Troubleshooting Common Control Valve Issues
Control valves are vital components in industrial systems, facilitating the precise regulation and distribution of flowing media. However, they are susceptible to various issues such as jamming, leakage, oscillation, and inactivity, which can compromise their functionality. In this comprehensive guide, we'll explore common problems encountered with control valves and effective solutions to address them, ensuring optimal performance and longevity.
01 Jamming:
Card blocking frequently occurs in control valves, especially during system initiation and after overhaul. This is caused by welding slag, iron filings, and other impurities clogging the throttle port and guide parts. Jamming disrupts medium circulation and can result from overly tight packing during valve overhaul, increasing friction and causing failure to respond to small signals and overreaction to large ones.
02 Control Valve Leakage:
Control valve leakage includes internal, packing, and spool/seat deformation leaks.
(1) Valve leakage occurs when the valve stem length is inappropriate, causing a gap between the valve spool and seat due to excessive stem length in air-open valves.
(2) Packing leakage occurs due to axial pressure from packing, resulting in uneven contact with the valve stem, leading to potential leakage, especially in high-temperature, high-pressure environments with high permeability fluid mediums.
(3) Spool and seat deformation leakage results from erosion and impact, causing mismatch and gaps over time, leading to shutdown and leakage issues.
03 Control Valve Oscillation:
Insufficient spring stiffness in control valves can lead to oscillation. When the control valve output signal is unstable and experiences sudden changes, resonance occurs between the control valve frequency and the system frequency, causing violent pipeline and base vibrations.
Improper control valve selection, particularly in small openings, can result in significant flow resistance and pressure fluctuations. If these changes surpass the control valve's stiffness, it leads to decreased stability and potential oscillation.
04 Control Valve Inactivity:
(1) Lack of signal or air source due to closed gas source, blockages in air ducts or pressure reducing valve failures.
(2) Air supply without signal, caused by positioner failures due to bellows rupture or converter malfunctions.
(3) Lack of air supply to the positioner due to clogging in filters or pressure reducers.
(4) Air supply to the positioner, but no output due to throttle hole blockages.
(5) Presence of signal, but no action, possibly due to detached valve spools or bent or broken valve stems.
05 Control Valve Operation Instability:
(1) Insufficient compressor capacity leading to unstable gas source pressure from pressure reducing valve malfunctions.
(2) Unstable regulator performance or inappropriate control system time constant resulting in unstable signal pressure.
(3) Excessive frictional resistance on valve stem movement causing blockages.
(4) Wear and tear on dirt-caused ball valve in the positioner amplifier, resulting in output oscillation, particularly during increased air consumption.
06 Regulating Valve Sluggish Action:
(1) Pneumatic diaphragm breakage leading to unilateral valve stem sluggishness.
(2) Weak response performance of the positioner.
(3) Rust or obstructions hindering axial movement.
(4) Poor quality or overly tight packing increasing friction resistance.
Control Valve Failure Solutions:
01 Addressing Control Valve Jamming:
Promptly open and close the valve or clamp the valve stem with a pipe wrench to exert external signal pressure and flare the stem to dislodge the spool. If the jamming persists, increase external pressure and drive power, or seek professional disassembly.
02 Managing Control Valve Leakage:
Adjust the valve stem length to eliminate internal leakage. Chamfer the stuffing box top, add a corrosion-resistant metal protection ring at the bottom, and ensure proper contact surface finishing between the metal ring and packing. Choose flexible graphite packing for high gas sealing, low friction, and durability.
For spool and seat deformation leakage, select high corrosion-resistant materials and replace severely damaged valves.
03 Handling Control Valve Oscillation:
Increase spring stiffness or support to eliminate violent pipeline and base vibrations. Replace the regulating valve if its frequency matches the system frequency.
04 Resolving Control Valve Inactivity:
Adjust the pressure reducing valve to meet gas source pressure requirements. Check and tighten or replace leaky positioner joint box pipelines and joints. Repair enlarged holes or throttle holes in the positioner.
05 Correcting Control Valve Operation Instability:
Inspect and seal signal pipeline, joints, and sealing rings to eliminate leaks. Tighten loose parts and lubricate high-friction areas.
