Selection and installation of control valve
In the automatic control of modern chemical plant, regulating valve is very important. By receiving the control signal from the control unit, the process parameters such as medium flow, pressure, temperature and liquid level can be changed. It can be said that the regulating valve is the last gate in the process loop. Therefore, how to correctly select the control valve to achieve the best control effect has become a key problem.
The selection of control valve should be based on the characteristics of control system, interference source and S (valve resistance ratio) value.
General selection principle
- 1. The linear flow characteristic should be selected for the small change of pressure difference on the valve, the small change of given value, the small change of main variables in the process, and the control object with s > 0.75.
- 2. For slow production process, when s ＞ 0.4, the linear flow characteristic should be selected.
- 3. When large adjustable range is required, the pressure loss of piping system is large, the opening change and the pressure difference on the valve are relatively large, the equal percentage flow characteristic should be selected.
- 4. For fast production process, when the dynamic process of the system is not well understood, the equal percentage flow characteristic should be selected.
- 5. Quick opening feature: it is suitable for two position action or when the maximum flow capacity of the control valve needs to be obtained quickly. When the regulator must be set in the wide proportional band, its control valve can also choose the quick opening feature.
1. According to the process variables (temperature, pressure, pressure drop, flow rate, etc.), fluid characteristics (viscosity, corrosiveness, toxicity, suspended solids or fiber, etc.), the requirements of regulating system (adjustable ratio, leakage and noise, etc.) and the connection form of regulating valve pipeline, the type of regulating valve shall be comprehensively selected.
2. In general, priority should be given to the straight through single seat, double seat control valve and common sleeve valve with small volume, large carrying capacity and advanced technology. Low S value energy-saving valve and fine small control valve can also be selected.
3. According to different occasions, the following types of control valves can be selected.
1) Straight through single seat valve
- It is generally applicable to the occasions with small leakage, small flow and small pressure difference before and after the valve. But the valve with diameter less than 20 mm is also widely used in the occasion of large differential pressure;
- It is not suitable for high viscosity or fluid containing suspended particles.
2) Straight through double seat valve
Generally, it is suitable for the occasions where the leakage is not strict, the flow is large and the pressure difference between the front and back of the valve is large;
But it is not suitable for high viscosity or fluid containing suspended particles.
3) Sleeve valve
It is generally applicable to the occasions where the fluid is clean and does not contain solid particles.
There may be large pressure difference between the front and back flash valve and liquid.
4) Ball valve
Suitable for high viscosity, containing fiber, granular and dirty fluid.
- The adjustable range of the regulating system is very wide (R can reach 200 ∶ 1; 300 ∶ 1).
- When the valve seat gasket is made of soft material, it is suitable for tight sealing.
- “O” type ball valve is generally applicable to two position cut-off occasions.
- “V” type ball valve is generally suitable for continuous regulation system, and its flow characteristics are similar to equal percentage.
5) Angle valve
It is generally applicable to the following situations:
- Fluid with high viscosity or suspended solids (if necessary, it can be connected to flushing liquid pipe);
- Gas liquid miscible or easily flashing fluid;
- Where the pipeline requires right angle piping.
6) High pressure angle valve
It is not only suitable for various occasions in 5), but also suitable for occasions with high static pressure and large pressure difference. But we must choose the material and structure of the valve internals reasonably to prolong the service life.
7) Valve body separate control valve
- It is generally suitable for high viscosity, granular, crystalline and fibrous fluids;
- When used for strong acid, strong alkali or strong corrosive fluid, the valve body shall be provided with corrosion-resistant lining, and the valve cover, valve core and valve seat shall be provided with corrosion-resistant pressure pad or corresponding corrosion-resistant material. Its flow characteristic is better than diaphragm valve.
8) Eccentric rotary valve
It is suitable for the occasions with large flow capacity, adjustable ratio (r up to 50 ∶ 1 or 100 ∶ 1), large pressure difference and tight sealing.
