Author : M Babb
05 September 2009
Product quality and process safety are important aspects in ensuring smooth plant operation. Repeating the same process to produce the exact same product requires high levels of equipment accuracy and process control. Calibration is a critical aspect of meeting this need.
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PHOTO at right: In this oil refinery, a Promass 80F Coriolis flowmeter is being verified using Fieldcheck, a handheld verification tool. Such demanding applications can require special configuration and more frequent monitoring and maintenance. By checking the instrument’s behaviour in the process, Fieldcheck offers user-oriented configuration and service. The verification data are transferred and stored on a computer. Using the FieldCare software, the measuring data are then presented graphically offering clear visualisation and analysis for diagnostics.
The criticality of each process and the required accuracy of the instrument under test are key factors in the selection of the most appropriate verification or calibration method and how frequently it should be carried out. On-site verification and calibration has seen considerable growth in recent years as it has presented substantial benefits in applications where the calibration process needs to be completed in the field.
Even in that case, however, there are different options to choose from with the use of handheld equipment continuously expanding throughout the process industries. Technological advances over the last years have made handheld calibration a practical proposition that meets flexibility requirements in various applications and can be tailored to the process to meet different needs.
PHOTO BELOW: Endress+Hauser’s Fieldcheck is a handheld tester/simulator for testing and verifying all the major functions of flowmeters on-site. That means longer intervals between wet calibration outages or— if drift affecting the secondary electronics is expected to be the main concern—it might even be possible to skip recalibration. Here, Fieldcheck checks the entire electronics chain of a Promag 53W in a water distribution line and simulates the various signals (measuring signal inputs, current outputs, pulse/frequency outputs, etc). Device parameters can be tested, such as coil current rise time and electrode integrity in the filled measuring tube. Modern testers of this type can simulate the safety concept of a production facility without running hot (without fluid flow), in order to assess situations such as a pipe blockage (jammed pumps) or upper/lower limit value exceeding.
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The handheld optionHandheld devices are typically used to calibrate parameters including flow, pressure, temperature and conductivity. Taken as a whole, the benefits of these systems are numerous and obvious in terms of time, cost and convenience.
Handheld equipment allows electronic verification and calibration in situations where inline calibration is essential but mobile rigs may be impractical. Internal procedures or official requirements and conformity reasons might also stipulate that certain instruments must be checked more frequently than others to verify that they are working correctly in the process.
For example, applications where abrasive and corrosive conditions demand frequent performance tests so that the device parameters can significantly minimise the risk of failure. In those cases, suspending operations while the instrument is sent away to a calibration centre or back to the manufacturer can be extremely time-consuming and incur considerable costs related to downtime or investment in spare equipment to keep the production line going. Here, handheld equipment is the preferred method.
There are four primary reasons for this:
1. No process interruption—A significant advantage of handheld calibration over other traditional methods is that it allows meter verifications to be carried out directly in the process without additional costs for removal of the instrument or process interruptions. As a result, downtime is minimised and critical processes can be verified and optimised efficiently. By supporting and facilitating regular on-site verification, handheld equipment helps users to quickly diagnose any failures and to swiftly remedy the situation.
2. Time and cost savings—Device verification using handheld equipment requires a maximum of 15-30 minutes per instrument. The device does not need to be sent away to the calibration centre and production can, therefore, resume faster than with any other method. What’s more, after the process has been completed, the direct uploading of the device parameters avoids time-consuming configuration. This method helps achieve optimum availability of plant equipment. The production does not need to be suspended resulting in considerable savings. What’s more, frequent test functions allow costly calibration cycles to be extended.
3. Complete on-site verification—Handheld electronic verification not only checks the accuracy of the device under test, but also performs a complete check of the entire measurement chain.
These systems allow verification of all major functions of flow and temperature instruments on-site, ensuring they are tested in the environment where they’re installed and allowing extended intervals between formal calibrations.
And while the use of electronic verification tools is not intended as a substitute for regular calibration, it may prevent instruments that have drifted out of specification from being used unwittingly.
Additionally, long-term testing of critical instruments allows a trend analysis of process-specific parameters for process optimisation. Comparing test and verification results makes it possible for a plant operator to analyse flowmeter performance over an extended period of time and to adapt maintenance intervals to the conditions at the metering point.
4. Simulation of the process—As safety during operation is considered a ‘must’ for plant operators, testing the safety and functionality of equipment in the process is often indispensable. Simulation of the measuring signals during calibration or verification processes can achieve that. Handheld calibrators can simulate process states, in flow applications for example, even without real flow.
In cases such as a time-critical commissioning, testing under ‘operating conditions’ can be done in a simple and cost-effective manner. Handheld verification devices can simulate different flow rates in the process, high and low limit values, receiving signal chains (operation of valves or control loops) and different flow behaviour in piping, for example, such as flow curves in bottling machines.
Traceable and accuratePerforming certified and traceable calibrations within the framework of ISO 9000 is a prerequisite that also applies to handheld methods. Working to Standard Operating Procedures (SOPs) helps achieve that and plays a crucial role in ensuring quality calibration procedures.
Factors of particular importance include:
* All calibration results both ‘as found’ and ‘as left’ (if different) including any failures or non-conformities of the Unit Under Test (UUT) should be recorded on the calibration certificate.
* The handheld reference instrument should be calibrated traceable to UKAS or equivalent standards.
* Once the UUT has been re-commissioned after the completion of the calibration, its function should be observed to ensure that it is measuring correctly and that the signal transmission is intact and the whole instrumentation loop is responding correctly. The customer benefits from that service, which eliminates the risk of potential failures after remote laboratory calibration and incurring costs of re-commissioning.
The futureWith the obvious benefits of on-site calibration, handheld in-situ verification and calibration methods are expanding their capabilities with technological advances in a wide range of applications. With many handheld verification and calibration devices featuring extended functionality and enabling the user to store and transfer calibration data, this method is becoming increasingly popular in many industries.
—contributed by Alan Sellars, Field Service Manager, Endress+Hauser
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PHOTO: The FieldCare software considers the configuration structure of the measuring instrument and works with known navigation aids (MS Explorer). Here, the service interface includes a PC connected to the service port in the terminal chamber of the measuring instrument. A fast Endress+Hauser protocol is used to gain quick access to configuration and measuring data. It is also possible to communicate and access configuration through HART, Profibus, and Foundation Fieldbus protocols.
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