POWER May 2023 - 37

OPERATIONS & MAINTENANCE
to avoid problems related to quenching
and cracking.
There are also threats to trim components,
such as seat surface erosion.
One common expression of such erocost.
Trim components that need to be
replaced due to heavy wear, such as gear
toothing, can be quite costly, especially
if their delivery needs to be expedited.
The worst-case scenario for a turbine byThe
worst-case scenario for a turbine
bypass valve is metal fatigue developing
in the valve body or diffuser, with
extreme repair procedures, or even a
complete valve replacement, required.
3. A routine turbine bypass valve health
check and full inspection is strongly advised.
Metal fatigue can develop in various welds
located on the valve itself, and in the inlet,
outlet, and water supply piping (circled areas).
Courtesy: Emerson
ity is that virtually every turbine bypass
valve will ultimately fail in some way.
Unfortunately, many of these valves are
installed in difficult-to-access locations,
are typically welded in place, and tend to
be heavily insulated. As a result, they are
often ignored until problems start to surface.
Steam leakage through the valve is
usually the first symptom noticed, but
much more significant and potentially
dangerous damage can occur.
The extreme pressure drops invariably
generate high noise and high vibration.
Over time these vibrations, coupled with
frequent temperature changes, fatigue
the metal in the valve, water connections,
and the piping itself (Figure 2).
Such metal fatigue can result in catastrophic
failure under pressure.
To avoid this problem, every turbine
bypass valve and its associated piping
should be routinely inspected. Some
type of non-destructive examination
should be periodically employed to detect
metal fatigue problems that may be
developing but are not yet visually apparent.
If a plant lacks the knowledge or
equipment to perform these inspections,
the valve vendor or authorized representative
may be utilized to perform turbine
bypass health check services (Figure 3).
Fabrication welds on the body and
water manifold, customer connection
welds, diffuser welds, and surrounding
piping can be inspected to identify any
developing problems before the equipment
is compromised. Water injection
nozzles and desuperheaters must be
appropriately inspected and maintained
May 2023 | POWER
sion, specifically on the plug, is often referred
to as " gear toothing. " This occurs
more commonly in flow down valves,
where the steam accelerates through
the cage holes and then strikes the trim/
seat area directly, causing excessive
wear and reducing service life. Over
time, this high-velocity steam, which
can contain water during startup conditions
and magnetite during any condition,
will erode and damage the valve
seat and trim (Figure 4).
Common maintenance items like soft
goods and spray nozzles can usually be
replaced relatively easily and at minimal
pass valve is metal fatigue developing in
the valve body or diffuser, with extreme
repair procedures, or even a complete
valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves currently
in service were installed during
the heyday of combined cycle plant
construction from 1998 to 2004. Most
of those valves and piping systems are
showing their age and often have significant
signs of metal fatigue. Others have
plenty of life left in them but could benefit
from upgrades in technology. Tech4.
Flow down valve body designs are prone to trim damage as the wet, erosive steam passes
through the cage and impacts the seat. Courtesy: Emerson
www.powermag.com
37
http://www.powermag.com
POWER May 2023

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