Answer / carl_ellis

I seen 10 to 20 causes for each of these categories, but
I've only listed one as an example.

Reasons for valve not closing or opening:

intentional or unintentional
- lack of energy - air/electric/hydraulic (Fuse blown)
- insufficient energy - (supply air regulator turned down)
- control system issue (control signal interlocked)
- positioner or I/P issue (locked in manual)
- actuator crippled (punctured diaphragm)
- positioner-to-actuator linkage/plumbing issue (tubing failed)
- actuator-to-valve linkage issue (sheared roll pin)
- valve trim damage
- foreign object blocking movement (how does all that junk
get in the piping in the first place?)
- change in design process conditions (actuator can't close
against increased line pressure)

Not all failures to actuate/move are faults - a valve could
be intentionally locked in place for safety reasons - access
to the piping/tank.

Tell 'em Carl sent you.

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