Have you ever seen fire ants excitedly swarming over a dropped sandwich? At first glance, you might believe that you were looking at a bunch of ants running around with no organization or direction to their movements. Take another look a few minutes later and you will see that the sandwich is noticeably smaller. Each of those ants has a purpose and an objective. They are working as a team to disassemble and transport the sandwich to a specific place. A unit shutdown has a similar appearance. First glance shows group of workers swarming over a piece of equipment with no organization or direction. But, like those ants, each worker knows what he is expected to do. Many hours of planning and preparation preceded the start of maintenance. By the time the workers swarm the unit, the job has been planned and organized down to the number of man-hours it will take to finish the task.
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.