A typical control system mainly consists of three parts: primary instruments, the DCS system, and actuators. At present, primary instruments and DCS systems are increasingly mature, with less maintenance required in actual production. Actuators, however, have attracted growing attention and become the focus of maintenance in field operation.

In the petroleum and chemical industries, most actuators are pneumatic control valves (dampers), which are important instrument equipment in petrochemical units. Therefore, the performance of control valves directly affects the operation of the DCS control system and the safe, stable, long-term, and optimal operation of process units. Control valves are a critical link in the control system; once a control valve fails, the entire control system malfunctions. For this reason, calibration and maintenance of control valves are extremely important in practice. The following describes several valve types:

1. Pneumatic Diaphragm Control Valve

a. Pneumatic Diaphragm Control Valve with Positioner

When calibrating this type of valve, first confirm the supply air pressure, then calibrate in accordance with procedures to ensure its linearity and sensitivity meet specified requirements. Special attention must be paid to valve positioner installation: when the valve is at 50% opening, the feedback lever of the positioner must be in the horizontal position.

b. Pneumatic Diaphragm Control Valve without Positioner

When calibrating pneumatic diaphragm control valves without a positioner, special attention must be paid to the signal range of the control valve, which is marked on the nameplate as the spring range. Calibration shall be performed according to this signal range.

2. Cylinder-Type Control Valve

Cylinder-type control valves feature high output power and sensitive action. During calibration, first confirm the supply air pressure, then calibrate per procedures.If poor linearity or jumping is observed, perform an air tightness test on the cylinder to check for leaks in the cylinder, piston, fittings, etc. If the fault persists, it may be caused by excessively tight valve packing; adjusting the packing will restore normal operation.

3. Damper Actuator

Damper actuators are special valves. During calibration, first confirm the supply air pressure, then calibrate per procedures. Although such valves are equipped with positioners, their linearity is generally not ideal, and they only need to meet process requirements.During routine inspection, check for air leaks at fittings and proper supply air pressure. Lubricate mechanical transmission parts regularly.

4. Governor

Unit governors are special regulating devices consisting of an actuator and a hydraulic motor. The instrument department is responsible for the actuator section.During calibration, first confirm the supply air pressure and air cushion pressure, then calibrate per procedures to meet process requirements. During inspection, pay close attention to air line leaks and handle them promptly if found.

5. Diaphragm Valve

Diaphragm valves are two-position valves. During calibration, first confirm the supply air pressure, then check whether the action is flexible and in-position. If abnormal, inspect whether the diaphragm is broken and whether the packing is in good condition.

6. Solenoid Isolation Valve

Solenoid isolation valves include unit shutdown solenoid valves and natural gas isolation valves in fuel systems. These valves are two-position. During calibration, pay special attention to normal supply voltage, flexible and in-position action, and absence of leakage.

7. Change of Air-Open and Air-Close Valves

Air-open and air-close modes are inherent characteristics of valves, determined during manufacturing. They can only be changed by modifying the actuator.For cylinder-type control valves, the mode can be changed by switching the signal ports of the upper and lower cylinders.Changing the action (direct/reverse) of the positioner or controller cannot change the air-open or air-close mode.

8. Vent Valves and Anti‑Surge Valves

Vent valves and anti‑surge valves are normally closed during normal process operation. During calibration, special attention must be paid to the force exerted by process medium pressure on the valve plug.The valve actuation pressure is determined by calculation. For air‑close valves, the actuation pressure is not necessarily 100 kPa, but may be 95 kPa or 90 kPa. For air‑open valves, it is not necessarily 20 kPa, but may be 40 kPa or 50 kPa. The purpose is to ensure reliable closing and proper operation under normal process conditions.

Although the actuation pressure should be determined by scientific calculation, this is difficult in practice. A conservative and reasonable value is usually set based on experience:

Air‑open valves: actuation pressure set at 25–35 kPa

Air‑close valves: actuation pressure set at 85–95 kPa