This article introduces the main application scenarios, sealing forms, and design points of O-ring rubber seals in safety valves, analyzes the common failure forms of O-ring rubber seals in practical engineering, and proposes suggestions for the use of O-ring rubber seals in safety valves.

1. Overview

Safety valves are essential safety accessories for various types of boilers, pressure vessels, and pressure pipelines, and are widely used in various fields such as petroleum, chemical, power plants, metallurgy, nuclear power, and national defense. The sealing of various parts of the safety valve can be divided into metal sealing and non-metallic sealing according to the contact type, and the O-ring rubber sealing ring (hereinafter referred to as the O-ring) is a typical structure of non-metallic sealing. Rubber is an elastic polymer material that can undergo significant deformation with minimal stress. This deformation can provide contact pressure, compensate for leakage gaps, and achieve the purpose of sealing. Therefore, rubber sealing is easier to achieve compared to other forms of sealing, and its sealing performance is excellent. Whether used for static or dynamic sealing, the leakage rate can be maintained to a small extent. Especially for dynamic seals, the friction resistance of O-ring movement is very small, making it suitable for pressure alternating situations.

2. Applicable working conditions

A safety valve is a type of safety protection valve that automatically opens and closes according to the working pressure of the system. It can be divided into direct acting and non direct acting according to its working principle. Spring loaded safety valves and pilot operated safety valves are representatives of these two forms, respectively. Take these two commonly used safety valves as examples to illustrate the main application scenarios of O-rings.

(1) Low pressure working condition

A safety valve is an automatic valve that seals without relying on external driving force (except for safety valves with auxiliary devices). The sealing load at the valve seat is mainly provided by the pre tightening force of the spring (directly loaded safety valve) or the pressure of the working medium (pilot type safety valve). The sealing load is smaller than other types of valves and 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. Other types of sealing forms are difficult to seal. Due to the high elasticity of the O-ring, its sealing mechanism can ensure sealing under small load conditions. Under low-pressure conditions, it is more reasonable to choose an O-ring rubber sealing ring for sealing at the valve seat.

(2) Dynamic sealing condition

A pilot operated safety valve consists of a main valve and a pilot valve, and relies on the pressure sensing system of the pilot valve to control the opening and closing of the main valve. The internal structure of the pilot valve is compact, the flow channel is narrow, the spatial position is limited, there are many relative moving parts, and the sealing part is tight. The main valve disc and guide sleeve are in a piston type structure. To ensure that the medium in the chamber formed by the valve disc and guide sleeve does not leak out, and to maintain the good motion characteristics of the valve disc when the safety valve is opened, the axial dynamic seal is mostly equipped with an O-ring.

(3) Operating conditions with strict leakage rate requirements

Compared to non-metallic sealing structures, metal sealing structures have a higher leakage rate. According to API 527, the maximum allowable leakage rate of metal sealed safety valve seats varies depending on the flow channel area and set pressure, with a bubble count of 20-100 per minute. The safety valve with non-metallic sealing structure can leak bubbles as low as 0 per minute. Obviously, non-metallic sealing structures are more suitable for situations with strict leakage rate requirements. For example, hydrogen belongs to small molecule gases, which are easy to escape, flammable and explosive, and very dangerous. Therefore, it is believed that strict control of leakage rate is necessary. In non-metallic sealing, the sealing structure of O-ring is relatively simple, easy to install, maintain, and replace, so O-ring is widely used for sealing at the valve seat in hydrogen containing conditions.

3. Main sealing forms and structural design

The O-ring belongs to the extrusion type seal, and the basic working principle of the extrusion type seal is to rely on the elastic deformation of the sealing element to create contact pressure on the sealing contact surface. If the contact pressure is greater than the internal pressure of the sealed medium, no leakage occurs; otherwise, leakage occurs. In order to ensure effective sealing, the performance of the material and the design of the sealing structure are crucial. For different types of sealing structures, the compression amount and groove design of the O-ring vary. The sealing forms of O-ring rubber seals in safety valves mainly include end face static sealing, reciprocating dynamic sealing, and valve seat sealing.

