Process manufacturing and other industrial facilities must often provide documented evidence of flowmeter performance to maintain compliance with various regulatory agencies, ensure product quality and optimize production. Today’s smart instrumentation helps plants accomplish these tasks through built-in verification techniques traceable to known metrology standards.
This article explains how self-verification works.
In the water and wastewater industry, typical flowmeter requirements are:
- Flowmeters must be verified at regular intervals.
- Verification must be performed by a qualified third party and with an accepted inspection method based on quality regulations such as the International Organization for Standardization (ISO) 9001.
- A test report must be provided for documented proof of verification.
In the pharmaceutical industry, quality risk management has become a mandatory regulatory requirement for drug manufacturers. The U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) publish guidelines and requirements for process instrumentation that customers and vendors are expected to follow. Guidelines such as "Process Validation: General Principles and Practices" by the FDA and Annex 15 issued by the EMA offer input to help drug manufacturers manage instrumentation correctly.
The chemical industry has requirements for proof testing per the International Electrotechnical Commission (IEC) 61508 and IEC 61511, while the oil and gas industry must adhere to contractual agreements between buyers and sellers, and comply with government agency mandates. For example, a company pulling oil from a well under an agreement with the Bureau of Land Management or the property owner may have to prove flowmeters at a determined frequency.
Calibration versus verification
Modern flowmeters operating based on a Coriolis, electromagnetic, ultrasonic, vortex or thermal measuring principle do not have moving parts subject to wear. But they can have problems that require recalibration, including failure of an internal part or degradation because of temperature effects on electronics — or corrosion, clogging or coating buildups in the flowmeter body.
Legal requirements are commonly fulfilled with wet calibrations, in which a flowmeter is removed from the process (see Image 1), taken to a flow lab or calibration rig, and tested by a formal comparison to a standard directly or indirectly related to national standards. Detected deviations between the displayed value and the reference value can be corrected after the calibration by adjusting the calibration factor. A calibration protocol is issued to document the findings, and is put on record for possible audits.
The downside of wet calibration is the instrument must typically be removed from the process. After calibration, the instrument is then sent back to the facility to be reinstalled. Damages during transport or handling can sometimes stay undetected and lead to a situation where a recently calibrated instrument does not perform per specifications.
An alternative way to fulfill legal requirements is onboard verification of the flowmeter. The flowmeter’s transmitter electronics run an onboard diagnostics program during which all relevant components of the instrument are checked to confirm and document that the instrument is still in calibration and none of the meter components have drifted outside original tolerances.
Verification of a flowmeter equipped with built-in capabilities can be performed without removing the instrument from the process. It may not even be required to interrupt the process because the verification tests can be performed in the background. This article uses Endress+Hauser’s Heartbeat Technology as an example.
How self-verification works
Several flowmeter manufacturers offer self-verification, and all work differently and have different capabilities. Verification in flowmeters equipped with Heartbeat Technology is based on continuous monitoring of all relevant internal parameters and mechanical, electromechanical and electronic components.
Typically, failure mode, effects and diagnostic analysis are used during the flowmeter’s design phase to identify critical components in the signal chain, starting at the process-wetted parts and followed by the electromechanical components, amplifier board, main electronic module and outputs. A proper margin of safety is then assigned to every critical path or component.
These tests include digital signal processing and continuous loop checks with the help of internal reference components. In the case of a Coriolis flowmeter — and other time-based measuring principles like vortex or ultrasonic — a frequency reference oscillator is used for analyzing the frequency of the measuring tubes. For electromagnetic flowmeters, this is a reference voltage since the measured value is determined by comparing the voltage on the electrodes to the reference voltage.
The primary reference is monitored by a second redundant reference system to guarantee it does not change during the device life cycle. The two reference signals from the primary and secondary references are compared against each other. A drift or deviation of the two systems from each other is immediately detected and reported by the device diagnostics.
For an internal component to be used as a diagnostic reference, it must fulfill special requirements such as factory traceability and exceptional long-term stability. For the most critical circuits, independent and redundant components are implemented, greatly reducing the possibility of undetected drift. Today it is possible to design instruments with a self-diagnostics coverage of 94 percent or higher (in accordance with IEC 61508) and correspondingly low rates of dangerous undetected failures.
Heartbeat Technology continuously monitors the entire signal chain for deviations within a tight band. The failure threshold is defined by the flowmeter’s specified accuracy. If the diagnostics detect an error, the testing solution sends an alarm message that conforms to NAMUR recommendation NE 107 (see Figure 1). The alarm is displayed on the flowmeter’s front panel and can be sent as a message over the interface to the automation system. The message also includes troubleshooting tips and remedial instructions.
Diagnostics to detect problems are performed continuously, but a verification is done on command from the automation system or at the instrument itself. In most cases, diagnostics continuously perform all of the same checks in the background as the verification performs on-demand. The verification, then, is simply a snapshot in time. Verification allows documented evidence to be generated from the device and saved for record keeping purposes.
For Coriolis devices, the technology tests electrodynamic excitation, electrodynamic sensors, temperature sensors, connectors, cables and the measuring tube (see Figure 2). It verifies the excitation system and the coil current for electromagnetic flowmeters, and checks the electrical and mechanical integrity of the transducer and the temperature sensors (as applicable). This enables the detection of systematic failures caused by factors such as fluid properties or process operating conditions.
The testing solution fully complies with the requirements for traceable verification according to DIN EN ISO 9001:2008, Section 7.6 a "Control of monitoring and measuring equipment." Performing regular verification on a flowmeter can extend calibration cycles by a factor of 10 or higher without jeopardizing the quality or the regulatory compliance. In some cases, it may even be possible to replace wet calibrations completely with verification.
Upon completion of a verification, a report is generated that summarizes the results. The report can be shown to an inspector in case of an audit.
Flowmeter faults during operation that go undetected by diagnostics can result in an unexpected plant shutdown, product loss or a reduction in product quality. This is particularly true in applications in which process-related faults during operation are expected because of demanding operating conditions such as multiphase media, buildup, corrosion or abrasion. Condition monitoring recognizes if the measuring performance or the integrity of the flowmeter are impaired.
The monitoring values described above are transmitted to an external condition monitoring systemsuch as Endress+Hauser’s PC-based FieldCare software. A condition monitoring system can be used to recognize trends in the secondary measured values and to evaluate relationships among individual parameters. Condition monitoring reduces the risks of an unexpected failure. Condition monitoring also makes it possible to display temporary, process-specific faults that neither calibration nor verification can detect, since the latter only takes a snapshot of the device status as opposed to continuous condition monitoring.
Built-in flowmeter verification can be initiated locally or remotely from the automation system, even during operation. Because the procedure is simple and noninvasive, the meter can be verified on a regular basis (e.g., daily or quarterly), drastically reducing the risk of degradation in meter performance between wet calibrations. In batch applications, a system check can be initiated from the automation system prior to starting the batch to ensure all flowmeters work properly, greatly reducing the risk for product loss because of instrument failures.
Nathan Hedrick has more than seven years of experience consulting on process automation. He graduated from Rose-Hulman in 2009 with a bachelor’s degree in chemical engineering. He began his career with Endress+Hauser in 2009 as a technical support engineer. In 2014, Hedrick became the technical support team manager for flow where he was responsible for managing the technical support team covering the flow product line. He has been flow product marketing manager since 2015.