As a kind of valve widely used in industrial and civil fields, the sealing performance of gate valve is directly related to the safe and stable operation of the system. We will introduce in detail the various parameters that affect the sealing performance of gate valves:
Sealing surface material
1. Metal material
Carbon steel is a common metal sealing surface material with high strength and hardness, and can withstand greater pressure and impact. For example, in some low-pressure industrial water systems, gate valves with carbon steel sealing surfaces can meet long-term stable sealing requirements.
However, carbon steel has relatively poor corrosion resistance and is prone to rust and corrosion in some corrosive media environments, affecting sealing performance.
Stainless steel has good corrosion resistance and oxidation resistance, and can adapt to a variety of corrosive media. For example, in the acid, alkali and other corrosive fluid transportation pipelines in the chemical industry, gate valves with stainless steel sealing surfaces can effectively prevent the medium from eroding the sealing surface and ensure sealing performance.
However, the cost of stainless steel is relatively high, and its sealing performance may be limited under some special working conditions of high temperature and high pressure.
Alloy steel has good comprehensive performance. By adding different alloy elements, higher strength, hardness, temperature resistance and corrosion resistance can be obtained. For example, in high-temperature and high-pressure steam pipelines, gate valves with alloy steel sealing surfaces can maintain good sealing performance under harsh working conditions and withstand the scouring and high pressure of high-temperature steam.
2. Non-metallic materials
Rubber is a commonly used non-metallic sealing surface material with good elasticity and flexibility. It can compensate for the unevenness and deformation of the sealing surface to a certain extent, thereby achieving a good sealing effect. Common nitrile rubber is suitable for oil media and has good oil resistance.
EPDM rubber has excellent weather resistance, ozone resistance and water resistance, and is often used in water systems and some non-oil medium pipelines. Gate valves with rubber sealing surfaces are usually used in low-pressure and normal temperature conditions, such as building water supply and drainage systems, which can effectively prevent water leakage.
Polytetrafluoroethylene (PTFE) has an extremely low friction coefficient, good chemical stability and corrosion resistance, and is resistant to almost all chemical media.
In some high-precision chemical fluid delivery systems, gate valves with PTFE sealing surfaces can ensure the reliability and stability of the seal, and its low friction coefficient also helps the opening and closing operation of the valve. However, PTFE has limited heat resistance, and performance may deteriorate at high temperatures.
Sealing surface processing accuracy
1. Surface roughness
The surface roughness of the sealing surface directly affects the fit between the gate and the valve seat. Generally speaking, the smaller the surface roughness value, the smoother the sealing surface, the smaller the gap between the gate and the valve seat, and the lower the possibility of medium leakage.
For example, for some gas transmission pipelines with higher requirements, the surface roughness of the gate valve sealing surface is usually required to reach Ra0.8 - Ra1.6μm to ensure the sealing performance in the high-pressure gas environment and prevent gas leakage.
2. Flatness
The flatness of the sealing surface is also one of the key indicators. If there is a flatness error on the sealing surface, the gate and the valve seat cannot fit evenly and comprehensively when closed, which will cause local leakage.
In some large industrial pipeline systems, such as heavy oil pipelines in petrochemical plants, due to the high viscosity and high pressure of the medium, the flatness requirements of the gate valve sealing surface are very strict, and the flatness error is usually required to be within ±0.05mm to ensure the reliability of the seal.
3. Coaxiality
The coaxiality of the gate and the valve seat also has an important influence on the sealing performance. If the two are not coaxial, the gate will be deflected during the closing process, causing uneven force on the sealing surface, which is prone to local wear and leakage. In some high-precision hydraulic systems, the coaxiality requirement of the gate valve is usually controlled within ±0.02mm to ensure good sealing performance under high-pressure and high-precision working environments.
Valve pressure level
1. Low-pressure gate valve
Low-pressure gate valves are usually suitable for working conditions with a nominal pressure of PN≤1.6MPa. The sealing structure of this type of gate valve is relatively simple, and generally soft sealing materials such as rubber or plastic can meet the sealing requirements. For example, in the water supply and drainage system of civil buildings, the commonly used DN50-DN200 low-pressure gate valves, whose sealing performance is mainly achieved by the close fit of the rubber sealing surface with the gate and the valve seat, can effectively prevent water leakage under normal water pressure.
2. Medium-pressure gate valve
The nominal pressure of the medium-pressure gate valve is generally between 1.6MPa < PN≤10.0MPa. For medium-pressure gate valves, the sealing surface material and structure need to have higher strength and sealing performance. For example, in some industrial steam pipelines, the medium-pressure gate valves used usually use metal sealing surfaces, and by optimizing the sealing structure, such as adding sealing gaskets, improving the matching method between the gate and the valve seat, etc., to ensure good sealing performance under the action of medium-pressure steam.
3. High-pressure gate valves
High-pressure gate valves are suitable for working conditions with a nominal pressure of PN > 10.0MPa. Under high-pressure conditions, the sealing surface of the gate valve is under tremendous pressure, and the requirements for sealing performance are extremely high. For example, in the wellhead device of oil extraction, the high-pressure gate valve needs to withstand pressures of tens of MPa or even higher. Its sealing surface is usually made of high-strength alloy steel and undergoes special surface treatments, such as hard alloy surfacing and nitriding treatment, to improve the hardness and wear resistance of the sealing surface. At the same time, multi-channel sealing structures, such as piston ring seals and bellows seals, are used to ensure the sealing reliability under high-pressure oil and gas media.
