Control valve

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1 Common faults of malfunctioning control valves or inadequate process performance and their solutions The control valve is installed on-site and comes into direct contact with the medium; it operates under conditions of high temperature and pressure, extreme cold and corrosivity, as well as susceptibility to crystallization, which result in harsh operating environments. Its quality and reliability not only affect the accuracy of regulation but also impact the safety of the production system and prevent environmental pollution. In some situations, automatic control is urgently needed, but it often cannot be implemented because control valves fail to meet the requirements on site. Common problems with control valves include sticking, membrane rupture, slow response, high leakage, insufficient thrust, poor valve sealing, limited adjustment range, vibration, humming, and external leakage. 　　Common control valves include single-seat valves, double-seat valves, sleeve valves, angle valves, three-way valves, butterfly valves, ball valves, and all-function ultra-lightweight valves. Special valves include high-pressure valves, high-temperature valves, corrosion-resistant valves, and quick-acting shut-off valves. 　　Often, the failure rate of control valves is higher than that of other instruments, and in order to analyze and diagnose faults, instrument technicians must be well aware of the common failures of control valves. Valve positioners and handwheels are part of control valves. 　　1. Problems with control valves (actuators V): 　　①. The valve core falls off, the valve stem is broken or bent out of alignment, or the pins of the butterfly valve are broken; 　　②. The valve core gets stuck to the valve seat or bushing, or there are foreign objects (or crystals) inside the valve body that cause obstruction; 　　③. The diaphragm is damaged, or air leaks from the cylinder, the exhaust hole on the diaphragm head is blocked, or the “O” rings are worn out and leak air; 　　④. The valve springs are not properly adjusted, are damaged, or the valve frame is broken; 　　⑤. The valve is already in its extreme position, leaving no room for adjustment; the wrong model was selected, or there is an opening in the process line valve; 　　⑥. High friction on the valve stem (due to tight packing or high temperatures) results in slow operation, as well as insufficient output force from the actuator; 　　⑦. The valve stem is either too long or too short, resulting in insufficient valve travel and excessive noise; 　　⑧. The valve has excessive leakage, and the guide sleeves are severely worn; 　　⑨. The valve core is severely eroded or corroded, and the connections to the valve stem are not secure or proper; 　　⑩. The valve is restricted in its movement, either because the handwheel isn’t released properly or because the solenoid valve hasn’t been reset correctly. 　　2. Valve positioner (VP) issues: 　　①. Problems with the air supply or input signal pressure – it may be too low or too high, and there may be severe leaks in the pressure reducing valve, connectors, pipes, and positioner itself; 　　②. Dirt, blockages, or malfunctions in the positioner’s nozzle dampers, throttle holes, amplifier, etc.; 　　③. Improper installation or connection of the feedback elements (pins), resulting in poor fixation, jamming, or detachment due to vibration; 　　④. Looseness, fatigue, aging, or oscillation of the internal mechanical components of the positioner; 　　⑤. The positioner has not been properly calibrated, leading to incorrect settings during operation. 　　5. Process-related issues: ①. Foreign objects (or medium crystals) inside the pipes can jam or lift the valve core, which is also caused by inadequate purging of the equipment’s pipelines during the process; ②. The process deviates from the designed operating conditions, preventing the control valve from functioning properly; ③. Abnormalities in the process system; ④. Improper process operations, such as incorrect settings for the bypass valves or improper use of the handwheel. 　　 　　2 Experience in the calibration and maintenance of various control valves (including dampers) and positioners Control valves are an essential part of a control system; once a control valve fails, the entire control system stops functioning. Therefore, it is necessary to carry out calibration and maintenance of control valves with care and attention. The following discusses several types of such valves. 　　1. Pneumatic diaphragm control valve 　　 a. Pneumatic diaphragm control valve with positioner 　　 When calibrating such valves, the air supply pressure is determined first, and then calibration is carried out in accordance with the relevant procedures to ensure that their linearity and sensitivity meet the specified requirements. It is particularly important to note that when installing the valve positioner, the feedback arm of the positioner must be in a horizontal position when the valve is at 50% open. 　　 b. Pneumatic diaphragm control valve without a positioner When calibrating a pneumatic diaphragm control valve without a positioner, special attention must be paid to the signal range of the control valve; the spring range, which is also the signal range, is indicated on the nameplate, and calibration is carried out according to this signal range. 　　