Pneumatic ball valve selection and maintenance and verification instructions

In the power plant automatic control system, the regulating valve is the most common type of actuator. The general automatic control system consists of objects, monitoring instruments, controllers, actuators and so on. Correct selection of the structural type, flow characteristics, flow capacity and actuator output torque, thrust and stroke of the regulating valve plays an important role in the safety, stability, economy and reliability of the automatic control system. If it is not properly selected, it will directly affect the performance of the control system, and even the automatic control will not be realized, which will affect the safe and economic operation of the entire unit. The survey found that about 95% of these cases were caused by improper selection, and the calculation error caused less than 5%. Practice has proved that compared with the selection, the selection is more difficult and there are more problems, so we should pay special attention to this.
1 control valve selection
1.1 Main factors that should be considered in selection
(1) To meet the temperature, pressure, liquid level and flow requirements of the production process;
(2) Valve leakage and sealing requirements;
(3) The working pressure difference of the valve < needs the pressure difference;
(4) Considerations for improving valve life and reliability;
(5) Consideration of valve operating speed and flow characteristics;
(6) consideration of the mode of action and flow direction of the valve;
(7) Consideration of actuator type, output torque, stiffness and spring range;
(8) Considerations about the material and valve economy (the improper selection price will differ by 3~4 times).
1.2 General principles of selection
Under the premise of meeting the process control requirements, the selected valve should be as simple, reliable, inexpensive, long life, convenient maintenance and timely and reliable source of spare parts. Try to avoid simply pursuing good structure, good materials, and multiple accessories, and neglecting the consideration of reliability and economy. From a reliability point of view, the simpler the structure, the higher the reliability; the material selection is too high, which will cause unnecessary price input.
1.3 Process parameters and system requirements to be provided
(1) Process parameters: temperature, pressure, pressure difference at normal flow rate and pressure difference at the time of cutting.
(2) Fluid properties: effects of corrosiveness, viscosity, and temperature changes on fluid properties.
(3) System requirements: leakage, adjustable ratio, speed and frequency of motion, linearity and noise.
1.4 Classification and selection of regulating valves
Regulating valves can be roughly divided into the following nine categories according to their structural characteristics:
(1) Straight-through single-seat control valve: This valve is the most widely used, with small leakage, small allowable pressure difference, complicated flow path and simple structure. It is suitable for clean media with strict leakage requirements and small working pressure difference. However, small size valves (DN300mm usually have butterfly valves to complete).
(8) Ball valve: When the “O”-shaped ball valve is fully open, it is an unobstructed regulating valve. The “self-cleaning” performance is the best, and it is suitable for the 2-severing occasion of particularly dirty and fibrous media. The "V" shaped ball valve has an approximately equal percentage of adjustment characteristics, and is suitable for adjustment situations where the dirtiness and fiber-containing medium are relatively large, and the ball valve is more expensive.
(9) Eccentric rotary valve: The valve is between the butterfly valve and the ball valve. It is “self-cleaning”, has good adjustment performance and can be cut off. Therefore, it is suitable for the adjustment of dirty media and small leakage requirements, but the valve is more expensive. .
The first six of these nine types are straight-stroke regulating valves, and the last three are quarter-turn regulating valves. As a user, you must understand its characteristics.
1.5 Several questions of correct choice
1.5.1 valve body material selection
(1) The pressure resistance grade, service temperature and corrosion resistance of the valve body should not be lower than the requirements of the process connection piping, and the stereotyped products are preferred.
(2) For steam and water-containing wet gas medium, when the ambient temperature is lower than -20 °C, it is not suitable to use cast iron valve.
1.5.2 Valve trim material selection
(1) Non-corrosive media generally use 1Cr18Ni9Ti or other stainless steel.
(2) In the rectangular coordinate which is more serious for cavitation and erosion, medium temperature and pressure difference, when the temperature is 300 °C and the pressure difference is 1.5 MPa, the area other than the connecting line should be made of wear-resistant materials, such as Cobalt-based alloys or surface-stacked Stellai alloys.
(3) For hard seal shut-off valves, in order to improve the reliability of the sealing surface, wear-resistant alloys should be selected. When the sealing requirements are very strict, soft seals such as PTFE and rubber should be used.
1.5.3 High and low temperature material selection
When the temperature of the medium is <-60 °C, copper or 1Cr18Ni9Ti is used; when the temperature is 450~600 °C, titanium and molybdenum stainless steel are used; when the medium temperature is >600 °C, high-temperature high-strength alloy (such as Inker) should be used.
