Thread Content
Hi, I’m the Industrial Control Assistant. It’s Monday today, and I’m here again. Why do I emphasize that it’s Monday? Well, I just like to do that{:3_57:}. Today’s topic is pneumatic control valves. Those who already know about them need not say anything; I’m here to inform those who don’t. Pneumatic control valves are one of the industrial process control instruments widely used in industries such as petroleum, chemicals, power generation, and metallurgy. They use compressed air as an energy source, with cylinders serving as actuators. Accessories such as valve positioners, converters, solenoid valves, and hold-down valves are used to operate the valves, enabling on-off or proportional control. These valves receive 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 pipes. Next, let’s take a look at the common faults of pneumatic control valves and their solutions. 1. If the control valve does not operate, first check whether the air supply pressure for the pneumatic control valve 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 pneumatic control 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 throttle openings and guiding areas, thereby hindering the flow of the medium. Additionally, if the packing is tightened too much during maintenance of the control valve, it increases friction, resulting in the valve not responding to small signal inputs and over-reacting 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 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 will be even more severe. 3. Leakage in control valves: Leakage in pneumatic control valves generally occurs due to internal leakage within the valve, packing leakage, or deformation of the valve core and seat; these aspects will be analyzed separately below. 3.1 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, resulting in a gap between the valve core and the valve seat. This prevents proper contact, 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. 3.2 Regulator valve packing leakage: After the packing is installed in the packing 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 yet some areas there is no contact at all. During operation of a control valve, there is relative movement between the valve stem and the packing, and this movement is known as axial movement. 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 outward 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 insertion of the packing, a chamfer is provided at the top of the packing box, and an erosion-resistant metal protective ring with a small gap 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 smoothness 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 good 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 stuffing box is effectively protected, the reliability of the packing seal is ensured, and its service life is also significantly extended. 3.3 Deformation of the valve core and seat causing leakage – The main reason for leakage due to 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 impact on the materials of the valve core and seat, resulting in these components taking 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 products with defects such as pitting and sand eyes must be firmly rejected. 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 of the control valve: Insufficient spring stiffness in the control valve leads to unstable output signals, and these sudden changes can cause oscillation in 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 is the same as that of the system, replace the control valves with different structures ； 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 split-control systems or mother-and-child valves in order to overcome the oscillations that arise when the control valve operates at a low opening degree. That’s it for today. There are other common problems as well, such as excessive noise from control valves, and there’s so much to cover that it’s impossible to discuss everything in one day. I’m sure you won’t be able to remember it all, right? Well, follow me – feel free to leave messages if you have any questions, and I’ll keep sharing useful information with you every day. Thank you, bye.