Self-verifying flowmeters: modern technology improves instrument management
By Gernot Engstler, Endress+Hauser Product Management
Monday, 05 May, 2014
The process industry is undertaking great efforts to ensure high levels of process reliability, consistent product quality and accurately priced billing of goods. There is also an increasing need to prove that operation is sustainable by meeting environmental regulations.
State-of-the-art process measuring technology is the key to ensuring reliability, quality and sustainability goals, by ensuring highly stable measurement results over a long period of time. Despite this, it is common practice to inspect quality or safety-related measuring points at regular intervals.
Modern flow sensors don’t have moving parts that are subject to wear and feature measuring electronics with self-diagnosis. These two aspects make it possible to significantly reduce effort in the management of these devices by preventing unnecessary maintenance work or calibration cycles, while at the same time supporting quick troubleshooting. Verification reports can also be created based on internal device tests which allow the device to document its own health condition.
Presently, individual device manufacturers have integrated diagnostics, monitoring and verification functions in the flowmeter so that they can be used in a uniform manner for the entire installed base. Consistent handling and uniform functionality allow end users to simplify their operational workflows by standardising operating procedures. Users only need to learn how to work with a method once, resulting in cost savings through increased efficiency.
A device with high long-term stability, which is tested using highly stable, internal references with a redundant design, is a basic requirement for reliable device-internal diagnostic and verification methods. For devices with internal verification, the traditional method of verification with traceable, external measuring instruments is no longer necessary. Often the intervals between labour-intensive recalibrations can be extended. The advantages of this process include the ease of use and the option for integrating into a higher-level control system or asset management system. All of this saves time and costs, while virtually eliminating the possibility of interference due to incorrect handling.
Individual manufacturers of process measuring technology recognised the trend to reducing costs through simplified management of assets and reduced specialisation years ago, and today offer comprehensive solutions for reducing complexity. The objective here is simplicity obtained by consistent and uniform solutions and by omitting specialised expertise.
Standardisation is based both on uniformity (doing the same thing the same way) and consistency (offering one solution for all instruments). Presenting functionality for all instruments and devices in a uniform manner makes handling safer and simplifies the learning curve while streamlining operational workflows and ensuring the sustainability of established processes and acquired knowledge.
The aim of seamless integration is to improve the flow of information between the device and its environment by perfect interaction between device and host. Diagnostics integrated in the device constantly deliver information about the device status or provide notification of events, such as if the current process conditions are interfering with measuring performance. Rapid and specifically targeted troubleshooting is possible since each diagnostic event on the system displays an additional corrective action.
Once the device itself generates and stores the information necessary for documenting the device inspection, it is automatically available to all operating and integration interfaces.
In addition, integration increases the safety of personnel since, under certain conditions, the operator or technician can call up information in the field without access to the measuring point.
Measurement with consistent quality
Measuring with consistent quality requires long-term stability, diagnostics, condition monitoring and verification.
Modern flowmeters, which operate based on a Coriolis, electromagnetic, ultrasonic, vortex or thermal measuring principle, are well known for highly stable measurement results over a long period of time.
The addition of an integrated self-monitoring system allows safety- or quality-related conditions to be identified in a timely manner if these measurement technologies are used in applications where the process conditions influence measuring performance or harm the integrity of the device.
That is to say, measurement technologies can provide other secondary measured variables in addition to the primary measured variable (flow), which are useful for monitoring and documenting the measuring point.
Diagnostics are primarily based on constantly monitoring the function of the device’s internal components during ongoing operation, allowing for a rapid response in a timely manner. These messages are typically interpreted in accordance with NAMUR recommendation NE 107 and are displayed by the device as a diagnostic event. This also includes direct instructions on what to do next - ensuring that the process can be up and running again quickly in the case of a shutdown, while preventing unnecessary maintenance measures.
Diagnostics enable quick and targeted responses to interruptions during operation in case of an error or a device failure. This is enough to guarantee safe, reliable operation for most applications. Faults during operation that go undetected, or are not detected in a timely manner, can result in an unexpected plant shutdown, product loss or a reduction in product quality. This specifically applies to applications where process-related faults during operation are to be expected due to the demanding operating conditions (formation of build-up, multiphase media) or where the device is subject to programmed wear (corrosion, abrasion). Condition monitoring is recommended for these types of applications, since it recognises if the process conditions, the measuring performance or the integrity of the device are impaired.
Verification can be used to take and store a snapshot of the device status. Verification is used to demonstrate that the flowmeter meets specific technical requirements defined by the manufacturer or end user (ie, the process application).
During verification, the current conditions of the secondary parameters are compared with their reference values, thereby determining the device status, resulting in a ‘pass’ or ‘fail’ result. A traceable and redundant reference, contained in the verification system of the device, is used to ensure the reliability of the results. In the case of a Coriolis flowmeter, this is an oscillator, which provides a second, independent reference frequency.
A verification report can be generated by means of a web server or asset management software. It can be implemented either as quality documentation (for compliance with ISO 9001) or, in safety-related applications, as documentation of the proof test (for functional safety - SIL).
