Troubleshooting and maintenance of faulty control valves

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What are the common causes of failure for pumps and control valves during the production process? How should control valves be maintained in actual production? Today, I will learn together with you the basic knowledge and common skills in this area. A control valve is the final actuator in an automatic control system; it regulates the production process by receiving control signals. Regulating valves feature high performance stability, low cost, and fire and explosion protection capabilities; they can be used in conjunction with pneumatic and electric control instruments, offering a high degree of automation. The advantages of control valves during use are: 1. They respond quickly, enabling them to carry out various adjustment commands in a timely manner ； 2. Used in combination with a large cylinder, it provides a high torque thrust force ； 3. It maintains stable performance in harsh working conditions and can operate normally ； 4. High safety performance. The proper functioning and responsiveness of control valves have a direct impact on production quality and efficiency. Therefore, it is particularly important to analyze the factors affecting failures of control valves during use and to develop corresponding solutions. Common faults and their influencing factors 1. Blockage – A frequent issue in control valves, often occurring in systems that have just been put into operation or those that are restarted after major repairs. This is caused by the accumulation of impurities such as slag and iron shavings inside the pipe at the throttle and guide areas, resulting in blockages. When clogging occurs, it can lead to poor flow of the medium, or excessive tightness of the packing during the maintenance of the control valve, which increases friction and results in the valve not responding to low signals while overreacting to high signals. 2. Leakage in control valves: Leakage in control valves generally includes internal leakage, packing leakage, and leakage caused by deformation of the valve core and valve seat. (1) Valve leakage occurs due to an inappropriate length of the valve stem; in the case of air-operated valves, the stem is too long, resulting in too short a distance for the stem to move upward or downward. This creates a gap between the valve core and the valve seat, preventing full contact and thus leading to internal leakage of the valve. (2) Packing leakage: Installing packing in the packing box exerts axial pressure on the control valve. Due to the plastic deformation of the packing, radial pressure is generated, resulting in close contact with the valve stem, though the contact is uneven. During operation, the valve stem and packing of the control valve undergo axial movement. In high-temperature and high-pressure environments, affected by highly permeable fluid media, filler leakage is likely to occur. (3) Deformation and leakage occur due to the deformation of the valve core and valve seat caused by erosion and impact; over time, a mismatch between the valve core and valve seat develops, creating a gap between them that results in poor sealing and leakage. 3. Oscillation of control valves: The causes of oscillation in control valves include insufficient spring stiffness of the valve, sharp fluctuations due to unstable output signals from the valve, a frequency of the valve that matches that of the system, and severe vibrations in the pipes and the base. There is also the issue of improper selection of control valves; when these valves operate at a low opening degree, severe changes in flow resistance, flow rate, and pressure occur. When such changes exceed the stiffness of the control valve, its stability is reduced, and in severe cases, oscillations of the valve may occur. 4. The control valve does not operate. The fault symptoms of a control valve that does not operate include: (1) No signal, no air supply. The reason for this is that the gas supply is not turned on or there is a fault with it, the air ducts are blocked, or the pressure reducing valve is malfunctioning due to blockages; additionally, there may be a fault with the compressor ； (2) There is an air supply, but no signal. The cause is that the locator malfunctions due to air leakage resulting from a ruptured bellows, or the converter malfunctions ； (3) The locator has no air supply. The reason is that the filter and pressure reducer are clogged and have become ineffective ； (4) The locator has an air supply but no output. The cause of the failure is a blockage in the throttle hole of the positioner ； (5) There is a signal, but no action. The cause is the valve core having fallen off ； The valve stem is bent or broken. 5. Unstable operation of the control valve: The reasons for unstable operation of the control valve are: (1) The compressor capacity is too low, which causes problems with the pressure reducing valve and results in unstable air supply pressure ； (2) The regulator is unstable, and the time constant of the control system is inappropriate, resulting in unstable signal pressure ； (3) The frictional resistance of the valve stem during motion is very high; stalling occurs during contact ； (4) The ball valve of the amplifier in the positioner becomes poorly sealed due to dirt and wear; this leads to increased air consumption and output oscillations. 