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1. The regulating valve does not operate
First, confirm whether the pressure of the air source is normal, and find out the fault of the air source. If the air supply pressure is normal, determine whether the amplifier of the positioner or electric/pneumatic converter has output; If there is no output, the constant orifice of the amplifier is blocked, or moisture in the compressed air accumulates at the amplifier ball valve. Use small fine steel wire to dredge the constant orifice, remove dirt or clean the air source.
If all of the above are normal and there is a signal but no action, then the actuator is faulty or the valve stem is bent, or the valve core is stuck. In this case, the valve must be removed for further inspection.
2. The regulating valve is blocked
If the reciprocating stroke of the valve rod is sluggish, there may be substances with high viscosity in the valve body, coking and blocking, or the packing is pressed too tightly, or the PTFE packing is aged, and the valve rod is bent and scratched. The blocking failures of the regulating valve mostly occur in newly put into operation systems and in the early stages of overhaul and commissioning. Due to the blockage of welding slag and rust in the pipeline at the throttle and guide parts, which results in poor flow of media, or the excessive tightening of the filler during the maintenance of the regulating valve, resulting in increased friction, leading to the phenomenon of small signal inaction and large signal excessive action.
In such cases, quickly open and close the auxiliary line or regulating valve to allow the dirt to be washed away by the medium from the auxiliary line or regulating valve. Alternatively, you can clamp the valve stem with a pipe wrench, and under the condition of applying signal pressure, forcefully rotate the valve stem in both directions to allow the valve core to flash past the clamp. If the problem cannot be solved, it can be solved by increasing the air supply pressure and increasing the driving power and repeatedly moving up and down several times. If it still cannot operate, it is necessary to disassemble the control valve. Of course, this work requires strong professional skills and must be completed with the assistance of professional and technical personnel, otherwise the consequences will be even more serious.
3. Valve leakage
The leakage of regulating valves generally includes internal leakage of regulating valves, leakage of packing, and leakage caused by deformation of valve core and valve seat. The following are analyzed separately.
(1) Valve leakage
The length of the valve rod is not suitable, the valve rod of the air opening valve is too long, and the upward (or downward) distance of the valve rod is insufficient, resulting in a gap between the valve core and the valve seat that cannot be fully contacted, leading to laxity and internal leakage. Similarly, if the valve stem of an air shutoff valve is too short, there may be gaps between the valve core and valve seat that cannot be fully contacted, resulting in tight shutoff and internal leakage. Solution: The valve rod of the regulating valve should be shortened (or extended) to make the length of the regulating valve suitable and prevent internal leakage.
(2) Packing leakage
After the packing is loaded into the stuffing box, axial pressure is applied to it through the gland. Due to the plastic deformation of the packing, it produces a radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are in loose contact, some parts are in tight contact, and even some parts are not in contact at all. During the use of a regulating valve, there is a relative movement between the valve stem and the packing, which is called axial movement. During use, due to the influence of high temperature, high pressure, and highly permeable fluid media, the control valve stuffing box is also a location where leakage occurs frequently. The main reason for filler leakage is interface leakage, and leakage can also occur for textile fillers (pressure media leaking outward along the tiny gaps between the filler fibers). The interface leakage between the valve rod and the packing is caused by the gradual attenuation of the packing contact pressure, aging of the packing itself, and other reasons. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve rod.
To facilitate packing installation, chamfer the top of the stuffing box and place a metal protective ring with a small erosion resistance gap at the bottom of the stuffing box. Note that the contact surface between the protective ring and the packing should not be inclined to prevent the packing from being pushed out by medium pressure. The surface of the contact part between the stuffing box and the filler should be finished to improve the surface finish and reduce the wear of the filler. Flexible graphite is selected as the filler because of its good air tightness, low friction, small changes in long-term use, small wear and burn, and easy maintenance. Moreover, the friction force does not change after the gland bolts are retightened, with good pressure resistance and heat resistance, and is not subject to corrosion by internal media. The metal in contact with the valve rod and stuffing box does not undergo pitting or corrosion. This effectively protects the sealing of the valve stem stuffing box, ensures the reliability of the packing seal, and greatly improves its service life.
(3) Deformation and leakage of valve core and valve seat
The main reason for valve core and valve seat leakage is that casting or forging defects in the production process of control valves can lead to enhanced corrosion. The passage of corrosive media and the flushing of fluid media can also cause leakage of the control valve. Corrosion occurs mainly in the form of erosion or cavitation. When corrosive media pass through the regulating valve, it will cause erosion and impact on the valve core and valve seat materials, causing the valve core and valve seat to form an oval shape or other shapes. Over time, it will lead to mismatching of the valve core and valve seat, resulting in gaps, and leakage if the valve is not tightly closed.
Control the material selection of valve core and valve seat. Select corrosion resistant materials and resolutely eliminate products with defects such as pitting and trachoma. If the deformation of the valve core and valve seat is not too serious, it can be ground with fine abrasive paper to remove traces and improve the sealing smoothness to improve sealing performance. If the damage is serious, replace the valve with a new one.
4. Oscillation
The spring stiffness of the regulating valve is insufficient, and the output signal of the regulating valve is unstable and changes rapidly, which can easily cause the regulating valve to oscillate. Also, the frequency of the selected valve is the same as the system frequency or the pipe or base vibrates violently, causing the regulating valve to vibrate accordingly. Improper selection of the control valve may cause severe changes in flow resistance, flow rate, and pressure when operating at a small opening. When the stiffness of the valve is exceeded, the stability becomes poor, and in severe cases, oscillation occurs.
