Discussion on maintenance of regulating valve and technical problems in application
Abstract: This paper mainly discusses several technical aspects and maintenance problems that need to be paid attention to in the application of regulating valves, and proposes solutions in combination with the actual situation.
Keywords: regulating valve selection installation application
Preface
In the highly automated refining and chemical control system, the regulating valve is used as the terminal execution device of the automatic regulating system, and receives control signals to adjust the refining and chemical process. As the control valve for the final control of various quality and safety production indicators of the medium, it plays an important role in stable production, optimization control, maintenance and maintenance cost control. Taking the refinery chemical plant as an example, with the high-load operation of the device, problems such as jamming, corrosion, erosion, wear, vibration, and internal leakage of the regulating valve are constantly occurring, resulting in shortened service life of the regulating valve and reduced operational reliability. Causes a decline in the production efficiency of process systems and devices. Therefore, selecting and installing the regulating valve in the original construction and maintaining the regulating valve in the future to operate the regulating valve at a high level will be a critical issue. Combine the actual construction and daily maintenance with a few personal opinions for your reference.
1 Selection and installation of regulating valve
When selecting a regulating valve, first collect the physical characteristics of the complete process fluid and the working conditions of the regulating valve, the composition of the main fluid, temperature, density, viscosity, normal flow, maximum flow, minimum flow, maximum flow and minimum flow. Import and export pressure, maximum pressure difference, etc. In the technical aspect, it mainly grasps and determines the structure, flow characteristics, rated flow coefficient Kv value, caliber size, process allowable pressure difference calculation, actuator selection, materials and installation of the regulating valve itself. The general principles to be followed when selecting and installing the regulating valve are as follows.
1.1 Selection of regulating valve
1.1.1 Structure of the regulating valve: It should meet the requirements of medium temperature, pressure, fluidity, flow direction, adjustment range and tightness.
1.1.2 Flow characteristics of the regulating valve: It should be able to meet the system characteristics for reasonable compensation.
The flow characteristic of the regulating valve refers to the relationship between the relative flow rate of the medium flowing through the valve and the relative displacement of the valve stem. (1) The mathematical expression is as follows:
Q/Qmax=f(l/L) (1)
In the formula, Q/Qmax is the relative flow rate, which is the ratio of the flow rate Q to the full open flow rate Qmax of the regulating valve at a certain opening degree; l/L is the relative displacement, and the spool displacement and full opening of the regulating valve at a certain opening degree The ratio of the displacement L.
The general principle of choice is that the flow characteristics of the regulating valve should be opposite to the characteristics of the regulating object and the characteristics of the regulator, so that the overall characteristics of the regulating system are close to linear. The choice of flow characteristics is usually done under the requirements of the process system, but the following actual conditions are also considered.
1) Linear flow characteristics Applicable range: 1 differential pressure change is small, almost constant; 2 the main parameters of the process flow change linearly; 3 system pressure loss is mostly distributed on the regulating valve (change opening degree, differential pressure change on the valve) Relatively small); 4 external interference is small, the set value changes little, and the adjustable range is small.
2) Applicable range of equal percentage characteristics: 1 The actual adjustable range is large; 2 The opening degree changes, the differential pressure on the valve changes relatively; 3) The pipeline system has large pressure loss; 4 The process system load fluctuates greatly; 5 The regulating valve is often small Run under opening.
3) In addition to the above two commonly used flow characteristics, there are also regulating valves for other flow characteristics such as parabolic characteristics and quick opening characteristics.
1.1.3 The diameter of the regulating valve: it should be able to meet the flow requirements of the process.
Based on the known fluid conditions, the necessary Kv values ​​are calculated and the appropriate regulator valve diameter is selected.
1.1.4 The material of the regulating valve should meet the requirements of temperature, pressure, differential pressure and corrosiveness of the medium.
(1) The effect of fluid pressure and temperature on the material.
(2) The influence of fluid corrosivity on the material.
(3) Cavitation of fluid or the influence of mud fluid on materials.
(4) From the structural point of view, whether there is a problem with the material assembly.
1.1.5
Regulating valve actuator output force: should meet the requirements of pressure difference in the field, its stiffness should meet the requirements of system stability; switch type should meet the requirements of safe operation of the system.
1.1.6
When selecting the regulating valve, it is necessary to consider that in special cases, the valve should meet the requirements of operation time and split control through auxiliary devices such as valve positioner, solenoid valve and limit switch.
1.2 Installation of the regulating valve
1.2.1 In the initial stage of commissioning and overhauling of the newly installed regulating valve, it is necessary to take measures to isolate or remove the process piping to prevent clogging caused by welding slag, rust, etc. in the pipeline at the throttle and guiding parts. The medium is not well circulated, and the filler in the imported pump Castel red wine or the regulating valve is too tight, which causes the friction to increase, resulting in the phenomenon that the small signal does not move and the large signal moves too far.
