Azbil VDC - Chapter 8: Trouble Shooting

VDC, VAC, VST, VAA - Diaphragm Control Valve 8-1
Chapter 8 : Trouble shooting
Symptom Cause / Verication / remedial measures
Valve Operation unstable
Hunting at fully closed
position
Supply air pressure
uctuates
Supply air pressure
uctuates
Signal pressure uctuates
Hunting continues even on
constant signal
Valve capacity is too large (Reduce Cv)
For single valve, direction of ow is wrong
Check if other line is using excessively large volume of air.
(Pipe capacity, restriction, air supply source capacity)
Check if other line is using excessively large volume of air.
(Pipe capacity, restriction, air supply source capacity)
Check for abnormal controller output
Hunting by positioner itself (Check, repair or replace
positioner)
Impact by process uid pressure (Insu󰀩cient torque of
actuator. Replace actuator with larger size one)
Valve vibrates
Vibration at any opening
(Abnormal noise noticed)
Intermittent vibration
(Sound noticed)
Check and see if the pipe on which the valve mounted is
vibrating (Enhance support)
Search for other source of vibration
Erosion of plug and guide (Replace parts)
Change in Process uid condition (Modify orice or Cv
coe󰀩cient)
Change plug type (Change of characteristics)
Valve operation sluggish
Valve fails to operate
Leakage on pneumatic pipe
Air leakage from actuator
Foreign matter adhering on plug’s guide
Hardening of gland packing (Increased hysteresis)
Faulty positioner (Try on other line’s positioner)