9) Butterfly valve
- Suitable for large diameter, large flow and low pressure difference;
- It is generally applicable to the occasions of dense and turbid liquid and fluid containing suspended particles;
- The rubber or polytetrafluoroethylene soft sealing structure should be used when tight sealing is required, and the corresponding corrosion-resistant lining should be used for corrosive fluid.
10) Three way valve
It is suitable for the split flow and confluence of the fluid temperature below 300 ℃, and for the simple ratio adjustment. The temperature difference between the two fluids should not be greater than 150 ℃.
11) Diaphragm valve
It is suitable for the fluid with strong corrosion, high viscosity or containing suspended particles and fibers, and the flow characteristics are not strictly required. Due to the limitation of diaphragm lining, it can only be used when the pressure is less than or equal to 1MPa and the working temperature is less than 150 ℃.
12) Bellows seal valve
It is suitable for vacuum system and occasions where the fluid is highly toxic, volatile, rare and precious.
13) Low temperature control valve
It is suitable for low temperature and deep freezing.
When the medium temperature is – 100 ~ 40 ℃, the valve with heat sink (heat absorption here) and flexible graphite packing can be selected.
When the medium temperature is between – 200 ℃ and – 100 ℃, long neck low temperature valve should be selected.
14) Low S value energy saving regulating valve
It is suitable for the situation that the process load changes greatly or when s value is less than 0.3.
15) Low noise valve
It is suitable for liquid flash, cavitation and gas flow velocity at the valve contraction surface is greater than the speed of sound, and the estimated noise is more than 95dB (a).
16) Quick cut off valve
It is suitable for emergency opening or closing of valve in case of failure of two position regulating system and process.
17) Self operated control valve
It is suitable for the occasions with small flow change, low regulation accuracy requirement or difficult instrument air supply.
The special control valve should be selected according to the use experience in the special process.
Material selection of control valve
General selection principle
- 1. The pressure resistance grade, temperature range, corrosion resistance and material of valve body shall not be lower than the requirements of process connecting pipe material. Priority should be given to the products approved by the manufacturer. Generally, cast steel or forged steel valve body is selected.
- 2. The cast iron valve body is not suitable for steam or wet gas with more water and flammable fluid.
- 3. Cast iron valve body should not be used when the ambient temperature is lower than – 20 ℃.
- 4. The valve internals shall be resistant to corrosion, fluid erosion and cavitation damage caused by throttling and flashing.
Material selection of valve trim
1. 1Crl8Ni9, 1Crl8Ni9Ti or other stainless steels are generally used as non corrosive fluids.
2. For corrosive fluid, the appropriate corrosion-resistant material should be selected according to the type, concentration, temperature and pressure of the fluid, as well as the oxidant content and flow rate of the fluid. Common corrosion resistant materials are 1Crl8Ni9Ti, 0crl8nil2m02ti, 20 alloy, Hastelloy and titanium steel.
3. Wear resistant materials should be selected for the condition of high velocity and serious erosion. Such as 9crl8 and 17-7ph after superheating treatment, Cr Mo steel and g6x with firm oxide layer, high toughness and fatigue strength.
4. Material selection for severe wear cases:
- In case of flash, cavitation and fluid containing particles, the surface of valve core and valve seat shall be hardened.
- When the fluid temperature is too high and the pressure difference is too large, the valve core and valve seat should be surface hardened. Such as surfacing Steller alloy.
Selection of leakage and flow rate of control valve
According to the requirements of the process for leakage, the valve types with different leakage levels are selected.
Generally, the leakage of straight through single seat valve shall be less than or equal to 0.01% of rated C value, and that of double seat valve shall be less than or equal to 0.1% of rated C value.
Choice of flow direction:
1. The flow direction of ball valve and ordinary butterfly valve is not required, and any flow direction can be selected.
