1 Overview

Safety valves are indispensable safety accessories for various boilers, pressure vessels and pressure pipelines, and are widely used in various fields such as petroleum, chemical industry, power station, metallurgy, nuclear power, and national defense. The seals of various parts of the safety valve are divided into metal seals and non-metal seals according to the contact type. O-shaped rubber seals (hereinafter referred to as O-rings) are typical structures of non-metal seals. Rubber is an elastic polymer material, which can produce large deformation when given a small stress. This deformation can provide contact pressure, compensate for leakage gaps, and achieve the purpose of sealing. Therefore, rubber seals are easier to achieve than other forms of seals. , and the sealing performance is excellent, no matter it is used for static sealing or dynamic sealing, the leakage rate can be kept at a small level. Especially for dynamic seals, the motion friction resistance of O-rings is very small, which can be adapted to the occasion of alternating pressure.

2. Applicable working conditions

The safety valve is a safety protection valve that automatically opens and closes according to the working pressure of the system. According to the principle of action, it can be divided into direct acting type and indirect acting type. Spring loaded safety valve and pilot operated safety valve are these two forms respectively representative. Take these two commonly used safety valves as examples to illustrate the main application occasions of O-rings.

(1) Low pressure condition

The safety valve is an automatic valve that does not rely on external driving force to seal (except for safety valves with auxiliary devices). The sealing load at the valve seat is mainly determined by the spring preload (spring direct load safety valve) or the working medium Pressure (pilot-operated safety valve) provides, the sealing load is smaller than other types of valves, and the sealing is more difficult. When the set pressure is low, the spring preload or the pressure of the working medium is correspondingly small, and the sealing load of the valve seat is small, and other types of sealing forms are difficult to seal. Due to the high elasticity of the O-ring, its sealing mechanism can ensure the sealing under a small load. Under low-pressure conditions, it is more reasonable to choose an O-ring for sealing at the valve seat.

(2) Dynamic sealing conditions

Pilot-operated safety valve consists of a main valve and a pilot valve, and the opening and closing of the main valve is controlled by the pilot valve sensing system pressure. The internal structure of the pilot valve is compact, the flow channel is narrow, the space is limited, there are many relative moving parts, and the sealing parts are tight. The valve disc of the main valve and the guide sleeve are piston-type structures. In order to ensure that the medium in the chamber formed by the valve disc and the guide sleeve does not leak out, and at the same time maintain good movement characteristics of the valve disc when the safety valve is opened, its axial Most of the dynamic seals use O-rings.

(3) Working conditions with strict leakage rate requirements

Compared with the non-metallic sealing structure, the leakage rate of the metal sealing structure is higher. According to the regulations of API 527, the maximum allowable leakage rate of the metal-sealed safety valve seat depends on the area of the flow channel and the set pressure, and the number of bubbles is 20 to 100 per minute. The leakage air bubbles of safety valves with non-metal sealing structure can be as low as 0/min. Obviously, the non-metallic sealing structure is more suitable for occasions with strict leakage rate requirements. For example, hydrogen gas is a small molecular gas, easy to escape, flammable and explosive, and very dangerous. Therefore, it is considered that the control of the leakage rate should be very strict. In the non-metal seal, the sealing structure of the O-ring is relatively simple, and it is easy to install, maintain and replace. Therefore, a large number of O-rings are used for the sealing of the valve seat in the hydrogen-containing condition.

O-ring rubber seal

3. Main sealing form and structure design

The O-ring belongs to the extrusion type seal. The basic working principle of the extrusion type seal is to rely on the elastic deformation of the seal to cause contact pressure on the sealing contact surface. If the contact pressure is greater than the internal pressure of the sealed medium, no leakage will occur, and vice versa. A leak occurs. In order to ensure an effective seal, the performance of the material and the design of the sealing structure are very important. For different types of sealing structures, the compression amount and groove design of the O-ring are different. The sealing forms of the O-shaped rubber sealing ring in the safety valve mainly include end face static seal, reciprocating dynamic seal and valve seat seal.

