(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

many existing combined cycle gas turbine plants need to move towards more
flexible operation than was initially envisaged at their design stage.
This move from what was initially a system engineered for base loading to one
with substantial requirement for two-shift operation has a detrimental impact
on plant reliability, specifically with regards to the HRSG. For the plant, the
main risk associated with the use of the HRSG under flexible operation is the
impact on the achievable lifetime of pressure part and non-pressure part
components such as tubes, headers, casing components etc. In general, a
reduction in lifetime is expected resulting from causes such as Low Cycle
Fatigue (LCF), localised header stresses and flow accelerated corrosion (FAC)
and from a combination of LCF and Stress Corrosion Cracking (SCC).
The challenge therefore faced by engineers is to design solutions that ensure
the reliability of the next generation of HRSGs to be installed and upgrade the
existing plants (by correctly sizing drains for example) before major problems
occur.
In order to do this companies have had to invest heavily in effective transient
thermal modelling capabilities that allow them to analyse thoroughly every
component of the HRSG and make design changes to limit thermal stresses.
One such example where new features have specifically been designed to
provide flexibility for the plant during thermal transients is with Foster
Wheeler’s Fort Meyers repowering project [61] . This novel feature is a “wet
bypass” unit which is designed to absorb the instantaneous power loss of a
steam turbine trip and enable the gas turbine to continue operating at a full
simple cycle load. In the event of the steam turbine tripping, main steam is
attemperated and its pressure reduced, before it is bypassed to the condenser.
Reheated steam is either bypassed to the atmosphere, when the condenser is
not available or also attemperated and reduced in pressure before passing to
the condenser dump. Thermal fatigue of the steam headers is greatly reduced
by this innovative process.
Likewise Alstom have adopted features specifically to reduce thermal stress at
welded joints albeit at a potential increase in capital outlay. Most natural
circulation HRSGs use a multiple–row harp-shaped design, comprising of one
horizontal upper header and one horizontal lower header joined by two or
three rows of vertical tubes. Alstom maintains that, the temperature of the
exhaust gas drops sufficiently as it passes through the multiple tube rows to
cause the individual tube rows to operate at different temperatures, inducing
differential thermal stress at the weld joints. Their solution is to form a single
row of tubes between headers to remove these differential stresses. As a single
row allows for smaller header diameters, circumferential temperature
gradients in the headers are also minimised. Alstom’s analysis concluded that
small-diameter headers reduce thermal stress by as much as 60% when
compared to headers used with multiple rows.
(80)