Installation and maintenance of control valve

Installation, maintenance and common faults of regulating valve

In the automation of coal chemical production process, regulating valve plays an important role in stabilizing production, optimizing control and maintenance. As the terminal execution device of the automatic adjustment system, the regulating valve accepts the control signal to realize the adjustment of the chemical process, which is directly related to the normal production of the whole device. To do this work well, it is necessary not only to fully understand its various characteristics in theory, but also to comprehensively analyze and judge in combination with actual use experience, so as to give full play to the role of the regulating valve.

1. Installation of regulating valve

The installation quality of the regulating valve directly affects the operating performance, safety, and cost of the regulating valve, ensuring that it plays a good control role in the control system. During installation, pay attention to the following issues:

1) The inspection before the installation of the regulating valve is a very important link to confirm whether the regulating valve meets the design requirements. The specifications and models of the regulating valve include: valve model, valve type, nominal diameter, valve seat size, valve core form, flow characteristics, leakage grade, valve body material, valve core material, valve seat material, CV value, flange standard grade Size and sealing surface form, etc.

2) Before installing the regulating valve, the dust accumulated during the storage of the valve must be carefully removed, because the dust and impurities will damage the valve seat and internal parts, usually a cover plate is installed at the end of the open flange or a plastic wrap is used to seal it.

3) The regulating valve should be installed vertically on the horizontal pipeline. If it must be installed inclined or horizontally, the support should be added. There should be straight pipes longer than 10 times the pipe diameter before and after the valve.

4) The flow direction of the fluid should be the same as the mark on the valve body.

5) The installation of the regulating valve is often disturbed by the vibration source, which will directly affect the performance of the regulating valve. Therefore, try to avoid installing in places with vibration sources.

6) The quality of the installation position of the regulating valve directly affects the level of maintenance costs, so the regulating valve should be installed in a position that is convenient for maintenance and repair.

7) In order to stabilize the air source and protect the instrument, the regulating valve is often installed and used together with the air filter pressure reducer and safety valve.

8) After installation, when the pipeline is pressure tested and blown off, the regulating valve should be removed and connected with the corresponding straight pipe section to prevent debris such as welding slag and iron chips from being stuck between the valve core and the valve seat. The end of the opening flange of the dismantled regulating valve should be firmly wrapped with plastic cloth.

2. Routine maintenance and common faults of regulating valve

2.1. Daily maintenance of regulating valve

The maintenance of the regulating valve is often overlooked by people. Due to improper maintenance, accidents that cause the entire control system to fail frequently occur. The regulating valve is a very important and indispensable part of the automatic control system. If the regulator is compared to the human brain, then the regulating valve is the hands and feet of the human being. We must pay attention to its maintenance work to ensure the normal operation of the regulating system. , reliable and efficient operation. The daily maintenance of the regulating valve is divided into two parts: patrol inspection and regular maintenance.

The contents of the patrol inspection are as follows: 1. Check the operation of the regulating valve from the process operator on duty; 2. Check whether the air supply pressure of the regulating valve or the positioner and the purification of the air source are normal; 3. Check whether the static and dynamic sealing points of the regulating valve are there or not. Leakage; ④ Check whether the connection part of the regulating valve is loose, whether the joint is loose, leaking or corroded; ⑤ Check whether the instrument operates within the prescribed stroke, and check whether the handwheel mechanism is placed in the "release" position; ⑥ Observe when the regulating valve is in the manual position Check whether the regulating valve stem still moves up and down; ⑦ Check whether the action of the regulating valve is flexible and whether it changes in time when the control signal changes; ⑧ Check the rain and corrosion protection of the actuator and regulating valve to avoid damage to the actuator due to water ingress and other reasons ; ⑨ Contact and deal with problems in time; ⑩ Make records of patrol inspections and file them.

The contents of regular maintenance work are as follows: ① Regularly clean the outside of the control valve; ② Regularly adjust the control valve stuffing box and other sealing components, and replace the sealing components if necessary to maintain the sealing of static and dynamic sealing points; ③ Regularly Check the wheel parts to keep the handwheel flexible and easy to use; ④ Regularly check the diaphragm, piston, spring, seal and other key components, and carry out anti-corrosion, lubrication or replacement; ⑤ Regularly add lubricating oil to the parts that need to be lubricated; Source or hydraulic filter system for sewage and cleaning work; ⑦ Regularly check the connection and corrosion of each connection point, and replace the connection parts if necessary.

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2.2. Common faults of regulating valve

The regulating valve is an important link in the automatic control system, and it is the control mechanism that directly contacts the process medium, and is subjected to the pressure, temperature, erosion, corrosion and wear of the fluid, and the accessories of the regulating valve are easily affected by the surrounding environment. The common faults of the regulating valve are:

2.2.1. Air source system failure

1) The instrument air line is blocked. Since the ball valve has a throttling effect at the end of the air line of the instrument branch, the dirt in the air line is easy to accumulate and block here. If the air pressure of the instrument is too low, the regulating valve cannot be fully opened and fully closed, or even the regulating valve does not act.