(1) End face static seal

The seals between the valve seat and the valve body and between the valve body and the cover plate of the pilot-operated safety valve belong to the static seal of the end face, and its structure is relatively simple. Under static sealing conditions, the O-ring is squeezed in the axial direction, and under pressure, it will produce radial creep. Therefore, the direction of the pressure needs to be considered when designing the O-ring groove. The O-ring must have a certain space in the groove, and the volume of the O-ring accounts for about 75% of the groove volume. When the inner diameter is stretched into the groove, in order to prevent excessive internal stress caused by excessive stretching, the stretching amount should be controlled at 1% to 3% of the wire diameter, and should not exceed 5%. The compression rate of the end face seal should be controlled at 20% to 30%. In order to prevent stress concentration, the bottom of the groove needs to be rounded, the radius is 0.2-0.5mm, and the roughness Ra in the groove is preferably 1.6μm.

(2) Reciprocating dynamic seal

The seal between the main valve disc and the guide sleeve of the pilot safety valve is a reciprocating dynamic seal. Since the safety valve is a normally closed valve, the action frequency is generally very low and the movement cycle is short. In normal working condition, the disc and the guide sleeve are axially statically sealed, and there is reciprocating movement only when the safety valve is opened and closed. When the safety valve is opened, the disc of the main valve moves upward. When the safety valve is closed, the disc of the main valve moves downward, which is different from the reciprocating dynamic sealing of general machinery, and the O-ring is squeezed in the radial direction. In order to ensure the initial sealing effect without affecting the action performance of the valve, the groove design of the O-ring needs to consider controlling the amount of radial deformation. The radial compressibility should be controlled within 10% to 15%, and the roughness of the groove should be controlled below Ra0.8μm. When reciprocating under high pressure, the O-ring is easily squeezed out, and it is recommended to install a retaining ring.

(3) Seat seal

The sealing surface formed by the valve seat of the safety valve, the disc and the O-ring is the seat seal. When the system pressure is lower than the setting pressure of the safety valve, the safety valve is in the closed state, and at this time the valve seat and the disc contact to form a static seal on the end face. When the system pressure is greater than the setting pressure of the safety valve, the disc and the sealing surface of the valve seat are separated, and the safety valve opens to release the system pressure. When the system pressure drops to a certain value, the safety valve returns to the closed state, and the valve seat and the disc contact again to form a static seal on the end face. During the opening and closing process of the safety valve, the sharp change of pressure will have a certain impact on the sealing surface, and the O-ring is easily blown out or damaged by the pressure accumulated inside the groove. Therefore, the O-ring groove design of the valve seat seal should not only consider the sealing performance, but also consider the reliability of the O-ring after the valve seat and the disc sealing surface are separated. A common form of valve seat seal is the dovetail groove type, and different structures have different schemes in design. The most important thing for seat sealing is to ensure the volume ratio of the groove and prevent the groove opening from being too large, causing the O-ring to be blown out.

4. Material selection

O-ring materials commonly used in safety valves generally include fluororubber (FKM), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), fluorosilicone rubber (FVMQ) and perfluoroether rubber (FFKM). For the temperature exceeding the usage limit of the O-ring, or the medium has an impact on the performance of the O-ring, it is generally not recommended to choose a safety valve that uses the O-ring as the seal. The selection of O-rings in safety valves is related to temperature, medium and pressure. In addition, the comprehensive influence of various factors needs to be considered.

(1) temperature

Temperature is one of the key factors affecting the use range of O-ring rubber seals. The use temperature is generally -60°C to 327°C. Special materials need to be selected for high or low temperature conditions to achieve effective sealing. Among the commonly used rubber materials, perfluoroether rubber has excellent high temperature resistance. The working temperature range of ordinary perfluoroether rubber is -25°C to 240°C, and special grades of perfluoroether rubber can withstand a constant temperature of 316°C or 343°C Intermittent high temperature does not cause hardening and embrittlement failure, so for high temperature conditions where other rubbers are not suitable, perfluoroether rubber can be used.

