Introduction

A pneumatic valve that does not close tightly leads to media leakage, energy waste and even safety accidents. This "internal leakage" problem looks simple, but its causes may lie in the actuator, the valve body, the positioner, the mechanical transmission, the control signal or even the installation. This article systematically analyzes the 6 main causes of a pneumatic valve not closing tightly and provides a full-process solution from on-site commissioning to component replacement.

1. The 6 Main Causes of Failure to Close Tightly

1. Insufficient actuating mechanism force

  • Insufficient air pressure: working pressure < 0.4 MPa (standard requires 0.4-0.7 MPa)

  • Spring failure: fatigue deformation of the return spring

  • Cylinder leakage: worn piston sealing ring causes a sharp drop in thrust

2. Failure of the valve body seal

  • Seat damage: hard particles scratch the sealing surface

  • Disc offset: wear of the guide bushing causes a concentricity deviation

  • Aged seals: the service life of a PTFE seat declines under high-temperature operating conditions

3. Positioner calibration deviation

  • Zero drift: the 4mA position deviates from the opening

  • Insufficient stroke: the valve opening does not reach fully open at a 20mA signal

  • Loose feedback lever: deviation between the actual position and the signal

4. Mechanical transmission jamming

  • Bent valve stem: the straightness error needs to be corrected

  • Packing too tight: excessive compression force increases friction

  • Foreign matter jamming: welding slag and coking deposits remain in the valve cavity

5. Abnormal control signal

  • Solenoid valve response delay: excessively long switching time causes lagging action

  • PLC output abnormality: insufficient pulse width of the control signal cannot drive the valve

  • Blocked air circuit: too small an inner diameter of the air pipe causes insufficient flow

6. Deformation caused by improper installation

  • Pipeline stress transfer: flange parallelism deviation

  • Insufficient support: the valve body weight causes slight deformation at the connection

  • Thermal expansion: the metal expansion under high-temperature conditions is not compensated

2. Step-by-Step Adjustment Guide (Butterfly Valve as an Example)

Step 1: Air supply pressure calibration

  • Adjust the pressure reducing valve so that the air pressure is stable at about 0.55 MPa

  • Test the full-stroke actuation time of the actuator (standard ≤3 seconds)

Step 2: Mechanical limit adjustment

  • Loosen the limit screw and manually close the valve to the zero position

  • Rotate the limit stop until it presses the micro switch

  • After tightening the screw, repeat opening and closing 3 times to verify

Step 3: Repair of the valve sealing surface

  • Remove the valve disc and grind the sealing ring with emery

  • The contact width must reach more than 30% of the seat diameter

  • Kerosene penetration test: no dripping for 5 minutes is acceptable

Step 4: Positioner calibration

  • Input a 4mA signal and adjust the zero screw so that the opening returns to zero

  • Input a 20mA signal and adjust the span screw to 100% opening

  • Three-point verification (opening deviation at 25%, 50% and 75% < 1%)

3. Targeted Solutions by Valve Type

Valve type Characteristic of failure to close tightly Key adjustment
Ball valve Internal leakage grows linearly Adjust the seat preload spring
Gate valve Leakage is obvious at the bottom Correct the wedge angle of the gate
Globe valve The disc and the stem are not concentric Replace the guide bushing
Butterfly valve Edge leakage Increase the sealing interference

4. Preventive Maintenance and Selection Advice

Daily maintenance

  • Monthly: drain the FRL unit, inject sealing grease into the valve stem seal

  • Quarterly: positioner repeatability test, actuating mechanism sealing test

Key selection parameters

  • Sealing class: prefer Class VI (low leakage rate)

  • Temperature resistance: select suitable seal material according to the operating conditions

  • Explosion-proof certification: ATEX/NEPSI certification is required in hazardous areas

5. Quick Answers to Common Questions

Q1: The valve can be closed manually, but the pneumatic actuator cannot close it tightly?

→ In 90% of cases the actuator thrust is insufficient; check whether the cylinder inner diameter matches (thrust F=0.785×D²×P).

Q2: Bubbles still appear after the valve is closed?

→ Use the soap solution bubble test; if the number of bubbles per minute exceeds the standard, repair is required.

Q3: What if internal leakage persists after adjustment?

→ It may be a casting defect in the valve body; a sealing test at 1.5 times the pressure is needed to determine this.

Conclusion

A pneumatic valve that does not close tightly is often the result of several factors - mechanical, pneumatic and electrical - acting together. During troubleshooting it is recommended to check the power first, then the seal, and finally calibrate the positioner, and to use valve position feedback and torque monitoring for diagnosis. For valves with continuous internal leakage, the leakage class should be determined according to the relevant standards to avoid running with a defect and causing a process accident. For technical support in troubleshooting and repairing internal leakage of pneumatic valves, please feel free to contact our professional service team.

Zhejiang Huaershi Auto-control Instrument Valve Co., Ltd.

Since 1989 · 30+ years focused on pneumatic actuators and pneumatic valves

Website: www.hesvalve.com