POWER June 2012 - 70

COMBINED CYCLES
Startup Purge Credit Benefits
Combined Cycle Operations
Combined cycle power plants use fuels and other materials that can cause fires or
explosions in the combustion turbine, ducting, or heat recovery steam generator.
Purging that equipment with ambient air to displace residual combustible
gases before starting is a normal safety practice. But when plants are cycled,
the disadvantages of purging often outweigh the advantages.
By David S. Moelling, PE and Peter S. Jackson, PE, Tetra Engineering Group Inc.
E
xplosions and fires in a combined cycle
(CC) plant may be rare, but when one
occurs, it normally results in an extended
outage for repairs. One cause of these undesirable
events is the accumulation of vapors
and liquids in the combustion turbine (CT)
and heat recovery steam generator (HRSG)
system during a shutdown that can combust
during unit startup. In the U.S., this potential
is addressed in National Fire Protection Association
(NFPA) 85: Boiler and Combustion
Systems Hazards Code (2011). The code sets
" design, installation, operation, maintenance,
and training " standards for " safe equipment
operation " for all but the smallest fired and
unfired steam generators.
NFPA 85 for many years has required a
fresh air purge of the CT, HRSG, and other
CT exhaust systems (for example, by bypass
stack) prior to CT startup and operation. The
purge rate is a mass flow requirement to provide
the required air velocity to ensure dilution
and removal of combustible gases prior
to light-off of the CT. This purge is specified
to be at least five volume changes and
no less than 5 minutes in duration. Typical
purge times range from 5 to 18 minutes, depending
upon the fuel selected, HRSG and
duct design, and guidance from the original
equipment manufacturer (OEM). The purge
cannot be less than 8% of the full power mass
flow of the CT.
The requirement to purge equipment,
from the CT to the exhaust stack, may protect
the CC system from the hazards of fire
and explosion, but complying with the code
also imposes performance and reliability
challenges. In CC plants, steam-side startup
is typically determined by the temperature
of key steam generation systems, which then
controls the steam turbine ramp rate up to
operating conditions (Figure 1). The exact
values vary from plant to plant, but typical
numbers are:
1. Producing condensate. This figure illustrates that when the combustion turbine exhaust
temperature (HRSG inlet temperature) during a cold air purge is below the saturation
temperature, enormous quantities of condensate are formed inside the HRSG that must be
removed through vents before startup can continue. Source: Tetra Engineering Group Inc.
1,400
1,200
1,000
800
600
400
200
Condensate production
Turbine exhaust temperature
Drain open
Drum pressure
Superheat temperature
Drain closed
Saturation
temperature
20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
4:19
70
4:33
4:48
5:02
Time
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POWER | June 2012
5:16
5:31
5:45
6:00
■ Cold starts: No pressure on steam system,
prior shutdown greater than 24 to 30
hours.
■ Warm starts: High-pressure steam pressure
is typically 60 to 200 psi, after a shutdown
of 10 to 20 hours.
■ Hot starts: High-pressure steam is typically
greater than 500 psi, after a shutdown of
8 to 10 hours.
For cold starts, the time required for purge
adds to the total time required for starting the
unit, from pushing the start button through synchronization
and ramp-up to full load. Added
time is required during warm and hot starts as
well, but in these two situations the purge puts
relatively cold airflow through a warm or hot
HRSG and its upstream and downstream duct
work. This relatively cold purge air quenches
the relatively hot HRSG and its ducting, which
produce lower metal and water temperatures
and, thus, steam pressure. As long as the temperature
of the incoming purge air is less than
the saturation temperature of the high-pressure
steam system, the system loses heat and superheater
panels act as large condensers for the
trapped condensed steam (Figure 2).
The amount of condensate produced can be
quite large. This condensate must be removed
prior to loading the CT to avoid water transport
to the steam turbine or other components.
2. Accumulating condensate. During
the purge period (shown in Figure 1), water
can accumulate inside the HRSG superheater
panels that must be vented. Source: Tetra Engineering
Group Inc.
Water
Temperature/pressure
Condensate (lb/hr)
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POWER June 2012

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