POWER September 2016 - 57

COMBINED CYCLE O&M
3. HP-IP economizer header alteration. This was the design
alteration that was installed in the five units. Courtesy: Tetra Engineering
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This review identifies components whose condition warrants a
change in priority for future maintenance and provides specific recommendations
for such actions. Several case studies follow from recent
projects.
Evaluating Operational Impacts of a New Contract
on a HRSG
As part of a forward-looking aspect of one HRSG condition assessment,
expected changes in plant operational modes were considered.
At this plant, new power purchase agreements were being negotiated
that required additional low-load and load-following operations. The
pressure part health evaluations showed that extending these ranges,
while continuing to conduct daily load changes and two shift cycling,
substantially increased the required changes for superheater life consumption
as well as other plant impacts.
The primary impacts of the proposed operation on variable operation
and maintenance costs (VOM) were:
■ Higher HRSG operating temperatures due to higher GT outlet gas temperatures,
resulting in higher superheater (SH) life consumption
■ Increased thermal stresses in HRSG due to load cycling
■ Increased wear and tear on HP/RH/LP bypass and steam conditioning
valves
■ Increased potential for condenser tube damage in bypass mode
■ Increased water consumption in cooling tower in bypass mode
■ Decreased generation (and VOM payments) in ancillary service
mode
The impact of this operation on VOM costs was assessed by analyzing
three operating cases:
■ Low load 2 x 1 automatic generation control (AGC) service (60%
to 100%) for 90 days per year
■ Low load 2 x 1 AGC service (60% to 100%) for 180 days per year
■ Low load 2 x 1 AGC service (60% to 100%) for 60 days per year
plus low load 2 x 1 bypass load for 30 days per year
The primary impact of these analyses was the increased repair costs
on each HRSG of HP SH #1 elements due to higher temperatures
and thermal stresses in load-following service and the cost impact of
reduced generation. In the bypass cases, the same factors occur, but
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with the addition of added water costs and allowances for potential
condenser damage. Table 1 shows the results.
The primary contributors to the change in costs are the impact
of lower generation and higher water/chemical use in the bypass
case. Secondarily, the earlier replacement/repair of HP SH 1 modules
in the HRSGs and budgeted condenser and valve repairs for
the bypass case contribute. One key factor is that 30 days of bypass
operation has the same cost impact as 100 days of non-bypass
AGC operation.
Long-Term Progression of Economizer Tube Leaks
A review of past tube leaks in a five-unit cogeneration plant HRSGs
showed an increasing trend of corrosion fatigue cracks in HP economizer
tubes. Figure 1 shows how the trend rapidly increased after
year 12. The cause was high stresses at tube bends due to cycling. An
example of the cracks that result is shown in Figure 2. This was made
worse by combined IP-HP economizer lower headers in the original
design. Tetra recommended splitting these to allow differential expansion.
This recommended major modification is shown in Figure
3. That modification lowered stresses and reduced the incidence and
progression of the tube leaks going forward.
Flow-Accelerated Corrosion Pressure Part Thinning
FAC is an issue throughout the North American fleet (Figure 4), as
are creep, thermal stress/low cycle fatigue, quench failures in hightemperature
panels (HP superheaters and reheaters), the potential for
cold-end corrosion damage to extended heat transfer surfaces (fins)
and water-side damage such as under-deposit corrosion in HP evaporators,
pitting, and general corrosion in the water side of the LP comwww.powermag.com
57
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POWER September 2016

Table of Contents for the Digital Edition of POWER September 2016
