Material Selection by Pressure: A Practical Guide to Seal Materials

In industrial equipment and fluid systems, effective sealing is critical for ensuring operational integrity and preventing media leakage. The selection of sealing materials, particularly their ability to withstand internal pressure, is a fundamental factor determining sealing success. Incorrect selection can lead to premature seal failure, leakage, and potential safety hazards. This guide outlines the primary sealing materials recommended for different pressure ranges.

​I. Low-Pressure Applications (0 - 5 MPa)​​

​Common Scenarios:​​ Pneumatic systems, low-pressure hydraulic systems, water treatment, food and beverage machinery, low-load reciprocating rod seals.

​Material Selection:​​

1. ​Nitrile Rubber (NBR):​​ The most economical and versatile choice for low-pressure applications. It offers excellent resistance to petroleum-based hydraulic oils, lubricants, fuels, and air, delivering outstanding value. Ideal for most low-pressure oil hydraulic and pneumatic uses.

2. ​Ethylene Propylene Diene Monomer (EPDM):​​ Excellent resistance to hot water, steam, coolant (glycol), ketones, and weak acids/bases. Not suitable for mineral oils or fuels. Primarily used for sealing water-based media and heat transfer fluids in low-pressure systems.

3. ​Polyurethane (PU/AU/EU):​​ Characterized by extreme abrasion resistance and high mechanical strength. Under low pressure, its superior extrusion resistance and wear properties make it an exceptional choice for reciprocating seals (e.g., piston and rod seals), significantly outperforming standard rubber in service life.

​Summary:​​ For low-pressure applications, prioritize media compatibility. NBR is the default versatile option, PU offers enhanced wear life, and EPDM is specialized for aqueous and polar media.

​II. Medium-Pressure Applications (5 - 30 MPa)​​

​Common Scenarios:​​ Construction machinery, injection molding machines, machine tools, medium-power hydraulic systems.

​Material Selection:​​

1. ​Polyurethane (PU):​​ The predominant choice for medium-pressure hydraulics. Its high mechanical strength, hardness, and exceptional extrusion resistance effectively combat pressure-induced deformation and gap extrusion, making it the preferred material for piston and rod seals.

2. ​Nitrile Rubber (NBR):​​ Reinforced NBR compounds can still be suitable for applications where pressure remains below 15-20 MPa and temperatures are moderate, particularly in static seal applications like O-rings. Its extrusion resistance, however, is considerably lower than PU's.

3. ​Fluoroelastomer (FKM/Viton®):​​ The preferred choice when the media involves high temperatures, fuels, or aggressive chemicals (e.g., acidic fluids), even within the medium-pressure range. FKM provides superior chemical resistance and high-temperature performance (up to 200°C+)

​Summary:​​ In medium-pressure scenarios, extrusion resistance is paramount. Polyurethane (PU) is the primary choice for dynamic seals, while Fluoroelastomer (FKM) is selected for demanding chemical and high-temperature environments.

​III. High and Ultra-High Pressure Applications (Above 30 MPa, up to 100 MPa+)​​

​Common Scenarios:​​ Hydraulic jacks, ultra-high pressure pumps, waterjet cutting, oil and gas wellhead equipment, pressure vessel testing systems.

​Material Selection:​​

1. ​Polyurethane (PU):​​ Specially formulated and engineered polyurethanes (e.g., cast polyurethane) remain a viable option for high-pressure dynamic seals, but require precise formulation and seal design, often mandating the use of anti-extrusion backup rings.

2. ​Aramid Fiber Composites / Engineering Plastics (PEEK, Filled PTFE):​​ These are critical materials for ultra-high pressure applications. They are not elastomers but high-performance plastics with exceptional mechanical strength and modulus.

   • ​Filled PTFE:​​ Adding fillers like glass fiber, copper, or carbon fiber to PTFE drastically improves its compressive strength and extrusion resistance. Commonly used for backup rings and seal rings to protect primary seals from extrusion and damage.

   • ​PEEK:​​ Offers (extremely high strength), stiffness, and temperature resistance, used for manufacturing seal rings and support rings in ultra-high pressure environments.

3. ​Metal Seals (Copper or Stainless Steel):​​ Under extreme pressure (e.g., >70 MPa), high temperature, or high shock pressure, elastomers and plastics reach their limits. Metal O-rings or C-rings become the ultimate solution. They seal via plastic deformation, offering extreme reliability but are typically single-use and require high installation preload.

​Summary:​​ For ultra-high pressure conditions, the strategy shifts from "elastic sealing" to "rigid containment." High-strength engineering plastics (reinforced PTFE, PEEK) and metals are essential, with designs focused on minimizing deformation and extrusion.

​IV. Key Additional Selection Factors​

Pressure is not the sole criterion; selection must involve a holistic assessment:

• ​Temperature:​​ The material's operational temperature range must fully encompass the system's temperature. High heat accelerates aging; low temperatures cause embrittlement.

• ​Media Compatibility:​​ This is the primary prerequisite. The selected material must not be corroded, swollen, or degraded by the sealed medium.

• ​Motion Type:​​ Static seal, reciprocating dynamic seal, or rotary seal? Each motion type imposes different demands on wear resistance, heat generation, and thermal conductivity.

• ​Hardware Compatibility:​​ System clearance design, surface finish, and hardness directly impact a seal's extrusion resistance and wear rate.

​Conclusion:​​

Selecting seal materials is a systems engineering challenge, with pressure serving as a core differentiator:

• ​Low Pressure: Focus on Media.​​ NBR/EPDM are the mainstays.

• ​Medium Pressure: Focus on Extrusion Resistance.​​ PU/FKM are the prime choices.

• ​High Pressure: Focus on Strength.​​ Composites and metals take charge.

The pragmatic selection principle is: while meeting media and temperature requirements, choose materials with adequate extrusion resistance and mechanical strength based on working pressure, and ensure proper seal groove and clearance design. The most expensive material isn't necessarily the best; the optimal choice is the one most suited to the specific operating conditions.

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