Maintenance and troubleshooting of control valves

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Maintenance and troubleshooting of control valves Classification number in the Chinese Library Classification system: TP214 ； TP207 Document Code: B Article Number: 1007-7324 (2000) 01-0072-04 There are 3 typical maintenance methods for control valves: 1) Reactive Maintenance ； 2) Preventive Maintenance ； 3) Predictive Maintenance. As is well known, the first two methods have been widely used in the field of industrial control for a long time, whereas the third method is a pattern that has only developed over the past decade or so. In particular, by leveraging computer technology, advanced expert diagnosis systems make predictive maintenance simpler and faster. Preemptive maintenance of control valves is a derived approach. It is based on the theory of Total Quality Control: investing some time in advance to address those issues that might arise, preventing them from occurring in the first place. Preventive maintenance is an important step when installing and adjusting control valves, and it is also one of the key factors in extending their service life. So what preliminary preparations should be done when installing and adjusting control valves, and what role do these preparations play? “\"Proactive maintenance\" will provide you with some information and answers, as shown in Table 1. Table 1: Pre-maintenance tasks, results, and purposes
– Protect the control valve assembly from external damage before installation.
– Remove any packaging materials from the valve, such as fixings, protective plugs, or covers.
– Flush the process piping and conduct a hydrostatic test before installing the control valve.
– Use appropriate flange gaskets.
– When installing the control valve, use fixing clamps to support it.
– Adopt standard methods for positioning and aligning the valve and its actuator.
– When installing flangeless or wafer-type valve bodies, pay attention to controlling the torque and evenness of tightening.
– Ensure that the rotary valve is in the open position, with the flow element above the axis.
– During installation and hydrostatic testing, protect the bellows from damage.
– Install insulation material where necessary.
– Purge the air lines to prevent leaks at the connection flanges, as well as leaks in the air lines; this also helps avoid reduced response speeds due to bending or crushing of the air lines.
– Prevent foreign objects from interfering with the proper operation of the control valve and slowing down its response speed.
– Avoid damage to the components inside the valve, internal leaks, and damage to the bellows.
– Prevent leaks at the flange connections.
– Prevent failures in the valve accessories due to gravitational forces, as well as air line leaks.
– Prevent premature damage to the shaft sleeves and seals ； It also makes maintenance, repair, and operation management simpler and more convenient. It prevents damage to the valve body and seals due to over-tightening or uneven tightening. It helps to cause debris to settle at the bottom of the valve, so that it does not get into the seals and bushings, thereby avoiding leaks through the bellows. It keeps the packing system cool, which helps prevent leaks through the bellows as well. It prevents debris from damaging the performance of the valve accessories and from affecting the valve’s stroke speed. It provides sufficient air supply pressure and volume. Install the electric/pneumatic converter as close as possible to the valve to avoid issues related to the switching points of the electric/pneumatic converter. Remove all packaging and protective plugs from the accessories. Ensure that there is enough air flowing through the exhaust lines. Adjust the bench settings correctly. 3) Adjust the valve’s stroke properly. Adjust the packing system correctly. Calibrate the electric/pneumatic converter and the valve positioner properly. Insufficient air supply can affect the valve’s stroke speed ； Insufficient supply pressure can result in the valve not being able to close fully, a reduced stroke distance, or a slower stroke speed. If the electro/pneumatic converter is too far away from the control valve, it will increase latency and reduce the stroke speed. 2) Multi-stage operation increases latency and reduces the stroke speed. Packaging materials and protective plugs can affect the performance of the accessories as well as the valve’s stroke speed. Insufficient air flow (poor exhaust) will also reduce the stroke speed. If it is not set correctly, this will lead to a reduced stroke distance, the valve not being able to close fully, and a slower stroke speed. For butterfly valves, overtravel can cause damage to the seals and the valve disc ； For rotary valves, an incorrect zero setting can lead to significant internal leakage ； Undertravel will affect the flow capacity of the valve ； For sliding-stem control valves, incorrect stroke adjustment procedures can lead to significant internal valve leakage. Improper packing installation can lead to reduced stroke, poor dynamic response, and excessive internal/external leakage through the packing system. Incorrect adjustment or configuration can result in a reduced stroke or significant internal valve leakage. Note: 