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Technical Selection and Application of Pressure Relief Valves for Hydraulic Applications

Pressure relief valves limit the maximum system pressure, protect components from overload, and stabilize the hydraulic circuit. For industrial hydraulics, two designs are relevant: direct-acting valves for compact, fast pressure limitation at low flow rates, and pilot-operated (reinforced) valves for higher flow rates and more precise pressure control. Selection criteria include the adjustable pressure range, flow characteristic, materials, connection type, sealing materials, and design (housing, inline, or cartridge type).

Materials, Surfaces, and Service Life

Common materials include structural steel (case-hardened, nitrided, or phosphated), stainless steel (1.4404/316L for corrosive environments), and aluminum alloys for lightweight applications. Steel housings offer the highest strength for high pressures up to 400 bar; stainless steel increases corrosion resistance, reduces wear due to damage, and is suitable for food and chemical applications. Surface treatments such as nickel plating or hard chrome improve wear resistance and sliding properties of the seat surfaces. In systems with recirculating media containing abrasive particles, hard coatings reduce cavitation effects and extend service life.

Seals: NBR vs. FKM – Selection Criteria

NBR (Acrylonitrile Butadiene Rubber) is suitable for mineral oils and standard hydraulic fluids up to approximately 100 °C, offering good abrasion resistance and sealing performance at typical operating pressures. FKM (Viton) tolerates higher temperatures up to approximately 200 °C and is resistant to many chemicals, fuels, and aggressive hydraulic additives. For applications with high temperatures, bio-hydraulic fluids, or hydrolyzing additives, FKM is recommended. NBR remains the more economical choice for standard applications. In cases of alternating conditions, consult compatibility data sheets from the seal manufacturer and consider shear stress, temperature cycling, and pressure surges.

Connections, Designs, and Mounting Types

Pressure relief valves are available as free screw-in valves (G-thread), flange valves, and cartridge valves. Cartridge valves offer space-saving integration into manifold blocks with low leakage rates, ideal for modular plates in mechanical engineering and series production systems. Inline models with SAE or BSP connections simplify retrofitting into existing pipelines. For high pressures and large volumes, flange solutions are the appropriate choice. Important connection sizes and standards: BSPP/BSPT, UN/SAE, ISO 1179, DIN 3852 for thread and sealing requirements. For cartridge designs, installation dimensions according to ISO 7789 or customer-specific installation dimensions must be observed. Mounting with torque limitation and sealant according to manufacturer specifications reduces labor effort and prevents damage to sealing surfaces.

Connection Systems and Ports

Valves are typically specified with the following ports: P (Pump/Pressure), T (Tank/Return), and A/B (Working Line). Pressure relief valves are primarily connected between P and T. For applications in directional control sections (e.g., cylinder protection), combined variants are available that also relieve the working line. For pilot operation, additional ports for pilot pressure and control pressure are required; correct piping of the pilot circuit prevents instabilities.

Functional Parameters and Performance Data

Important performance data include nominal flow rate (Q), set pressure (pset), pressure stage accuracy, repeatability, and pressure stability under load. Direct-acting valves are typically used economically up to 20–30 l/min, while pilot-operated variants handle several hundred l/min. Adjustment ranges vary depending on the design, from a few bar to over 400 bar. Pay attention to the valve characteristics: opening behavior, pressure increase as a function of flow, and pressure drop when exceeding nominal flows.

Control Behavior, Hysteresis, and Damping

The accuracy of pressure limitation depends on the spring characteristic, seat geometry, and damping. Pilot-operated valves allow for finer control and lower hysteresis, especially with changing loads. Damping orifices and damping inserts prevent short, sudden pressure peaks. In systems with highly pulsating flows, damped or electrically monitored valves are recommended.

Practical Application Examples

Example 1: In a stamping press, a pilot-operated cartridge valve with FKM seals and a stainless steel housing protects the piston hydraulics from pressure peaks when the punch wheel retracts. The valve is integrated into the manifold block, the pump port is connected via an SAE connection module; the pilot pressure is secured by a filtered pilot line filter to prevent malfunctions due to contamination. Example 2: In mobile hydraulics for agricultural machinery, a direct-acting valve with NBR seals ensures rapid pressure limitation during boom movements. The valve is mounted as an inline version with BSPP connections, and the housing is phosphated to minimize corrosion in outdoor areas. Regular checking of spring tension and visual inspection of the seals reduce unplanned downtime. Example 3: In a hydraulic test stand for quality control of components, pilot-operated pressure relief valves are operated in combination with electronic pressure sensors. The valves are preset to actively relieve pressure when certain test pressures are reached; the return line to the tank is routed through a cooling and particle filter to prevent thermal loads and contamination. For continuous test cycles, valves with wear-resistant seats and hard coatings are used.

Installation, Commissioning, and Maintenance

Installation instructions: Before installation, clean lines, exclude chips and foreign particles, tighten connections with the prescribed torque, and use sealant as appropriate. Keep pilot lines clean and short, install additional filters (3–10 µm) in the pilot circuit. For cartridge mounting, check installation dimensions, lightly grease O-rings before installation to prevent nicks and sealing failures. Commissioning: Slowly raise the operating pressure, perform a functional check by triggering several times, visually inspect for leaks, and check settings with an accurate manometer. Documenting the set pressure values and spring travel facilitates later adjustments. Maintenance: Regular visual and leak inspections, replacement of seals (NBR/FKM) according to manufacturer specifications or in case of trace elements in the oil, cleaning or replacement of filters, checking spring force and seat surfaces. If the control characteristic significantly deteriorates or leakage oil increases, the seat and poppet should be checked for wear. For safety-relevant systems, a documented test cycle including pressure and leak testing is recommended.

Integration into System Architecture and Control

Pressure relief valves can be used alone for limitation or as part of complex control strategies with pressure sensors, electronics, and valve islands. For modern industrial applications, a combination of mechanical overpressure protection and electronic pressure monitoring with logical linkage to PLC systems is recommended. This way, mechanical protection remains the last line of defense, while the control system preventively detects pressure limit conditions and adjusts operating parameters. More information on technical details and applications can be found on our technology page: https://maku-industrie.de/technik and in specific practical examples at https://maku-industrie.de/anwendungsbeispiele.

Testing and Certification Aspects

Pay attention to pressure tests according to EN ISO 3457 or corresponding standards for hydraulic components; material-related certificates for stainless steel (e.g., 3.1 or 2.2 test certificate according to EN 10204) are desirable for sensitive applications. Electronically monitored systems should be CE-compliant and consider relevant safety standards such as the Machinery Directive. For food-grade or pharmaceutical applications, documentation of materials and seals is required.

Selection Aid – Overview of Criteria

* Adjustment range and accuracy, nominal flow rate, design (cartridge, inline, flange), material (steel/stainless steel/aluminum), seal (NBR/FKM), connection standard, wear protection (coating), pilot vs. direct operation, ease of maintenance, and standard certificates.

FAQs

Which seal is better for high temperatures and aggressive additives: NBR or FKM?

For higher temperatures and chemically demanding media, FKM is preferable; NBR remains economical for standard mineral oils up to approximately 100 °C.

When is a pilot-operated pressure relief valve more sensible than a direct-acting valve?

If larger flow rates, higher pressure accuracy, or lower hysteresis with changing loads are required, a pilot-operated valve is the better choice; direct-acting valves are suitable for compact systems with low flow rates.

How often should pressure relief valves be maintained?

Maintenance intervals depend on operating conditions; typical intervals include semi-annual visual inspections and annual seal and functional checks for continuous industrial use. Shorter intervals are required for abrasive media or high cycle rates.