Flow Calibration – Five Things You Need to Know

Know your options regarding flow calibration to make the best operational decisions A flow meter that does not take accurate measurements can lead to many problems. For example, in a buying and selling application involving crude oil or natural gas, even the smallest error can cost an operator millions of dollars annually. Although calibration may not be the largest or most important item on your to-do list, it can have a significant impact on your facility as a whole. When performed by experts, calibration positively affects your production and compliance and reduces the risk of quality or safety deviations. Here are 5 things you need to know when considering flow calibration:

1. Some measuring devices can be calibrated easily by yourself in the field, but flow meters are not one of them. Calibrating flow meters is much more complex than other measurement instruments (e.g. temperature). The greater complexity means that performing it yourself is more costly. Professional calibration laboratories use pipe clamp measurement chambers to adapt to different diameters. The chambers are usually large, which makes them costly for operators who want to perform calibration themselves.

To conduct calibration within the facility, one or more traceable flow reference devices with high cost and traceability must be obtained. This adds additional cost and complexity to your processes. Another difficulty in being able to perform calibration economically within the facility is technical expertise. Ensuring that personnel receive adequate training to perform successful calibrations and retaining this trained personnel can be costly.

2. Performing flow calibration in the field can be more cost-effective than sending devices to a calibration laboratory. Flow meters are often removed from the system and sent to a calibration laboratory for calibration purposes. There is a widespread belief that the required calibration accuracy can only be guaranteed under laboratory conditions. However, this is only partially true.

Flow meters and many other measurement devices can be calibrated directly in the field. This has many advantages:

• Facility availability increases since the device is calibrated in place or even directly on the line.
• The source of error can be identified and eliminated in the field, whereas the possibility of achieving the same result in a calibration laboratory is limited. Field calibration technicians can detect errors during installation and identify blockages or contamination in pipes.
• Cost savings increase through faster completion, reduced downtime, and elimination of spare equipment inventory.
• There is no need to disassemble and transport contaminated devices. Contact with hazardous substances can be reduced and costly decontamination procedures can be avoided.

1. Three-point calibration offers the best cost-benefit ratio. Each flow meter covers a specified measurement range (e.g. from 0 kg/h to 200 kg/h). However, in practice, rarely the entire range is utilized. Generally, only a portion of the entire measurement range is used. For example, a flow meter capable of measuring in the 0-200 kg/h range may only be operated in the 70-100 kg/h range with an average flow of 80 kg/h.

Instead of testing the entire measurement range during calibration, using the three-point calibration technique reduces the required measurements to application-based values: 70 kg/h (min.), 80 kg/h (average), and 100 kg/h (max.). Thus, both minimum and maximum values of the measurement range are tested, and the average value is used to test the linearity of the measurement curve. As an extension of this method, an additional point can be tested. The zero point of the measuring device is typically used for this.

Three-point calibration is a widely used standard method because it:

• keeps costs low
• provides information on both point accuracy and linearity of the measurement curve
• offers an excellent cost-benefit ratio.

2. Fully professional calibration requires highly trained specialists. While you may be able to calibrate some measurement points yourself, other points present a challenge that should not be underestimated even by experts. In cases covering numerous flow meters, calibration requires project planning. Personnel performing these calibrations must consider: minimizing facility downtime, dismantling and reinstalling devices, technical knowledge related to calibration, and the use of tools and equipment. Additionally, personnel must always be familiar with current regulations to ensure that the correct calibration interval is observed and documentation can be completed in compliance with regulations. In the end, calibration provides evidence and documentation that permitted measurement errors are being met and plays an important role in inspections and certifications.
3. Flow calibration in the field saves time. Calibration requires time. Ideally, calibration is conducted as part of planned downtime so production is not affected. However, experience shows that planned downtime periods are becoming shorter. Instrumentation usually comes last in priority. Delays in prioritized tasks can shorten the time allocated for instrumentation and calibration. Therefore, calibration should always be performed with time optimization in mind.

On-site calibration reduces coordination efforts and eliminates the need to disassemble a device and send it to a calibration facility. While a device may take days or weeks to return from a calibration laboratory, field calibration can be completed within a few hours. Of course, traceability is required in both cases. In some cases, necessary adjustments or repairs can be made to the device by the field calibration technician.

Chemical and petrochemical companies place great emphasis on safety – for both people and the environment. Third-party technical specialists provide maximum service reliability without compromising safety, as the risk of error from dismantling and reinstalling devices is eliminated. Summary In situations where quick turnaround and keeping production running are important, flow meters can be calibrated in the field cost-effectively. Depending on facility topology, many instrument measurement points can be calibrated quickly with minimal interruption. This increases facility availability while reducing process complexity. Field calibration also reduces risks for instruments and personnel. When performed by technicians using traceable mobile equipment and providing comprehensive documentation, it ensures compliance.

Glossary of terms

Calibration is the process of determining and documenting the deviation between the measurement data of a measuring device and the measurement data of another (reference) measuring device or a standard measurement called a calibration standard. Traceability means that the results of a measurement can be traced through an unbroken chain of calibration back to national or international calibration standards. It also verifies that a measuring device operates within the maximum error permitted by calibration standards. Traceability of calibration can generally only be guaranteed through formal accreditation performed by a recognized authority. Correction includes changes made to the measurement system. Calibration is purely an analytical process. If the deviation of a measuring device from the reference is unacceptably high, the measuring device is adjusted to bring it back to the "true" value of the reference.