Flow Transfer Standards: Versatile, Cost-effective Solutions for In-house Flowmeter Calibrations
21 October 2009
For many end-users, flowmeter calibration means shutting down the process, removing the meter from the line, and sending it to an outside calibration lab. The concept of portable ‘flow transfer standards’ offers a cost-effective solution for in-house flowmeter calibrations.
Figure 1. Calibration system manufacturers have utilised recent advancements in computer technology to develop portable flow transfer standards.
Many chemical plants and refineries have large numbers of flow measurement instruments, such as magnetic, Coriolis, positive displacement and turbine meters, requiring periodic calibration in order to maintain their accuracy and traceability to recognised industry standards.
Increasingly, flowmeter owners are seeking cost-effective solutions for performing on-site calibrations to help maintain their equipment in optimal working order.
Flow measurement suppliers have answered this demand by developing portable flow transfer standards that serve as an alternative to costly primary standard calibrations. This system combines special electronics and software, with master flow meters, to be used for in-house calibrations.
Reasons to calibrate
In the past 10 – 15 years there have been low maintenance electromagnetic, Coriolis, and ultrasonic flowmeters introduced to the market. The perception of some instrument engineers is that these meters can go for a long time between calibrations, even up to ten years. However, this may not necessarily be true.
With ultrasonic or magnetic flowmeters, electronic performance depends on adjustment and cleanliness of the pipe and electrodes. Corrosion within the pipe or on the magnetic electrode can make it difficult for these meters to deliver a correct output for the actual flow rates.
With buildup or corrosion in a pipe section, Coriolis mass flowmeters, for example, may shift the vibration frequency, which in turn shifts their calibration—indicating increased mass flow.
Only routine calibrations of flow measurement instruments can help companies manage their application within specifications. Without accurate calibrations, money may be, literally, going down the drain.
Most companies follow their quality control guidelines or have written ISO 9000 procedures for the intervals of their flowmeter calibrations. The most common interval is an annual calibration. However, many end-users have their flowmeters calibrated semi-annually or even quarterly depending on the nature of the application.
Taking the calibrator into the field
For most companies, calibration of flowmeters involves removing the meter from the line and sending it to a flow calibration laboratory where it is calibrated against a primary standard.
A primary standard measurement is made using fundamental components (mass, length, time, etc.). An instrument is considered a primary standard if it is not ‘characterised’ by the same method it is being used for. The most common examples of primary standard calibrators include positive displacement, continuous flow loop, and time-weigh systems. These calibrators involve passing a known volume or mass of a fluid during a certain amount of time and calculating the exact flow rate to within ±0.05%.
Figure 2. Flow transfer standard software merges output information from a master flowmeter, temperature sensor, pressure sensor, and the device under test (DUT) and generates a calibration data sheet in volumetric or mass units, which can be stored
This method is, obviously, costly and time-consuming.
For this reason, calibration system manufacturers have developed portable flow transfer standard (FTS) systems that can be used on site as a field calibration device, or as a tool to monitor how a particular industrial line or application is functioning (See Fig. 1).
There are three main components in the FTS system. First, there is the electronics and software package, which is carried in a suitcase. Then there is the master flowmeter, which is a flowmeter that has been calibrated on a primary standard. And thirdly, there is the flowmeter that is being calibrated, referred to as the device under test (DUT).
Instrument engineers at some plants have built carts that carry the electronics and several different types of master flowmeters, making them highly versatile and portable.
A typical installation may have one or more master flowmeters used in the FTS system. These flowmeters are periodically sent out to a calibration laboratory for primary calibration. But they are the only ones shipped from the company out to the laboratory; all the other flowmeters in the plant remain installed in their positions in the pipes.
The engineers take the FTS equipment to the flowmeter to be calibrated, on the factory floor. They temporarily install the master flowmeter in line with the DUT. The actual flow rate is derived from reading the master meter. In some cases other application inputs, such as temperature and pressure, must also be used. The FTS electronics and software connects the master flowmeter and the DUT so the engineers can compare their readings and save them on the computer.
When engineers use a master meter that has been calibrated on a primary flow standard, high accuracies are obtainable. Of course, secondary standard calibration uncertainty increases with the introduction of additional inputs to derive the flow rate and repeatability of the master meter. In many applications, this uncertainty is sufficient to meet the user’s acceptance specifications. In most cases, a 4-to-1 accuracy increase on the primary or secondary standard is acceptable to complete a calibration on the device under test (DUT).
One great advantage of an FTS system is that it performs in-line calibration of meters using the actual process fluid, not the fluid that is used in a calibration laboratory.
Calibrations for all types of meters
Today’s universal FTS design can calibrate or prove all principal meter types, including mass, turbine, vortex, magnetic, ultrasonic, and positive displacement. This system offers greater versatility by incorporating virtually any input for the master meter and DUT. Users are no longer required to purchase separate modules for RF inputs, pulse inputs, etc. The calibrator can accept pulse, magnetic, sine wave or RF signals, as well as analogue inputs to 0-5 volts, 0-10 volts, 4-20 mA or 10-50 mA.
The FTS employs an interface box that takes inputs from a master meter, temperature sensor, and pressure sensor, and supplies these outputs to the system software. Based on the inputs, the software can calculate the flow rate. The software merges the output information from the master meter, temperature sensor, pressure sensor, and DUT and generates a calibration data sheet in volumetric or mass units, which can be stored for future reference (See Fig. 2).
