Flowmeter accuracy is a central concern in the custody transfer process. As the cash register in fiscal accounting, ensuring and maintaining accurate measurements is essential for profitable, efficient operations, as well as fair transactions. The choice of proving solution to establish a meter’s accuracy is therefore important, and the global meter proving market may be worth as much as $180 to 200 million a year. Not all of that money is necessarily well spent.
Companies have a range of options of varying quality. The weaknesses of some of these are increasingly recognised. Low costs for master meters, for example, are offset by recurrent uncertainty due to meter drift and errors that go undetected, as well as the need to calibrate with an approved technology. Likewise cans, a low-cost solution offering good levels of accuracy rely on a slow labour-intensive process. The disadvantages of other methods, however, are less immediately obvious.
In particular, ball or pipe provers remain a popular choice despite more modern, flexible and sophisticated alternatives. In part, this can be attributed to habit on the part of both EPC contractors and the consultants drafting specifications for the end users. More significantly, contractors tend to focus on upfront costs, where ball provers can be competitive. Other costs may only become apparent in the long-term.
However, their use also reflects that ball provers are often a successful solution. With suppliers globally, they are able to deal with high flow rates (over 20 000 BPH), can prove manufactured pulses produced by microprocessor-based flow-meters such as Coriolis and ultrasonic equipment, and – at least initially – offer a rugged and reliable solution. There is little argument that ball provers work.
Against this, however, they are bulky and demonstrate higher life cycle and maintenance costs compared to small volume provers (SVPs). They are also potentially less accurate and stable than alternatives, as well as being less versatile. Again, this last point will not bother contractors not focused on lifecycle costs. As long as the prover is suitable for its current application, they are happy. Nevertheless, owners and operators may need to think longer term when it comes to a key asset that may be used for 20 years.
The demands over that period are an unknown. Just as fuels over time have become more dirty and corrosive – think not only of crude oil but ethanol in gasoline, which has caused significant compatibility problems in the last decade – the demands on the prover will continue to evolve. While a ball prover is static, its lining cannot be changed and the solution has a limited turndown ratio. SVPs on the other hand are designed for flexibility: firmware can be upgraded, they can easily be moved around, and seal materials can be switched.
At the same time SVPs are now a mature technology themselves, tested and approved by both regulators and oil companies for proving all types of meters, including Coriolis and ultrasonic meters. Much smaller, they offer great versatility and reliable operation in many applications.
Flow-meter accuracy is right at the heart of the custody transfer process. Readers interested in a complete evaluation of the various meter proving solutions can download the full white paper at http://instrumentation.co.za/+J1257
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