Ultrasonic
Leak
Detection
By Phillip Twaddle, Exelon Generation.
Phillip Twaddle
Phillip Twaddle works for Exelon
Generation Corporate
Engineering,
where he has been
the Component
Programs Safety
Relief Valve
(SRV) Engineer
for seven years.
He has oversight
responsibility for all
Class 1/2/3 SRVs at
seventeen Exelon
Nuclear power
plants.
Twaddle has a
B.S. in Chemical
Engineering from
the University
of Illinois at
Champaign – Urbana, 1982.
The described innovation was a 2013
Equipment Reliability Award Winner.
Nuclear Energy Institute’s Top Industry
Practice (TIP) Awards highlight the
nuclear industry’s most innovative
techniques and ideas.
The team members who participated
included: Phillip Twaddle, Corporate
Engineering Component, SRV Specialist,
Exelon Generation; Rick Lack, Peach
Bottom System Engineer, Exelon
Generation; Dan Nugent, Limerick
System Engineer, Exelon Generation;
Marie Murphy, Limerick CMO
Valve Engineer, Exelon Generation;
Mike Desai, Corporate Engineering
Welding Specialist, Exelon Generation;
Steve Scheeren, Business Manager,
Scientech; Brad Schulte, President, NWS
Technologies.
Summary
Many Limerick (LGS) and Peach
Bottom (PB) Target Rock (TR) 3-stage
Safety Relief Valves (SRVs) over the past
10 years have passed the cold bar test
after refurbishment only to develop leaks
while in-service that require unscheduled
maintenance outages to replace these
same valves prior to a refuel outage.
The MSRV as-left certification test is an
ASME method called the cold bar test
which is used as the current acceptance
criteria for verifying that the refurbished
valves have been repaired properly
for installation in the Nuclear power
plants during refuel
outages. The cold
bar test is inserted
into the valve, and
any steam leaking
past
the
seat
collects as drops of
condensation on the
surface of the cold
bar. The cold bar
test is a pass/fail test
where the absence
of water droplets
on the metal surface
of the cold bar is
considered a pass.
Since the cold bar
test is a qualitative
test, there is no
information known about how close the
valves were to becoming leakers.
The
Curtiss-Wright
Scientech
ultrasonic leak detection sensors have
been used in mechanical applications
that monitor rotating equipment, such
as pumps or impeller bearings on cruise
ships. Phillip Twaddle was searching
for a means to acquire quantitative test
results on “seat tightness” during as-left
certification testing of main steam TR
3-stage SRVs, and Phillip thought of and
initiated an opportunity to use Scientech
ultrasonics to test a PB TR 3- stage SRV
with a severe leak that was removed
during an unscheduled maintenance
outage during 2008. Another PB TR
3-stage SRV with recently replaced pilot
and 2nd stage seats was tested using
Scientech ultrasonic as a Control of a
“good” SRV per Phillip’s request and the
test results were satisfactory. The shear
wave energy (SWE) test results that were
recorded showed that a good TR 3-stage
SRV with tight seats have much lower
SWE values on all three seats than a bad
TR 3-stage SRV with one or more bad
seats that have much higher SWE values.
During the last 6 months of 2011 and
12 months of 2012, more than 96 main
steam valves were steam tested using the
Scientech ultrasonic sensors attached to
the valves during as-left certification at
NWS Technologies. The average SWE
values and standard deviations were
calculated for each seat after all three of
the as-left certification lifts. These SWE
data results are being compared to the
average delay times and the cold bar test
results.
The results of the Scientech test
are being used in conjunction with the
cold bar test results to determine if the
refurbishment results are satisfactory or
more repairs are needed.
Safety
The Scientech ultrasonic leak
detection method will eliminate leaking
TR 3-stage SRVs between the 2-year
refuel cycles and allow the LGS Site
Management to transition from shopping
all 14 Target Rock 3-stage SRVs (100%)
in each unit every refuel outage to only
removing and refurbishing 7 Target
Rock 3-stage SRVs (50%) in each unit
every refuel outage. The cost saving of
refurbishing 7 less TR 3-stage SRVs per
year is equivalent to $750,000 savings/
year and a reduction in dose of 7 TR
3-stage SRVs not refurbished each refuel
outage times 7 person-rem is equal to 49
person-rem per year. The cost and dose
savings is expected to be implemented
within 2 years in conjunction with the
Limerick EPU changes.
Cost Savings
There has been an unscheduled
maintenance outage to replace a leaking
Target Rock 3-stage SRV at either Peach
Bottom or Limerick each year for the past
five years: 2008 -PB, 2009 -PB, 2010-
PB, 2011-LGS, and 2012-LGS (2 valves
at once). Using the Scientech ultrasonic
leak detection method should save
$7,000,000/year for the next four years =
$28,000,000.
Innovation
The Scientech ultrasonic leak
detection method has never been used to
quantitatively determine the acceptability
of refurbished valves. This application
(Continued on page 44)
Nuclear Plant Journal, September-October 2013 NuclearPlantJournal.com
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