Figure 3. For in-situ calibration with a flow transfer standard, the application should be equipped with ball valves both upstream and downstream of the spool piece prior to the device under test. This is for the a process that can be stopped
With this FTS, overall system accuracy is dependent on several factors, including master flowmeter calibration, repeatability and primary standard accuracy; as well as temperature and pressure sensor accuracies. This fundamentally allows the user to tailor the accuracy of the system to meet the company’s needs through the level of master sensors they choose to utilise.
In other words, if the company needs to have very high accuracies, it can invest in a Coriolis flowmeter to use as the master meter. If the accuracy only needs to be within a few percent, then a vortex or electromagnetic flowmeter might do.
The key benefit of the FTS lies in its versatility: the system can be installed into an existing application utilising the flow source and actual conditions of the process liquid. In addition, a bypass system can be built into the application for ease of installing the master meter with flow straighteners for calibration of the DUT. Once the calibration is complete, the master flow meter can be replaced with a spool piece. The bypass approach maximises accuracy while minimising expensive downtime in production schedules.
Putting the system to use
The latest flow transfer standards are designed for improved portability, convenience and ease of use; in most cases, on-site flowmeter calibrations or validations can be performed in 10 minutes or less with the proper preparations and test configuration.
In addition to selecting a suitable master flowmeter, plant personnel performing an in-situ meter calibration must know: 1) the process fluid to be measured, and 2) the minimum and maximum flow rate for the application.
The user populates the FTS software with the absolute viscosity vs. temperature and density vs. temperature parameters; enters data for the process fluid, master flowmeter and meter under test; and selects the appropriate reading units, such as litres. The aforementioned parameters can be completed in the field or at the office prior to the calibration.
There are basically two ways that master flow meters can be installed in an application to complete a calibration or validation:
1. If the application can be shut down for a few minutes, a section of pipe (150 to 300 mm) can be removed and two ball valves installed with a spool piece placed between them (See Fig. 3). This arrangement can be left permanently in place, with the two ball valves locked in wide open position.
When ready to perform a calibration, the user must temporarily shut down the application by closing the ball valves. He removes the spool piece, and attaches flexible lines to the master flowmeter. Specific lengths of flexible line may be needed to accommodate the need for straight pipe before and after the flowmeter.
At this point, the ball valves are reopened and the calibration/validation is performed using the actual process fluid and pumps available upstream of the DUT. After the calibration, the installation process is reversed, leaving the ball valves and spool piece in line for the next calibration.
2. If the application cannot be shut down, a bypass may be installed (Fig 4) upstream from the DUT. In this case, the valves in the bypass, upstream and downstream of the spool piece, are closed while removing the spool piece and during this time the master meter is installed. Once the master meter is installed the valves in the bypass are reopened and the main bypass valve, which is on the main line (at the top in the diagram) is closed, allowing all of the flow to travel through the bypass and through the master meter. After flowing through the master meter it proceeds to pass downstream through the DUT. With the master meter and the DUT simultaneously reading the same flow, the calibration is performed. Upon completion of the calibration, the installation process is reversed and the spool piece is re-installed into the bypass pipe.
Figure 4. For in-situ calibration with a flow transfer standard for continuous flow, the application should be equipped with a bypass system where the master flow meter can be installed without disrupting the process
The calibrator system software is then used to export calibration data into a spreadsheet program such as Microsoft Excel, where it is formatted in data sheet templates for archiving and future analysis.
Benefits to end-users
Thanks to current flow transfer standard technology, industrial plants can utilise portable calibration devices in the field to monitor a process or perform multiple calibrations to trend a flowmeter.
End-users can reduce calibration costs by only having to calibrate, for example, three to five master flowmeters to cover almost all the liquid flow applications inside the plant. The master meters can be used to perform traceable calibrations or validations on-site, thus minimising the need to send meters away to a calibration lab for service. Furthermore, this solution eliminates the need to rotate calibrated meters in order to keep the process in uninterrupted operation.
The use of an FTS also allows plants to implement regularly scheduled meter calibrations on production lines. Maintenance personnel can install the master flowmeter, calibrate the device under test and print a calibration data sheet within just minutes. This compares with the normal 2-3 week turnaround necessary to remove the flowmeter from the installation and send it out to a remote site for calibration service.
In addition, the portable FTS calibration solution allows plants to schedule more frequent flowmeter calibrations. It can also serve as a tool for troubleshooting errors on meters in production. As a result, users can trend the actual meter in the process to identify production issues, equipment malfunctions or rejected products, as well as plot calibrations year over year in order to schedule any type of maintenance that might be required.
Lastly, the FTS is a valuable asset for documenting compliance with quality standards found in pharmaceuticals, biotechnology, food and beverage and other regulated industries. Such calibrators provide electronic filing of calibration documentation to the user’s hard driver or record-keeping network. Calibration certificates can be prepared in spreadsheet format with individual flowmeter certification number, model number, serial number and asset number.
The FTS solution
Today’s flow transfer standards enable flowmeter owners to perform accurate, traceable calibrations without the cost and downtime associated with removing their instruments from service. Plus, calibrations can be done on-site—with the meter installed in the actual process—at a fraction of the cost of sending equipment out to a remote, primary standard calibrations lab. For a plant with at least 30 flowmeters calibrated on an annual basis, the investment in an FTS system is typically recouped within a year or less. But even if the facility has fewer flowmeters, and FTS system will, in the long run, prove to be more cost effective.
Clearly, not every plant or process requires a primary standard calibration offering accuracy in the range of ±0.05%. Many flowmeter users find secondary calibrations at ±0.2% of reading to be suitable for their application. In these cases, a flow transfer standard provides a versatile, cost-effective solution for a wide range of calibration needs.
—Mark Evans, Applications Engineer, Flow Technology, Inc
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