Working Principle and Fault Analysis of Pneumatic Control Valves

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Overview of Pneumatic Control Valves Pneumatic control valves are one of the industrial process control instruments widely used in industries such as petroleum, chemicals, power, and metallurgy. A pneumatic control valve is typically formed by connecting and installing a pneumatic actuator, a valve, a positioner, etc., after proper adjustment. A pneumatic control valve uses compressed air as its power source, with a cylinder serving as the actuator. It relies on accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves to operate the valve, enabling on-off or proportional control. The pneumatic control valve receives control signals from industrial automation systems in order to regulate various process parameters such as the flow rate, pressure, and temperature of the fluid flowing through the pipeline. Working principle of pneumatic control valves: A pneumatic control valve uses compressed air as its power source, with a cylinder serving as the actuator. It relies on accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves to operate the valve, enabling on-off or proportional control. It receives control signals from industrial automation systems in order to regulate various process parameters of the fluid flowing through the pipeline, such as flow rate, pressure, and temperature. Working Principle of Pneumatic Control Valves (Diagram) Pneumatic control valves typically consist of a pneumatic actuator and a control valve, which are connected and adjusted together. Pneumatic actuators can be of either single-acting or double-acting type; single-acting actuators have a return spring inside them, while double-acting actuators do not have such a spring. Among them, the single-acting actuator can automatically return to the initial open or closed state set for the valve in the event of a loss of power or a sudden failure. Pneumatic control valves are divided into two types based on their operating mode: air-open and air-close, which are also known as normally open and normally closed types. The air-open or air-close operation of pneumatic control valves is usually achieved through the opposite actions of the actuator and different assembly methods of the valve structure. Operation mode of pneumatic control valves: The air-operated (normally closed) type operates in such a way that as the air pressure on the diaphragm increases, the valve opens further; when the maximum input air pressure is reached, the valve is in its fully open position. Conversely, as the air pressure decreases, the valve moves in the direction of closure, and when no air is supplied, the valve closes completely. Generally, we refer to air-operated control valves as fail-closed valves. 　　The operating direction of the air-shut type (normally open type) is exactly the opposite of that of the air-open type. As air pressure increases, the valve moves in the direction of closure ； When the air pressure decreases or is absent, the valve moves in the opening direction until it is fully open. Generally, we refer to air-operated control valves as fail-open valves. 　　The choice between gas on and gas off is made from the perspective of ensuring safety in the production process. When the air supply is cut off, is the pneumatic control valve in a closed position for safety or in an open position for safety? 　　For example, in the combustion control of a heating furnace, a control valve is installed on the fuel pipeline to regulate the fuel supply based on the temperature of the furnace chamber or the temperature of the material being heated at the outlet of the furnace. At this time, it is safer to use a pneumatic on-off valve, because once the gas supply is interrupted, it is more appropriate for the valve to be in a closed position than in an fully open one. If the gas supply is interrupted with the fuel valve fully open, it can lead to dangerous overheating. Another example is a heat exchange device cooled by cooling water; the hot material exchanges heat with the cooling water inside the heat exchanger to be cooled. A control valve is installed on the cooling water pipe, and the amount of cooling water is regulated based on the temperature of the material after heat exchange. In the event of a disruption in the air supply, it is safer for the control valve to be in the open position, so an air-operated shut-off (i.e., FO) type control valve is recommended. Valve positioner: A valve positioner is a key accessory for control valves, used in conjunction with pneumatic control valves. It receives the output signal from the controller and uses this signal to control the pneumatic control valve. When the control valve moves, the displacement of its stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce the delay in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome the frictional forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. Actuators can be divided into pneumatic actuators and electric actuators, and further categorized into linear stroke and rotary stroke types. Used to automatically or manually open and close various types of shutters, baffles, etc. Installation principles for pneumatic control valves: (1) The installation location of the pneumatic control valve should be at a certain height above the ground, with sufficient space above and below the valve to facilitate its disassembly, assembly, and maintenance. For