On-site Calibration, Installation, and Commissioning of Pneumatic Control Valves (with Valve Positioners)

In a control system, control valves serve as actuators. The valve's movement is controlled by the regulator, and at the same time, it directly affects changes in process parameters. Therefore, except for control valves equipped with bypass lines on-site, which can be calibrated in a bypassed state, all other valves can only be calibrated when the system is shut down. To enhance control performance, control valves are often equipped with valve positioners, and in general, they are calibrated together with the valve positioners.

The correct installation of a control valve is as important as its proper selection. The quality of installation is of utmost importance as it relates to operational performance, safety, and cost.

Control valves must be installed strictly in accordance with process requirements and control system requirements. Once a valve malfunctions, it can lead to serious accidents such as fires, explosions, poisoning, and personal injuries, resulting in incalculable losses.

Before installing a control valve, its quality should be inspected, starting with a visual check. Then, check for any missing parts or accessories during transportation and verify the presence of a certificate of conformity. If necessary, conduct static characteristic checks or special inspections.

Usually, a control valve installation involves a control valve group, consisting of an upstream valve, a bypass valve, a downstream valve, and the control valve. Figure 8-3 shows six common combination forms of control valves and describes their respective characteristics.

It should be noted that the installation of shut-off valves (upstream valve and downstream valve) and bypass valves should be close to the tee to reduce dead spaces.

Generally, control valves are required to be installed vertically. When vertical installation is not possible, a control valve with a flange fixed by four bolts can have four installation positions: tilted upward by 45°, tilted downward by 45°, horizontally installed, and vertically installed downward. A control valve with a flange fixed by eight bolts can have nine installation positions, each separated by 22.5°.

Among these installation positions, vertical upward installation is the most ideal and should be given priority. The next best positions are those tilted upward, followed by positions at 22.5°, 45°, and 67.5°. Vertical downward installation is the next option. The least suitable position is horizontal installation, and positions tilted downward by 67.5° close to horizontal are generally not adopted. Figure 8-4 provides a comparative schematic of installation positions.

Due to the harsh operating environment, control valves are prone to certain failures during use. Therefore, it is essential to have a good understanding of control valve maintenance and repair.

The main contents of daily maintenance for pneumatic control valves should include the following:

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

- (2) Valve seat: Pay attention to the fact that the inner surface of the threads used to fix the valve seat is prone to corrosion due to medium infiltration, which may loosen the valve seat. For high-pressure applications, also check whether the valve seat sealing surface has been eroded.

- (3) Valve core: Carefully check whether various parts of the valve core have been corroded or worn. In high-pressure differential situations, valve core wear is more severe and should be given special attention. Also, check whether the valve stem has similar phenomena or is loosely connected to the valve core.

- (4) Diaphragms, O-rings, and other sealing gaskets: Check for aging and cracking.

- (5) Sealing packing: Check whether the PTFE packing and sealing lubricating grease have aged and dried up, and whether the mating surfaces have been damaged