Key Points for Selecting Chemical Valves
Sep 27, 2023
1.1 Selection of valve types
①Determine the corresponding nominal pressure based on the three conditions, namely temperature, pressure, and materials, which must meet the requirement for the nominal pressure value of the selected valve.
②The leakage amount of the determined valve type should meet the process requirements.
③The working pressure difference of the determined valve type should be less than the allowable pressure difference of the valve.
④The medium temperature is within the operating temperature range of the valve, and the ambient temperature should meet the requirements.
⑤Consider anti-blocking the valve according to the medium conditions.
⑥Consider the corrosion resistance of the valve according to the chemical properties of the medium.
⑦According to the pressure difference and medium containing hard objects, consider the erosion and wear resistance of the valve.
⑧When considering cost-effectiveness, several issues need to be considered such as simple structure, easy maintenance, availability of spare parts, service life, and price.
1.2 Selection of executing agency
①The simplest is the pneumatic diaphragm type, followed by the piston type, and finally the electric type.
②The main advantage of electric actuators is that the driving source (power supply) is convenient, but the price is high; the reliability, waterproofing and explosion-proof are not as good as pneumatic actuators, so pneumatic actuators should be preferred.
③There are many varieties and specifications of piston actuators, so a new lightweight structure should be selected.
1.3 Selection of materials
①The pressure rating, operating temperature and corrosion resistance of the valve body should not be lower than the requirements of the process connecting pipelines.
②Cast iron valves should not be used for water vapor or wet gases containing more water and flammable and explosive media.
③When the ambient temperature is below -20°C, it is not advisable to use cast iron valves, especially in northern China.
④For highly corrosive media, when choosing corrosion-resistant alloys, appropriate corrosion-resistant materials must be selected based on the type, concentration, temperature, and pressure of the medium.
⑤The valve body and valve core are treated separately. The throttling speed of the inner wall of the valve body is small and a certain amount of corrosion is allowed; the corrosion rate is slow; the valve core is scoured at high speed and is easily corroded, which will cause an increase in leakage, and the corrosion rate is fast.
⑥When selecting lining materials such as rubber and plastic, the temperature, pressure, and concentration of the working medium must meet the use range of the material and the physical and mechanical damage such as shear damage to the valve during operation must be considered.
⑦ Vacuum valves should not be lined with rubber or plastic.
⑧The two-position shut-off valve of the water treatment system should not be lined with rubber.
⑨The universal corrosion-resistant material is polytetrafluoroethylene. Therefore, PTFE corrosion-resistant valves should be selected first, and alloys should be selected secondly when the temperature is greater than 180°C and the engineering pressure 1.6MPa.
1.4 Selection of flow characteristics
① When working with a small opening and there is a great change in unbalanced force, valves with equal percentage flow characteristics should be selected.
②When the required adjusted parameters change rapidly, valves with linear flow characteristics should be selected. When the changes are slow, valves with equal percentage flow characteristics should be selected.
③Valves with linear flow characteristics should be selected for pressure-regulating systems and liquid-level regulating systems.
1.5 Selection of flow directions
At the orifice, the flow of the medium in the opening direction of the valve core is called open flow, and the flow in the closing direction of the valve core is called closed flow. The main choice for flow direction is single-seal regulating valves, which are divided into three categories, namely single-seat valves, angle valves, and single-seal sleeve valves. Others are specified flow direction such as double-seat valves and free-flow O-ring ball valves. When DN is greater than 15mm, open flow is usually selected; when DN is less than and equal to 15mm, small-diameter valves, closed flow can be used especially for high-pressure valves to increase the service life.
1.6 Selection of packing
Commonly used in regulating valves are polytetrafluoroethylene V-shaped packing and graphite O-shaped packing. PTFE packing has a small friction coefficient, but has poor temperature resistance and short service life; graphite packing has a great friction coefficient, but has good temperature resistance and long service life; graphite packing is recommended for high temperatures and valves with positioners.
1.7 Selection of accessories
The accessories of the control valve mainly include positioners, pressure-reducing valves, filters, lubricators, travel switches, feedback devices, solenoid valves and handwheel mechanisms. Accessories have supplementary functions and ensure the operation of the valve and can be provided when necessary.
Titanium can resist hydrochloric acid corrosion of less than 10% and can resist hydrochloric acid corrosion of 3% at 50°C. Therefore, titanium can be chosen when chlorate decomposes, and the concentration of hydrochloric acid should be controlled. Since titanium reacts greatly with chlorine in a dry state, which may even cause a huge accident; titanium equipment cannot be used with chlorine in a dry state. To ensure good application effects of titanium materials, the mass fraction of water in chlorine should be controlled to be above 1.5%. Carbon steel has good stability in dry chlorine gas environments below 90°C, so it can be used as materials for chlorine compressors and some pipelines to transport cooled dry chlorine gas. Strictly control the temperature below 90°C. In some facilities where dry chlorine gas and circulating water exchange heat, to avoid the corrosion of Cl- in the circulating water, the surface of this part of the pipeline is usually coated to improve the corrosion resistance. In addition, rubber is also a commonly used anti-corrosion material and is often used as the lining of some metal equipment. This is mainly because rubber has excellent anti-penetration and anti-corrosion properties.