control valves equipped with pneumatic valve positioners and handwheels, it is necessary to ensure easy operation, observation, and adjustment. (2) The control valve should be installed on a horizontal pipe, perpendicular to the pipe both above and below; it is generally necessary to provide support under the valve to ensure stability and reliability. For special occasions where the control valve needs to be installed horizontally on a vertical pipe, it should also be supported (except for control valves with small diameters). During installation, avoid applying additional stress to the control valve). (3) The operating temperature for pneumatic control valves should be within the range of (–30 to +60), with a relative humidity not exceeding 95%. (4) There should be straight pipe sections before and after the pneumatic control valve, with a length of not less than 10 times the pipe diameter (10D), to prevent the short straight pipe sections from affecting the flow characteristics of the valve. (5) When the diameter of the control valve differs from that of the process pipeline, a reducer should be used for connection. When installing small-diameter control valves, threaded connection can be used. The fluid direction arrow on the valve body should be in line with the flow direction of the fluid. (6) A bypass pipe should be installed. The purpose is to facilitate switching or manual operation, allowing the control valve to be serviced without stopping the machine. (7) Before installing the control valve, all foreign objects in the pipeline, such as dirt and slag, must be thoroughly removed. Common faults of pneumatic control valves and their solutions: 1. The pneumatic control valve does not operate. First, check whether the air supply pressure is normal and identify any issues with the air supply. If the gas supply pressure is normal, check whether the amplifier of the positioner or the electro/pneumatic converter is producing an output ； If there is no output, the constant-throttle orifice of the amplifier is blocked, or moisture in the compressed air has accumulated at the ball valve of the amplifier. Use a thin wire to clear the constant throttle orifice, remove debris, or clean the air supply. 　　If everything else is normal but there is a signal with no action, then the actuator is faulty, the valve stem is bent, or the valve core is stuck. In such a case, the valve must be removed for further inspection. 2. Clogging of the control valve 　　If the reciprocating motion of the valve stem is sluggish, it may be due to the presence of viscous substances inside the valve, coking and blockages, overly tight packing, aging of the polytetrafluoroethylene packing, or bending and scratches on the valve stem. Blockage faults in control valves occur most frequently in systems that have just been put into operation or at the beginning of operation after major repairs. This is due to slag, rust, and other debris in the pipes blocking the throttling areas and guiding components, thereby hindering the flow of the medium. Additionally, excessive tightness of the packing during valve maintenance can increase friction, resulting in the valve not responding to small signal inputs and overreacting to larger signal inputs. 　　In such situations, the bypass line or control valve can be quickly opened or closed to allow the contaminants to be carried away by the fluid through the bypass line or control valve. Alternatively, pipe wrenches can be used to clamp the valve stem; under the action of an external signal pressure, the valve stem is rotated in both directions to move the valve core past the jammed area. If the problem cannot be resolved, increasing the air supply pressure and the driving power, and moving up and down several times, can solve the issue. If it still cannot be operated, the control valve will need to be disassembled. Of course, this task requires high levels of technical expertise, and it must be carried out with the assistance of qualified professionals; otherwise, the consequences could be severe. 3. Leakage in pneumatic control valves 　Leakage in control valves can occur due to internal leakage within the valve, packing leakage, or deformation of the valve core and seat; these different causes will be analyzed below. (1) Internal leakage of the valve: The length of the valve stem is inappropriate; in the case of air-operated valves, the stem is too long, and the distance between the stem and the upward (or downward) direction is insufficient. This results in a gap between the valve core and the valve seat, preventing proper contact and leading to leakage. Similarly, if the stem of a gas shut-off valve is too short, it can also result in a gap between the valve core and the valve seat, preventing proper contact and leading to poor sealing and internal leakage. Solution: The valve stem of the control valve should be shortened (or lengthened) to adjust its length appropriately, so as to eliminate internal leakage. (2) Packing leakage After the packing is installed in the stuffing box, axial pressure is applied to it via the gland. Due to the plastic deformation of the filler, radial forces are generated, causing it to make close contact with the valve stem. However, this contact is not uniform; in some areas the contact is loose, in other areas it is tighter, and in some areas there is no contact at all. During operation of a control valve, there is relative motion between the valve stem and the packing, and this motion is known as axial motion. During operation, due to the effects of high temperature, high pressure, and fluid media with strong permeability, the packing gland of control valves is also a part where leaks occur frequently. The main cause of filler leakage is interfacial leakage; in the case of textile fillers, seepage also occurs (the pressure medium leaks out through the tiny gaps between the filler fibers). Interfacial leakage between the valve stem and the packing is caused by the gradual decrease in the contact pressure of the packing, as well as aging of the packing itself; under such conditions, the pressurized medium leaks outward through the gap between the packing and the valve stem. 　　