1.2.2 When installing the regulating valve, pay attention to the air opening and air closing of the regulating valve to prevent the valve from leaking due to the length and discomfort of the valve stem. The valve valve stem is too long and the valve stem of the gas shut-off valve is too short, which may cause a gap between the valve core and the valve seat, which may not be fully contacted, resulting in a tight seal and internal leakage.
1.2.3 In order to prevent cavitation, the following points should be noted in the original selection and installation:
(1) Install the regulating valve as far as possible at the lowest position of the system, so that the pressure of the regulating valve inlet P1 and the outlet P2 can be relatively increased.
(2) Install a shut-off valve or orifice plate upstream or downstream of the regulating valve to change the original installation pressure drop characteristics of the regulating valve (this method is generally effective for small flow conditions).
(3) It is also effective to prevent flashing or cavitation with special anti-cavitation internals, which can change the flow rate of the fluid in the regulating valve, thereby increasing the internal pressure.
(4) Try to use a harder material control valve, because in the case of cavitation, for such a regulating valve, it has certain erosion resistance and wear resistance, and cavitation can exist under certain conditions, and The internals of the regulator valve will not be damaged. On the contrary, for the soft material regulating valve, due to its poor erosion resistance and wear resistance, when cavitation occurs, the internal components of the regulating valve are quickly worn and thus cannot be cavitation. Under normal work.
1.2.4 Regulating valves should normally be installed upright and should be supported if they are to be tilted.
1.2.5 The flow direction of the medium must be consistent with the arrow of the valve body.
1.2.6 The installation position of the regulating valve should be easy to observe, operate and maintain.
1.2.7 The signal management camp of the actuator has sufficient flexibility to prevent the actuator from acting.
2 Factors and countermeasures affecting the safe operation of the regulating valve
2.1 Regulating valve leakage
2.1.1 Valve leakage, the length of the valve stem is not suitable.
The reason for the leakage inside the valve: the valve is open, the valve stem is too long, the valve stem of the gas shutoff valve is too short, and the distance of the valve stem is upward (or downward) is insufficient, causing a gap between the valve core and the valve seat, which cannot be fully contacted. The result is a tight and leaky leak.
Solution: The valve stem should be shortened (or extended) so that the stem length is appropriate so that it is no longer leaky.
(1) Packing leak
Reasons for packing leakage: 1 After the packing is loaded into the stuffing box, axial pressure is applied to the packing. Due to the plasticity of the packing, it creates a radial force and is in intimate contact with the valve stem, but this contact is not very uniform. Some parts are loosely contacted, some parts are tightly contacted, and some parts are not in contact with each other; 2 During the use of the regulating valve, there is relative movement between the valve stem and the packing, and the fluid medium with high temperature, high pressure and permeability The influence of the control valve stuffing box is also the part where the leakage phenomenon occurs. 3 The main cause of the leakage of the packing is the interface leakage, and the leakage of the textile packing (the pressure medium leaks outward along the small gap between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the contact pressure of the packing and the aging of the packing itself. At this time, the pressure medium leaks outward along the contact gap between the packing and the valve stem.
Solution: 1 In order to facilitate the filling of the packing, chamfer the top of the stuffing box, and place a metal guard ring with a small erosion-resistant gap at the bottom of the stuffing box (the contact surface with the packing cannot be inclined) to prevent the filler from being medium. Pressure is introduced; 2 the metal surface of the parts of the stuffing box and the contact part of the packing should be finished to improve the surface finish and reduce the wear of the packing; 3 The flexible graphite is selected for the packing, because it has good air tightness, low friction and long-term use. Small, wear and tear is small, easy to maintain, and the frictional force does not change after the gland bolt is retightened. Through the above points, the reliability and long-term stability of the packing seal can be ensured.
(2) Regulating valve spool and seat deformation leakage
The main cause of leakage of the valve core and seat is the corrosion enhancement due to casting or forging defects in the production process of the regulating valve. The passage of the corrosive medium and the flushing of the fluid medium can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will cause erosion and impact on the valve core and the valve seat material, so that the valve core and the valve seat are elliptical or other shapes. As time passes, the valve core and the valve seat are not formed. Matching, there is a gap, and the leak is not strict.
Solution: The key is to select the quality of the valve core and the valve seat, and the quality is off. Choose corrosion-resistant materials and resolutely eliminate defects in products such as pitting and trachoma. If the valve core and the valve seat are not too deformed, they can be ground through fine sandpaper to eliminate the marks and improve the sealing finish to improve the sealing performance. If the damage is serious, replace the valve with a new one.
