Flow measurement: getting it right

04 November 2019

Understanding, monitoring and controlling flow rate, as part of industrial processes, are essential elements to the viable operation of production systems. Brendan Robson discusses what is involved in getting such measurements right and the hazards of getting it wrong.

Fundamentally, the selection of appropriate measurement technology and its cost, accuracy and proper use will affect the result. For example, a recent BBC report estimated that the UK exported 1.2 billion bottles of Scotch whisky in one year, which equates to approximately £4.36 billion in sales (and associate tax revenue, of course). A measurement uncertainty of 0.5% on volume across this industry would result in a financial exposure of around £21.8 million. 

A recent measurement audit of an alcohol bottling plant found that classic manual procedures were in place for both sampling and for offline measurement of density, subsequently used to infer the Alcohol By Volume (ABV) of the product. Unknown to the plant operators, the expensive Coriolis meters being used to measure flow rate were also capable of measuring density. Using these Coriolis meters to measure both flow and density would allow for immediate online monitoring of alcohol content along with prompt action and enhanced product control.

The reluctance to change is a powerful barrier throughout industry. However, this should be balanced with an understanding of the available technology, including the importance of flow measurement, and its assessment in a cost-benefit analysis according to requirements.  When operating a process that relies on measurement systems for monitoring productivity, control, or safety, the ability to prove the accuracy of the measurement system is vital. This requires an understanding of measurement uncertainty, calibration and traceability, as well as a management system that incorporates a measurement policy and a maintenance schedule. This leads to confidence in the measurement process and the final result.

To achieve the target accuracy, which is so critical to trade and commerce, most countries have a dedicated regulatory framework which supports the national measurement infrastructure and is designed to facilitate and regulate good measurement practice. In the UK, this system is known as the National Measurement System (NMS), which is delivered through the UK Government’s Department for Business, Energy & Industrial Strategy (BEIS). 

Metrology traceability, including flow determination, plays a vital role in national infrastructure since accurate results and confidence in measurement are impossible to achieve without it.  Traceability is the technical proof that a measurement device has the appropriate pedigree, normally through calibration records, referenced back to a national standard. 

The traceability chain
As we move up the traceability chain towards a given measurement standard, the uncertainty in measurement reduces (becomes more accurate). However, to achieve lower uncertainty it is necessary to invest more money in the system by way of increased capital, maintenance costs and experienced staff. This is important to consider, as the most accurate system is not always the correct solution for a given application.
Owing to the cost penalty associated with achieving and maintaining low uncertainty, the requirements for a given application need to be considered prior to system design and component selection. 

Despite the work that is invested in maintaining and regulating the NMS, when performing a measurement audit, a common finding relates to instruments that are not installed or operated in accordance with the requirements, or in some instances are no longer traceable to the appropriate standard. Furthermore, it is not uncommon to find flow meters in service where the operator has no record of when the device was calibrated and no planned maintenance for the system.

So, what is restricting industry from investing in such powerful diagnostic tools and traceability? A key factor is the required level of measurement uncertainty. If we consider the oil and gas petrochemical industries, the needs are usually well established. These organisations typically have dedicated metering departments to support the measurement of flow for hydrocarbon-based products of high value, where even small uncertainties can lead to large financial exposure over short timescales. Another key factor is that these industries are regulated much more tightly than others, due to the fiscal value of the metered product. The same degree of stringent regulation does not often apply to flow measurements in other industrial environments. So, companies in sectors such as food and beverage, power or chemical may rarely undertake complete flow measurement audits and instrumentation is often under-utilised. Conversely, the development and application in medical and pharmaceutical areas are potentially a matter of life or death.

Key considerations
Regardless of the industry in question, a key consideration is: Do you understand the uncertainty in the measurement systems that you require for your business? And can you prove it? Furthermore, how does your measurement system perform over time? How frequently do you calibrate? And do you have any past performance data that would allow you to improve the performance or establish an optimal calibration period? Crucially, are these points recognised in your quality system?

Traceable and good measurement practice is critical in achieving accurate and repeatable flow measurement. However, selecting the appropriate solution depends upon understanding the operation and the measurement uncertainty required for a given application. This requires a fundamental understanding of metrology as well as the process to which it is applied. Additionally, given the sophistication and capability of modern flow measurement technology, such systems may not simply measure flow rate but also provide powerful diagnostic capability and valuable data. This can provide insight into both the efficiency of the measurement system and the effectiveness of the process, which has the potential to yield substantial commercial benefits including improved monitoring and performance.

Brendan Robson is a project engineer at TÜV SÜD National Engineering Laboratory.

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