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Tips to extend the service life of control valves – Tips for extending lifespan: 1. Operate at full opening: Have the control valve operate at its maximum opening degree from the start, such as 90%. In this way, damages such as turbidity and erosion occur on the head of the valve core. As the valve core is damaged, the flow rate increases, causing the valve to close a little more; this process of continuous damage and gradual closure ensures that the entire valve core is utilized to the fullest extent, until the root of the valve core and the sealing surface are damaged and the valve can no longer be used. At the same time, operating at a large opening results in a larger throttle gap, which reduces erosion; this increases the valve’s lifespan by more than 1–5 times compared to operating it at intermediate or small openings from the start. If a chemical plant adopts this method, the service life of the valves is doubled. 2. Reducing the valve resistance ratio (or pressure drop ratio). Decreasing S means increasing the losses in the system other than those occurring across the control valve; this, in turn, reduces the pressure drop allocated to the valve. To ensure that the desired flow rate passes through the control valve, its opening must be increased. Meanwhile, the reduced pressure drop across the valve also diminishes cavitation and erosion. Specific methods include: installing a orifice plate behind the valve to create pressure drop through throttling ； Close the manual valve connected in series on the pipeline until the control valve reaches an optimal operating position. For valves that initially operate at a small opening degree, this method is very simple, convenient, and effective. 3. Reduce the diameter and increase the operating opening: The operating opening can be increased by reducing the diameter of the valve. The specific methods include: ① Replacing the valve with one of a smaller diameter, such as changing from DN32 to DN25 ； ②The valve body remains unchanged; only the valve spool and seat with a smaller diameter are replaced. During a major overhaul of a chemical plant, replacing the throttle element dg10 with dg8 doubled its service life. 4. Shift the location of damage: Move the areas that are most damaged from critical positions to less important ones, in order to protect the sealing surfaces and throttling surfaces of the valve core and valve seat. It can increase the service life of the valve. 5. Change in flow direction: In the flow-out type, the fluid flows in the direction of the valve core’s opening; cavitation and erosion mainly affect the sealing surfaces, causing rapid damage to the root of the valve core as well as the sealing surfaces of the valve seat ； In the flow-blocking type, flow moves in the direction of closure; cavitation and erosion occur after throttling, below the valve seat sealing surface, which protects the sealing surface and the root of the valve element, thereby extending its service life. Note: Even when changing from flow open to flow closed, a jumping phenomenon occurs (when the control valve is opened). The effect of vortices during the adjustment process prevents the control system from making smooth adjustments. This method must be adopted with caution and after comprehensive consideration. 6. Use of special materials: To resist cavitation (damage in the form of honeycomb-like spots) and erosion (streamlined grooves), special materials resistant to cavitation and erosion can be used to manufacture the throttle components. Such special materials include 6YC—1, A4 steel, stainless steel, and cemented carbide. To resist corrosion, materials that are more resistant to corrosion and possess certain mechanical and physical properties can be used instead. This material is divided into two categories: non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metallic materials (such as Monel, Hastelloy, etc.). 7. Proper selection of valve structure: The lifespan of valves can be increased by altering their structure or by choosing valves with a longer service life, such as labyrinth valves, multi-stage valves, anti-cavitation valves, and corrosion-resistant valves. The advantages of control valves during use are: (1) they respond quickly and can execute various control commands in a timely manner ； (2) Used in combination with a large cylinder, it provides a high torque thrust ； (3) It maintains stable performance in harsh working conditions and can operate normally ； (4) High safety performance. The proper operation 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 in control valves during use and to develop corresponding solutions. Common faults and their influencing factors https://mmbiz.qpic.cn/mmbiz_jpg/vSABwyURAVhJaHLHS1ToapmrI2utclw5dcOU1ibe5u9AaMicW187ibdfKs8K1ibECCeGtCKFWIA8WiaQf7c9FvA1Fzw/640?wx_fmt=jpeg 1. Clogging – This is a common problem in control valves, often occurring in systems that have just been put into operation or after being overhauled and restarted. This is caused by the accumulation of impurities such as slag and iron filings inside the pipeline at the throttle and guide areas, resulting in blockages. After a blockage occurs, it can impede the flow of the medium; or if the packing of the control valve is tightened excessively during maintenance, friction increases, resulting in the phenomenon where the valve fails to respond to small signals while overreacting to large signals. 