POWER January 2021 - 29

OPERATIONS & MAINTENANCE
The Steam Cycle
Mechanical Relationship to
Chemistry and Its Influence on
Heat Rate and Reliability
It is well-known that a conventional boiler or heat recovery steam generator (HRSG)
and related steam turbine have innumerable pieces all working in conjunction to
produce the end product of electric power. In today's competitive power market, every
aspect of the steam generation and turbine equipment needs to be monitored for
proper operation. Several examples dealing with steam cycle chemistry abnormalities
that are rooted in mechanical issues, and mechanical issues that have a superficial
appearance to a chemistry-related problem, are presented in this article.
Kurt Bayburt
A
s the number of facilities having a
dedicated onsite chemist are becoming
fewer with each passing
year, it becomes increasingly important
for power plant operations, maintenance,
and engineering groups to understand
the cause-and-effect relationship between
water chemistry and the macrofunctionality
of the boiler, turbine, and
balance-of-plant equipment. The payoff
to this understanding is detecting seemingly
innocuous events that can have a
significant effect on heat rate or maintenance
costs, ultimately affecting the cost
of generation.
Pump Mechanical Seals
The Electric Power Research Institute
(EPRI)-recommended maximum condensate
dissolved oxygen (DO) for bimetallic
and all-ferrous systems are 10 ppb and
20 ppb, respectively. If the condensate
system is air-tight, these values should
be easily achieved.
Often-overlooked sources of air inleakage
that contribute to elevated DO
are the mechanical seals of condensate
pumps and heater drain pumps (if so
equipped). A mechanical seal consists
of two opposing mating surfaces (the rotating
face, secured to the pump shaft;
and the stationary face, secured to the
seal cover flange) that are precisionmachined.
Under normal operating conditions,
the stuffing box in which the
seal resides is under pump discharge
conditions. The pressurized condensate
forces itself between the rotating and
January 2021 | POWER
stationary faces, which, due to heat of
friction, causes it to evaporate and create
a steam film that forms the seal
against atmosphere. Secondly, a cushion
between the faces is created, helping
minimize wear.
The seal's influence on chemistry occurs
when the pump is offline. This applies
to redundant pump setups with the
standby pump, or to heater drain pumps
in the scenario of all pumps being offline
due to low-load condition. In an offline/standby
condition, the stuffing box
is under suction conditions-with condensate
and heater drain pumps, this
is a vacuum condition. In order to keep
the stuffing box, and ultimately the process
liquid, sealed against atmosphere
and prevent oxygen ingress resulting in
elevated DO levels, supplemental seal
water needs to be applied to the atmospheric
side of the seal (Figure 1).
Typically, this seal water comes from
1. An assembled mechanical seal is shown
here ready for installation on a vertical turbine
pump. Courtesy: Kurt Bayburt
on the atmospheric side. Oftentimes,
this is not the case. In many situations,
it is applied to the process side (internal
to the stuffing box). With substantial
vacuum in this area, atmospheric oxygen
can still be pulled across the seal face
and dissolve in the stuffing box, eventually
being pulled into the suction side of
the pump.
Problems can arise from
poorly/improperly configured seal
water piping, or no seal water at all.
an adjacent operating pump (sourced
from the common discharge header).
Problems can arise from poorly/improperly
configured seal water piping, or no
seal water at all. Furthermore, if seal water
is indeed applied, it must be applied
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Applying a very low-pressure/low-flow
seal water (5 psig/0.5 gpm) to the atmospheric
side of the seal will prevent
oxygen ingress. This may require replacing
the seal flange cover with one that
has proper internal ports and external
29
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POWER January 2021

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