To facilitate the filling of the packing, a chamfer is provided at the top of the packing box, and an erosion-resistant metal protective ring with a small clearance is placed at the bottom. It is important that the contact surface between this protective ring and the packing not be inclined, in order 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 to improve surface finish and reduce packing wear. Flexible graphite is chosen as the packing because it offers good airtightness and low friction, shows minimal changes over time, suffers little wear or degradation, is easy to maintain, and its friction level remains unchanged after the gland bolts are tightened again. It also possesses excellent pressure and heat resistance, is not affected by internal media, and does not cause pitting or corrosion in the metals that come into contact with it within the valve stem and stuffing box. In this way, the sealing of the valve stem packing box is effectively protected, the reliability of the packing seal is ensured, and its service life is also greatly extended. (3) Leakage due to deformation of the valve core and seat The main cause of leakage in the valve core and seat is the enhancement of corrosion resulting from casting or forging defects during the production of control valves. The passage of corrosive media, as well as the scouring by fluid media, can also cause leakage in control valves. Corrosion mainly occurs in the form of erosion or cavitation. When corrosive media pass through the control valve, it causes erosion and wear on the materials of the valve core and seat, resulting in these components taking on an oval or other shape. Over time, this leads to a mismatch between the valve core and seat, the creation of gaps, and leakage as a result of poor sealing. Pay close attention to the material selection for the valve core and valve seat. Choose corrosion-resistant materials, and firmly reject products with defects such as pitting and sand eyes. If the valve core and seat are not severely deformed, they can be polished with fine sandpaper to remove any marks and improve the smoothness of the seal, thereby enhancing its sealing performance. If the damage is severe, a new valve should be replaced. 4. Oscillation 　　Insufficient spring stiffness in the control valve, as well as unstable and rapid changes in the output signal of the control valve, can lead to oscillation of the control valve. Also, the frequency of the selected valve is the same as the system frequency, or the pipes and base vibrate violently, causing the control valve to vibrate as well. Due to improper selection, the control valve operates at a low opening degree, resulting in significant changes in flow resistance, flow rate, and pressure. When these changes exceed the valve’s stiffness, its stability deteriorates, and oscillations may occur in severe cases. 　　Since there are various reasons for oscillations, each case must be analyzed on its own. For those with mild vibration, the stiffness can be increased to eliminate it, such as by using control valves with high-stiffness springs or switching to a piston actuation mechanism ； Severe vibration in pipes and bases can be eliminated by adding supports to reduce vibration interference ； When the frequency of the valve matches that of the system, replace the control valve with a different structure ； The oscillations that occur when the control valve operates at a low opening degree are caused by an inappropriate selection; specifically, it is due to an excessively high value for the valve’s flow capacity C. It is necessary to select a valve with a lower flow capacity C, or to use stage control or mother-and-child valves in order to overcome the oscillations that arise when the control valve operates at a low opening degree. 5. High noise from control valves When fluid flows through a control valve, if the pressure difference before and after the valve is too large, cavitation can occur in components such as the valve core and seat, resulting in noise generated by the fluid. If the flow capacity value is set too high, it is necessary to select a control valve with an appropriate flow capacity value in order to eliminate the noise caused by the control valve operating at a low opening degree. The following are several methods for reducing noise. (1) Resonance noise elimination method: Intense noise of over 100 decibels is generated only when the control valve resonates, due to energy buildup. Some exhibit strong vibration with low noise, while others have weak vibration but very high noise levels ； Some have significant vibration and noise. This noise produces a single-toned sound, with a frequency generally ranging from 3000 to 7000 hertz. Obviously, by eliminating resonance, the noise disappears as well. (2) Method