1.6 packing and bonnet type selection
(1) Under normal circumstances, when the medium temperature is 400 °C, a heat-dissipating bonnet and graphite packing are required.
(4) In order to increase the reliability of the stem seal, a double-layer packing structure can be selected.
1.7 Locator selection
The positioner should be used in the following cases:
(1) When the electric instrument controls the pneumatic valve and is a slow response system.
(2) Where it is necessary to increase the output force of the film actuator.
(3) The slow process needs to increase the response speed of the regulating valve, such as temperature, liquid level and analysis parameters.
(4) It is necessary to overcome the friction force and reduce the excessive hysteresis to cause poor adjustment quality, such as low temperature or regulating valve with flexible graphite packing.
(5) When the regulator has a wide proportional band, but requires a small signal response from the valve, a system with no spring actuator adjustment is used.
For valves with positioners, springs of 20~100kPa are usually used, but in order to increase the output force, the air source pressure PS=250kPa can be selected. For the air opening valve, a spring of 60~180 kPa can be selected to increase the output force of the starting point actuator. A 20~100 kPa spring can be used for the gas-closed valve to increase the output force of the actuator when closed.
2 problems with the correct installation of the regulating valve
2.1 General requirements for installation
(1) The regulating valve should be installed vertically and vertically on the horizontal pipe. It is nominally passed through the regulating valve with Dg ≥ 50. It is better to have a permanent bracket on the pipe before and after the valve.
(2) The installation position of the regulating valve should be convenient for operation and maintenance, so that personnel can carry out maintenance and operation, and the platform should be set if necessary.
(3) There should be enough space in the upper and lower parts of the regulating valve to remove the actuator and trim and the lower flange and plug of the valve during maintenance.
(4) When the regulating valve is installed in a vibration occasion, anti-vibration measures should be considered.
(5) The valve of the valve positioner is not installed, and a small pressure gauge indicating the control signal is preferably installed on the film head.
(6) The regulating valve should be checked first and installed after the pipe is purged.
2.2 Consideration of security issues
(1) Valves should first pay attention to the safety of personnel and equipment in all aspects of operation (ie installation, testing, operation and maintenance).
(2) After the valve is cut off, the pressure in the valve can be maintained for a period of time, and there should be safety measures for reducing pressure, such as installing a vent valve or a discharge valve.
(3) For liquid medium, a venting valve capable of restricting flow should be installed to prevent the damage caused by water hammer when the vent valve is opened too quickly.
(4) For the steam line, the upper and lower ends of the control valve should be kept warm.
(5) Where the pressure fluctuations are severe, a line buffer should be installed.
2.3 Considerations for the performance of the regulating valve
(1) The pipe diameter should be as close as possible to the valve diameter.
(2) The length of the straight pipe section of the inlet of the regulating valve shall not be less than 10 times the passage of the pipe.
(3) The straight pipe section of the regulating valve outlet should have 3 to 5 times the pipe through.
(4) The pressure point of the inlet and outlet of the regulating valve is 2 times the pipe through the valve and 3 times the pipe diameter after the valve.
(5) The regulating valve must be installed in accordance with the flow direction arrow to avoid excessive installation stress.
2.4 Considerations for manual operation
(1) The valve installation position should be easy to operate and allow the operator to see the parameters displayed on the indicator (eg level gauge).
(2) Consideration should be given to removing the side space of the adjustment valve handwheel mechanism, positioner and other accessories.
(3) For large-diameter, high-altitude regulating valves, the working position of the operator during maintenance should be considered.
2.5 Adjustment valve signal piping and wiring
(1) The piping and wiring scheme of the regulating valve should meet the requirements of the regulating system.
(2) The D6×1mm copper tube should be used for the regulating valve piping; the D8×1mm copper tube should be used for the large diaphragm head regulating valve and pneumatic valve.
3 on-site inspection of the main performance of the regulating valve
There are many performance indicators for the regulating valve. The following items should be tested and adjusted.
(1) Basic error: smoothly increase or decrease the signal of 20~100kPa into the input air chamber (or positioner), measure the stroke value corresponding to each point, and calculate the relationship between the "signal-stroke" relationship and the theoretical value. The error of each point is the basic error. The test point shall be 5 points, 25%, 50%, 75%, 100% of the signal range. The basic error of the measuring instrument shall be limited to 1/4 of the basic error limit of the tested valve.