Rubber will become brittle at low temperature and lose its sealing effect, so it is generally not recommended for use in low temperature environments. Silicone rubber has the best low temperature resistance among rubber materials. The low temperature grade grade can be used up to -100°C, but its tensile strength is low and its wear resistance is weak. Generally, it cannot be used for dynamic seals, and is not suitable for safety valve seals. pieces. Fluorosilicone rubber, which is second only to silicone rubber in low temperature resistance, can be used in a temperature range of -60°C to 177°C. On the basis of maintaining the low temperature resistance of silicone rubber, it strengthens the chemical resistance and mechanical properties. In low temperature conditions, fluorosilicone rubber is preferred.

(2) Medium

Safety valves are widely used in petrochemical, energy and power industries. For different media, appropriate materials need to be selected. Fluorine rubber has flame retardancy, excellent air tightness, ozone resistance, weather resistance, good aging resistance and extensive corrosion resistance. It is suitable for inorganic acids, fuel oil, pure oxygen, silicon tetrachloride, etc., in safety valves widely used in. Ethylene-propylene rubber has excellent resistance to water, steam and superheated water, and is suitable for safety valves in high-temperature steam conditions in the power industry. Perfluoroether rubber is suitable for organic solvents containing aromatic hydrocarbon compounds, and those containing wet hydrogen sulfide media.

(3) Pressure

Commonly used O-rings have a hardness of 50-90 Shore. According to the sealing principle of the O-ring, when the system pressure is low, it is recommended to use an O-ring with a lower hardness. When the system pressure is high, choose O-ring with higher hardness. When the working pressure exceeds 10MPa, it is best to use a retaining ring to prevent the O-ring from being squeezed into the sealing gap and deformed under high pressure. In addition, when the pressure is high, the O-ring is prone to "implosion", that is, the O-ring is under high pressure for a long time, and the high-pressure gas molecules penetrate into the inside of the O-ring. When the external pressure of the O-ring drops instantly, the internal high pressure Gas molecules are easy to break through the O-ring due to rapid diffusion. Especially the O-ring at the seat seal, when the safety valve is opened, the instantaneous release of pressure can easily cause the O-ring to "implode", so under high-pressure conditions, it is recommended that the O-ring here It is best to use a high-density O-ring or an implosion-proof O-ring.

5. Failure analysis and treatment

(1) Medium incompatibility failure

In actual engineering, the selected O-ring is incompatible with the medium, and problems such as corrosion, swelling, deformation, and fracture will occur, resulting in failure of the safety valve seal.

(2) Fatigue failure On a natural gas hydrogenation unit, the set pressure of the safety valve is 26MPa, and the working pressure is 24.7MPa. After a period of on-line operation, leakage occurs. Disassembly inspection of the safety valve revealed a fractured O-ring at the seat seal. After analysis, because the actual working pressure fluctuation range exceeds the expected range of the system, and the system working pressure is very close to the set pressure, the safety valve frequently takes off, reaching the life limit of the O-ring, resulting in O-ring fatigue fracture failure. The solution is to control system pressure fluctuations through process improvement, and regularly replace the O-shaped rubber seal ring at the valve seat, so that the safety valve operates normally after it goes online.

(3) Deformation failure

Compared with other types of non-metallic sealing materials such as plastic and graphite, rubber has lower hardness and better resilience, but it is prone to deformation under high temperature, high pressure or both conditions, resulting in sealing fail. For a natural gas pipeline, the set pressure of the safety valve is 21MPa, and an O-ring with a hardness of 75 Shore is selected. After one year of operation, the surface of the valve is smooth, but the cross section of the O-ring at the seal of the valve seat has been deformed into a rectangular groove shape, which loses its resilience and causes the seal to fail. When replaced with a 90 Shore hardness O-ring, the seal works well.