On site flow calibration is painful but necessary

Some new product introductions have raised doubt about what in-situ calibration for flowmeters is, and whether it can be duplicated with simulators and calibrators with expanded diagnostics.

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An engineer in a northern USA cement plant says that his plant has “a few magnetic flowmeters that measure water flows, but there are no calibration procedures for them. Our stack gas flowmeters are calibrated by the manufacturer’s personnel, but I do not know if they are wet tested or not.”

In-situ Calibration Equipment
With the exception of flowmeter provers, most in-situ calibration equipment is designed to calibrate the flowmeter secondary, or transmitter. Some of these calibrators are designed to transfer calibration information to/from other systems such as maintenance management systems. Automating these information transfers can speed the availability of information, improve worker productivity, and reduce the number of errors that inherently occur in a manual system.

John DuBay, Product Manager, Meriam Process Technologies, Cleveland, Oh., says, “Our calibrators address the calibration of the flowmeter secondary. The calibration procedures can be set up and scheduled using PC-based software and then downloaded to a handheld calibrator. The calibration can then be performed and automatically documented, yielding results which are then uploaded back into the PC-based software.” Other vendors produce similar systems. Documentation of the calibration can be paperless, and depending upon the system details, integration with computerized maintenance management systems and messaging systems may be possible.

DuBay continues, “Electronic document control for the FDA can be implemented by associating a user with the handheld calibrator that performs the calibration.” This is accomplished by means of a pass code and user ID that must be entered to invoke the rights and allow validation of the handheld operator’s actions. This signing information, along with the calibration results, is stored in an audit trail that conforms to 21 CFR Part 11 requirements.

Unfortunately, suppliers do not always offer a clear distinction between in-situ calibration and expanded diagnostic coverage. Expanded diagnostic coverage can improve flowmeter availability by continuously monitoring certain parameters and comparing them with historical values to detect and diagnose certain flowmeter problems in a timely manner. Implementing expanded diagnostics can be beneficial to help justify an increase in the time between calibrations. However, performing expanded diagnostic coverage does not provide an in-situ flowmeter calibration (as some suppliers might imply), so it is not a substitute for an in-situ flowmeter calibration and should not be represented as such. 

Expanded Diagnostic Coverage

PERIODIC FLOWMETER verification and/or calibration are commonly used to maintain the availability of flowmeters installed in operating facilities. These approaches typically detect flowmeter problems after they occur, and often only after the flowmeter had a detrimental effect on the process.

To detect potential problems before they occur, the transmitter often contains self-diagnostics that verify its operation. While this approach may provide significant diagnostic coverage for the transmitter, it generally provides only rudimentary diagnostic coverage of the flowmeter primary.

However, some flowmeter suppliers are embedding diagnostics that verify the integrity of the flowmeter primary. In general, certain parameters associated with the flowmeter primary are measured in the field and compared with measurements made at the factory. As long as the difference between the factory values and field measurements remain within tolerance, the calibration of the flowmeter primary is presumed to have not shifted. Some expanded diagnostics are continuously monitored, whereas others are performed periodically.

Note that this approach is not a flowmeter calibration, but rather an in-situ flowmeter verification check that implies (but does not definitively determine) that the calibration of the flowmeter primary has not shifted. It can be likened to checking to verify that the condition and dimensions of an orifice plate have not changed, thereby implying that its calibration has not changed. However, this orifice plate check would not detect an out-of-round pipe, coating, plugging, or an obstruction that could change the calibration of the orifice plate flow measurement system. Even using physical inspection of the primary cannot detect out-of-round pipes, welding slag and burrs, a glove caught on the valve upstream, or other serious problems. Only an in-situ flowmeter calibration can do all that and make sure the flowmeter system is accurate and reliable.

Despite its limitations, expanded diagnostic coverage is an in-situ flowmeter verification tool that can improve flowmeter availability and extend intervals between routine calibrations/verifications. It cannot completely replace the actual calibration of a flowmeter system, either in-situ or at the lab.

For example, Micromotion (Emerson Process Management) recently announced the availability of expanded diagnostic coverage for its Coriolis mass flowmeters. They have found that verifying the density calibration by periodically filling the flowmeter with known fluids (typically water and air) implies that the flowmeter calibration has not changed due to product build-up or corrosion.

Some flowmeter suppliers such as ABB, Krohne, and Siemens can provide portable equipment to periodically check their magnetic flowmeter primaries for insulation resistance and other internal electrical parameters that are indicative of flowmeter primary problems. Krohne offers a transmitter with embedded expanded diagnostics that can perform these checks continuously.

 About the Author
David W. Spitzer is a principal in Spitzer and Boyes LLC, which offers consulting services for the process industries. He can be reached at 845/623-1830 or at spitzerandboyes.com.

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