1) Valve trim components refer to the removable internal parts that come into contact with the process medium, namely the valve disc, seat, stem, valve disc guides, bushings, and cage-type valve discs, as well as the packing box (including the packing compression ring, springs, liquid seal ring, and packing bottom ring); these do not include the packing itself, the valve cover, the bottom flange, nor the gaskets between the valve cover and bottom flange and the valve body. 2) Sometimes, for the convenience of maintenance, inspection, and calibration, several electric/pneumatic converters are installed together on a single track, and then connected to control valves located at different distances. But if the distance is too great, it will affect the quality of the regulation system. Based on experience, if a valve positioner is connected after the electric/pneumatic converter, the distance should be less than 30 ft (about 9 m) ； If the electric/gas converter is connected directly to the actuator without going through a valve positioner, the distance should be less than 10 ft (about 3 m). 3) Also known as Actuator Benchset, it is a setting related to components such as the actuator spring and diaphragm (or piston). The quality of the setting will directly affect important performance parameters of the valve, such as the seat load, the valve’s stroke range, the stroke speed, and the seat leakage rate (the tightness level when the valve is fully closed). Sometimes, adjustment can be made using the actuator’s spring adjuster (located beneath the actuator’s spring seat). It should generally be set before the control valve leaves the factory or before it is installed. If the control quality of the control valve is poor or it malfunctions, it should be inspected and repaired promptly to ensure the normal operation of the process and safeguard human life safety. But how can one quickly and accurately identify the problem and resolve it? “The “Troubleshooting Guide” may be able to provide you with some hints and assistance ； See Table 2. Table 2: Troubleshooting Guide – Symptoms and Causes, Solutions
1) Packing leakage
a) Surface finish of the valve stem: There are issues with its smoothness.
b) The valve stem is bent.
c) The packing is not packed tightly enough (or insufficiently packed).
d) The type or configuration of the packing is incorrect.
e) Excessive buildup of packing material (in the case of graphite).
f) Corrosion resulting in pits and cavities (in the case of graphite).
g) The packing compression ring (or packing gland gland) is worn or deformed.
a) Clean and polish the valve stem to achieve a surface finish of 4rms.
b) Straighten the valve stem; its linear accuracy over the full range of motion should be less than 0.02 inches (approximately 50 μm).
c) Tighten the nuts on the packing box cover again or replace the packing with new one.
d) Check the type and configuration of the packing based on actual usage; re-pack if necessary.
e) Install adjusting spacers to reduce the height of the packing, then repack it.
f) Use sacrificial washers.
4) If the valve’s operation remains sluggish after 2 to 3 weeks, then the graphite packing should be replaced. g) Inspect and replace all damaged components, such as the packing compression flange, nuts, and packing rings, to keep the entire packing system in optimal condition. 2) The valve does not respond to control signals: a) There is no supply of instrument air or the air pressure is insufficient ； or a leak in the actuator b) The solenoid valve on the air supply inlet line is closed c) No control signal is being sent in d) The air supply line is bent, curled, crushed, or damaged e) There is a leak in the fittings on the air supply line f) The flow direction is incorrect, resulting in an excessive load on the valve stem g) The air line connections are incorrect h) Components of the packing system restrict the movement of the valve stem or shaft i) The valve positioner or electro/pneumatic converter is damaged (faulty) j) The packing is too tight k) The components inside the valve are damaged ； The valve spool is stuck inside the valve seat a) According to P & IDs. Check whether all the gas supply valves are open? Measure and verify whether the supply pressure meets the requirements ； Listen for the sound of air leakage at each sealed terminal joint or diaphragm. Repair or replace the damaged components. b) Power on to energize and drive the solenoid valve. If it is found that the solenoid valve is faulty and does not function, repair it. c) The absence of a control signal input may indicate that the fuse has blown. Replace it d) Check all the air supply pipelines to verify whether they are bent (crumpled, crushed) or damaged Repair or replace damaged pipelines e) Check whether all fittings are leaking? If there is a leak, tighten it or replace it. f) If the valve is merely installed on the pipeline, check the flow direction arrow on the control valve to ensure that the process medium flows in the correct direction; if conditions permit, reverse the flow direction. g) Check the air inlet and outlet paths of the piston actuator to confirm that the air supply line is not connected incorrectly to the exhaust port (or that the exhaust port is not connected incorrectly to the air supply