Hydrocarbon Processing - November 2022 - 40

Maintenance and Reliability
Due to concerns of production interruptions
from downtime, many steam
turbine areas can look like a fog zone,
which is the result of multiple open steam
bleeders as an attempt to prevent damage
or imbalance. Even in such instances, turbines
can experience incidents because
the systems are either improperly designed
or inadequately maintained.
One professional method to mitigate
FIG. 3. Dry steam cannot be distributed in a steam system. Steam is either superheated or wet.
risk is to follow the principles of API RP
580/581.1,2 FIG. 2 shows a typical risk
mitigation matrix. Risk is the product
of the probability of a failure event multiplied
by the consequence of the event
(typically in monetary terms). While the
consequence of an event usually cannot
be lessened without a change in design,
multiple actions can be taken to reduce
the probability of failure. This article provides
focus on some of the possible mitigation
designs that can help reduce the
probability of failure relative to the steam
system, water entrainment in steam, and
condensate removal.
Notice the blue arrows in the matrix
FIG. 4. If not superheated, steam contains
substantial moisture that should be
disentrained and drained.
shown in FIG. 2 representing the reduction
of risk deeper into an acceptance
zone for two assets (such as turbines). It
is recommended and possible to incorporate
risk mitigation practices for all
turbines in a plant, as well as other critical
pieces of equipment, which lowers the
combined risk of unwanted events.
FIG. 6. A collecting leg must be adequately
sized for high-velocity condensate to
drain into it.
FIG. 5. All steam systems can have
condensate flowing along the bottom
of the pipe, and moisture is expected in
flowing wet steam supply.
rating best practice design recommendations
to help reduce ejector nozzle/throat
erosion and precipitate buildup. These
actions can help optimize and sustain
benchmark vacuum pressures and surface
condenser operation for longer periods.
Properly draining pedestal-mounted
vacuum steam turbines can be especially
challenging, as pumping (rather than
trapping) systems may be required to
remove condensate from the sub-atmo40
NOVEMBER 2022 | HydrocarbonProcessing.com
spheric turbine case and exhaust piping.
If drainage is inadequate, significant
damage to internal turbine components,
as well as a reduction in performance of
downstream surface condensers, may occur.
Each of these elements are reviewed
in detail in the following sections.
Reducing risk. Steam turbines, even
small ones, are sophisticated pieces of
equipment (FIG. 1). It is common that
turbines are the unnoticed, unsung heroes
of the plant, functioning properly
for extremely long periods of time-in a
sense, forgotten until there is damage that
can result in significant maintenance cost
and service or production interruptions.
Steam quality. Steam that drives the
turbines can be either superheated, near
saturation or wet.
It
is important
to
have steam without wetness as the drive
source, and this is often the reason superheated
steam is often selected for the design
(FIG. 3).
There is a common misconception
that plant steam is either superheated or
saturated, but this is inaccurate because
dry saturated steam cannot be sustained
in a plant's steam system.3
significant wetness
has
Boiler steam
if not passed
through a superheater, and usually much
more wetness if generated from a waste
heat boiler (FIG. 4). Steam quality without
wetness is so important to turbine reliability
that systems that are not supplied
with superheated steam are recommended
to have a best practice steam trapping
design just before the turbine entry point,
in addition to a steam separator and drain
combination between that trapping location
and the turbine entry point.

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Hydrocarbon Processing - November 2022

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