HRSG Circulation System Design
In the design of Babcock & Wilcox
Power Generation Group, Inc’s
(B&W PGG) heat recovery steam
generator (HRSG), the evaporator
circulation system is an important
aspect that must be considered
for successful operation. An
improperly or inadequately
designed circulation system can
limit the HRSG’s availability during
rapid startup and shutdown
conditions, and during extreme
rates of load change; result in drum
level excursions or persistent water
level trips; and contribute to flowassisted corrosion. To compensate
for an undersized or improper
circulation system design, many
OEM suppliers require a circulation
system that includes a larger-thannormal steam drum with a large
water-holding capacity.
B&W PGG’s design philosophy is
to provide the proper circulation
system for the design conditions
the boiler is expected to experience
during typical operation. Through
circulation design analysis, the boiler
components can be sized properly
and more economically for the
expected operational conditions.
unit through the use of larger
steam drums. Analyzing for flow
excursions during load changes
and transient operation can be
beneficial by eliminating larger
and/or additional supply and riser
connections to the heat generating
surface. An optimized circulation
system design provides a reliable
HRSG, which will eliminate most of
the operational issues that result
from an improperly designed unit.
B&W PGG has developed an
extensive set of design requirements
for HRSGs to meet the demanding
conditions required for today’s
combined cycle power generation
operation. Our circulation analysis
goes beyond the typical steadystate circulation analysis and
investigates the transient operation
of the boiler. This type of analysis
reduces the need for special and
costly features, like additional
water holding capacity for the
At the heart of B&W PGG’s
circulation evaluation is a boiler
circulation analysis computer
program which utilizes highly
advanced techniques for
calculation of heat transfer and
single- and two-phase fluid flow
parameters. With this computer
program, a circulation model of
each individual circuit of an entire
boiler is evaluated.
Circulation analysis
Circuits within the HRSG are subject
to a variation in heat transfer across
its width and depth. The computer
program manages this by allowing
the designer to divide each circuit
into zones. The designer is then
able to determine the balanced
water and steam flow in each zone
by solving the energy, mass, and
momentum equations. At the
balanced flow condition, design
criteria are examined allowing the
designer to determine whether
circulation characteristics are
adequate.
For each condition analyzed,
adjustments are made to individual
circuits, when necessary, to bring
their flow characteristics within
acceptable limits. These limits are
based on B&W PGG’s experience,
both in the field and through
testing at our research center in
Barberton, Ohio. Adjustments
to improve circuit circulation
can include, but are not limited
to, determining the appropriate
number of steam separators in the
steam drum or incorporating a
vertical separator; adding orifices
at the entrance or within individual
circuits; altering the number of
supplies and/or risers; and lowering
feedwater temperature entering
the steam drum. Each circulation
analysis is unique and the
requirements to achieve acceptable
circulation characteristics are casespecific to the boiler and operating
conditions being examined.
Details that B&W PGG considers
when it designs and analyzes an
HRSG include exit quality, saturated
water head, saturated velocity,
sensitivity, stability, departure from
nucleate boiling (DNB) and drum
internals.
Exit quality
Maximum exit quality from a circuit
is a function of the drum operating
pressure. Keeping a circuit’s exit
quality below the maximum exit
quality assures that boiling in
the circuit will be restricted to
nucleate boiling, and that the tube
metal temperature will be close to
saturation temperature.
Saturated velocity
Adherence to the required
minimum circuit saturated velocity
helps assure that steam blanketing
does not occur and reduces the
possibility of solids deposition.
Both steam blanketing and solids
deposition can cause tube failure.
B&W PGG limits on saturated
velocity are a function of tube
orientation, tube location and type
of firing. Circuit velocities must also
be below the maximum limit to
avoid flow-assisted corrosion.
Sensitivity
In a correctly designed natural
circulation system, steam-water
mixture flow increases with
increased heat input, until a point
where maximum flow is reached. If
more heat is added, mixture flow
then begins to decrease as specific
volume and frictional pressure
losses offset the pumping head
due to increased heat absorption.
Beyond the maximum circuit
flow point at a specific load, heat
input, and quality, circuits become
unstable and may flow upward,
stagnate, or flow backward. For a
range of heat inputs, sensitivity is
the steam-water mixture flow trend
(i.e., increasing or decreasing) of
circulation capability relative to
the maximum continuous steam
flow rating. B&W PGG carefully
evaluates both steady state and
transient operation to assure
reliable circulation.