6. The control valve operates slowly because the pneumatic diaphragm gets damaged or leaks during operation, resulting in sluggish movement of the valve stem ； The weak response performance of the locator leads to sluggish operation of the control valve ； There are obstructions such as rust inside the control valve, causing blockages that result in slow operation of the control valve during its axial movement ； Poor quality of the packing or overly tight packing increases frictional resistance, which in turn causes the control valve to respond slowly. Fault analysis of control valves 1. How should one deal with a stuck control valve? When a sticking issue occurs in the control valve, it is necessary to quickly open and close the valve, using the fluid medium to flush out the blockage ； It is also possible to use pipe wrenches to clamp the valve stem; under the action of an external signal pressure, the valve stem can be moved back and forth to push the valve core past the jammed area. If the jamming issue cannot be resolved at this point, the external pressure and driving power can be increased appropriately to carry out repeated movements; if the device fails to resume operation, a professional must disassemble the control valve. 2. How should leaks in control valves be dealt with? After internal leakage occurs in the control valve, the length of the valve stem should be adjusted appropriately so that it is suitable and no further internal leakage takes place ； After the packing leaks, in order to facilitate handling of the packing, it is necessary to chamfer the top of the packing box, and to install a corrosion-resistant metal protective ring with a small gap at the bottom of the packing box. The contact surface between the metal protective ring and the packing should not be inclined, so as to prevent the packing from being pushed out by the pressure of the medium. The surface of the part of the stuffing box that comes into contact with the packing should be finely processed, which improves surface smoothness and reduces packing wear. Flexible graphite is the best choice for fillers, as it offers high airtightness, low friction, minimal wear over time, and is easy to maintain. It also possesses high pressure and heat resistance. Leakage can occur due to deformation of the valve core and seat; therefore, it is essential to carefully select the materials for these components, choosing those with high corrosion resistance. Products with defects such as pitting or sand holes must be excluded, to ensure that the valve core and seat have a high level of sealing smoothness. When the valve core and seat are severely worn or damaged, it is necessary to replace them with new ones. 3. How to deal with oscillation in control valves? The control valve oscillates; when the oscillation is mild, the stiffness of the spring can be increased, for example by selecting a spring with greater stiffness for the control valve ； When the pipes and bases vibrate violently, additional supports should be added to eliminate the vibration interference ； If the frequency of the valve is the same as that of the system, the control valve needs to be replaced. 4. What to do if the control valve does not operate? When the control valve does not function, the pressure regulator must be adjusted to bring the pressure to the specified supply pressure ； Inspect the leakage points in the pipelines of the locator junction box, and tighten or replace the joints and pipelines ； Inspect the magnifying hole and throttle hole of the locator and carry out repairs. 5. How to deal with unstable operation of control valves? If the control valve does not operate stably, it is necessary to inspect the signal pipelines, connectors, sealing rings, etc., and eliminate any leaks ； Find a loose component and tighten it ； Apply lubricant to components with high friction. 6. How to deal with slow operation of control valves? The control valve responds slowly; inspect the components with high friction and reassemble them ； Adjust the packing gland and replace the packing again. How should control valves be maintained? If a control valve fails during operation, it can cause significant harm to the normal and stable functioning of the system; therefore, it is necessary to maintain these control valves in order to reduce the incidence of failures. 1. Passive maintenance: When a control valve fails, it is necessary to inspect the valve, determine the cause of the failure, and carry out timely repairs. 2. For preventive maintenance, the control valve should be cleaned regularly to maintain its hygiene and ensure the integrity and durability of all its components. Pipelines should also be flushed periodically; care must be taken to remove any blockages or debris within them. Additionally, the fasteners securing the control valve should be inspected regularly to ensure good anti-corrosion properties and lubricity of the components. When installing control valves, minimize forced pulling and bending during installation to reduce pressure and deformation. Maintain a distance between the control valve and the compressor to reduce vibrations from the source. Regulating valves should be inspected regularly; close attention should be paid to their operation, and it is necessary to check for leaks in the packing as well as to apply lubricant to it on a regular basis. 3. Predictive maintenance utilizes smart meters or other diagnostic devices to obtain operational data on control valves, thereby improving their control quality and diagnostic capabilities. Through diagnosis using smart meters or other diagnostic devices, issues with control valves can be identified in a timely manner, allowing for prompt implementation of corrective measures for maintenance. Increase the development and use of intelligent valve positioners. Related article: Official website of Shanghai Jisheng Valve Co., Ltd