Introduction to the calibration and maintenance ex

2022-08-22
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Introduction to the calibration and maintenance experience of actuators in the petrochemical industry

generally, a regulation system is mainly composed of primary instrument, DCS system and actuator. Among them, the 2015 chronology of primary instrument and DCS system are becoming more and more mature, and the maintenance amount is becoming smaller and smaller in actual production, while the actuator is more and more valued by people, and has become the focus of maintenance in actual production

in the petroleum and chemical industry, actuators are mostly pneumatic control valves (dampers), which are important instrument equipment in petroleum and chemical plants. Therefore, the operation of the control valves is directly related to the operation of DCS control system and the "safe, stable, long-term and excellent" operation of process devices. The regulating valve is an important part of the regulating system. Once the regulating valve fails, the whole regulating system will fail. Therefore, in practice, the calibration and maintenance of regulating valves are very important. The following describes several types of valves:

1, pneumatic diaphragm regulating valve

A. pneumatic diaphragm regulating valve with positioner

when calibrating such valves, first determine the air source pressure, and then calibrate according to the procedures to ensure that its linearity and sensitivity meet the specified requirements. Special attention is paid to that when the valve positioner is installed, when the valve is at 50% opening, the feedback arm of the positioner must be in the horizontal position

b. pneumatic diaphragm regulating valve without positioner

when calibrating the pneumatic diaphragm regulating valve without positioner, pay special attention to the signal range of the regulating valve, mark the spring range, that is, the signal range, on the nameplate, and then calibrate it according to the signal range

2. Cylinder type regulating valve

cylinder type regulating valve has the advantages of high output power and sensitive action During calibration, first determine the air supply pressure, and then calibrate according to the regulations. If you find poor linearity and jumping phenomenon, please conduct air tightness test on the cylinder to check whether the cylinder, piston, joint, etc. have air leakage. If the fault has not been eliminated, it may be caused by the too tight packing of the valve, and the packing can be handled normally

3. Damper actuator

damper actuator is a special valve. When verifying this kind of valve, first determine the air source pressure, and then verify according to the regulations. This kind of valve comes with a positioner, but the linearity is not very good, as long as it meets the process requirements. During patrol inspection, pay attention to whether each joint leaks air, whether the air source pressure is normal, and regularly add lubricating oil to the mechanical transmission part

4. Governor

the governor of the unit is a special regulating device, which is divided into two parts: actuator and hydraulic motor, and the instrument is responsible for the actuator. During the verification, first determine the air source pressure and air cushion pressure, and then verify according to the procedures to meet the process requirements. During the inspection, pay special attention to whether there is leakage in the gas pipeline, and deal with it in time once it is found

5. Diaphragm valve

diaphragm valve is a two position valve. During calibration, first determine the air supply pressure, and then check whether the action is flexible and in place. If there is any abnormality, please check whether the diaphragm is broken and whether the packing is normal

6. Electromagnetic shut-off valve

electromagnetic shut-off valve includes unit shutdown solenoid valve and natural gas shut-off valve of fuel system. This kind of valve is of two position type. During calibration, special attention should be paid to whether the power supply voltage is normal, whether the action is flexible and in place, and whether there is leakage

7. Change of gas on valve and gas off valve

gas on valve and gas off valve are the inherent attributes of the valve, which have been determined when the valve is made. Only changing the actuator can change the gas on and gas off; If it is a cylinder type regulating valve, you can change the position of the upper and lower cylinder signals. If you change the positive and negative effects of the positioner or regulator, you cannot change the air on and air off

8. Vent valve and anti surge valve

vent valve and anti surge valve are closed when the process is normal. When checking these valves, special attention should be paid to the force of process medium pressure on the valve core. The starting value of the valve is calculated. The starting value of the gas closing valve is not necessarily 100KPA, but may be 95kpa or 90kpa, and the gas opening valve is not necessarily 20KPa, but may also be 40kpa or 50kPa, Its purpose is to ensure that the valve closes reliably and operates well when the process is normal. The method of determining the starting value should be scientific calculation, but it is difficult in practice. It is often based on experience to determine a more conservative and reasonable starting value, and it is often set as 25 ~ 35kpa for the gas valve; 85 ~ 95kpa is set as the empirical starting value for the air shutoff valve

example: a pneumatic high-pressure regulating valve with positioner, input 20 ~ 100KPA, output innovation in industry, technology and products 40 ~ 200KPa, pressure difference before and after the valve is 30000kpa, valve core diameter 8mm, valve stroke 20mm, membrane head effective area 280cm2, valve friction 30kgf. Check this valve under normal pressure, and ask what degree it can work normally after it is put into operation

solution: (1) calibrate the travel. That is, input 20 ~ 100KPA, make the output 40 ~ 200KPa, and the valve has a full stroke of 20mm

(2) verify the starting pressure (flow open)

po= (△ p × π r2+f friction +fe)/ae

= (300 × π(0.4 × 0.4)+30+280 × 0.05)/280

=70kpa

in the formula, Fe is the pressing force of the valve seat. Generally, take fe=0.05ae

to calculate the starting value, and then adjust it according to the following method:

(1) input signal 20 ~ 100KPA, adjust the zero point of the positioner, so that the output is zero (40kpa)

(2) compress the spring with high added value, and the input slowly increases from 20KPa until the output is 70kpa, and the valve starts to start

generally, petroleum and chemical plants need to be overhauled annually after years of operation. The regulating valve is an important instrument equipment in petroleum and chemical plants, so it is the focus of maintenance. After maintenance, it is necessary to ensure the purpose of "safe, stable and long-term excellent" operation of the plant. According to experience, in addition to routine maintenance requirements, the overhaul of regulating valve equipment should pay attention to the following points in practice:

· maintenance personnel must carry out relevant safety education before entering the maintenance site After entering the maintenance site, you must wear labor protection clothes and labor protection appliances in strict accordance with the requirements

· please indicate the position number of the regulating valve for all replacement parts in the maintenance process, and put them into special sealed plastic bags for safekeeping

· the regulating valve shall be completely disassembled, cleaned and overhauled

· for regulating valves with high pressure difference and working in corrosive media, Dekkers said that Bayer has affirmed the following strategic directions of the group:

· pressure resistance and corrosion resistance of the inner wall of the valve body

· whether the valve seat is corroded or scoured

· whether the valve core is corroded, washed, worn, and whether the connecting parts are loose

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