line). Of course, all air connection pipes should also be checked. h) Check the sealing rings of the packing system (Cland). Improper use of the sealing collar is a major factor restricting the movement of the valve stem or shaft. Replace the components of the packing system as necessary, and wipe the components inside the valve clean. i) Check the valve actuator and the electro/pneumatic converter to see if their output values can be changed manually. If not, it indicates that they are faulty; repair or replace them. j) If the guides are installed or assembled too tightly, it will cause significant frictional resistance, thereby hindering the operation of the valve. Loosen the packing system to lubricate and rotate the valve stem, then tighten it again. k) Replace or repair any damaged components inside the valve; damaged parts may be able to be ground and polished before being reused ； Pull the valve core out of the valve seat using manual or mechanical methods in order to repair or replace damaged components. 3) The valve cannot be opened to its full stroke: a) Insufficient supply air pressure; b) Leaks in the actuator or accessories; c) Incorrect calibration of the valve positioner or electro/pneumatic converter; d) Incorrect adjustment of the stroke; e) The rated spring range (also known as spring stiffness) of the actuator spring is incorrect; f) Incorrect settings for the base; g) The valve stem or shaft is bent; h) Internal components of the valve are damaged; i) There are debris inside the valve components; j) Incorrect flow direction; k) The actuator is too small (insufficient effective acting area); l) Excessive frictional resistance in the packing system; m) Incorrect positioning of the manual mechanism or stroke stopper. a) Check whether there is sufficient supply air pressure; b) Fix all leaks in the actuator, air lines, fittings, and accessories; c) Correctly calibrate the valve positioner and electro/pneumatic converter; d) Readjust the valve stroke; e) Replace the actuator spring; f) Adjust the base settings; g) Replace the bent valve stem or shaft; h) Replace the damaged internal components of the valve; i) Clean the internal components of the valve; j) Reverse the flow direction; k) Replace the actuator; l) Loosen the packing system to lubricate and rotate the valve stem, then tighten it again; m) Readjust the positioning of the manual mechanism or stroke stopper. 4) The valve’s stroke movement is slow or sluggish: a) Excessive frictional resistance in the packing system; b) The valve stem or shaft is bent; c) Insufficient supply air pressure; d) Insufficient air supply volume; e) Accessories do not meet the required specifications; f) Excessive internal frictional resistance in the piston-type actuator; g) Excessive frictional resistance in the sealing sleeve; h) The valve positioner responds slowly. a) Readjust or replace the packing; b) Replace the bent valve stem or shaft; c) Increase the supply air pressure; d) Use an air supply pipe with a larger diameter (recommended diameter: 1/4 inch to 3/8 inch; 1 inch = 25.4 mm), or increase the flow capacity of the air supply valve; e) Increase the flow capacity of the accessories; f) Clean and polish the inner walls of the piston cylinder, and remove any excess lubricating oil; g) Repair or replace the damaged sealing sleeves; h) Repair or replace the valve positioner. 5) The valve’s stroke movement is jerky: a) Packing seals or sealing sleeves restrict the stroke movement; b) The bypass of amplifier 7 may need to be adjusted; c) The valve positioner may be faulty; d) The gain of the valve positioner may be too high. a) Loosen the packing system to allow for lubrication. Replace or repair the sealing components and bushings. b) Adjust the amplifier’s bypass circuit. c) Repair or replace the valve positioner. d) Adjust the gain amplification factor or switch to a valve positioner with lower gain. 6) The rotary valve does not rotate – in addition to the problems mentioned in points 2) and 3), there are also the following specific issues: a) The stop settings of the rotary vane actuator are incorrect. b) The shaft is broken. c) Contamination or erosion of the valve seat can cause damage to the valve stem or the cam elements of the valve. d) The process operating conditions have changed. a) Readjust the actuator’s stop settings. b) Replace the shaft. c) Replace or clean the relevant components. d) Recheck the specifications of the actuator and the operational range of the valve. 7) Poor flow control (for valves with rotating or sliding stems) – see also points 4) and 5) regarding “low response sensitivity” and “jumping movements”. a) The cage element is deformed. b) The piston rings are damaged. c) Erosion, corrosion, and cavitation can damage the components inside the valve. d) The flow direction of the control valve may be incorrect. e) The flow characteristics were not selected properly. f) The performance of the control valve itself is poor. a) Replace the cage element. b) Replace the piston rings. c) Address the causes of these damages and replace the damaged components. d) Install the control valve in the correct orientation on the pipeline. e) Correct the flow characteristics. f) Select an appropriate control valve based on the control requirements