Although
Coriolis mass flow meters are not always included in the planning of a shut down,
this may be a good time to perform some preventative maintenance on the critical
flow meters. You may have heard that Coriolis meters are so dependable that they
should work forever with no attention. In reality, as long as man makes Coriolis
meters using man-designed machines there will be a few that perform a little outside
factory specifications. Shut downs are an opportunity to check and calibrate your
critical flow meters. The best way to calibrate a Coriolis meter is to remove
the meter, clean it and send it to a facility that has a gravimetric calibration
flow laboratory. In place "proving" may be acceptable for applications
that do not require great accuracy, but for a critical measurement, there is no
substitute for direct mass-to-mass calibration. Master meter comparators and "inferred-mass"
volumetric provers cannot approach the accuracy of a gravimetric facility. Mass
Flow Technology in Baytown, Texas has a gravimetric flow calibration laboratory
with 0.052% system uncertainty. Some factories have equivalent facilities for
calibrating production meters and may provide certified calibration services for
customer meters. If your process fluid is likely to coat or plug,
check the meter for internal deposits. Deposits on the inner flow tube walls will
degrade meter accuracy. Decontaminate the flow element and use a bore scope to
check for deposits inside the flow tubes. If deposits are found, a good hydro-cleaning
company can clean the flow tubes. Mass Flow Technology has had considerable success
is cleaning Coriolis flow meters that are plugged with set-up concrete. You
don't have to wait for a shut down to keep up with basic and periodic maintenance.
Several valuable checks can be made on Coriolis meters during normal operating
times. Flow meter zero (the flow meter output during non-flowing conditions) can
be checked any time the process flow can be blocked for a few minuets. When process
flow is blocked, the flow meter should indicate zero flow. The procedure is simple.
Close the upstream and downstream valves and read the flow rate. The best time
to check the meter zero is immediately following a batch, not before the batch.
The process should be stabilized to operating conditions and entrainment should
be purged. Also, make sure any parameters that determine a flow cutoff threshold
is set to "0.0" before checking the meter zero. After checking the meter
zero, return the original cutoff threshold parameter. Periodic checks
can be a valuable indicator for conditions that gradually grow from nothing into
a big problem. Most manufacturers have test points that can be measured and compared
to previous checks made under similar conditions. Make a chart for recording these
test points and compare the most recent checks to past checks. This may show a
trend.
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