2. Cylinder-type control valve The cylinder-type control valve has advantages such as high output power and rapid response. During calibration, the air supply pressure is determined first, and then calibration is carried out in accordance with the relevant procedures. If poor linearity or jumping is observed, perform a gas-tightness test on the cylinder to check for leaks in the cylinder, piston, connectors, etc. If the problem persists, it may be due to the packing of the valve being too tight; adjusting the packing will resolve the issue. 　　3. Damper actuator 　　 A damper actuator is a special type of valve. When testing such valves, the air supply pressure is determined first, and then testing is carried out in accordance with relevant procedures. These valves are equipped with positioners, but their linear performance is not very good; it is sufficient as long as the requirements of the process are met. During inspections, check whether there is any air leakage at the joints, ensure that the air supply pressure is normal, and apply lubricant to the mechanical transmission parts on a regular basis. 　　4. Governor The governor of a power unit is a special control device that consists of an actuator and a hydraulic motor; the instrument is responsible for the actuator part. During verification, the pressure of the gas supply and the pressure of the air cushion are first determined; thereafter, verification is carried out in accordance with the relevant procedures to ensure that the process requirements are met. During inspections, special attention should be paid to whether there are any leaks in the gas pipelines, and any such issues must be addressed promptly once detected. 　　5. Diaphragm valve 　　 The diaphragm valve is a two-position valve; during testing, the air supply pressure is determined first, and then it is checked whether the valve operates smoothly and reaches its intended position. If there are any abnormalities, please check whether the diaphragm is damaged and whether the packing is in good condition. 　　6. Electromagnetic shut-off valves The electromagnetic shut-off valves include the valve for shutting down the unit, as well as the natural gas shut-off valve in the fuel system. These valves are two-position types; during testing, special attention should be paid to whether the power supply voltage is normal, whether the valves operate smoothly and reach their intended positions, and whether there is any leakage. 　　7. Changes in air-open valves and air-close valves 　　 Air-open valves and air-close valves are inherent properties of valves, determined during their manufacture; only by changing the actuator can it be altered whether a valve operates in air-open or air-close mode. In the case of cylinder-type control valves, this can be achieved by changing the positions of the signals for the upper and lower cylinders. Changing the forward and reverse actions of the locator or regulator will not alter between air-open and air-close operation. 　　8. Bleed valves and anti-surge valves 　　 Both bleed valves and anti-surge valves are in a closed state when the process is operating normally. When testing such valves, special attention must be paid to the force exerted on the valve element by the pressure of the process medium. The operating value of the valve is determined through calculations; for air-actuated shut-off valves, this value is not necessarily 100 KPa – it could be 95 KPa or 90 KPa. For air-actuated open valves, the operating pressure is not necessarily 20 KPa – it might be 40 KPa or 50 KPa. The purpose of this is to ensure that the valves remain closed reliably and function properly during normal operation. The method for determining the starting value should be based on scientific calculations, but this is somewhat difficult in practice; usually, a conservative and reasonable starting value is determined based on experience, with a starting value of 25–35 KPa often being set for air-operated valves ； An empirical starting value of 85–95 KPa is set for the air shut-off valve. 　　Example: A pneumatic on-off high-pressure control valve equipped with a positioner. The input pressure ranges from 20 to 100 KPa, while the output pressure ranges from 40 to 200 KPa. The pressure difference across the valve is 30,000 KPa. The diameter of the valve stem is 8 mm, the valve stroke is 20 mm, the effective area of the diaphragm head is 280 cm2, and the frictional force on the valve is 30 Kgf. This valve is to be tested at normal pressure; to what extent must it be tested before it can function properly once in operation? 　　Solution: (1) Check the route. That is, by inputting 20–100 KPa, the output is set to 40–200 KPa, with the valve moving through a full stroke of 20 mm. 　　 (2) Starting pressure for verification (flow-opening type):
Po = (△P × πr² + F摩 + Fe) / Ae
= (300 × π(0.4 × 0.4) + 30 + 280 × 0.05) / 280
= 70 KPa

In the formula, Fe represents the clamping force on the valve seat; generally, Fe is taken as 0.05Ae. Once the starting value is determined, it can be adjusted as follows:
(1) Input a signal of 20–100 KPa, and adjust the zero point of the positioner so that the output is zero (40 KPa).