2. Three way valve, venturi angle valve and double seal sleeve valve with balance hole have specified a flow direction, which can not be changed generally.
3. For single seat valve, angle valve, high pressure valve, single seal sleeve valve without balance hole and small flow control valve, the flow direction of control valve should be selected according to different working conditions.
1) For DN ≤ 20 high pressure valve, due to high static pressure, large pressure difference and serious cavitation erosion, flow closed type should be selected; when DN > 20, good stability should be selected as the condition to determine the flow direction.
2) Angle valve for high viscosity, containing solid particles medium requirements “self-cleaning” performance is good, should choose flow closed type.
3) Single seat valve, small flow control valve generally choose flow open type, when the erosion is serious, can choose flow closed type.
4) The single seal sleeve valve generally selects the flow open type; when there is a “self-cleaning” requirement, the flow closed type can be selected.
5) Two position control valves (single seat valve, angle valve, sleeve valve, quick opening flow characteristics) shall be of flow closed type; in case of water hammer and surge, the flow open type shall be selected.
6) When the flow closed type is selected and DS < D (DS – stem diameter; D – seat diameter), the stability of the valve is poor, the following points should be noted:
- The minimum working opening is more than 20% ～ 30%;
- Choose the spring with high rigidity;
- The equal percentage flow characteristics are selected.
Selection of accessories for regulating valve
Valve positioner application
- 1. it is used to overcome friction or need to improve the speed of regulating valve.
- 2. the situation where the open and closed modes of the air are required to be changed for the split control and regulating valve.
- 3. the situation where the flow characteristics of the regulation need to be changed.
- 4. the regulator has a wide proportional band, but also requires the valve to respond to small signals.
- 5. the situation where the non spring actuator or piston actuator is required to realize proportional action.
- 6. use standard signal to operate the actuator of non-standard spring (spring range outside 20-100kpa).
Application of pneumatic relay
- 1. in case of rapid process, the response speed of regulating valve needs to be improved, and the distance between the regulating valve and regulator is more than 100m.
- 2. the situation where the output signal of pneumatic regulator needs to be improved.
Three solenoid valve
1. it is applicable to remote control, program control, interlocking system, automatic closing of air circuit and opening or closing of regulating valve.
- The direct solenoid valve is used for two position regulation and remote control. According to the logic relation of program control, the “normally closed” or “normally open” solenoid valve can be selected.
- Two position three-way solenoid valve: it is generally used to control the single acting cylinder actuator, pneumatic film actuator, pneumatic regulating valve and other control systems for automatic switching control or interlocking program control of air circuit.
- Two position four (five) way solenoid valve: it is generally applicable to control the double acting cylinder and the regulating valve with piston actuator, and realize automatic switching and program control in the automatic control system using the cut-off ball valve.
2. when large capacity is required to shorten the operation time, the solenoid valve is used as the pilot valve and the large capacity pneumatic relay.
3. in the explosion hazard place, explosion-proof solenoid valve, intrinsically safe type solenoid valve or switch type electrical converter shall be selected.
Four position valve
It is suitable for the situation where the pressure of the air source is lower than the given value, and the regulating valve is required to be kept in a certain position.
Five electrical converters
- 1. the control system is composed of electric instrument and pneumatic regulating valve.
- 2. change the electrical signal into gas signal.
- 3. for the quick adjustment system, the electric converter should be selected.
Six valve position transmitter
- 1. in important occasions, valve position transmitter should be selected.
- 2. the electric actuator shall be equipped with valve position transmitter.
Seven handwheel mechanism
- 1. if no bypass regulating valve is set, handwheel mechanism shall be set in the following situations; however, for emergency vent valve for process safety production interlock and regulating valve installed in dangerous area prohibited from entering, handwheel mechanism shall not be set.
- 2. occasions where valve opening is required to be limited.
- 3. for large diameter and the occasions where precious metal pipes are selected.
Eight pneumatic three way control valve
It is suitable for remote control or program control system, which makes its regulating valve or pneumatic ram valve open or closed.