①Determine the corresponding nominal pressure based on the three conditions, namely temperature, pressure, and materials, which must meet the requirement for the nominal pressure value of the selected valve.
②The leakage amount of the determined valve type should meet the process requirements.
③The working pressure difference of the determined valve type should be less than the allowable pressure difference of the valve.
④The medium temperature is within the operating temperature range of the valve, and the ambient temperature should meet the requirements.
⑤Consider anti-blocking the valve according to the medium conditions.
⑥Consider the corrosion resistance of the valve according to the chemical properties of the medium.
⑦According to the pressure difference and medium containing hard objects, consider the erosion and wear resistance of the valve.
⑧When considering cost-effectiveness, several issues need to be considered such as simple structure, easy maintenance, availability of spare parts, service life, and price.
1.2 Selection of executing agency
①The simplest is the pneumatic diaphragm type, followed by the piston type, and finally the electric type.
②The main advantage of electric actuators is that the driving source (power supply) is convenient, but the price is high; the reliability, waterproofing and explosion-proof are not as good as pneumatic actuators, so pneumatic actuators should be preferred.
③There are many varieties and specifications of piston actuators, so a new lightweight structure should be selected.
1.3 Selection of materials
①The pressure rating, operating temperature and corrosion resistance of the valve body should not be lower than the requirements of the process connecting pipelines.
②Cast iron valves should not be used for water vapor or wet gases containing more water and flammable and explosive media.
③When the ambient temperature is below -20°C, it is not advisable to use cast iron valves, especially in northern China.
④For highly corrosive media, when choosing corrosion-resistant alloys, appropriate corrosion-resistant materials must be selected based on the type, concentration, temperature, and pressure of the medium.
⑤The valve body and valve core are treated separately. The throttling speed of the inner wall of the valve body is small and a certain amount of corrosion is allowed; the corrosion rate is slow; the valve core is scoured at high speed and is easily corroded, which will cause an increase in leakage, and the corrosion rate is fast.
⑥When selecting lining materials such as rubber and plastic, the temperature, pressure, and concentration of the working medium must meet the use range of the material and the physical and mechanical damage such as shear damage to the valve during operation must be considered.
⑦ Vacuum valves should not be lined with rubber or plastic.
⑧The two-position shut-off valve of the water treatment system should not be lined with rubber.
⑨The universal corrosion-resistant material is polytetrafluoroethylene. Therefore, PTFE corrosion-resistant valves should be selected first, and alloys should be selected secondly when the temperature is greater than 180°C and the engineering pressure 1.6MPa.
1.4 Selection of flow characteristics
① When working with a small opening and there is a great change in unbalanced force, valves with equal percentage flow characteristics should be selected.
②When the required adjusted parameters change rapidly, valves with linear flow characteristics should be selected. When the changes are slow, valves with equal percentage flow characteristics should be selected.
③Valves with linear flow characteristics should be selected for pressure-regulating systems and liquid-level regulating systems.
1.5 Selection of flow directions
At the orifice, the flow of the medium in the opening direction of the valve core is called open flow, and the flow in the closing direction of the valve core is called closed flow. The main choice for flow direction is single-seal regulating valves, which are divided into three categories, namely single-seat valves, angle valves, and single-seal sleeve valves. Others are specified flow direction such as double-seat valves and free-flow O-ring ball valves. When DN is greater than 15mm, open flow is usually selected; when DN is less than and equal to 15mm, small-diameter valves, closed flow can be used especially for high-pressure valves to increase the service life.
1.6 Selection of packing
Commonly used in regulating valves are polytetrafluoroethylene V-shaped packing and graphite O-shaped packing. PTFE packing has a small friction coefficient, but has poor temperature resistance and short service life; graphite packing has a great friction coefficient, but has good temperature resistance and long service life; graphite packing is recommended for high temperatures and valves with positioners.
1.7 Selection of accessories
The accessories of the control valve mainly include positioners, pressure-reducing valves, filters, lubricators, travel switches, feedback devices, solenoid valves and handwheel mechanisms. Accessories have supplementary functions and ensure the operation of the valve and can be provided when necessary.
Titanium can resist hydrochloric acid corrosion of less than 10% and can resist hydrochloric acid corrosion of 3% at 50°C. Therefore, titanium can be chosen when chlorate decomposes, and the concentration of hydrochloric acid should be controlled. Since titanium reacts greatly with chlorine in a dry state, which may even cause a huge accident; titanium equipment cannot be used with chlorine in a dry state. To ensure good application effects of titanium materials, the mass fraction of water in chlorine should be controlled to be above 1.5%. Carbon steel has good stability in dry chlorine gas environments below 90°C, so it can be used as materials for chlorine compressors and some pipelines to transport cooled dry chlorine gas. Strictly control the temperature below 90°C. In some facilities where dry chlorine gas and circulating water exchange heat, to avoid the corrosion of Cl- in the circulating water, the surface of this part of the pipeline is usually coated to improve the corrosion resistance. In addition, rubber is also a commonly used anti-corrosion material and is often used as the lining of some metal equipment. This is mainly because rubber has excellent anti-penetration and anti-corrosion properties.