A flowmeter is expected to demonstrate constant and therefore unchanged measuring performance throughout its entire lifespan. This is necessary to:
- guarantee safe plant operation
- ensure high product quality
- increase system availability and productivity
A number of requirements must be met to enhance operational reliability. These requirements are met best by comprehensive diagnostics of the measuring point in running operation and methods for condition-based maintenance. As we have seen, condition monitoring and verification provide efficient methods for measuring point maintenance during the system’s life cycle.
Traceability and long-term stability
To ‘measure’ means to compare an actual value to a reference value. In a self-verifying flowmeter, the actual value reported from the sensor is compared to a reference value in the transmitter electronics. In order to produce accurate measuring results, the reference value needs to be reliable. For this purpose, integrated self-monitoring of the reference value is applied.
To be effective, such integrated self-monitoring must be based on a traceable reference system with proven long-term stability. This allows a high level of stability - even without verification by external measuring instruments.
Verification of flowmeters
To ensure the conformity (quality) of the product, the ISO 9001 requires that:
In order to ensure valid results, the measuring equipment shall be calibrated and/or verified at specified intervals or prior to use, against measurement standards traceable to international or national measurement standards. Records must be kept on the results of the calibration or verification.1
ISO 9001 requirements also provide the impetus for today’s common practice of requiring an independent reference system for device inspection through verification. However, this does not verify the primary measured variable (flow), but rather the device function.
In practice, reliable verification of flowmeters can be fulfilled in two ways: either via an external verifier whose references can be traced along the life cycle by recalibrating the verifier at periodical intervals or by an internal verification which is based on factory-traceable references that are stable in the long term.
In the past, a method to assure the long-term stability of an internal verification system has not been available. Now, with the latest generation of flowmeters, a reliable internal verification technique has become available for the very first time.
Evolution to internal verification
External verification is a very complex procedure that requires access to the measuring point in the field. During verification, the transmitter is opened to input external signals using a special testing adapter. Verification is carried out by a skilled technician and requires approximately 30 minutes. The process requires specific knowledge and relies on the assembly and maintenance of infrastructure. This is why external verification is usually always implemented in the form of a service, eg, as part of a service contract.
Internal verification is based on the ability of the device to verify itself based on integrated testing, which is carried out on demand. This is why the most common question is: How can internal verification system achieve the same reliability and test coverage as an external verifier specifically created to do so?
Integrated self-monitoring replaces the need for external test equipment only if it is based on factory traceable and redundant references. The reliability and independence of the testing method is ensured by traceable calibration or verification of the references at the factory and the constant monitoring of their long-term stability during the life cycle of the product.
By eliminating additional components for inspection and preventing errors during handling, internal device inspection proves to be more reliable than external inspection when the inspection process is viewed as a whole.
Advantages of integrated verification
The results of internal verification are the same as with external verification: verification status (pass/fail) and the recorded raw data. However, since verification is now a part of the device technology, data acquisition and interpretation are also done in the device. This has the advantage of making the functionality available for all operating interfaces and system integration interfaces. The verification procedure depends on the measuring principle and can last anywhere from a few seconds up to approximately 10 minutes. The true time saving, however, comes from the ease of use, since no complex interaction with the device is necessary to carry out the verification.
Safety and quality
Verifying the measuring point is done on demand and via all operating interfaces (local display/HMI or web server) as well as the system integration interfaces, such as the fieldbus. The verification process can also be started via a higher-level system (asset management software or PLC) and reliably reports the device status. Therefore, access in the field is unnecessary, which minimises the risks for personnel. The quality of the verification results will also improve, as there will be less chance for human error.
Verification can be carried out much more often - including daily or before starting a production batch - since the function is so easily accessible and the entire process lasts only a few minutes without interrupting operation.
Higher plant availability
Devices with internal verification should be capable of storing multiple verification results in the transmitter. This is the case not only for the verification status (pass/fail), but also for the measured data. This has the advantage of making the data available for later documentation and makes it possible to create verification reports offline for quality documentation. Furthermore, by comparing the data of multiple consecutive verifications, trends can be detected and systematically tracked during the life cycle of the measuring point.
Flowmeters that include self-monitoring offer the highest reliability. This benefits the customers in three ways:
- Continuous self-monitoring is used for diagnostics, in order to react quickly to a device defect or an application problem. Since the diagnostics delivers specific messages and corrective actions to the device and its functions, quick troubleshooting is possible.
- If the information identified as part of self-monitoring is exported from the device, it can be used for condition monitoring. This continuous observation of the device and process status also allows proactive measures through early identification of trends, thereby preventing unplanned maintenance or plant shutdown.
- Reliable methods of self-monitoring are based on factory-traceable references and have high long-term stability. Only methods fulfilling these criteria are suitable for internal verification of flowmeters and can be used to create proven documentation in the areas of quality (ISO 9001) and safety, and to verify metrological requirements.
In order to fulfil the prerequisites of the most widely varying applications and requirements in the life cycle of a measuring point, all three features are needed.
1. Source: EN ISO 9001: 2008; 7.6 ‘Control and Monitoring of Measurement Equipment’.
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