On several occasions, I’ve discussed the subject of flow measurement and custody transfer. My WatchThatPage email spy service alerted me to a great new article by Emerson’s N.K. Chaudhary. He’s a member of the flow group based in Singapore.
His article, Improving Custody Transfer, describes the role of Coriolis direct mass flow measurement and some tips when using them in a custody transfer application. In describing the importance of good measurement in custody transfer, I’ll borrow N.K.’s words:
Whenever liquid product such as refined petroleum changes custody from one supplier or distributor to the next, it must be accurately measured and scrupulously accounted for.
There are many types of flow technologies. The article describes the three basic categories including inferential volumetric flow, direct volumetric flow and direct mass flow. Each has advantages and disadvantages. Inferential flow measurement devices include magnetic, ultrasonic, differential pressure and turbine-based flow meters. Positive displacement (PD) technology fits in the direct volumetric flow category.
The bulk of the article describes direct mass flow measurement. The best examples of these are Micro Motion Coriolis flowmeters. N.K. describes how these meters arrive at a volumetric flow rate:
To determine a volumetric flow rate, a mass flow meter must also know the density of the fluid, which is accomplished by measuring the natural frequency of tube vibration. The fluid’s flowing density is proportional to the square of the period of vibration of the flow tubes (inversely proportional to the frequency squared).
Coriolis flowmeters were approved by the American Petroleum Institute (API) in custody transfer applications in 2002 (API MPMS 5.6). N.K. cites a number of reasons that Coriolis technology has been widely accepted in custody transfer flow measurement:
…longstanding high accuracy and repeatability, versatility, reliability, tolerance of solid particles, and more recently low pressure drop and high performance.
N.K. offers some installation guidelines such as to avoid installing the Coriolis sensor at the highest point in the pipe. This is where gas is most likely to separate out. As I mentioned in an earlier entrained gas post, digital signal processing can filter out signal disturbance caused by slug flow conditions.
Unlike many of the other flow measurement technologies, Coriolis meters can be installed without long, straight pipe runs upstream and downstream which can simplify the installation. In applications with very high flow rates, it may make sense to install multiple Coriolis flowmeters in parallel. The total flow measured is the sum each output.
The article also describes OIML approval standards and proving methods to meet regulatory requirements. If you are considering alternatives for flow measurement in custody transfer applications, this article might help in your analysis.