2. Control valve leakage: Control valve leakage generally includes internal leakage, packing leakage, and leakage caused by deformation of the valve core and 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, which results 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 into the stuffing box applies axial pressure to the control valve. Due to the plastic deformation of the packing, radial pressure is generated, resulting in close contact with the valve stem, although the contact is uneven. During operation, the valve stem and packing of a 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 seat caused by erosion and impact; over time, this leads to a mismatch between the valve core and seat, creating a gap between them that results in poor sealing and leakage. 3. Oscillation of the control valve: Oscillation of the control valve occurs due to insufficient spring stiffness of the valve, unstable output signals that result in sharp changes, a frequency of the control 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, significant 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 causes include the gas source not being turned on or experiencing a malfunction, blocked air ducts, or a pressure reducing valve becoming inoperative due to blockage; compressor malfunctions may also be a cause ； (2) There is a gas 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) No air supply to the positioner. 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 the 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 gas consumption and output oscillations. 6. The control valve operates slowly – damage or leakage in the pneumatic diaphragm during operation causes the valve stem to move slowly when acting in one direction ； The poor response performance of the positioner can cause the control valve to operate sluggishly ； 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 packing or packing that is packed too tightly increases frictional resistance, resulting in sluggish operation of the control valve. Fault analysis of control valves https://mmbiz.qpic.cn/mmbiz_jpg/vSABwyURAVhJaHLHS1ToapmrI2utclw537TUB4sicDpj06SWqySlgOG8vwWxq1bGwbSyqkjRgzqicN6b7rC3dyHg/640?wx_fmt=jpeg 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 a pipe wrench to clamp the valve stem. Under the action of an applied signal pressure, apply force in both directions to move the valve stem, thereby allowing the valve core to get past the stuck point. 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 normal operation is not restored, 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 must 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 gas-sealing properties, low friction, minimal wear over time, is easy to maintain, and 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, so as to ensure a high level of sealing quality for the valve core and seat. 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 operate, the pressure reducing valve must be adjusted so that the pressure reaches 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 a slow-acting control valve? The control valve responds slowly; check the components with high friction and reassemble them ； Adjust the packing gland and replace the packing again. How should control valves be maintained? https://mmbiz.qpic.cn/mmbiz_jpg/vSABwyURAVhJaHLHS1ToapmrI2utclw5ZeiaHVJNVORUiaQhzSHKYmh2PribKQuVvd6tm6xDS5NIJuZ5qO8VIHkHg/640?wx_fmt=jpeg If a control valve fails during operation, it can cause significant harm to the proper 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, it is necessary to clean the control valves regularly to maintain their cleanliness and ensure the integrity and durability of all their components. The pipes should also be flushed periodically to remove any blockages or debris inside them. The fixing fittings of the control valves need to be inspected regularly to guarantee good corrosion resistance and lubricity of those components. When installing control valves, minimize forceful pulling and bending during installation to reduce pressure and deformation. Maintain a distance between the control valve and the compressor to reduce vibrations originating from the vibration source. Regular inspections of the control valve should be conducted. Close attention must be paid to its operating conditions; check whether there are any leaks in the packing, and regularly apply lubricating oil to the packing. 3. Predictive maintenance obtains operating information of control valves through smart meters or other diagnostic devices, thereby enhancing the regulation quality and diagnostic capabilities of the control valves. Through diagnosis using smart meters or other diagnostic devices, issues with control valves can be detected in a timely manner, allowing for prompt identification of solutions and maintenance actions. Increase the development and use of intelligent valve positioners.