POWER June 2012 - 71

COMBINED CYCLES
Excessive condensate in the HRSG can cause
blockage of steam flow through tubes, thermal
distortion and shock, and many other problems.
Draining the condensate in a timely fashion
requires drain systems with adequately sized
pipes, valves, and discharge. The drains also act
to reduce operating pressures and, thus, temperatures
in the HRSG steam systems. In daily
or two-shift cycling CCs, the thermal impact of
the purge cycle can result in damage to pressure
parts, causing leaks and a reduction in the remaining
life of parts (Figure 3).
Over the past few years, HRSG purge issues
have been extensively discussed at industry
conferences. Some-in particular, a large segment
of CC suppliers and operators-suggest
eliminating the purge requirement on cycling
units. However, the safety need for eliminating
combustible gases remains. A concept for
modifying the purge requirements to credit prior
purges for cycling CCs was first introduced to
the NFPA Technical Committee for Heat Recovery
Steam Generators in 2003. The NFPA
Boiler and Combustion Systems Hazards Code
Committee established a task group in 2006 to
pursue the idea and to enable CC operators to
safely implement procedures that provide for a
fast-start capability. The change in the code was
finally issued in the 2011 Edition of NFPA 85 as
the Combustion Turbine Purge Credit.
Calculating Purge Credits
The purge credit, simply defined, is when, during
a normal CT shutdown, there is sufficient CT
post-purge airflow to remove any residual combustible
materials from the CT and HRSG system.
If gases cannot seep into the ductwork, then
the post-purge meets the NFPA requirements
and a redundant start purge is not required.
In this scenario, the only concern is that leakage
from the CT, duct burner, and/or the ammonia
injection systems could travel into the CT and
HRSG system after shutdown. To ensure that
this extremely unlikely scenario never occurs,
additional sealing valves and components are
installed in the fuel systems. The final safety step
3. Draining condensate. Many combined
cycle plant owners have retrofitted their
superheater panels with high-capacity drains
and added multiple drain connections to speed
condensate removal and therefore quicken plant
startup. Source: Tetra Engineering Group Inc.
Lower superheater headers
Fuel supply
Multiple drain connections to
prevent condensate flooding
of lower headers and tubes.
Large drain manifold
sized to pass maximum
condensate flow
Drain line sized for
rapid gravity drainage
To high-capacity drain valves and
below-grade blowdown tank
June 2012 | POWER
Air or inert
gas supply
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71
V1
Vent
V2
V3
in the design is to provide instrumentation and
procedures to monitor the status of the isolation,
assuming that the isolation is not inadvertently
breached during unit shutdown. When these enhanced
system isolation upgrades are completed,
the revised NFPA 85 (2011) provides a purge
credit that allows startup to proceed without an
additional purge prior to CT light-off.
For traditional isolation of fuel supplies (gas
and liquid fuels) in NFPA 85, a double block
valve and vent piping arrangement has been
required. The two block valves provided a positive
isolation and the vent of the piping between
them allowed gas to escape the isolated section.
For implementing a purge credit, a triple block
and vent valve arrangement is required. Three
options are provided in NFPA 85 (2011):
■ System 1 for gaseous fuels: Use triple block
and vent valves with continuous monitoring
of valve position, pressures in the two double
block and vent valve pipe sections, and block
valve validation for leak tightness by a valveproving
system (Figure 4).
■ System 2 for gaseous fuels: Use triple block
and vent valves with pressurized pipe sections.
The two pipe sections are filled with an
inert gas or air at sufficient pressure to prevent
fuel gas from entering. Continuous monitoring
of valve positions and pressurized pipe
section pressures is required (Figure 5).
■ System 3 for liquid fuels: Use a triple block
and double drain valve arrangement filled
with pressurized inert gas or air. Continuous
monitoring of valve positions and pressurized
pipe section pressures is required (Figure 6).
Comparing Isolation Options
The NPFA 85 purge systems 1 and 2 for gaseous
fuels have different advantages and
disadvantages. System 1 does not require a continuous
supply of pressurized air or inert gas,
but additional testing and monitoring must be
performed (valve proving) to ensure system integrity.
In addition, the purge credit can only be
maintained for eight days, after which a purge
must be performed to renew the credit. System
2 can maintain the purge credit indefinitely, but
a continuous pressurized air or inert gas supply
must be provided. If the supply is lost, the credit
is no longer allowed. In both systems, the credit
is lost if continuous monitoring of valve positions
and pipe pressures is not maintained.
Purge system 3 can be used only for liquid
fuels. It has pressurization and monitoring re4.
Purge system 1. Gaseous fuel triple block and double vent valve arrangement. Source:
NFPA 85, Figure A.8.8.4.6.1
Vent
Vent
V4
V5
PT
Fuel supply
V1
V2
V3
5. Purge system 2. Gaseous fuel with pressurized pipe section. Source: NFPA 85, Figure
A.8.8.4.6.2
Vent
V4
DPT
PT
To combustion
turbine or duct burner
Vent
V5
PT
To combustion
turbine or duct burner
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POWER June 2012

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