Pneumatic Control Valves and Electric Control Valves

I. Installation Principles of Pneumatic Control Valves and Electric Control Valves

Installation Principles of Pneumatic Control Valves

1. The installation position of the pneumatic control valve shall be at a certain height from the ground, and sufficient space shall be reserved above and below the valve to facilitate its disassembly, assembly and maintenance. For control valves equipped with pneumatic valve positioners and handwheels, it shall be ensured that operation, observation and adjustment are convenient.
2. The control valve shall be installed on horizontal pipelines and perpendicular to the pipeline at both the upper and lower connections. Generally, support shall be provided under the valve. For special occasions where the control valve needs to be installed horizontally on vertical pipelines, the control valve shall also be supported (except for small-caliber control valves). During installation, additional stress on the control valve shall be avoided.
3. The working environment temperature of the control valve shall be within the range of (-30 ~ +60)℃, and the relative humidity shall not exceed 95%.
4. Straight pipe sections shall be reserved at the front and rear of the control valve, with a length not less than 10 times the pipe diameter (10D), to prevent the short straight pipe section from affecting the flow characteristics.
5. If the caliber of the control valve is inconsistent with that of the process pipeline, a reducer shall be used for connection. When installing small-caliber control valves, threaded connection can be adopted. The fluid direction arrow on the valve body shall be consistent with the actual fluid direction.
6. A bypass pipeline shall be installed for the convenience of switching or manual operation, which allows maintenance of the control valve without stopping the system operation.
7. Before installation, foreign matters in the pipeline, such as dirt and welding slag, shall be completely removed.

Installation Principles of Electric Control Valves

1. The installation position, height and inlet-outlet direction of the valve must comply with the design requirements, and the connection shall be firm and tight.
2. The valve can be connected to pipelines through various end connections, among which the main connection methods include threaded connection, flange connection and welded connection. For flange connection, if the temperature exceeds 350℃, high-temperature resistant bolt materials shall be selected due to creep relaxation of bolts, flanges and gaskets.
3. Before installation, the valve must undergo visual inspection, and its nameplate shall comply with the provisions of the current international standard GB 12220 General Valves – Marking. For valves with a working pressure greater than 1.0 MPa and those used for shut-off on main pipelines, strength and tightness tests shall be carried out, and the valves can only be put into use after passing the tests. Other valves need not be tested separately and can be inspected during the system pressure test.
4. During the strength test, the test pressure shall be 1.5 times the nominal pressure, and the duration shall not be less than 5 minutes. No leakage shall be allowed at the valve body and packing.
5. During the tightness test, the test pressure shall be 0.3 MPa, which shall remain stable during the test duration as specified in Table 2. The test is qualified if there is no leakage at the valve disc sealing surface.
6. Nominal diameter: DN15–500

II. Common Faults of Pneumatic Control Valves and Electric Control Valves

III. Common Faults and Causes of Pneumatic Control Valves

(1) Valve Fails to Actuate

1. No signal and no air supply: ① Air supply not turned on; ② Air pipeline blocked due to icing of water contained in the air supply in winter, or filter and pressure reducing valve blocked and malfunctioning; ③ Compressor failure; ④ Leakage in the main air supply pipeline.
2. Air supply available but no signal: ① Regulator failure; ② Diaphragm leakage of the positioner; ③ Damaged regulator diaphragm.
3. No air supply to the positioner: ① Filter blocked; ② Pressure reducing valve failure; ③ Pipeline leakage or blockage.
4. Air supply available to the positioner but no output: Throttle orifice of the positioner blocked.
5. Signal available but no actuation: ① Valve core detached; ② Valve core stuck to the valve stem or valve seat; ③ Bent or broken valve stem; ④ Valve seat and valve core frozen or blocked by coked deposits and dirt; ⑤ Actuator spring seized due to long-term inactivity.

(2) Unstable Valve Actuation

1. Unstable air supply pressure: ① Insufficient compressor capacity; ② Pressure reducing valve failure.
2. Unstable signal pressure: ① Improper time constant of the control system; ② Unstable output of the regulator.
3. Stable air supply pressure and signal pressure but unstable valve actuation: ① The ball valve of the amplifier in the positioner is worn by dirt and does not seal tightly, resulting in excessive air consumption and output oscillation; ② The nozzle and baffle of the amplifier in the positioner are not parallel, and the baffle cannot cover the nozzle; ③ Leakage in the output pipeline or wires; ④ Insufficient rigidity of the actuator; ⑤ High friction resistance during valve stem movement and jamming at contact parts.

(3) Valve Vibration

1. Valve vibrates at any opening: ① Unstable support; ② Vibration source nearby; ③ Severe wear of the valve core and bushing.
2. Valve vibrates when approaching the full-closed position: ① Oversized valve selected, often used at small openings; ② Medium flow direction of the single-seat valve is opposite to the closing direction.

(4) Sluggish Valve Actuation

1. Sluggish valve stem movement in one direction only: ① Damaged and leaking diaphragm in the pneumatic diaphragm actuator; ② Leakage of the O-ring seal in the actuator.
2. Sluggish valve stem movement in both reciprocating directions: ① Valve body blocked by viscous substances; ② Deteriorated and hardened PTFE packing or dried lubricating oil in the graphite-asbestos packing; ③ Over-tightened packing leading to increased friction resistance; ④ Bent valve stem resulting in high friction resistance; ⑤ Pneumatic control valve without a positioner can also cause sluggishness.

(5) Increased Valve Leakage

1. Excessive leakage when the valve is fully closed: ① Worn valve core causing severe internal leakage; ② Improper valve adjustment leading to incomplete shut-off.
2. Valve fails to reach the full-closed position: ① Excessively high medium pressure difference and insufficient actuator rigidity resulting in incomplete shut-off; ② Foreign matter inside the valve; ③ Sintered bushing.

(6) Reduced Flow Adjustable Range