Medium properties
1. Temperature
High-temperature media will change the performance of sealing materials. For example, rubber sealing surfaces will gradually age, harden, and lose elasticity at high temperatures, resulting in decreased sealing performance; while metal sealing surfaces may be thermally deformed at high temperatures, increasing the gap between the sealing surfaces and causing leakage.
For example, in the high-temperature steam pipeline of a thermal power plant, when the steam temperature exceeds 400°C, ordinary rubber-sealed gate valves cannot work properly. High-temperature resistant metal-sealed gate valves are required, and corresponding insulation and cooling measures are taken to ensure sealing performance.
Low-temperature media may make the sealing material brittle, reducing its toughness and sealing performance. In some low-temperature refrigeration systems, such as liquid nitrogen delivery pipelines, special low-temperature gate valves are required. The sealing surface material is usually made of low-temperature resistant rubber or plastic, and the valve is insulated to prevent the sealing surface from being damaged by low temperature.
2. Viscosity
For high-viscosity media, such as heavy oil, asphalt, etc., the sealing performance of the gate valve will be affected to a certain extent. When the valve is closed, high-viscosity media may form a thick liquid film between the sealing surfaces. Although this liquid film helps sealing to a certain extent, it will also increase the opening and closing resistance of the valve.
At the same time, if the viscosity of the medium is uneven, it may cause uneven force on the sealing surface, affecting the sealing effect. In this case, it is usually necessary to select a drive device with a larger torque to ensure the normal opening and closing of the valve, and to make special designs for the sealing surface, such as using a wide sealing surface, increasing the roughness of the sealing surface, etc., to improve the sealing performance.
3. Corrosiveness
Corrosive media will erode and damage the sealing surface material of the gate valve, resulting in a decrease in sealing performance. Different corrosive media have different requirements for the sealing surface material.
For example, in the pipeline of the sulfuric acid production workshop, sulfuric acid is highly corrosive, and gate valves with polytetrafluoroethylene lining or special corrosion-resistant alloy sealing surfaces are required to resist the corrosion of sulfuric acid and ensure sealing performance.
For some alkaline media, such as sodium hydroxide solution, although the corrosiveness is relatively weak, it is also necessary to select suitable alkali-resistant sealing surface materials, such as stainless steel or specially treated carbon steel, to prevent the sealing surface from being corroded.
Valve opening and closing speed
1. Fast opening and closing
When the gate valve is opened and closed quickly, a large impact force will be generated between the gate and the valve seat. This impact force may cause wear, deformation or even damage to the sealing surface, thus affecting the sealing performance.
For example, in some emergency cut-off occasions, such as the emergency cut-off valve of the gas pipeline, if the valve is opened and closed too quickly, the impact force of the gate and the valve seat when they are closed instantly may cause cracks or pits on the sealing surface, making it impossible for the valve to achieve a good seal when it is closed next time.
2. Slow opening and closing
Although slow opening and closing can reduce the impact force between the gate and the valve seat, if the opening and closing speed is too slow, the flushing time of the medium between the sealing surfaces will be prolonged, which will also cause wear on the sealing surface and affect the sealing performance.
For example, in some medium pipelines containing particulate impurities, such as the tailings conveying pipeline of the mine, when the gate valve is slowly closed, the particulate impurities will continue to rub and flush between the sealing surfaces, causing the sealing surface to gradually wear and the sealing performance to decrease.
Installation and maintenance
1. Installation
The installation position has an important influence on the sealing performance of the gate valve. If the valve is installed in a place where vibration is easy to occur, such as on a pipeline near a pump or compressor, the vibration will cause the sealing surface of the gate valve to be subjected to periodic impact force, which may easily cause the sealing surface to loosen and wear, affecting the sealing performance. At this time, it is necessary to install a shock absorber before and after the valve to reduce the impact of vibration on the valve.
Inaccurate pipeline alignment will also bring additional stress to the gate valve. When the connecting flanges of the pipeline and the valve are not coaxial or the parallelism deviation is large, the valve will be subjected to twisting or tensile stress during installation. This stress will change the fit between the gate and the valve seat and affect the sealing performance.
Therefore, when installing the gate valve, the centering accuracy of the pipeline and the valve must be strictly guaranteed. Generally, the coaxiality deviation of the connecting flange is required to be no more than ±0.5mm, and the parallelism deviation is no more than ±0.2mm.
2. Maintenance
Regularly cleaning impurities on the sealing surface is an important measure to maintain the sealing performance of the gate valve. In actual operation, impurities such as particles and dust in the medium may adhere to the sealing surface, destroying the flatness and finish of the sealing surface, causing sealing leakage.
For example, in some industrial water systems with poor water quality, impurities such as mud and rust in the water are easily accumulated on the sealing surface of the gate valve, and the valve needs to be disassembled and cleaned regularly to ensure the sealing performance.
Timely replacement of worn parts is also the key to maintaining sealing performance. The gate valve's gate, valve seat, sealing ring and other parts will inevitably wear out after long-term use. When the wear reaches a certain degree, these parts need to be replaced in time to restore the sealing performance of the valve.
For example, in some industrial pipelines that are frequently opened and closed, the sealing ring of the gate valve may age, deform and wear after a period of use. If it is not replaced in time, it will cause valve leakage and affect the normal operation of the system.
The sealing performance of the gate valve is affected by a combination of multiple parameters. In practical applications, it is necessary to reasonably select the type, material and parameters of the gate valve according to the specific working conditions, and pay attention to installation and maintenance to ensure that the gate valve has good sealing performance and ensure the safe and stable operation of the system.