| Case Studies
TRACERCO Diagnostics™ Flow Study
The use of radioisotope tracer measurements to optimize
the operating cycle of a steam turbine
A steam turbine was used to compress gas prior to the reaction
section of an ethylene oxide plant. The flow of process
gas to the reactor was measured by an installed flow meter,
with an output signal from the flow meter fed back to a
control valve on the inlet steam line to the turbine. In
this way the flow of steam to the turbine was controlled
so as to maintain a constant throughput of process gas.
During the course of a routine maintenance shutdown, it
was observed that the blades of the turbine had suffered
significant metal losses. These were attributed to a combination
of (a) corrosion, brought about by deposition on the blades
of minute traces of sodium salts present in water entrained
in the steam and (b) erosion caused by impingement of the
entrained water on the corroded blades.
From the magnitude of the metal loss, it could be directly
inferred that the efficiency of the turbine had been severely
impaired and this in turn, implied that steam utilization
must have been excessively high.
From the evidence in hand, it was possible to predict the
rate at which the metal loss and therefore the loss of efficiency,
had taken place. To measure, and take steps towards preventing
loss of efficiency some means of measuring steam utilization
was required. It was not possible to fit a conventional
flow meter as the configuration of the steam lines inlet
and exit the turbine did not offer an appropriate location
for an installation. It was decided to employ Tracerco to
undertake a programme of regular steam-flow measurements
using the TRACERCO Diagnostics™ Flow Study
technique, to study the performance of the turbine as a
function of time.
The TRACERCO Diagnostics™ Flow Study technique
helped to ensure that the plant manager ‘got it right
first time’ - and every time. The regular measurements
of efficiency enabled him to optimise the operating schedule
of his plant, balancing temporary loss incurred by shutdown
against savings from avoidance of steam losses. Additionally,
the radioactive tracer measurements provided timely warning
of the onset of efficiency deterioration so that corrective
action could be taken before the turbine blades suffered
serious, and possibly catastrophic damage.
The absence of appropriate pipe-runs meant that, a pulse-velocity
technique was not feasible. It was therefore decided to
employ a dilution flow method, injecting on the inlet line
to the turbine, sampling on its exit, and relying upon the
turbulence in the machine to produce the requisite good
mixing. Since the steam was known to be partially wet, Tracerco
decided that a tritium dilution technique would be most
appropriate.
A metering pump was coupled to the steam inlet line via
a pressure gauge tapping and was used to inject tritiated
water at a constant and measured rate into the steam.
Samples were extracted from the steam exit line using a
cooling coil in an ice bath to condense the steam. The sampling
rate was approximately one per minute throughout the 16-minute
duration of the injection. The samples were returned to
the Tracerco laboratory and 10g aliquots of each were assayed
using a liquid scintillation counter. Samples of the injected
material, after appropriate dilutions were also assayed
in a similar manner.
Typically, the accuracy of the steam flowrate measurement
was+/- 1% or better. The first measurement was made approximately
4 months after the plant had been brought on line and, though
data for the first four months were not available it could,
nevertheless, be inferred from the measurements that steam
utilization, for the first 10 months of operation was constant.
Thereafter, steam utilization increased until, after 16
months on line it was judged to be appropriate to temporarily
shut down the plant and bring on-line a standby turbine
rather that to continue at greatly reduced steam efficiency.
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