1、 Field calibration of regulating valve (with valve positioner)

The regulating valve is an actuator in the regulating system. The action of the valve is controlled by the regulator. At the same time, the action of the valve also affects the change of process parameters in a straight line. Therefore, except for the regulating valve installed on the site, which can be cut out of the state through the auxiliary line for calibration, the rest can only be calibrated under the shutdown state. In order to improve the regulating performance, the regulating valve is often equipped with a valve positioner. In general, the regulating valve is calibrated together with the valve positioner.

1. Calibration of regulating valve with pneumatic valve positioner (Wenzhou Niwei Valve Co., Ltd.)

(1) Wiring

Connect the pipeline according to Figure 8-1. Connect the air source to adjust the set-point device so that its output (digital pressure gauge 1) is 20kPa, and observe whether the valve stroke is at the starting position (limited to the large stroke position). Adjust the output of the set-point device to 100kPa and observe whether the valve stroke is large (starting position). The digital pressure gauge 2 in the figure is used to monitor the output of the positioner.

(2) Step

A. Select five points of input signal pressure 20kPa, 40kPa, 60kPa, 80kPa and 100kPa for calibration.

B. The corresponding valve position indication shall be 0, 25%, 50%, 75% and 100.

C. Calibrate in both positive and negative directions. If the valve position indicator multiplies the scale percentage by the full stroke (mm), the number of millimeters of the stroke can be obtained.

2. Adjustment of regulating valve with electric valve positioner

(1) Wiring

Connect the pipeline according to Figure 8-2. The digital pressure gauge in the figure is used to monitor the output of the positioner.

First send 4mA input signal and observe whether the digital pressure gauge is 20 kPa and whether the valve stroke is at the starting position (stroke position). Then adjust the input signal to 20mA, and observe whether the digital pressure gauge is 100 kPa, and whether the valve stroke is large (starting position).

(2) Step

A. Select the input signal pressure of 4 mA, 8 mA, 12 mA, 16 mA and 20 mA for calibration.

B. The corresponding valve position indication shall be 0, 25%, 50%, 75% and 100.

C. Calibrate in both positive and negative directions. If the valve position indicator multiplies the scale percentage by the full stroke (mm), the number of millimeters of the stroke can be obtained.

During the calibration of the control valve, because of the on-site calibration, the valve has been installed in the use position, so some items such as air tightness test cannot be carried out. During the linkage calibration of the positioner and the regulating valve, if the positioner is found to work abnormally, the positioner shall be removed for separate calibration.

2、 Installation of pneumatic control valve

1. Installation of regulating valve

The correct installation of the regulating valve is as important as the correct selection, and the installation is the top priority related to the operation performance, safety degree and cost.

The regulating valve must be installed in strict accordance with the process requirements and the control system requirements. Once the valve fails, it will cause serious accidents and personal injuries such as fire, explosion, poisoning, etc., and the loss cannot be calculated.

Before installing the regulating valve, the quality of the regulating valve should be checked, starting from the appearance inspection, and then the parts and accessories should be checked for loss and conformity certificate during transportation. If necessary, the static characteristics should be checked or special inspection should be carried out.

2. Installation principle

(1) The installation position of the pneumatic control valve shall be at a certain height from the ground, and a certain space shall be reserved at the top and bottom of the valve to facilitate the disassembly and repair of the valve. For the regulating valve equipped with pneumatic valve positioner and handwheel, it must be convenient to operate, observe and adjust.

(2) The regulating valve shall be installed on the horizontal pipeline, and the upper and lower parts shall be vertical to the pipeline. Generally, it shall be supported under the valve to ensure stability and reliability. For special occasions, when the regulating valve needs to be installed horizontally on a vertical pipe, the regulating valve should also be supported (except for small diameter regulating valve). During installation, it is necessary to avoid bringing additional stress to the control valve, such as the pipe is not concentric with the valve or the flange is not parallel.

(4) The working ambient temperature of the valve shall be - 30~+60 ℃, and the relative humidity shall not be greater than 95%. Because rubber parts such as diaphragm and sealing ring of regulating valve are easy to harden and become brittle at low temperature and aging at high temperature, attention should be paid to the installation position during installation, leaving the heating furnace and high temperature pipeline.

(5) There shall be straight pipe sections at the front and back of the regulating valve, and the length of the straight pipe sections at the front and back of the valve shall not be less than 10 times the pipe diameter (10D), so as to prevent the flow characteristics from being affected by the short straight pipe sections of the valve.

(6) If the diameter of the regulating valve is different from that of the process pipe, the reducing pipe shall be used for connection. When installing the small diameter regulating valve, screw connection can be used. The flow direction arrow on the valve body shall be consistent with the flow direction.

(7) Bypass pipe shall be set. The purpose is to facilitate switching or manual operation. The regulating valve can be maintained without stopping.

(8) Before the installation of the regulating valve, the foreign matters in the pipeline, such as dirt, welding slag, etc. shall be completely removed. After installation, use normal temperature water for test run. During test run, fully open the valve or open the bypass valve. During the test run, attention shall be paid to the tightness of the joint between the valve body and the pipeline.

3. Installation form

The installation of regulating valve usually includes one regulating valve group, i.e. upstream valve, bypass valve, downstream valve and regulating valve. Figure 8-3 shows six common combinations of control valves, and describes their characteristics.

It should be noted that the block valve (upstream valve, downstream valve) and bypass valve should be installed close to the tee to reduce the dead angle.

4. Selection of installation position of regulating valve

Generally, the regulating valve requires vertical installation. When the vertical installation cannot be met, the regulating valve fixed with four bolts on the flange can have four positions: 45 ° upward tilt, 45 ° downward tilt, horizontal installation and downward vertical installation. The regulating valve fixed with 8 bolts on the flange can have 9 installation positions, 22.5 ° apart.

In these installation positions, the ideal is to install vertically upwards, which should be preferred; The upward tilt position is the second, 22.5 °, 45 ° and 67.5 ° in turn; Vertical installation downward is the second position; The inappropriate position is horizontal installation, which inclines 67.5 ° downward from the near-horizontal installation, and is generally not accepted. Figure 8-4 shows the comparison diagram of installation positions.

3、 Maintenance of regulating valve

Due to the harsh operating environment of the regulating valve, certain faults are unavoidable in use. Therefore, it is necessary to master the maintenance and repair of the regulating valve.

The main contents of daily maintenance of pneumatic control valve shall include the following:

(1) Inner wall of valve body. Check its pressure and corrosion resistance.

(2) Valve seat. Due to medium infiltration, the inner surface of the thread used to fix the valve seat is easy to be corroded, which causes the valve seat to loosen. Attention should be paid during inspection. For high-pressure occasions, check whether the sealing surface of the valve seat is washed out.

(3) Valve element. Carefully check whether all parts of the valve core are corroded and worn, especially under the condition of high pressure difference, and pay attention to it. In addition, check whether the valve rod has similar phenomenon or loose connection with the valve core.

(4) Diaphragm, O-ring and other gaskets. Check whether it is aged or cracked.

(5) Sealing packing. Check whether the PTFE packing and sealing lubricating grease are aged and dry, and whether the mating surface is damaged.