Hydrocarbon Processing - July 2022 - 57

Process
Optimization
B. BUECKER, ChemTreat, Lawrence, Kansas;
and J. POWALISZ, Sentry Equipment Corp.,
Milwaukee, Wisconsin
HRSG water/steam sampling: Do it right
or face potential consequences
Many large industrial facilities-including
refineries, and chemical and petrochemical
plants-are increasingly turning
to co-generation for process steam
and power production. Popular for many
applications, including the power industry,
are combined-cycle units with heat recovery
steam generators (HRSGs). Many
HRSGs operate at high temperatures and
pressures, where the harsh conditions
can transform seemingly minor chemistry
upsets into major problems. Unit outages
and employee safety are two primary
concerns of poor water/steam chemistry.
Online water/steam chemistry monitoring
is critical not only for normal chemistry
control, but also to detect upsets.
However, if samples are not extracted
and conditioned properly, the data may
give false indications of actual conditions
in the unit. This article examines three
primary issues regarding HRSG water/
steam monitoring, including the:
* Selection of critical measurements
to maintain normal operating
conditions, establish chemical feed
control and detect chemistry upsets
* Importance of proper sample
extraction and conditioning
to ensure that online
instrument data is accurate
* Benefits of iron monitoring,
as these measurements provide
direct data on the efficacy
of steam-generator chemical
treatment programs.
Regarding the last item, iron monitoring
is particularly important for tracking
flow-accelerated corrosion (FAC), which,
over the last 30 yr, has caused several serious
accidents-some with fatalities-
FIG. 1. Authors' reproduction of the basic flow path of the most common type of HRSG,
the feed-forward low-pressure HRSG. Other than a small amount of steam generation,
the low-pressure evaporator heats feedwater for the IP and high-pressure circuits.
Hydrocarbon Processing | JULY 2022 57
at several conventional power plants. The
tight radius elbows in HRSG low-pressure
evaporators and economizers can be
particularly susceptible to FAC.
Important monitoring parameters.
The authors have seen many HRSG
sampling system design specifications in
which some important chemistry analyzers
were omitted while others were overspecified.
The following text offers a basic
outline of the critical instrumentation for
typical multi-pressure HRSGs, with the
understanding that every project should
be evaluated per the HRSG design and
operating conditions. Much of this information
is condensed and updated from
literature.1,2
In addition, references cited
within this article offer expanded discussions
on these topics.
Compared to coal-fired units, HRSGs
have a radically different design. Most
notably, the various waterwall tube and
superheat/reheat panels (i.e., harps) are
aligned in sections along the flue gas
path. The water tubes in a coal unit basically
form the walls (thus the name
" waterwall " ) of a box that surrounds the
combustion zone. This design is necessary
to control the ash that forms during
coal combustion. A positive aspect of the
coal boiler configuration is that the tubes
are reasonably accessible for repair in
the event of a failure. The tight packing
of tubes in an HRSG can make repairs
very difficult, which further enhances the
need for good corrosion control.
The most popular high-pressure
HRSG design has three steam-generating
networks (FIG. 1), where the low
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