When the electrical components attached to the regulating valve, such as the positioner, solenoid valve and electrical converter, are used for explosion-proof occasions, the explosion-proof grade shall conform to the relevant explosion-proof design regulations.
Selection principle of air opening and closing of regulating valve: when the instrument air supply system fails or the control signal is interrupted suddenly, the opening of the regulating valve shall be in the position to make the production device safe.
Installation of regulating valve
- 1. the regulating valve should be installed vertically and vertically on the horizontal pipe. Permanent supports shall be set on the front and rear pipes of regulating valves with nominal diameter DN ≥ 80mm.
- 2. the installation position of regulating valve shall be convenient for operation and maintenance. Platforms shall be set up if necessary.
- 3. the regulating valve group piping shall be compact in combination, which is convenient for operation, maintenance and liquid discharge.
- 4. there should be enough space for the upper and lower parts of the regulating valve to remove the actuator and valve internals, and the lower flange and plug of the valve during maintenance.
- 5. when the regulating valve is used for high viscosity, easy crystallization, easy vaporization and low temperature fluid, measures of heat preservation and antifreeze shall be taken.
- 6. the ambient temperature of regulating valve is not higher than 60 ℃ and not lower than – 40 ℃.
- 7. when the valve is installed in the case of vibration, vibration prevention measures shall be taken into consideration.
- 8. for regulating valve without valve positioner, small pressure gauge indicating control signal shall be installed on membrane head.
- 9. when the regulating valve is used for containing suspended solids and high viscosity fluid, flushing pipeline shall be provided.
- 10. during the installation of regulating valve, it should be noted that the medium flows through the direction of valve body calibration arrow.
- 11. the regulating valve shall be checked and calibrated first and installed after the pipeline is purged.
Bypass of regulating valve
1. bypass shall be set in the following cases:
- Corrosive fluid;
- Severe wear of valve internals;
- Other important occasions, such as boiler feed water regulating valve.
2. bypass can not be set in the following situations:
- Clean the fluid;
- The nominal diameter DN > 80mm;
- The situation where the regulating valve fails or repairs, which will not cause process accidents;
- The process is not allowed or can not use bypass valve operation. For example: emergency interlock vent valve and slurry and easy to crystallize fluid, etc.
3. the connection form of regulating valve shall conform to the provisions of the product description book of the manufacturer.
4. the eccentric expanding (shrinking) pipe is preferred for the regulating valve, concentric and eccentric expansion (contraction) pipes.
5. regulating valve piping and wiring
- The piping and wiring scheme of regulating valve shall meet the requirements of the regulating system.
- The regulating valve should be equipped with φ 6 × 1 copper pipe or PVC sheathed copper pipe, and the large film head regulating valve and pneumatic gate valve should adopt φ 8 × 1 copper pipe or PVC sheathed copper pipe.
- The wiring of electrical components used for regulating valve in explosion-proof area shall conform to the relevant provisions of code for design of electric equipment in explosion and fire hazard places (GBJ 58-83).
6. the pressure level of compressed air used for regulating valve shall meet the requirements of product specification. The quality of compressed air shall meet the requirements of national standard “pressure range and quality of air source for industrial automation instruments” (GB 4830-84)).
7. precautions for installation of self-propelled regulating valve
Pressure type pressure regulating valve with command valve, filter shall be installed in front of valve. The distance between pressure taking point and regulating valve shall not be less than 10 times of pipe diameter.
Temperature regulating valve, when the detector is bimetal, the detector shall be installed vertically on horizontal pipe.
When the detector is a temperature pack, install it vertically as far as possible. If conditions do not allow, they can also be installed in an inclined manner, but the angle to the horizontal pipe shall be greater than 45 °
In principle, the self-propelled regulating valve is not equipped with bypass valve, if it is necessary to set the bypass valve.
Source: China Control Valves Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)