Installation and Maintenance of Pneumatic Control Valves - COVNA Valve

Overview
1.1 Main content: This maintenance guidebook specifies the safety precautions, technical requirements, and implementation steps that should be taken into account during the daily maintenance, troubleshooting, and replacement of the pneumatic control valve used online. Other types of control valves can also refer to this guidebook for use.

1.2 Basic components: The pneumatic control valves consist of two parts: the pneumatic diaphragm actuator and the valve body components. The actuator is composed of upper and lower diaphragm covers, corrugated diaphragms, trays, brackets, pushrods, springs, and regulating parts. The valve body section consists of a valve body, valve core, valve seat, valve stem, flange, etc.

1.3 Working principle of the pneumatic control valve: When the signal pressure is input into the diaphragm chamber, it generates thrust on the corrugated diaphragm, causing the pushrod to move and compress the spring until the movement of the pushrod is balanced with the reactive force of the spring, which is the travel of the pneumatic actuator. The working mode of the pneumatic diaphragm actuator can be divided into positive action and reverse action. Positive action: when the signal pressure is high, the pushrod moves downward. Reverse action: when the signal pressure is high, the pushrod moves upward.

1.4 Classification of control valves: According to the structural form, the control valve can be classified into: straight-through single-seat valve, straight-through double-seat valve, angle valve, three-way valve, diaphragm valve, butterfly valve.

1.5 Applicable scope: This maintenance guidebook is applicable to all instrument personnel in the electrical and instrument workshop.



Maintenance Objectives
The purpose of this maintenance is to ensure that the regulating valve can be used properly, with no internal leakage or jamming, flexible and smooth switching action, and no leakage at all connection points. This ensures that the regulating valve can effectively regulate and control the medium in the pipeline and equipment at various positions, playing a role in stabilizing production.

Preparatory Work before Maintenance
Personnel Allocation:
a) Maintenance Manager: Based on the fault phenomena of the instrument components, determines the maintenance projects, responsible for the quality of the maintenance work, confirms whether the quality of the spare parts to be replaced is qualified, whether the replaced valve positioner and other components are applicable to the regulating position, and ensures that the valve is in good condition after maintenance or replacement, ensuring the quality and quantity of the maintenance work.

b) Safety Officer: Responsible for safety supervision during maintenance, ensuring the completeness of safety measures, the availability of protective equipment, compliance with safety regulations, and reminding the maintenance manager of matters to be aware of during maintenance. Ensure that safety measures are in place to ensure the safe and smooth completion of maintenance work.

Working Time:
Maintenance Tools: Two 12-inch adjustable wrenches, pliers, screwdrivers, tape, fine sandpaper, signal generator.
Maintenance Spare Parts: Regulating valve, valve positioner, diaphragm, spring, and related accessories.
Documents and Certifications: A maintenance notification form, a maintenance task book, a process handover form (according to specific actual situations), and a high-altitude work permit are required when working at heights.


Requirements during maintenance
(1) Firstly, ensure the spare parts required for maintenance are available and consistent with the specifications, models, materials, nominal pressures, and operating modes of the adjusted valve and its components that need to be replaced or repaired.

(2) Prepare appropriate, intact, and complete tools for use.

(3) Ensure that all relevant certificates and permits are obtained and that construction can only begin after obtaining the approval of the dispatchers, operators, and relevant personnel.

(4) Before dismantling or repairing the valve components, confirm that the process treatment is qualified and have on-site supervision by the operator.

(5) When repairing the valve positioner and other components, ask the operator to close the valve tightly before and after the valve, and wait for the operator to stabilize the process adjustment with the bypass valve. Obtain the operator's consent, and confirm that the process treatment is qualified together with the operator. Valve switch actions do not affect production, and the valve body has no residual medium or pressure before maintenance and adjustment can be carried out.

