After reading this, the control valve will last two more years!

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In the automatic control of modern factories, control valves play a very important role, as the production in such factories depends on the proper distribution and control of the flowing media. Whether it’s energy exchange, pressure reduction, or simply filling containers, these controls require certain final control elements to be carried out. Tips to extend service life 1: Operate at full opening – Have the control valve operate at its maximum opening 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 closing 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, when operating at a large opening, the throttle gap is larger, reducing erosion; this increases the lifespan by more than 1–5 times compared to operating the valve 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) S: By decreasing S, the losses in the system other than the control valve increase, which leads to a lower pressure drop across the valve. To ensure that flow passes through the control valve, it is necessary to increase its opening degree. At the same time, the reduced pressure drop across the valve also results in less cavitation and erosion. Specific methods include: installing an orifice plate after the valve to dissipate 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 valve size to increase the operating opening. The operating opening can be increased by reducing the valve’s size. Specific methods include: ① Replace the valve with one of a smaller size; for example, replace a DN32 valve with a DN25 valve ； ②The valve body remains unchanged; replace the valve core and seat with ones having a smaller seat diameter. During a major overhaul of a chemical plant, replacing the throttle element dg10 with dg8 doubled its service life. 4 Transfer of damage locations: Move areas with severe damage from critical locations to less important ones, in order to protect the sealing surfaces and throttling surfaces of the valve stem and seat. It can increase the service life of the valve. 5 Change in flow direction: The flow moves in the direction that opens the valve core; 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 core and its seat ； In the flow-blocking mode, 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 adjusting smoothly. This method must be used 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 type of 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 structures: The goal of extending the valve’s lifespan can be achieved by modifying the valve structure or selecting 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, enabling them to carry out various adjustment commands in a timely manner ； (2) Used in combination with an atmospheric cylinder, it provides a large torque thrust ； (3) It maintains stable performance in harsh working conditions and can operate normally ； (4) High safety performance. Whether control valves operate normally and their responsiveness 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 1. Clogging: This is a frequent problem 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 cause poor flow of the medium, or excessive tightness of the packing during the maintenance of the control valve, which increases friction and leads to the situation where small signals do not trigger any action while large signals cause overreaction. 2. Leakage in the control valve: Leakage in control valves 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, 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 applies axial pressure to the control valve. Due to the plastic deformation of the packing, a radial pressure is generated, which comes into close contact with the valve stem, but the contact is uneven. During operation, the valve stem and packing of a control valve undergo axial movement. Under high-temperature and high-pressure conditions, influenced 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: The oscillation of a control valve is caused by insufficient spring stiffness of the valve, unstable output signals that result in sharp changes, a frequency of the control valve that matches the system’s frequency, and severe vibrations in the pipes and 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 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 air supply, but no signal. The cause is a failure of the locator due to air leakage resulting from a ruptured bellows, or a failure of the converter ； (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. The control valve does not operate stably. The reasons for the instability 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 movement is very high, resulting in obstruction 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. The control valve operates slowly, and the pneumatic diaphragm may get damaged or leak during operation, causing the valve stem to move slowly when acting in one direction ； The weak response performance of the locator results in sluggish operation of the control valve ； There are obstructions such as rust inside the control valve, causing blockages that make its axial movement sluggish ； 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 1: How to 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 normal operation is not restored, a professional must disassemble the control valve. 2 How to deal with leakage in control valves? 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, for ease of packing installation, a chamfer must be made at the top end of the stuffing box. Additionally, a corrosion-resistant metal protective ring with a small clearance should be placed at the bottom of the stuffing box. The contact surface between this metal ring and the packing must not be sloped, so as to prevent the packing from being pushed out under 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 precision 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 ； The frequency of the valve is the same as the system frequency; therefore, the control valve must 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 magnification hole and throttle hole of the locator and carry out repairs. 5 How to deal with unstable operation of the control valve? 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 lubricating oil to components with high friction. 6 How to deal with slow operation of the 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? 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 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. Pipes should also be flushed periodically to remove any blockages or debris inside them. The fixing fittings of the control valves should be inspected regularly to ensure 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 from the 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: 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 detected in a timely manner, allowing for prompt identification of solutions and maintenance actions. Increase the development and use of intelligent valve positioners.