(1) End face static seal

The sealing of the end faces between the valve seat and valve body, as well as between the valve body and the cover plate of the pilot operated safety valve, belongs to the static sealing of the end faces, and its structure is relatively simple. Under static sealing conditions, the O-ring is compressed axially and undergoes radial creep under pressure. Therefore, when designing the O-ring groove, the direction of pressure needs to be considered. The O-ring must have a certain amount of space inside the groove, and the volume of the O-ring should account for about 75% of the groove volume, which is more suitable. When the inner diameter is stretched into the groove, to prevent excessive stretching from causing excessive internal stress on the material, it is advisable to control the stretching amount within 1% to 3% of the wire diameter and not exceed 5%. The compression rate of the end face seal should be controlled between 20% and 30%. To prevent stress concentration, the bottom of the groove needs to be rounded with a radius of 0 2-0 5mm, the roughness Ra in the groove is taken as 1 six μ M is appropriate.

(2) Reciprocating seal

The sealing between the main valve disc and the guide sleeve of the pilot operated safety valve is a reciprocating sealing. Due to the fact that safety valves are normally closed valves, their operating frequency is generally low and their movement cycle is short. In normal working condition, the valve disc and guide sleeve are in an axial static sealing state, and there is only reciprocating motion during the opening and closing process of the safety valve. When the safety valve is opened, the main valve disc moves upwards. When the safety valve is closed, the main valve disc moves downward, which is different from the reciprocating sealing situation of general machinery. The O-ring is compressed radially. To ensure the initial sealing effect without affecting the valve's performance, the groove design of the O-ring needs to consider controlling the radial deformation. The radial compression ratio should be controlled within 10% to 15%, and the roughness of the groove should be controlled at Ra0 eight μ Below m. When reciprocating under high pressure, the O-ring is easily squeezed out. It is recommended to install a retaining ring.

(3) Seat sealing

The sealing surface formed by the seat, disc, and O-ring of the safety valve is the seat seal. When the system pressure is less than the set pressure of the safety valve, the safety valve is in a closed state, and at this time, the valve seat contacts the valve disc to form a static seal on the end face. When the system pressure exceeds the set pressure of the safety valve, the valve disc and valve seat sealing surface detach, and the safety valve opens to release the system pressure. When the system pressure drops to a certain value, the safety valve returns to a closed state, and the valve seat and disc come into contact again, forming a static seal on the end face. During the opening and closing process of the safety valve, a sharp change in pressure can have a certain impact on the sealing surface, and the O-ring is easily blown out or damaged by the accumulated pressure inside the groove. Therefore, the design of O-ring grooves for valve seat sealing should not only consider sealing performance, but also the reliability of the O-ring after the valve seat and valve disc sealing surface detach. A common form of valve seat sealing is dovetail groove, and different structures have different designs. The most important thing for valve seat sealing is to ensure the volume ratio of the groove and prevent the groove opening from being too large, which may cause the O-ring to be blown out.

4. Material selection

The commonly used O-ring materials in safety valves include fluororubber (FKM), nitrile rubber (NBR), ethylene propylene rubber (EPDM), fluorosilicone rubber (FVMQ), and perfluoroether rubber (FFKM). It is generally not recommended to choose a safety valve with an O-ring as the sealing element when the temperature exceeds the O-ring usage limit or the medium has an impact on the performance of the O-ring. The selection of O-ring in safety valves is related to temperature, medium, and pressure, and also needs to consider the comprehensive influence of various factors.

(1) Temperature

Temperature is one of the key factors affecting the range of use of O-shaped rubber seals, and its operating temperature is generally between -60 ℃ and 327 ℃. Special materials need to be selected for high or low temperature conditions to achieve effective sealing. Among commonly used rubber materials, perfluoroether rubber has excellent high-temperature resistance. The working temperature range of ordinary perfluoroether rubber is -25 ℃ to 240 ℃, and special grades of perfluoroether rubber can withstand a constant temperature of 316 ℃ or intermittent high temperature of 343 ℃ without hardening and brittle failure. Therefore, for high-temperature working conditions that are not suitable for other rubbers, perfluoroether rubber can be selected.

