CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptx

Gasværksvej 24,3 DK 9000 Aalborg www.R-R-Consult.com www.LinkedIn.com/company/R&R-consult Info@R-R-consult.com
CFD Simulation of By-pass Flow in a
HRSG module

HRSG: A Heat Recovery Steam Generator is
an energy recovery heat exchanger that
recovers heat from a hot gas stream, such
as a combustion turbine or other waste gas
stream. Its primary function is to produce
steam, which can then be utilized in
various industrial processes or to power a
steam turbine.
Our partner, Tetra Engineering, was asked to solve a
performance issue at a 1509 MW natural gas-fired power
plant: The HRSG failed to deliver the expected amount of
steam, which resulted in inadequate electricity output.
During an inspection of the HRSG, it was discovered that a
significant volume of hot flue gas inadvertently bypassed the
boiler tubes. Instead, it flowed along the sides of the boiler
and through two central bypasses between three individual
modules. If the bypass was significant, that could explain the
reduced performance.
To understand whether the bypass was the only cause, we
first had to determine the extent of the bypassing flow. To
find out, we conducted a CFD analysis of a single module.
This allowed Tetra to perform a thermal analysis to assess
whether the bypass flow was the sole cause of the reduced
performance or whether there might be other explanations.
Otherwise, it would also be difficult to find the right solution.
HRSG plant, picture from Tetra Engineering
# 2
Background and introduction

Gasværksvej 24,3 DK 9000 Aalborg www.R-R-Consult.com www.LinkedIn.com/company/R&R-consult Info@R-R-consult.com # 3
An inspection of the boiler
revealed areas of possible by-
pass of the boiler tubes in
each module.
(See the yellow areas of the
sectional drawing.)
Picture of By-pass in the central part
of the 1st boiler module.
Sectional drawing 1st. boiler module
Flow
direction
Possible reason for reduced steam production
idetified

Results – Velocity distribution
# 4
The velocity distribution from the CFD simulation clearly
showed areas with by-pass flow, seen as areas with
accelerated flow. These areas are seen in red.
Velocity distribution [m/s] on a cross-
sectional plane through boiler module 1
Zoom picture of velocity distribution with directional vector

Results – Temperature distribution
# 5
Temperature distribution [K] on a cross-sectional plane
The temperature distribution clearly revealed areas where
the exhaust gas passed through the module almost
untouched.
The CFD analysis concluded that some of the exhaust gas
passed through the bypasses without transferring any heat.
However, it was still unclear whether the quantity was
significant enough to explain the reduced steam production.
Zoom picture of temperature distribution showing the center by-pass

Conclusion and solution
# 6
1.2 %
2.2%
2.2%
1.3%
=6.9%
1.1 %
1.8%
1.8%
1.1%
=5.8%
1.2 %
1.9%
1.9%
1.2%
=6.2%
1.2 %
1.9%
1.9%
1.2%
=6.2%
1.2 %
1.9%
1.9%
1.2%
=6.2%
Calculated part of flow that by-passes
The CFD calculation showed a clear
bypass flow and documented the extent:
On average, 6.3% of the exhaust gas
passed through the boiler tubes with
limited heat transfer.
Tetra used the results as input for a
thermal analysis of the boiler. The
analysis showed that the bypass flow in
itself explained the decreased steam
output.
Plans were immediately made to install
covering plates to reduce the by-pass
flow.
Covering plate at the side wall
Covering plate at centre
The CFD calculation was a
significant contribution in
making an informed decision
about which solutions to test to
address the issue of reduced
steam output.
More examples of our work here

CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptx

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CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptx