PROCESS EQUIPMENT UPDATE
However, as soon as you re-install it the process might be depositing dirt back on it. It has been calibrated for a perfect installation and is almost immediately imperfect. In this scenario, calibration is completely pointless.
LAB CONDITIONS VERSUS INSTALLATION CONDITIONS Next, it’s hard to compare installation to installation. All calibration laboratories pride themselves on making adequate provisions for calibration, especially good installation practice. If they’re testing a turbine meter, for example, then they should have a long length of correctly sized piping before the meter – and
Depending on the meter type, once installed, the original calibration may be valid – or it may not be
THE ROLE OF TEST FLUID Next you should ask whether it is the right fluid. Unless your process is running clean water or, maybe a calibration fluid, then your average lab will not be able to calibrate with the same fluid. For some flowmeter types this may not be important. For example, if you fluid is a weak acid with a viscosity of 1.2cP and the meter is an electromag, then the calibration with water will be perfectly valid. Contrarily, if you have 10cP process fluid and it’s a turbine meter then it could be very important that the test fluid is in the 9 to 11cP range to adequately represent the effect of viscosity on meter performance at lower flows.
WHAT ABOUT TRACEABILITY? Traceability is next on the list. If you have been able to clear the hurdles above then it’s important you pick a lab that has the right traceability for you. If your process demands an indication of flow within +/- 4% then there’s little point on getting a UKAS- accredited laboratory with an uncertainty level of 0.22%. We’re regularly asked ‘how often should it be
The low flow rig delivers automatic calibration of flowmeters and instrumentation within a flow range of 0.0006 to 50 l/hr to an accuracy of 0.2%
a length after, too. This eliminates swirl, if it’s long enough, to generate a flat flow profile and present optimum conditions to the meter. Most labs have this setup for horizontal installation – so if you have a vertical install, then watch out. Likewise, if you don’t have a long length of
correctly sized pipe, or perhaps a connector that necks the diameter down a few percent, then don’t bother. The results they give you will be meaningless.
calibrated?’ Recalibration periods of flowmeters are based on industry standards. In industrial applications, depending on the industry, periods of six to 12 months are recommended. We advise the user to seek out data relating to the process, other components within the process and the usage of the meter. If the measurement is critical then the recalibration should be more frequent than a non-critical, rarely used device. In the absence of any other data we advise an annual check and to vary the future calibration periods depending on results. If it has remained unused then no recalibration may
be necessary, depending on the meter type. It is wise to check that no fluid has settled in the meter that might alter the way the meter works or even cause corrosion. In the event of any doubt then the manufacturer is always your best source of advice. n
For more information ✔ at
www.engineerlive.com/process
Charles Wemyss is CEO of Litre Meter.
www.litremeter.com
www.engineerlive.com 11
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