Stability
Stability is based on the pressure
drop in a circuit between common
pressure points. B&W PGG has
established limits of circuit pressure
drop allowed at a given steam flow
and pressure. Circuits with heat
absorption rates that vary across
the different tube rows are most
susceptible to instability.
The stability analysis is a measure
of the tendency of a circuit to flow
in the desired direction. Figure 1
shows possible pressure differential
characteristics for a furnace circuit.
Line dP2 is the maximum attainable
pressure differential for downflow.
By forcing the pressure differential
to be greater than line dP2, the
circuit will flow upward. The
graph indicates that for pressure
differentials less than line dP2,
there are three different flows
that may yield the same pressure
differential: two downflow and one
upflow. In practice, the flow can
also oscillate or stall. Therefore,
Saturated water head
The saturated water head criterion
is the relationship between (a) the
actual density and pressure drop
characteristics of the steam and
water mixture, and (b) those of a
static column of saturated water of
the same vertical height between
common pressure points. Circuits
operating at a lower percentage
saturated water head could have
problems with recirculation
between parallel paths within
the same circuit. B&W PGG has
established limits for percentage
saturated water head preventing
this problem from occurring.
Fig. 1 Relationship between boiler tube pressure loss and direction of flow.
a circuit operating along line dP1,
which does not meet the stability
criteria, may operate at point C
indefinitely and would be an upflow
case. Unfortunately, the inherent
nature of an unstable system means
that it is sensitive to boundary
conditions. The result is that
system disturbances or changes,
sometimes even small ones, may
cause the operating point to be
moved from one upflow solution
to the potential for either upflow
or downflow. Therefore, it is always
desirable to operate above line dP2,
the stability limit.
of cooling liquid near the heated
surface of the tube, causing the
temperature of the tube to increase
rapidly, as seen between points
D and S in Figure 2. The point at
which the boiling mechanism is
interrupted (point D) is called the
DNB point. DNB must be avoided
in circulation design because it
produces serious overheating and/
or rapid corrosion of boiler tubes,
which leads to subsequent tube
failures. In general, higher boiler
tube absorption rates at higher
loads and/or higher circulating
(drum) pressures tend to cause DNB
problems.
Departure from nucleate
boiling
Drum internals
HRSG evaporator tubes are cooled
by nucleate boiling, which is
characterized by a very high heat
transfer coefficient. At certain
conditions, the boiling mechanism
is suddenly impaired, due to a lack
Steam and water flow rates to
each cyclone separator, as well
as the pressure drop through
each separator, are checked
against B&W PGG-established
limits. Conformance with B&W
PGG standards reduces the risk of
unacceptable carryunder levels of
steam into the downcomers and
carryover of water out of the drum.
B&W PGG’s patented low-pressuredrop cyclone separators improve
flow stability, increase drum water
level control range, and minimize
water carryover and steam
carryunder for the highest possible
steam purity.
What separates B&W PGG
from others?
Boiler circulation design is one of
many technical capabilities B&W
PGG offers that separates us from
the competition. We have more
than 145 years of experience in the
boiler industry. This experience,
combined with the company’s
state-of-the-art technology,
have allowed us to offer superior
products and services to customers.
B&W PGG is considered a leader
in the boiler circulation thermal
hydraulics field because of our:
• Boiler circulation analysis
computer program, which was
developed and benchmarked
using laboratory field data, and
features:
-- Benchmarking across a
wide pressure range
(15 - 4000 psig)
-- Easy evaluation of gas-side
unbalances
• Proprietary two-phase flow
correlations
• Accurate flow stability
predictions
• Flow sensitivity analyses at
steady-state and transient
conditions
• Technical knowledge of steam
separation equipment and its
performance
• Low-pressure-drop drum internals
and economizer cyclones
Fig. 2 Boiling conditions in evaporator tubes.
www.babcock.com
Babcock & Wilcox Power Generation Group, Inc.
20 S. Van Buren Avenue
Barberton, Ohio 44203 USA
Phone: 330.753.4511
Fax: 330.860.1886
Babcock & Wilcox Power Generation Group, Inc. is a subsidiary of
The Babcock & Wilcox Company (B&W). Established in 1867, B&W is
a world leader in providing steam generating and emissions control
equipment, nuclear operations and components, and defense program
management services.
The information contained herein is provided for general information
purposes only and is not intended nor to be construed as a warranty, an
offer, or any representation of contractual or other legal responsibility.
For more information, or a complete listing of our sales and service
offices, call 1-800-BABCOCK (222-2625), send an e-mail to
info@babcock.com, or access our website at www.babcock.com.
© 2012 Babcock & Wilcox Power Generation Group, Inc.
All rights reserved.
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