06 Addressing Slow Action of Control Valve:
Check and reassemble friction parts, adjust the packing gland, and replace packing as needed.
01 Jamming:
Card blocking frequently occurs in control valves, especially during system initiation and after overhaul. This is caused by welding slag, iron filings, and other impurities clogging the throttle port and guide parts. Jamming disrupts medium circulation and can result from overly tight packing during valve overhaul, increasing friction and causing failure to respond to small signals and overreaction to large ones.
02 Control Valve Leakage:
Control valve leakage includes internal, packing, and spool/seat deformation leaks.
(1) Valve leakage occurs when the valve stem length is inappropriate, causing a gap between the valve spool and seat due to excessive stem length in air-open valves.
(2) Packing leakage occurs due to axial pressure from packing, resulting in uneven contact with the valve stem, leading to potential leakage, especially in high-temperature, high-pressure environments with high permeability fluid mediums.
(3) Spool and seat deformation leakage results from erosion and impact, causing mismatch and gaps over time, leading to shutdown and leakage issues.
03 Control Valve Oscillation:
Insufficient spring stiffness in control valves can lead to oscillation. When the control valve output signal is unstable and experiences sudden changes, resonance occurs between the control valve frequency and the system frequency, causing violent pipeline and base vibrations.
Improper control valve selection, particularly in small openings, can result in significant flow resistance and pressure fluctuations. If these changes surpass the control valve's stiffness, it leads to decreased stability and potential oscillation.
04 Control Valve Inactivity:
(1) Lack of signal or air source due to closed gas source, blockages in air ducts or pressure reducing valve failures.
(2) Air supply without signal, caused by positioner failures due to bellows rupture or converter malfunctions.
(3) Lack of air supply to the positioner due to clogging in filters or pressure reducers.
(4) Air supply to the positioner, but no output due to throttle hole blockages.
(5) Presence of signal, but no action, possibly due to detached valve spools or bent or broken valve stems.
05 Control Valve Operation Instability:
(1) Insufficient compressor capacity leading to unstable gas source pressure from pressure reducing valve malfunctions.
(2) Unstable regulator performance or inappropriate control system time constant resulting in unstable signal pressure.
(3) Excessive frictional resistance on valve stem movement causing blockages.
(4) Wear and tear on dirt-caused ball valve in the positioner amplifier, resulting in output oscillation, particularly during increased air consumption.
06 Regulating Valve Sluggish Action:
(1) Pneumatic diaphragm breakage leading to unilateral valve stem sluggishness.
(2) Weak response performance of the positioner.
(3) Rust or obstructions hindering axial movement.
(4) Poor quality or overly tight packing increasing friction resistance.
Control Valve Failure Solutions:
01 Addressing Control Valve Jamming:
Promptly open and close the valve or clamp the valve stem with a pipe wrench to exert external signal pressure and flare the stem to dislodge the spool. If the jamming persists, increase external pressure and drive power, or seek professional disassembly.
02 Managing Control Valve Leakage:
Adjust the valve stem length to eliminate internal leakage. Chamfer the stuffing box top, add a corrosion-resistant metal protection ring at the bottom, and ensure proper contact surface finishing between the metal ring and packing. Choose flexible graphite packing for high gas sealing, low friction, and durability.
For spool and seat deformation leakage, select high corrosion-resistant materials and replace severely damaged valves.
03 Handling Control Valve Oscillation:
Increase spring stiffness or support to eliminate violent pipeline and base vibrations. Replace the regulating valve if its frequency matches the system frequency.
04 Resolving Control Valve Inactivity:
Adjust the pressure reducing valve to meet gas source pressure requirements. Check and tighten or replace leaky positioner joint box pipelines and joints. Repair enlarged holes or throttle holes in the positioner.
05 Correcting Control Valve Operation Instability:
Inspect and seal signal pipeline, joints, and sealing rings to eliminate leaks. Tighten loose parts and lubricate high-friction areas.
06 Addressing Slow Action of Control Valve:
Check and reassemble friction parts, adjust the packing gland, and replace packing as needed.
