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Pressure Relief Valves: Selection, Application, and Technical Details for Hydraulic Applications

This category features pressure relief valves for hydraulic circuits with various connections, materials, and sealing options. Pressure relief valves regulate the maximum system pressure, protect components from overload, and enable defined pressure holding or pressure-controlled switching. For industrial applications, reliable materials, suitable connection types, and compatible seals are crucial. Precise pressure settings, minimal leakage, and durable material resistance to hydraulic oil and environmental conditions are important.

Design Types and Configurations

Pressure relief valves are available as cast housing/flange variants, inline elements, and cartridges. Flange and screw-in models are suitable for fixed pipelines and systems with standardized connections, while cartridges allow space-saving direct mounting in manifold blocks. Distinction by operating principle: direct-acting valves are compact and react without pilot pressure, suitable for low to medium flow rates; pilot-operated relief valves handle higher pressures and large volume flows, offering finer control and lower pressure peaks during rapid load changes.

Materials and Surfaces

Housings and internal components are typically made of malleable cast steel, quenched and tempered steel, or stainless steel. For corrosive environments and food/pharmaceutical areas, stainless steel variants (e.g., 1.4404/316L) are recommended. Surfaces are often ground, phosphated, or painted to reduce wear and ensure corrosion protection. For highly dynamic applications, hardened steel with precise seat machining is recommended to minimize leakage rates and increase service life.

Seals: NBR vs. FKM and Seal Design

Seal material determines temperature resistance, chemical compatibility, and aging resistance. NBR (nitrile) is economical, suitable for mineral hydraulic oils and temperatures up to approx. +100 °C. NBR offers good abrasion resistance but limited resistance to high temperatures and certain additives. FKM (Viton) offers significantly higher temperature and chemical resistance (up to ~+200 °C) as well as improved resistance to oxidizing agents, glycols, and aromatic components. For applications with increased temperature requirements, high cycle numbers, or special hydraulic fluids, FKM is the better choice. Seal design typically includes O-rings, poppet seals, or special constructions with backup rings to prevent extrusion at high pressure differentials.

Connections, Sizes, and Standards

Common connection types are BSPP/BSPT, NPT, SAE, and metric screw-in threads according to ISO/DIN, as well as flanges according to SAE/ISO. Cartridge and block valves often follow ISO standards for installation dimensions (e.g., ISO 7368). When selecting, the correct thread is as crucial as the appropriate nominal flow rate (Q) in l/min and the pressure rating. Valves are available in pressure ranges from low bar ranges to >400 bar; most hydraulic systems operate at 63–315 bar. Test specifications and production tests usually comply with DIN, ISO, and, if applicable, ATEX/CE directives for potentially explosive atmospheres.

Functional Parameters and Characteristic Values

Key parameters include setting pressure range, hysteresis, repeatability, flow characteristic, and leakage rate. Direct-acting relief valves have a fast response time but higher pressure differentials during volume changes. Pilot-operated variants provide more stable pressure holding at large volume flows and lower pressure differentials. When designing, the viscosity of the hydraulic medium must be considered; air or particle content changes the control behavior. Precision valves with adjustable preload via an adjusting screw or fine adjustment via a hexagonal spindle with a scale for reproducible setting are recommended.

Temperature and Medium Compatibility

It is important to match seals and material to the hydraulic medium (mineral oils, esters, synthetic fluids). Temperature ranges vary with seal choice: NBR temperature range typically -30 °C to +100 °C, FKM from -20 °C to +200 °C. At low temperatures, special cold rubber compounds should be selected; at high temperatures, thermally stable seals and, if necessary, metal-to-metal seals should be used. Additives in the oil and oxidation products influence aging; regular fluid analyses extend service life and ensure sealing functions.

Mounting, Adjustment, and Installation Notes

Before installation, the connection thread must be cleaned and provided with suitable sealants (PTFE tape or liquid sealants, depending on manufacturer specifications). Tightening torques are manufacturer-dependent and must be observed to avoid deformation and leaks. For cartridge installations, mating surfaces must be cleaned and the insertion depth in the manifold block checked. Adjustment is carried out during operation: slowly increase pressure, carefully adjust the adjusting screw, and consider temperature and viscosity changes. After adjustment, a lock nut or securing device must be used to prevent unintentional adjustment. For pilot lines, the cross-section, length, and connections must be chosen to avoid delays or pressure losses.

Maintenance, Wear, and Spare Parts

Maintenance intervals depend on operating hours, operating conditions, and the degree of oil contamination. Filter fineness, particle count, and water content in the oil are critical influencing factors. Typical wear patterns include seal abrasion, erosion at seat surfaces, and spring fatigue. Spare parts include seal kits (O-rings, poppet seals), springs, seat rings, and threaded inserts. For scheduled maintenance, complete seal kits should be kept in stock; for critical processes, a replacement interval for seal kits depending on operating hours and fluid analysis is recommended. Detailed spare part numbers and assembly instructions can be found in the product data sheets and, if required, at https://maku-industrie.de/technik.

Practical Examples

Example 1: In a hydropneumatic press, pilot-operated pressure relief valves limit the operating pressure to 250 bar. The valve is installed as a cartridge in the manifold block. During rapid load changes, the pilot control ensures stable pressure holding, prevents pressure surges, and protects cylinders and hose lines. FKM seals were chosen here due to elevated oil temperatures up to 120 °C. Example 2: In a coolant return of an injection molding machine, a direct-acting screw-in valve limits the return pressure to 35 bar and protects the pump in case of blockages. The medium used was mineral hydraulic oil, NBR seal, BSPP thread. Due to its compact design, the valve could be mounted close to the unit, keeping line lengths short and improving reaction times. Example 3: In a lubrication circuit supply of a rolling mill, stainless steel valves with FKM seals were installed. A combination of flange valves and cartridges in a manifold plate allows modular maintenance without shutting down the entire system. Control is via pre-adjustable screws with a lock nut, documented in the maintenance log.

Selection Criteria at a Glance

* System pressure and flow, medium and temperature, connection type, design (cartridge/inline), seal type (NBR/FKM), material (steel/stainless steel), control accuracy and response behavior, maintainability and spare part availability For detailed technical data sheets, application examples, and support for design, please also see our technology page: https://maku-industrie.de/technik and specific practical cases at https://maku-industrie.de/anwendungsbeispiele. A product overview and ordering options for the pressure relief valves shown can be found directly in this category.

FAQs

1. Which sealing material is suitable for high temperatures and aggressive hydraulic fluids?
For high temperatures and chemically demanding media, FKM is the preferred choice. FKM offers resistance up to approximately +200 °C and to many additives, while NBR ages faster at higher temperatures. 2. When is a pilot-operated pressure relief valve necessary?
A pilot-operated valve is necessary for high flow rates, fluctuating loads, or when very precise pressure holding under dynamic conditions is required. It reduces pressure peaks and allows better control than direct-acting valves. 3. Which testing and maintenance steps ensure long-term function?
Regular oil analyses (contamination, water content), filter inspection, visual inspection for leaks, replacement of seal kits after operating hours, and functional testing of the adjustment mechanism ensure long-term function. Documented adjustment protocols and spare parts inventory for seal kits/springs minimize downtime.