Page 258 - Intro Predictive Maintenance

Process Parameters 249

10.9 STEAM TRAPS
Most of the failure modes that affect steam traps can be attributed to variations in
operating parameters or improper maintenance. Table 10–16 lists the more common
causes of steam trap failures.

Operation outside the trap’s design envelope results in loss of efficiency and may result
in premature failure. In many cases, changes in the condensate load, steam pressure
or temperature, and other related parameters are the root-cause of poor performance
or reliability problems. Careful attention should be given to the actual versus design
system parameters. Such deviations are often the root-causes of problems under
investigation.

Poor maintenance practices or the lack of a regular inspection program may be the
primary source of steam trap problems. It is important for steam traps to be routinely
inspected and repaired to ensure proper operation.

10.10 INVERTERS
Table 10–17 lists the common symptoms and causes of inverter problems. Most of
these problems can be attributed to improper selection for a particular application.
Others are caused by improper operation. When evaluating inverter problems, careful
attention should be given to recommendations found in the vendor’s operations and
maintenance manual. These recommendations are often extremely helpful in isolating
the true root-cause of a problem.

10.11 CONTROL VALVES
Although there are limited common control valve failure modes, the dominant prob-
lems are usually related to leakage, speed of operation, or complete valve failure. Table
10–18 lists the more common causes of these failures.

Special attention should be given to the valve actuator when conducting a root-
cause failure analysis. Many of the problems associated with both process and
fluid-power control valves are really actuator problems. In particular, remotely con-
trolled valves that use pneumatic, hydraulic, or electrical actuators are subject to
actuator failure. In many cases, these failures are the reason a valve fails to properly
open, close, or seal. Even with manually controlled valves, the true root-cause can
be traced to an actuator problem. For example, when a manually operated process-
control valve is jammed open or closed, it may cause failure of the valve mechanism.
This overtorquing of the valve’s sealing device may cause damage or failure of the
seal, or it may freeze the valve stem. Either of these failure modes results in total valve
failure.