(2) Hysteresis: The experimental method is the same as above. The maximum difference between the forward and reverse strokes measured on the same input signal is the hysteresis.
(3) Always point deviation: The experimental method is the same as above. The basic error at the upper limit of the signal (starting point) is the starting point deviation; the basic error at the lower limit (end point) of the signal is the end point deviation.
(4) Leak test: Generally, the test medium is normal temperature water. When the pressure difference of the valve is less than 350 kPa, the experimental pressure is made at 350 kPa. When the working pressure difference of the valve is greater than 350 kPa, the allowable pressure difference is used. The experimental medium shall flow into the inlet valve according to the regulations. The valve outlet may directly connect to the atmosphere or connect the outlet to the atmospheric low-pressure head measuring device. The leakage amount can only be measured after the confirmation valve and the downstream connecting tubes are completely filled with the medium. For the main valve, a strong pressure test is also required.
(5) The valve of the matching positioner should be debugged on site before installation and commissioning.
4 on-site maintenance of the regulating valve
Since the regulating valve directly contacts the process medium, its performance directly affects the system quality and environmental pollution. Therefore, the regulating valve must be regularly maintained and regularly inspected, especially for the use of harsh conditions and important occasions. It focuses on checking the maintenance site:
(1) For the use of regulating valves in the case of high pressure difference and corrosive media, the inner wall of the valve and the diaphragm are often subjected to the impact and corrosion of the medium, and the pressure resistance and corrosion resistance should be checked.
(2) The thread for fixing the valve seat, the inner surface is susceptible to corrosion and the valve seat is loose. The part should be inspected. The valve working under the high pressure difference should also check whether the sealing surface of the valve seat is eroded or cavitation.
(3) The valve core is most seriously washed and corroded by the medium. It is necessary to carefully check whether it is corroded or worn during maintenance. Especially in the case of high pressure difference, the valve core is more worn due to cavitation.
(4) Check if the diaphragm, “0” ring and other gaskets are cracked or aged.
(5) It should be noted that the PTFE filler, sealing grease is aging, the mating surface is damaged, and should be replaced if necessary.
5 control valve common faults and on-site treatment
The common problems at the on-site of the regulating valve are that it is not dead, can not be opened, has large backlash, large leakage, vibration, oscillation, etc. The treatment methods are as follows:
(1) The valve core is not dead: the solution to the gas-closed valve is to increase the pressure of the air source or to adjust the pre-tightening force of the spring (ie, reduce the pressure at the starting point of the gas chamber). The solution to the air valve is to increase the spring preload and increase the origin pressure.
(2) The push rod is slow or does not move: check whether the diaphragm, the rolling diaphragm, the gasket are aging, and the rupture causes air leakage.
(3) Large return difference: Whether the push rod is bent and the packing gland is pressed too tightly, especially if the graphite packing and the valve core guide are damaged. The solution is to change the valve stem, change the packing, increase the guiding clearance, and change the powerful actuator.
(4) The full stroke of the valve is not enough: Loosen the stem connecting nut and rotate the stem outward or inward. Make the full stroke deviation value exceed the allowable value and tighten the nut.
(5) The valve has small stability and poor stability: the site first checks whether the flow is reversed, or the valve is too large. The solution is to open the installation and reduce the size of the valve core.
(6) The valve is unstable: the positioner is faulty, the output line is leaking, the actuator stiffness is too small, and the fluid pressure changes cause insufficient thrust. The solution is to repair the positioner and the pipeline and switch to a more rigid actuator.
(7) Large leakage: First check whether the sealing surface is injured, whether the valve seat and the stem connecting thread are loose, and whether the pressure difference is greater than the output force of the actuator when the valve is closed. The solution is to replace the sealing surface, tighten the valve seat, and replace the actuator with high output force.
(8) Oscillation phenomenon: It is caused by the valve being operated at a small opening or the flow direction being a flow-closed type. The solution is to avoid small opening work and change the open work.
6 Conclusion
As the capacity of the unit increases and the degree of automation increases, the importance of regulating valves becomes more and more important. A satisfactory regulating valve can not only stabilize the operation of the automatic control system, but also reduce the labor intensity of the operator, and can effectively reduce the deviation of the operating parameters of the unit and improve the economy and power quality of the unit. Regulator valves often have design and manufacturing quality issues in use, but more problems involve use and maintenance.