The water recovery industry is making efforts to ensure high levels of process reliability, consistent quality and accurate billing of water use. There is also an increasing need to prove that operations are economically and environmentally sustainable. One of the keys to meeting these goals is ensuring flowmeters provide accurate data, which requires periodic flow calibration and verification of proper operation.
The generally accepted method of traceable flow calibration — using calibration rigs accredited to ISO 17025 — can be expensive and sometimes infeasible, mainly due to the labor and logistics involved in removing the flowmeter from the piping system. However, many modern flowmeters now come equipped with hardware and software allowing in-situ verification that meets all the requirements of ISO 9001.
In the water recovery industry, typical flowmeter requirements are:
- Verification must be performed at regular intervals by a qualified third party with an accepted inspection method based on quality regulations such as ISO 9001.
- A test report needs to be provided for documented proof of evidence.
The water recovery industry often uses large pipe sizes, and recalibration of these flowmeters is costly. In some cases, a certified local reference standard, typically a mobile calibration rig accredited according to ISO 17025, is not available. And in many cases, any interruption of service or supply of water is not acceptable, making it difficult to remove the flowmeter from service for calibration (see Figure 2).
Reliable verification of flowmeters can be fulfilled in two ways: via an external verification tool with references that can be traced along the life cycle by recalibrating the tool at periodical intervals, or by an internal verification, which is based on traceable references that are stable in the long term.
Verification of flowmeters has traditionally been carried out using an external verification tool (see Figure 1). During the verification process, the verification tool is connected to the flowmeter via test interfaces, and a functional test is carried out by simulating calibrated reference signals and observing the system response.
The reference signals for the flowmeter transmitters are supplied via a simulation box, and the sensor signals are sent via a sensor test box. In both cases, electrical characteristics of the system are tested. The transmitter and sensor signals are simulated automatically and independently from each other. The response from the flowmeter is measured and automatically interpreted by the verification tool. If the response is within factory limits, the algorithm produces a "pass" statement.
However, limitations and challenges arise with external verification techniques. During verification, the transmitter is opened to the atmosphere, and the meter cannot be used for measurement and control of the process for the duration of the test. External verification must also be performed by a trained technician. Additionally, the verification tool itself is defined by ISO 9001 as test equipment, which means that it must periodically undergo traceable calibration.
The status of verification and its data are subsequently used for documenting the results in a verification report. Modern external verification tools carry out the entire process automatically by controlling the flowmeter, simulating the measured values and documenting the results for further processing.
Even when modern tools are used, external verification is a complex procedure requiring access to the measuring point in the field. During verification, the transmitter is opened to receive external signals using a special testing adapter. Verification requires approximately 30 minutes and specific product knowledge, which is why external verification is usually implemented as part of a service contract.
Modern flowmeters have integrated this verification procedure internally to the device. Internal verification is based on the ability of the device to verify itself and does not require a skilled technician. A few device manufacturers have built diagnostics, monitoring and verification functions into the flowmeter. An example of this is Proline flowmeters from Endress+Hauser with integrated self-monitoring using Heartbeat Technology.
During flowmeter verification, the current conditions of secondary parameters are compared with their reference values to determine the device status. Heartbeat Technology produces a pass or a fail statement based on the tests, which is performed by traceable and redundant internal references. The individual tests and test results are automatically recorded in the flowmeter and used to print a verification report.
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 an electromagnetic flowmeter, this is a voltage reference that provides a second, independent reference value.
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 by the constant monitoring of the flowmeter’s long-term stability during the lifecycle of the product.
Several flowmeter suppliers offer external or internal in-situ verification tools, and their procedures differ somewhat. However, not every supplier’s verification methodologies use traceable references. This article explains how to perform a traceable, internal verification for an Endress+Hauser Promag 400 electromagnetic flowmeter.
Verifying a flowmeter
Modern flowmeters incorporate self-testing that was developed as an integral part of the device from the beginning. This concept embeds diagnostics tests in all electronic modules of the device. The entire signal chain from sensor to output modules is included in the flowmeter’s self-testing verification.
The verification procedure for the example flowmeter can take anywhere from a few seconds to about 10 minutes depending on the flowmeter type. The process can be done locally as shown in Figure 3. Alternatively, if the flowmeter has a permanent Ethernet or other digital bus connection to the plant network, the procedure can be performed remotely from a PC located in the maintenance department or the plant’s control room.
These tests are part of continuous self-monitoring and are also used for flowmeter diagnostics. They provide an immediate diagnostics alert, which allows maintenance to react quickly and target a device defect or an application problem.
On-demand verification is a test procedure, which briefly interrupts flow measurement but continues to send the last measured value to the control system throughout the duration of the test. These additional tests increase the overall test coverage within the flowmeter. The example device implements this concept so that the resulting test coverage is comparable to or higher than that of external verification. The crucial factor for this is the total test coverage (TTC), which indicates the efficiency of the tests.
The TTC is expressed by the following formula for random failures (calculation based on FMEDA as per IEC 61508):
TTC = (λTOT – λdu) / λTOT
λdu is the rate of dangerous failures (dangerous undetected).
λTOT is the rate of all theoretically possible failures.
Electronics failures labeled "dangerous" are those that would distort or interrupt the measured value output. The integrated self-monitoring of Proline flowmeters generally detects more than 94 percent of all potential failures. This test coverage is relevant for the documentation of tests in quality-related applications.
Upon completion of the test, a formatted verification report can be printed, signed and dated by the technician performing the verification. The verification data may also be transferred to an asset management system, a historian or other software for archiving and trend analysis. In addition to the verification result (pass/fail), the software logs the actual measured values for all tested parameters. This data can be used for tracking trends, allowing for timely conclusions regarding flowmeter health to prevent unexpected failures.
The results of an internal verification are the same as with an external verification — verification status (pass/fail) and recorded raw data. However, because verification is now a part of the device technology, data acquisition and interpretation are also performed in the device. This makes the functionality available for all operating and system integration interfaces.
The biggest advantage of in-situ internal verification is its ability to perform flowmeter verification without removing the device from the pipe and interrupting the process. Other advantages include reduced complexity and training requirements for plant personnel, verification without opening the device enclosure, and remote verification. These advantages reduce costs, mitigate risk and increase uptime.
Nathan Hedrick has more than six years of experience consulting on process automation. He graduated from Rose-Hulman Institute of Technology 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 recently taken on the position of flow product marketing manager.