| Case Studies
TRACERCO Diagnostics™ Leak Study
Determining Evidence of Gas By-Passing Through IS Reactor
A TRACERCO Diagnostics™ Leak Study was employed
to look for evidence of any process gas by-passing through
the Intermediate Shift Reactor (ISR), or gas leaking through
the ISR by-pass valve and to identify a cause for this effect
on a chemical plant in India. A variety of process measurements
and analysis had indicated that the ISR was performing at
a reduced efficiency. The most likely cause of this effect
was a mechanism for the process gases to by-pass this system.
Two possible scenarios had emerged for the cause of this
gas bypassing effect.
- The bypass line isolation valve was leaking allowing
unreacted gas into the exit line.
- Some mechanical defect has occurred within the reactor
allowing a percentage of gas to channel through unreacted.
The TRACERCO Diagnostics™ Leak Study is based
on the principle that if a pulse of a gamma-ray emitting
radiotracer is injected into the process stream feed to
a particular process component (in this case the ISR), then
any leakage or by-passing within the system will be seen
as a ‘double peak’, by sensitive radiation detectors
positioned externally at a strategic position on the gas
exit line.
This ‘double peak’ is caused by a detector
response firstly to the ‘bypassing or the channelling’
radiotracer then a secondary response due to the tracer
which has followed the normal route through the system.
In addition to these detectors, additional ones were placed
at various locations to monitor the progress of the tracer
in the system. Detectors were placed downstream of the gas
bypass valve and their response to the radiotracer would
provide indications of any leakage through this valve.
Also rings of detectors were placed circumferentially at
two elevations on the reactor itself. The timing, comparative
magnitude and shape of their response would provide additional
information to the passage of the radiotracer through the
reactor.
The responses from these detectors were relayed via co-axial
cables to a data processing unit. A computer connected to
the processing unit is used to log the data on a dedicated
software package, which was also used to analyse the response
data. The test was performed utilising a suitable radioactive
gas tracer, injected via a ¼” instrument nozzle,
into the reactor gas feed line approximately two metres
upstream of the by pass valve T-piece. The radiotracer pulse
was then monitored using externally mounted radiation detectors
as it passed through the system. Results from the TRACERCO
Diagnostics™ Leak Study showed that as bypassing
was detected during four separate tracer injections and
was calculated to be of the order of approximately 15-18%
of the total gas flowrate through the reactor.
The mean bypassing residence time through the reactor was
measured at approximately 5.6 seconds. The bulk gas residence
time through the reactor was approximately 20.2 seconds.
There was no indication of any leakage detected across the
ISR bypass valve. Some signal variations were apparent from
the circumferentially mounted rings of detectors, on the
vessel, which were at two different elevations. However,
there is not sufficient evidence at this time to conclude
that any of the gas is bypassing down the outer vessel shroud.
Critically, a very small response from the North East detector
mounted in the lower ring of detectors arrived at the same
time as the bypassing had been detected on the first exit
detector. Therefore, some degree of bypassing cannot be
ruled out for the North East quadrant of the reactor. No
similar effect was observed from the top ring of detectors.
Channelling of gas through catalyst bed appears unlikely.
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