(6) When dismantling and replacing the regulating valve, wear safety protective equipment such as gas masks and protective gloves. First, remove the diagonal connecting screws, slowly loosen the flange between the regulating valve body and the pipeline connection flange, and confirm that there is no medium pressure in the regulating valve before dismantling.

(7) When installing the regulating valve, make sure the arrow on the valve body is consistent with the flow direction of the medium. When replacing a small-diameter regulating valve with a threaded connection, use a detachable movable connector. Fix the regulating valve firmly.

(8) The calibration method of the ordinary valve positioner is as follows:
① Connect the air directly to the membrane chamber of the actuator through the air filter pressure reducing valve reserved separately, and adjust the air pressure to make the push rod of the actuator move to the middle position of the full stroke.

② When the push rod stops in the middle position of the full stroke, adjust the positioner to make the feedback lever and the positioner perpendicular.

③ Connect the gas source to the input port of the pressure reducing filter of the positioner, and connect the output port of the positioner to the membrane chamber.

④ Input 4MA signal, rotate the zero adjustment screw to make the actuator start exactly.

⑤ Adjust the output signal to 20MA, make the actuator complete the full stroke. If the stroke is insufficient, loosen the stroke adjustment locking screw. After adjustment, lock the locking screw.

⑥ Repeatedly adjust to make the starting and ending points of the actuator within the allowable error range.


(9) Calibration method for intelligent positioner of regulating valve:

① Set the input signal of the AVP (i.e. positioner) to DC18±1MA.

② Use a flat-head screwdriver to rotate the zero-full-scale adjustment screw clockwise until it can no longer turn, 90° in total.

③ Maintain this position until the valve starts to move (about 3 seconds), then the automatic setting program will be initiated. Release the screwdriver.

④ The valve will move from fully closed to fully open twice, then stop at 50% and hold for 3 minutes.

⑤ Confirm whether the automatic setting program is completed by changing the input signal. The entire automatic setting program takes about 3 minutes.

⑥ If the input signal drops below 4MA while the automatic setting program is being executed, the automatic setting will fail, and the automatic setting program needs to be restarted. After the automatic setting program is completed, maintain the signal (power supply) of at least 4MA for at least 30 seconds to save data and parameters to the AVP EEPROM.

(10) When dismantling and inspecting the regulating valve, check the corrosion and wear of the valve body, valve seat, valve core (valve stem), and check whether the diaphragm or cylinder O-ring in the actuator has aged or cracked; check the sealing of the packing and the integrity of other accessories, and replace them if necessary.

(11) After the calibration of the regulating valve is completed, the scale and valve opening indicator should be consistent at any opening of the valve.

5、Quality requirements for maintenance and repair:
The installation of the repaired or replaced regulating valve and its accessories should be standardized and firm, with no leakage at each connection, the valve opening and closing should be flexible, the action should be stable, the zero-point and range calibration should meet the requirements of use, and the indication should be accurate. The accessories should be complete and the materials should be correct. All components should be clean, and verification and repair records should be made, and waterproof measures should be taken.

6、Requirements after maintenance and repair:
(1) After the maintenance task is completed, clean up the site in time and keep the workplace clean.

(2) The operator should open the front and rear valves of the regulating valve, observe that there is no leakage at each connection, and the regulating valve is in good condition before it is put into use.

7、Daily inspection and maintenance:
(1) Check the appearance of the regulating valve, whether there is any leakage at each connection, whether each connection component is loose, and whether each accessory is complete and intact.

(2) Check whether the signal lines are loose or worn.

(3) Check whether the cylinder or diaphragm leaks, and whether there is any leakage at the threaded connection of each air source.

(4) Lubricate and anti-corrosion the moving parts and bolts regularly.

(5) Verify and inspect at any opportunity.

(6) Check whether all protective tubes are intact, and whether the waterproof measures at each inlet are intact.

(7) Check the cleanliness of the membrane of the air filter regulator for valve positioners, the ball valve of the air source, and the exhaust pipe of the air source pipeline once every six months.

(8) All air source pressure gauges should be intact and accurate in indication.