Due to the various reasons for oscillation, it is necessary to analyze it on a case by case basis. If the vibration is slight, it can be eliminated by increasing the stiffness, such as selecting a regulating valve with a large stiffness spring, changing to a piston actuator structure, etc; Severe vibration of pipes and bases can be eliminated by adding support; When the frequency of the valve is the same as the frequency of the system, replace the regulating valve with a different structure; The oscillation caused by working at a small opening is caused by improper selection. Specifically, due to the excessive flow capacity C value of the valve, it is necessary to select a new type. Select a valve with a small flow capacity C value, or use split range control or use a sub mother valve to overcome the oscillation caused by working at a small opening of the regulating valve.
5. The regulating valve is noisy
When the fluid flows through the regulating valve, excessive differential pressure between the front and rear will cause cavitation phenomena on the valve core, valve seat, and other components, making the fluid generate noise. "If the flow capacity value is selected too high, it is necessary to reselect a control valve with a suitable flow capacity value to overcome the noise caused by the control valve operating at a small opening. Below are several methods for eliminating noise.".
(1) Resonance noise elimination method
Only when the regulating valve resonates can there be energy superposition that produces a strong noise of more than 100 decibels. Some are characterized by strong vibration and low noise, while others are characterized by weak vibration and very high noise; Some have high vibration and noise. This noise produces a single tone sound with a frequency typically between 3000 and 7000 Hz. Obviously, by eliminating resonance, the noise naturally disappears.
(2) Cavitation noise elimination method
Cavitation is the main source of hydrodynamic noise. During cavitation, bubbles rupture and generate high-speed impacts, causing strong turbulence in local areas and generating cavitation noise. This noise has a wide frequency range and produces a grating sound similar to the sound produced by the presence of sand in the fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise.
(3) Using the thick wall pipeline method
The use of thick wall tubes is one of the methods for acoustic path treatment. The use of thin walls can increase the noise by 5 decibels, while the use of thick wall tubes can reduce the noise by 0 to 20 decibels. The thicker the wall of the same pipe diameter, the larger the pipe diameter of the same wall thickness, and the better the noise reduction effect. For example, when the wall thicknesses of DN200 pipes are 6.25, 6.75, 8, 10, 12.5, 15, 18, 20, and 21.5 mm, the noise can be reduced to - 3.5, - 2 (i.e., increased), 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 material method
This is also a relatively common and effective sound path processing method. Sound absorbing materials can be used to wrap the noise source and the pipeline behind the valve. It must be pointed out that since noise can propagate over long distances through fluid flow, the effectiveness of noise elimination ends wherever sound absorbing materials are packaged and thick walled pipes are used. This method is suitable for situations where the noise is not very high and the pipeline is not very long, as it is a more expensive method.
(5) Series muffler method
Suitable for use as an aerodynamic noise suppressor, it can effectively eliminate noise inside the fluid and suppress noise levels transmitted to the solid boundary layer. This method is effective and economical for places with high mass flow or high pressure drop ratio before and after the valve. The use of absorption type series silencers can significantly reduce noise. However, from an economic perspective, it is generally limited to attenuation to about 25 decibels.
(6) Soundproof box method
Use sound barriers, houses, and buildings to isolate noise sources inside, reducing the noise from the external environment to an acceptable level.
(7) Series throttle method
When the pressure ratio of the regulating valve is high (△ P/P1 ≥ 0.8), the series throttling method is used to disperse the total pressure drop between the regulating valve and the fixed throttling element behind the valve. For example, using diffusers and multi hole flow restrictors is an effective way to reduce noise. In order to achieve the efficiency of the diffuser, it is necessary to design the diffuser (physical shape and size) based on the installation of each piece, so that the noise level generated by the valve and the diffuser are the same.
(8) Select low noise valve
The low noise valve decelerates gradually according to the zigzag flow path (multi orifice, multi groove) of the fluid through the valve core and valve seat to avoid generating supersonic speeds at any point in the flow path. There are various forms and structures of low noise valves (some designed for special systems) for use. When the noise is not very high, select a low noise sleeve valve, which can reduce the noise by 10-20 decibels. This is an economical low noise valve.
Valve positioner failure
Common positioners operate using the mechanical force balance principle, namely nozzle baffle technology, and there are mainly the following types of failures:
(1) Due to the use of mechanical force balancing principle, there are many movable components, which are easily affected by temperature and vibration, resulting in fluctuations in the control valve;
(2) Using nozzle baffle technology, the positioner cannot work properly due to the small nozzle hole, which is easily blocked by dust or unclean air sources;
(3) Using the principle of force balance, the elastic coefficient of the spring can change in harsh environments, resulting in nonlinear control valves and reduced control quality.
(4) Intelligent positioners are composed of microprocessors (CPUs), A/D, D/A converters, and other components. Their working principles are different from ordinary positioners. The comparison between the given value and the actual value is purely an electrical signal, not a force balance. Therefore, the disadvantage of force balance of conventional positioners can be overcome. However, when used in emergency parking situations, such as emergency shut-off valves, emergency vent valves, etc., these valves require to be stationary at a certain position, and only when an emergency occurs, reliable action is required. Staying at a certain position for a long time can easily cause the electrical converter to lose control and cause a dangerous situation where small signals do not operate. In addition. Due to the on-site operation of position sensing potentiometers for valves, the resistance value is prone to change, resulting in dangerous situations where small signals do not operate and large signals are fully opened. Therefore, in order to ensure the reliability and availability of intelligent locators, they must be tested frequently.