2.2 Regulating valve wear failure
The valve body or internal valve parts are abraded.
The main cause of abrasion: the liquid speed is too high, there are particles in the liquid or flash.
Solution: increase the size of the valve or valve to reduce the fluid velocity; the valve body is changed to the flow structure to reduce the impact of the fluid; the valve body and the trim material increase the hardness; and the valve trim structure is changed to reduce the flow rate; Avoid air belt action, use low pressure recovery valve; weld with stainless steel material.
2.3 Oscillation
Reasons for oscillation: 1The spring stiffness of the regulating valve is insufficient, the output signal of the regulating valve is unstable and the sharp change is easy to cause the regulating valve to oscillate; 2The pipe and the base vibrate violently, so that the regulating valve vibrates; 3 improper selection, the regulating valve works There is a sharp flow resistance, flow rate, and pressure change at a small opening. When the valve stiffness is exceeded, the stability is deteriorated, and when it is severe, oscillation occurs.
Solution: Since the cause of the oscillation is multi-faceted, the specific problem is specifically analyzed. A slight vibration to the vibration can increase the stiffness to eliminate. 1 If a large stiffness spring is used, the piston is used to perform the structure; 2 the pipe and the base are vibrated by adding support to eliminate the vibration interference; 3 the oscillation caused by the small opening is the selection of the improper flow capacity Kv value, must Re-selection of the flow capacity Kv value is small or use split-range control to solve.
2.4 Valve positioner failure
The common positioner works with the mechanical force balance principle, that is, the nozzle baffle technology, and the following fault types mainly exist:
2.4.1 Due to the mechanical force balance principle, there are many movable parts, which are easily affected by temperature and vibration, causing fluctuations of the regulating valve.
2.4.2 Using the nozzle baffle technology, since the nozzle hole is small, it is easily blocked by dust or an unclean gas source, and the positioner cannot work normally.
2.4.3 With the principle of balance of force, the spring coefficient of the spring changes under the harsh scene, causing the nonlinearity of the regulating valve to cause the control quality to drop.
2.4.4 Because the electrical interface is not tightly sealed, water flows into the positioner from the threading pipe, causing the coil to be short-circuited or broken, and the positioner cannot work normally.
Solution: Fix the positioner firmly during installation, seal the electrical interface tightly, improve the quality of the purified air, and regularly discharge the sewage;
2.5 regulating valve jam
The reason for the jam: In the initial stage of the new commissioning system and overhaul, the slag, the rust, etc. in the pipeline are blocked at the throttle and the guiding part, so that the medium is not well circulated, or the packing in the regulating valve is too tight, causing friction. The force increases, causing the small signal to not move and the large signal to overshoot.
Solution: The secondary line or regulating valve can be quickly opened and closed to allow dirt to be washed away from the secondary line or the regulating valve. Another method is to clamp the valve stem with a pipe wrench. When the signal pressure is applied, the valve stem is rotated forward and backward to allow the spool to flash past the card. If not, increase the air supply pressure and increase the drive power repeatedly to move up and down several times to solve the problem. If it still does not work, it needs to be dismantled.
2.6 Regulating valve action failure
2.6.1 The valve does not move.
Causes: no gas source or gas source pressure is insufficient; actuator failure leakage; regulating valve no output signal; gas supply pipe breakage, deformation, joint damage leaking; flow direction is incorrect, excessive force, valve falling off, off Rod; the valve trim at high temperature is stuck and damaged.
Solution: Check the air supply, disassemble the valve, reconfigure the clearance between the shaft and the bushing, re-correct the direction, and repair.
The valve cannot reach the rated stroke.
Cause: The positioner is not calibrated, the stroke adjustment is improper; the actuator spring rating is too small; the manual mechanism limit position is not allowed.
Solution: Calibrate the positioner and replace the spring.
The valve is slow or slow.
Cause: The friction of the packing is large, the deterioration is aging; the friction of the piston actuator is too large; the friction of the bearing is large.
Solution: Replace the repair, re-adjust the damaged parts or clean the grinding cylinder and piston.
4 Conclusion
The selection and application of the regulating valve is a systematic work with a strong professionalism and a wide range of technical fields involved. To do this work, it is necessary not only to improve the selection and installation, but also to comprehensively analyze and judge based on the actual experience. To achieve a scientific combination of theory and practice. Proper handling and improvement methods, mastering its performance, and improving maintenance levels will greatly reduce the instrument failure rate, and have an important role in improving the production efficiency and economic efficiency of the process and the reduction of energy consumption, which can effectively improve the regulation system. Quality to ensure long-term operation of the production unit.
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