Page 221 - Root Cause Failure Analysis
P. 221
Control Valves 209
Motor
Actuator
Figure 17-8 High-torque electric motors can be used as actuators (Higgins and Mobley
1995).
Repeatability Repeatability, perhaps, is the most important performance criteria of
a process-control valve. This is especially true in applications where precise flow or
pressure control is needed for optimum performance of the process system.
New process-control valves generally provide the repeatability required. However,
proper maintenance and periodic calibration of the valves and their actuators are
required to ensure long-term performance. This is especially true for valves that use
mechanical linkages as part of the actuator assembly.
Installation
Process-control valves cannot tolerate solids, especially abrasives, in the gas or liq-
uid stream. In applications where high concentrations of particulates are present,
valves tend to experience chronic leakage or seal problems because the particulate
matter prevents the ball, disk, or gate from completely closing against the stationary
surface.
