POWER March 2015 - 45

COMBINED CYCLE GAS TURBINES
6. Cracked. This example of cracked
Grade 91 hot reheat latrolet was caused by
fatigue and Type IV creep damage Courtesy:
Tetra Engineering
8. The low-down. Low-load operation has recirculation of low-pressure (LP) steam flows
at the exit of the LP section of the steam turbine. This can result in trailing edge blade erosion.
Courtesy: Tetra Engineering
7. Hammered. Water hammer damage
to large-bore HP steam piping supports can
be significant. Courtesy: Tetra Engineering
pipe spool pieces, destruction of pipe supports
(Figure 7), and a resulting piping system that
is no longer operating within the maximum
allowable stresses specified by ASME Code
design. The result is generally premature and
costly inspections and repairs.
than predicted by design analysis.
For Grade 91 components, Type IV creep
cracking, enhanced by fatigue loads is a predominant
damage mechanism leading to macroscopic
cracks (Figure 6) and, in some cases,
leaks. This damage is more likely for components
with inadequate metallurgical properties
but is an issue of concern for all components,
especially those subjected to higher stresses in
1 x 1 operation than expected under the original
design. CCGTs in low-load operation require
a comprehensive approach to assess and maintain
power piping condition, which should be
an integral part of the Covered Piping System
Program in accordance with the recent ASME
Code Section B31.1 Power Piping, Chapter
VII, Operations and Maintenance.
Preventing Water Hammer. Water hammer
is a well-known issue for CCGT plants.
The more common types of damage at low
load will be caused by inadequate drain capacity
downstream of attemperator spray stations
and attemperator system malfunctions,
including controls logic inadequacies. At
low-load operation, there is increased likelihood
of condensate and spray water accumulation;
therefore, it is essential that drain
capacity be capable of removing water from
HP and hot RH steam piping that accumulates
after or during shutdown.
Damaged or inadequately maintained nonreturn
and stop valves will contribute to higher
risks of water hammer damage. Water hammer
events are generally severe, with yielding of
March 2015 | POWER
Steam Turbines
Establishing a minimum floor pressure for
HRSG operation at low loads is essential.
The trade-offs are that at low pressures, steam
flow increases, which can be helpful for LP
steam turbine operation but raises steam velocities
in HRSGs and piping. Low pressures
for HRSG operation can also reduce stability
in evaporator circulation.
At low steam flows, the performance of
the LP turbines is key. Low steam flows (and
enthalpies) result in poor turbine efficiencies.
Internal flow distribution and recirculation
can cause power loss and local heating.
Steam is pushed to the outer regions of
the turbine blades, and a recirculation flow
is established (Figure 8). This windage heating
can be reduced by using exhaust hood
sprays. These sprays can result in blade erosion
if caught up in the recirculation. At these
conditions the average temperature of the LP
turbine rotor is increased, which increases
the rotor expansion.
Excessive expansion of the rotor is a critical
operational limitation on low-load operation,
both limiting the absolute lower load
and limiting the time that low-load operation
can be maintained. These are site-specific
impacts that are assessed in assessments of
low-load operations.
Condensers
The use of hood sprays at low loads to cool
windage-heated LP steam raises the risks of
droplet impingement and damage to tubes.
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Good maintenance and monitoring of sprays
is essential to preventing condenser damage.
Many low-load contracts require the capability
of running in 100% bypass of steam
from the steam turbine to the condenser. In
this way dispatched power is less but fuel
consumption is the same as for low-load GT
operation without bypass. Extended bypass
raises risks of damage to internal baffles,
dummy and live condenser tubes and piping,
as well as the steam conditioning valves. In
general, the increased maintenance costs for
long-duration bypass can be substantial. Few
plants expect to run in this mode, but the capability
is necessary.
Careful Attention Is Essential
It's a given that low-load operation is becoming
a familiar fact of life in an increasing
number of markets. To ensure you get
the most reliable, long life out of your unit,
you need to understand the potential effects
of low-load operation on the steam cycle and
the tradeoffs involved in mitigating them. In
most cases, enhanced maintenance and inspection
programs may be required. ■
-Dave Moelling (dave.moelling@tetraeng.com)
is chief engineer at Tetra Engineering
Group, consults on HRSG thermal
design evaluations, and leads low-load
operations assessments. Peter Jackson
(peter.jackson@tetra-eng.com) is director
of field services at Tetra Engineering
Group, responsible for HRSG field
services, power piping, balance of plant,
and leading root cause failure analysis
and fitness-for-service assessments. Jim
Malloy (jim.malloy@tetra-eng.com) is
managing director at Tetra Engineering
Europe and is responsible for managing
CCGT engineering services for Europe,
Middle East, and Africa.
45

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POWER March 2015

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