By David W. Spitzer, PEPERIODIC FLOWMETER
verification and/or calibration are commonly used to maintain availability and proper operation of installed flowmeters. However, these approaches typically detect flowmeter problems after they occur, and usually only after the flowmeter has a detrimental effect on the process.
To detect and diagnose potential problems sooner, flowmeter transmitters often contain self-diagnostics that verify their operation. Integrating microprocessors into transmitters enabled development of software diagnostics that could verify operation of transmitter electronics. In addition, some technologies allow transmitters to perform limited diagnostics on parameters associated with the flowmeter primary.
For example, many microprocessor (and some analog) magnetic flowmeters can verify that the electrodes are part of a complete electrical circuit. An incomplete circuit is a condition that likely indicates that the pipe is empty, which could generate an alarm calling for operator or maintenance attention. In general, this particular diagnostic coverage is relatively straightforward, and has been offered for some time.
There is an underlying trend to reduce instrumentation maintenance requirements, while simultaneously increasing availability of the process. Fortunately, process availability often can be improved by increasing the instrumentation availability. One way to accomplish this is by identifying and fixing instrument problems before they affect the process.
However, a major difficulty in accomplishing this goal is that self-diagnostics typically verify transmitter operation, but generally don’t provide extensive diagnostics relating to the flowmeter primary. If you want to confirm this, examine the list of diagnostic errors associated with one of your typical flowmeters. You likely will find many errors relating to transmitter electronic “health” and few or no diagnostic errors relating to the flowmeter primary.Status Checking
However, expanded diagnostics applicable to some flowmeter technologies are becoming available to check more of the status relating to the primary element. In some flowmeters, primary element status is used to infer that flowmeter calibration hasn’t shifted. This inference isn’t a substitute for checking calibration, but when properly designed and interpreted, this status information does increase the probability that the flowmeter is operating properly and that the flowmeter calibration hasn’t shifted.
A brief review of calibration here may help clarify implications of information in the previous paragraph. Flowmeters can be calibrated by comparing measurements of the same fluid through a flow laboratory calibration standard and the “meter under test” (MUT). This can be done by circulating the liquid (See Figure 1, top),
diverting the liquid to a weigh tank (middle),
and then circulating again (bottom).
By comparing measurements from the standard and the MUT (while diverting), the MUT can be adjusted (calibrated) to provide measurements that mimic the standard. FIGURE 1: PRIMARY FLOWMETER LABORATORY
Calibrating flowmeters can be done by comparing measurements of the same fluid through a flow laboratory calibration standard and the "meter under test" (MUT). This is accomplished by first circulating the liquid (top), diverting the liquid to a weigh tank (middle), and then circulating again (bottom). By comparing measurements from the standard and the MUT (while diverting), the MUT can be adjusted (calibrated) to provide measurements that mimic the standard.
Calibration also can be performed by comparing measurements from the MUT with a flow laboratory with a master flowmeter in series with the MUT. In contrast, in-situ calibration is performed on the installed flowmeter in the field, where its measurements are compared with a standard such as a portable prover.
Note that these are wet calibrations because the actual fluid flows through the MUT and the standard. Alternatively, dry calibrations are often used to infer that the flowmeter is calibrated. For example, differential pressure flow transmitters are often wet calibrated using a pressure source; however, its orifice plate flowmeter primary may be dry calibrated by measuring its diameter and visually inspecting it for damage. Many times, it is not examined at all, so it remains uncalibrated and unchecked.
On the other hand, using a calibrator to verify operation of a magnetic flowmeter performs a wet calibration on the transmitter, but doesn’t check calibration of the magnetic flowmeter primary at all. This procedure is commonly called a “calibration.” Note that it’s entirely possible that the magnetic flowmeter primary could be damaged or even non-functional before, during, and after this type of “calibration.” Fewer Calibrations = Less Costs
Consequently, expanded diagnostics for the flowmeter primary are not a substitute for a wet calibration. However, performing expanded diagnostics does increase the probability that the flowmeter primary is still operational and functioning properly. Instruments with a higher probability of proper operation need not be calibrated as often as instruments subject to frequent calibration shifts or failure. Therefore, adding expanded diagnostics to a flow measurement system can increase the time between calibrations and can reduce the overall cost of calibrating the flowmeter.
Flowmeters that don’t have expanded diagnostics generally require some human intervention to locate and diagnose problems. In many plants, the expertise needed to diagnose the problem may not be readily available. Expanded diagnostics can reduce labor costs by more clearly identifying the problem quickly without human intervention. In some cases, problems can be detected and identified, while the problem may still be intermittent and occur well before the process is affected. This can improve process availability and save significant amounts of money in some applications.