ABB 800xA Operation page 108

Final Control Element Diagnostics
corresponding indices dealing with valve movement. An important prerequisite is
the handling of noise, which is not related to valve movement.
FCE Leakage
Gasket leaks can occur when valves are removed or replaced. These leaks are best
detected by visual inspection. If the leaks are severe enough, they can be detected
through a noticeable loss of pressure or flow in the valve system. A systemic
decrease in packing friction, manifested by decreased stroke time, is an indicator
that the packing is wearing out.
Leaks in the air system such as loose fittings or dry-rotted air lines also might occur.
This scenario can be simulated by diverting part of the actuator air supply through a
needle valve controlled vent. These leaks manifest themselves in the profile by the
drifting of the valve stem from the set point. In addition to using the profile to detect
the failure, plants can use positioners that are capable of performing a leak test.
FCE Size
Valves are usually designed to operate outside the extreme levels of open or closed
positions. If a valve operates within these ranges for a longer period of time, it can
be an indication of a wrongly dimensioned valve, which can result in a nonoptimal
loop performance. The diagnosis is based on the estimation of time where the valve
operates in the extreme positions close to 0 or 100 percent. This value is delivered
by the valve dimension index.
FCE Stiction/Backlash
Valve stiction is present in many control loops that involve control valves. The
diagnosis of valve stiction is primarily related to clear oscillations in the control
loop. Typically, in order to identify valve stiction without oscillations present, set
point changes must be performed manually. Valve stiction diagnosis involves quite a
few indices, among them are automatic mode, nonlinearity analysis, and stiction
indices. This is necessary since stiction is based on several preconditions. It also
helps avoid false alarms as much as possible.
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Section 8 Control Loop Asset Monitor (CLAM)
3BUA000150-600