Page 46 - Nuclear Plant Journal Digital Edition: January - February 2013
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Prevention
of Turbine
Rupture
Disc Air
Inleakage
By Lee Machemer, Jonas, Inc.
Lee Machemer
Lee Machemer is President of Jonas,
Inc. and has worked for the company
for 17 years as a
water chemistry
and corrosion
consultant. He has
been involved with
the development of
several products
used in fossil-
fired, nuclear,
and geothermal
power generation
facilities. Lee
received a Bachelor
of Chemical
Engineering degree
from the University of Delaware and is a
Professional Engineer.
A common problem on low-pressure
(LP) condensing steam turbines is air in-
leakage through leaking turbine rupture
discs. Turbines are equipped with rup-
ture discs (Figure 1) to protect the turbine
casings from deforming or rupturing due
to an accidental increase of the internal
steam pressure. They are an intentional
weak spot meant to fail should the sys-
tem get over-pressurized allowing it to
vent out safely. During normal opera-
tion, the exhaust ends of LP turbines are
under vacuum because they directly con-
nect to condensers. Due to stress, corro-
sion, and other causes, the lead discs are
susceptible to fatigue cracking and corro-
sion. Evaluation of several rupture discs
by one nuclear plant found high cycle
fatigue cracking at a location where the
discs were being deformed by the rup-
ture disc housing assembly. Cracks in
the discs were caus-
ing the assemblies to
no longer being air-
tight, which resulted
in a major path for
air leakage into the
vacuum part of a
steam system.
EPRI and other
industry guidelines
and
the
Heat
Exchange Institute
recommendations
allow air inleakage
to be only 1 SCFM
(standard
cubic
foot per minute) per 100 Megawatts of
installed capacity. When a rupture disc
is leaking, this requirement often cannot
be met. Subsequently, water and steam
chemistry and cycle corrosion protection
goes out of control and, in a case of severe
air in-leakage, condenser vacuum and
cycle output and efficiency are reduced.
When severe, this problem can result in
unit shutdowns to disassemble the rupture
disc assemblies and replace leaking
diaphragms. For a large unit, the outage
costs can be up to $100,000 per hour and
over one million dollars per repair.
There are several preventative and
corrective measures that are commonly
used to improve the rupture disc
reliability including ensuring the steel
plate is centered in flange opening so that
the disc is not exposed to an excessive
gap, coating the disc with a sealant during
installation, and reducing the torque on
retaining ring fasteners. Even with these
actions, rupture disc reliability is still
rather poor.
During operation, the loss of rupture
disc integrity is difficult to correct and
severe air in-leakage can be a safety
hazard for operators and maintenance
personnel. When leakage through a
rupture disc is suspected, attempts to
stop the leak with sealants can reduce
the leakage enough to get through to the
next maintenance outage, however this
method is not reliable and is not a long
term solution. Because of the frequency
of maintenance due to the failing rupture
discs and the lack of a reliable temporary
fix to the leaking discs during operation,
a new solution was developed for
preventing air inleakage. This solution
was the installation of a Turbine Rupture
Disc Cover.
These Covers are made from nylon
reinforced rubber and neoprene and slide
over the top of the rupture disc housing
to form an air-tight seal. The Covers
are custom designed to fit most rupture
disc housing designs, installation takes
approximately 20 minutes and does
not require the unit to be shutdown.
The material of the Rupture Disc
Cover is highly resistant to the ambient
environment for both indoor and outdoor
turbine installations.
During operation of the turbine,
if depressions appear on the top of
the Cover, this is an indicator that the
rupture disc is leaking. The rupture disc
can either be replaced during the next
outage, or the Cover can be left in place
permanently. In addition to sealing the
rupture disc to prevent air inleakage,
the Covers protect the rupture disc from
corrosive atmospheric contaminants.
Contact: Lee Machemer, Jonas,
Inc. 4313 Nebraska Court, Pomfret,
MD 20675; telephone: (301) 934-5605,
email:
.
Figure 1- Typical Turbine Rupture
Disc Housing
46
Nuclear Plant Journal, January-February 2013