of eliminating cavitation noise Cavitation is a major source of hydrodynamic noise. During cavitation, the bursting of bubbles creates high-speed shock waves, resulting in intense local turbulence and thus cavitation noise. This noise has a wide frequency range and produces a rattling sound, similar to the noise generated by sand and gravel in the fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise. (3) Use of thick-walled piping Using thick-walled pipes is one of the methods for acoustic path treatment. Using thin walls can increase noise by 5 decibels, while using thick-walled tubes can reduce noise by 0 to 20 decibels. The thicker the wall for a given diameter, and the larger the diameter for a given wall thickness, the better the noise reduction effect. For a DN200 pipe, when the wall thicknesses are 6.25, 6.75, 8, 10, 12.5, 15, 18, 20, and 21.5 mm respectively, the noise reduction is -3.5, -2 (i.e., an increase), 0, 3, 6, 8, 11, 13, and 14.5 decibels respectively. Of course, the thicker the wall, the higher the cost. (4) Using sound-absorbing materials This is also a common and effective method for handling sound paths. The noise source and the pipelines behind the valve can be wrapped with sound-absorbing materials. It should be noted that since noise can travel over long distances through fluid flow, the effectiveness of noise reduction ends where sound-absorbing materials are applied and where thick-walled pipes are used. This method is suitable for situations where the noise level is not high and the pipeline length is not long, as it is a relatively expensive approach. (5) Series silencer method: This method is suitable for reducing aerodynamic noise; it can effectively eliminate noise within the fluid and suppress the noise level transmitted to the solid boundary layer. For applications with high mass flow rates or a high pressure drop ratio before and after the valve, this method is effective and economical. Using absorptive series silencers can significantly reduce noise. However, from an economic perspective, it is generally limited to attenuating to about 25 decibels. (6) Soundproof box method: Soundproof boxes, houses, and buildings are used to enclose the noise source, thereby reducing the noise from the external environment to an acceptable level. (7) Series throttling method: In cases where the pressure ratio of the pneumatic control valve is high (△P/P1≥0.8), the series throttling method is used, which involves distributing the total pressure drop across both the control valve and a fixed throttling element located after the valve. Using diffusers or porous flow restrictors are among the more effective methods for reducing noise. To achieve a relatively ideal diffuser efficiency, it is necessary to design the diffuser (its shape and size) based on the installation conditions of each unit, so that the noise level generated by the valve is the same as that generated by the diffuser. (8) Use low-noise valves. Low-noise valves work by gradually reducing the flow velocity of the fluid as it passes through the valve core and seat via complex flow paths (multiple channels or grooves), thereby preventing supersonic flow at any point within these paths. There are various forms and structures of low-noise valves (some designed for specific systems) available for use. When the noise level is not very high, using a low-noise sleeve valve can reduce noise by 10–20 decibels; it is a cost-effective option for reducing noise. Valve positioner failure 　　Conventional positioners operate on the principle of mechanical force balance, namely the nozzle damper technique. The main types of failures associated with them are as follows: (1) Due to the use of this mechanical force balance principle, they have numerous moving parts, which make them susceptible to the effects of temperature and vibration, resulting in fluctuations in the performance of the control valve ； (2) The nozzle baffle technology is used; since the nozzle holes are very small, they can easily get blocked by dust or dirty air, preventing the locator from functioning properly ； (3) By applying the principle of force balance, the elastic coefficient of the spring changes in harsh operating conditions, resulting in non-linearity of the control valve and a decline in control quality. (4) The intelligent locator is composed of components such as a microprocessor (CPU), A/D, and D/A converters. Its working principle is completely different from that of ordinary locators; the comparison between the set value and the actual value takes place in the form of electrical signals, rather than through force balance. Thus, it can overcome the drawback of force balance in conventional positioners. However, when used in emergency shutdown applications, such as emergency shut-off valves and emergency vent valves, these valves are required to remain in a certain position; they only need to operate reliably in emergency situations. Staying in one position for an extended period of time can lead to the loss of control over the electrical converters, posing a risk that minor signals will not be acted upon. Furthermore. The position sensing potentiometer used for valves, operating in the field, has its resistance value prone to changes, which can lead to the risk of small signals not triggering any action while large signals cause the valve to open fully. Therefore, to ensure the reliability and availability of intelligent locators, they must be tested frequently.