Rubber will become brittle at low temperatures and lose its sealing effect, and is generally not recommended for use in low-temperature environments. The silicone rubber has the best low-temperature resistance among rubber materials. Its low-temperature grade can be used up to -100 ℃, but its tensile strength is low and its wear resistance is weak. It is generally not suitable for dynamic sealing and is not suitable as a safety valve seal. Fluorosilicone rubber, second only to silicone rubber in low temperature resistance, has a temperature range of -60 ℃ to 177 ℃. While maintaining the low temperature resistance of silicone rubber, it enhances its chemical and mechanical properties. In low-temperature working conditions, fluorine silicone rubber is preferred.

(2) Media

Safety valves are widely used in industries such as petrochemicals, energy, and electricity. For different media, it is necessary to choose appropriate materials. Fluoroelastomer has flame retardancy, excellent air tightness, ozone resistance, weather resistance, good aging resistance and extensive corrosion resistance. It is suitable for inorganic acid, fuel oil, pure oxygen, tetrachlorosilane, etc., and is widely used in safety valves. Ethylene propylene rubber has excellent resistance to water, steam, and superheated water, making it suitable for safety valves in high-temperature steam conditions in the power industry. Perfluoroether rubber is suitable for organic solvents containing aromatic compounds, as well as wet hydrogen sulfide media.

(3) Pressure

The hardness of commonly used O-ring is 50-90 Shore. According to the sealing principle of O-rings, it is recommended to choose O-rings with lower hardness when the system pressure is low. When the system pressure is high, choose an O-ring with higher hardness. When the working pressure exceeds 10MPa, it is best to choose a retaining ring in combination 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", meaning that after a long period of time under high pressure, high-pressure gas molecules infiltrate into the inside of the O-ring. When the external pressure of the O-ring suddenly decreases, the high-pressure gas molecules inside the O-ring are more likely to burst due to rapid diffusion. Especially for the O-ring at the valve seat seal, when the safety valve is opened, the instantaneous release of pressure can easily cause "implosion" of the O-ring here. Therefore, under high-pressure conditions, it is recommended to choose a high density O-ring or an anti implosion O-ring for this O-ring.

5. Failure analysis and handling

(1) Media incompatibility failure

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

(2) On a certain natural gas hydrogenation device, the set pressure of the safety valve is 26 MPa and the working pressure is 24 After running online for a period of time at 7MPa, leakage occurred. The disassembly inspection of the safety valve shows that the O-ring at the valve seat seal is broken. After analysis, it was found that due to the actual working pressure fluctuation range exceeding the expected range of the system, and the system working pressure being very close to the set pressure, the safety valve frequently jumped, reaching the service life limit of the O-ring, leading to the fatigue fracture failure of the O-ring. The solution is to control the fluctuation of system pressure through process improvement, and regularly replace the O-ring rubber seal at the valve seat, so that the safety valve can operate normally after going online.

(3) Deformation failure

Compared to other types of non-metallic sealing materials such as plastic and graphite, rubber has lower hardness and better resilience. However, under high temperature, high pressure, or a combination of both, it is prone to deformation, causing sealing failure. A certain natural gas pipeline, with a safety valve set pressure of 21MPa, uses an O-ring with a 75 Shore hardness. After one year of operation, the surface of the valve is smooth, but the O-ring section at the valve seat seal has deformed into a rectangular groove shape, losing its resilience and causing seal failure. When replaced with an O-ring with a 90 Shore hardness, the sealing ring performs well.

6. Conclusion

When selecting and designing O-ring seals, comprehensive consideration should be given to the effects of operating conditions such as temperature, medium, and pressure on the O-ring, as well as various factors such as groove structure, installation, and maintenance, in order to extend the service life of the O-ring and ensure the normal operation of the safety valve online.