In high pressure equipment (such as hydrogen energy storage system, pressure vessels, petrochemical equipment), the sealing performance of screw holes is crucial to the safety of the equipment. Failure of the screw hole seal may lead to media leakage, cause safety risks, and even cause equipment damage. Therefore, how to optimize the screw hole design to ensure the sealing performance under high faforce is a problem to be further studied.

• 1. Challenge of screw hole sealing for high-pressure equipment
• 1.1 Influence of high tightening force on sealing performance
• The high tightening force of the bolt is one of the key factors to realize the screw hole sealing. However, excessively high tightening forces may cause the following problems:
• Thread deformation: the plastic deformation of the thread surface, affecting the integrity of the sealing surface.
• Seket overoverload: If the seal washer is used, excessive pretension force may cause washer extrusion or premature failure.
• Stress concentration: In high pressure equipment, the stress concentration area is easy to form around the screw hole, which may cause thread fatigue or crack.

• 1.2 Screw hole machining error and seal failure
• Screw hole machining error may lead to seal failure, mainly in:
• Excessive thread tolerance: the insufficient thread accuracy of the screw hole will affect the engagement tightness of the bolts and reduce the sealing performance.
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• Coaxial degree deviation: the coaxial degree deviation of the thread hole and the seal surface causes local poor sealing.
• The influence of surface roughness: the roughness of the sealing surface will reduce the contact area and reduce the sealing performance.

• 1.3 Working environment impact
• The screw hole sealing of high pressure equipment is affected not only by mechanical factors, but also challenged by environmental factors:
• High / low temperature environment: temperature change will lead to different expansion coefficient of bolts and screw holes, resulting in attenuation of pretension force.
• Corrosive medium: high pressure gas such as hydrogen is easy to cause hydrogen embrittlement of bolts and screw holes.
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• • Vibration and shock: the vibration of mechanical equipment will loosen the bolts and affect the stability of the seal.
  • 2. Optimized design of the screw hole seal
  • 2.1 Thread structure optimization
  • Select the appropriate thread type:
  • Fine tooth threads (such as ISO 2901) can improve the sealing performance.
  • The trapezoidal thread (such as DIN 103) has good tensile strength and is suitable for high pressure seals.
  • Increase the thread engagement length:
  • In general, the thread engagement length is recommended to be 1.5-2 times the bolt diameter to improve the sealing capacity.
  • Optimize the thread surface treatment:
  • Phosphorized, nickel or silver plating coating is used to reduce the friction coefficient and improve corrosion resistance.
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  • 2.2 Selection of sealing auxiliary materials
• Metal washer (such as copper pad, nickel base alloy pad): suitable for extreme high pressure occasions, high temperature resistance, corrosion resistance.

Flexible sealing ring (e. g. fluorine rubber, PTFE): suitable for lower pressure sealing, improve the micro sealing effect.

Seal coating (e. g. liquid sealant): Fill in thread clearance to provide additional seal protection.

• 2.3 Pretension force control
• Screw hole seal depends on suitable pretension control, common methods include:
• Torque control method (per ISO 898-1 standard): select appropriate tightening torque according to bolt specification and material.
• Hydraulic tensile method: suitable for large diameter bolts to ensure the uniform force.
• Ultrasonic measurement method: used for high-precision equipment, real-time monitoring of pretension force.

3. Design verification and testing

3.1 Finite Element Analysis (FEA)

The finite element analysis (such as ANSYS, Abaqus) is used to simulate the force distribution of screw holes under high pressure conditions to optimize the sealing design.

3.2 Experimental test

Air tightness test (such as helium leak test): verify that the screw hole seal is compliant.

​Pressure fatigue test: simulate the long-term operation of the equipment and test the stability of the sealing performance.

Corrosion resistance test (eg. salt spray test): assess the performance of sealing materials in harsh environments.

The screw hole sealing design in high pressure equipment should comprehensively consider the factors of thread structure, sealing material, fastening force control and environmental impact. By optimizing the thread geometry, selecting high-performance sealing materials, combined with high-precision processing and strict testing and verification, the sealing performance of the screw holes can be effectively improved to ensure the safe operation of the equipment under extreme working conditions.