards, Tests, and Safety Aspects

Valves should be tested according to relevant standards; relevant standards include DIN, ISO, and EN standards for hydraulics, material, and pressure equipment directives. Documented leak and service life tests, test protocols for trigger values, and CE conformity are essential for industrial applications. Pay attention to correct labeling of adjustment ranges and manufacturer specifications for assembly and run-in procedures. Further information on hydraulic technology and application examples can be found at Technik and Anwendungsbeispiele.

FAQs

1. What are the differences between direct-acting and pilot-operated pressure relief valves?
Direct-acting valves operate purely mechanically and are suitable for lower flow rates and fast response. Pilot-operated valves allow higher flow rates and more precise pressure regulation, as the main valve is controlled by a smaller pilot chamber and the main force is transmitted hydraulically. 2. Which seal design is recommended for high oil temperatures and additive-containing hydraulic oils?
For high temperatures and aggressive additives, FKM seals or PTFE-based seals are to be preferred. NBR is suitable for standard oils and moderate temperatures but offers lower resistance to high temperatures and some additives. 3. How often should pressure relief valves be inspected and maintained?
Inspection intervals depend on operating hours, cycles, and environmental conditions. A practical recommendation is a visual and functional inspection every 3–6 months for intensive use, along with documented pressure trigger tests annually. Critical systems require shorter intervals and extensive leakage tests.