2) The air filter pressure reducing valve is faulty. The air filter pressure reducing valve has too much dirt when used for a long time, the pressure reducing valve leaks air, and the set output pressure of the pressure reducing valve is too low, so that the output instrument wind pressure is less than the specified pressure, so that the regulating valve moves slowly and cannot be fully opened or closed. Not even moving.

3) Copper pipe connection failure. The copper pipe is aging and leaking, the joint connection is loose or the dirt blocks the copper pipe, so that the air pressure of the instrument signal is low, so that the regulating valve does not move and cannot be fully opened or closed.

2.2.2. Power system failure

1) The terminals of the power cord are loose and short-circuited. Due to the on-site vibration, the loose wiring or too much dust caused the wiring to be loose due to the on-site vibration, so that the signal from the control room to the site is sometimes absent, resulting in the chaotic operation of the regulating valve and the regulation oscillation. The equipment is short-circuited due to water or damp or other reasons, so that the control valve cannot be fully opened and closed.

2) The middle connector of the power cord or the damaged part in the middle is faulty. The power cord is subject to environmental vibration, external force pulling, the failure of insulating tape, the decline of insulation performance, and the high temperature baking of the connector, which makes the power cord connector loose or seems to be broken, short circuit between power lines or short circuit to ground, and the terminal or the power cord is broken. As a result, the action of the regulating valve is discontinuous, and it cannot be fully opened, fully closed or even not moved.

3. Valve positioner failure

3.1. Mechanical electrical valve positioner

1) The zero point and range are inaccurate. Due to inaccurate debugging during the installation of the positioner or on-site vibration, temperature changes, changes in the stroke of the control valve stem, changes in the position of the feedback rod, etc., the minimum and maximum openings of the control valve are inconsistent with the signal in the control room. As a result, the signal output by the valve positioner cannot make the control valve fully open and fully close, resulting in large leakage and limited quantities. In the on-site adjustment of the positioner, first ensure that the regulating valve operates well, the feedback system is firmly installed and operates well, and then adjusts through the standard signal. Make the stroke of the regulating valve consistent with the control signal.

2) The orifice is blocked. Because the instrument air is not clean, the dirt blocks the orifice, so that the positioner has no output signal, resulting in the control valve not moving.

3) There is dirt between the nozzle and baffle. Affected by the on-site environment, a layer of dust will adhere to the positioner after a period of use, which will affect the back pressure of the nozzle baffle, thereby affecting the output of the positioner. The state of the regulating valve is unstable, resulting in vibration.

4) The sealing is not good. Various fastening nuts and sealing gaskets of the positioner used for a long time are prone to loosening and aging, resulting in air leakage of the positioner, so that the regulating valve cannot be fully opened and closed, and the valve position is unstable, resulting in adjustment oscillation.

5) The feedback lever is faulty. During long-term operation, the tightening nut of the feedback rod gradually loosens or even falls off, which causes the feedback rod to become loose, skewed, collide with the fixing parts, and fall off, which makes the control valve move slowly, fluctuate frequently, and the control valve is limited or even out of control. The limit spring on the feedback board falls off, or the feedback rod comes out of it, resulting in poor contact between the feedback rod and the feedback board, resulting in a lag, resulting in frequent movements of the regulating valve, making it difficult to stabilize the controlled parameters, especially when the regulating valve action requires accurate temperature control have a greater impact.

6) The fixing nut is loose. If the fixing nut of the positioner is not installed firmly, it will be loose, which will cause the positioner to be skewed, which will affect the action of the feedback rod, resulting in the phenomenon of jamming; The tightening screws of various springs in the positioner are loosened under the vibration environment, which changes the preload of the spring and affects the tension and state of the spring; the zero-point range of the positioner is changed, and the positioner is not linear, so that the regulating valve cannot be fully When the switch is fully closed, the action of the regulating valve is not linear.

3.2. Smart Locator

1) The feedback lever is faulty. The tightening nut of the feedback rod is loose or even falls off, causing the feedback rod to be loose, skewed, stuck with the fixed parts, and falling off, making the control valve move slowly, fluctuate frequently, and the control valve is limited or even out of control. If the positioner is not fixed firmly, it will be skewed and loose, which will affect the movement of the feedback rod, resulting in the phenomenon of jamming and limiting the regulating valve. The limit spring on the feedback board falls off, or the feedback rod comes out of it, resulting in poor contact between the feedback rod and the feedback board, resulting in a lag, resulting in frequent movements of the regulating valve.

2) The electronic feedback device is faulty. When the electronic feedback device (or feedback potentiometer) runs repeatedly for a long time, or runs within a certain range for a long time, the linearity of the potentiometer becomes worse or fluctuates, which seriously affects the normal use of the positioner. After running for a period of time, try to check the potential The actuator operates linearly or over the full travel range for a period of time.

3) The control circuit is faulty. Due to the environmental temperature, humidity, sealing and other conditions of the positioner during operation, the electronic components of the control circuit are aged and deteriorated due to excessive temperature, low temperature, excessive humidity or corrosion, etc., which directly lead to the failure of the positioner and lead to production accidents.