(2) Compress the spring, gradually increasing the input pressure from 20 KPa until the output reaches 70 KPa, at which point the valve starts to operate. 　　 　　 　　Precautions for the calibration and maintenance of various instruments in the 3-interlock alarm system 　　 1. Transmitters, sensors, thermocouples (resistive) instruments 　　 In the interlock alarm system, field transmitters, sensors, and thermocouples (resistive) convert process parameters into analog electrical signals that are sent to the DCS system. Interlock protection and alarms are implemented in the DCS system using sequential control functions; therefore, when maintaining such instruments, the following tasks must be carried out: 　　 a. Obtain an interlock deactivation work order to keep dispatch and process personnel informed of the situation. 　　 b. Remove the existing corresponding interlock on the CRT. 　　 c. Be familiar with the relationship between the instruments being maintained and other control systems to prevent malfunction. 　　 d. Set the instruments under maintenance to MAN and CAL mode on the CRT. 　　 e. Repair and calibrate the on-site instruments. 　　 f. After the verification and maintenance are complete, and once it is confirmed that the K5 value is within the normal range, remove the CAL mode on the CRT and set the system to AUT mode. 　　 g. After confirming that the parameters are normal, issue the interlock activation form to activate the interlock. 　　 2. Pressure switches and float-type instruments When calibrating and maintaining pressure switches and float switches on-site, follow these steps: a. Obtain a work order for disabling interlocks. 　　 b. Remove the corresponding interlock on the CRT. 　　 c. Perform calibration and maintenance on the on-site switches. 　　 d. After verification confirms everything is correct, issue the interlock commissioning ticket to activate the interlock on the CRT. 　　 3. Inspection and maintenance of interlock solenoid valves 　　 Interlock solenoid valves are divided into those that remain energized at all times and those that do not remain energized at all times. 　　 a. Continuously energized solenoid valves: When inspecting and maintaining such solenoid valves, since they are continuously energized, a temperature rise occurs in them, which can be felt by touch. If the temperature rise is severe, it indicates a fault in the solenoid coil, which needs to be replaced promptly. When replacing the solenoid, make sure the valve is in a safe position. 　　 b. Electromagnetic valves that are usually not charged: When maintaining such electromagnetic valves, it is difficult to detect problems with the wiring and coil sections; actual measurements must be carried out. Testing includes checking for continuity in the wiring and conducting insulation tests, and it is essential to ensure that the valve is in a safe position during these tests. 　　 4. Maintenance of interlock systems without interlocked switching switches Whether \"hard measures\" are taken for the input signals or for the side that carries out the actions, it is first necessary to be familiar with the principles of interlocking; one must be extremely careful during operation and cannot afford any carelessness! 　　 In interlock systems that lack interlock switching switches, to prevent accidental operation, \"physical measures\" must be taken on site, such as using solenoid valve pins, setting limits on control valve handwheels, or securing the valve stems ； Set the motor to the MAN position, or take \"hard measures\" on the input signal side – for example, using a short-circuit then open-circuit method based on normally open or normally closed contacts – before proceeding with maintenance.