Custody Transfer Flow Measurement

If you have custody transfer applications in your plant and you’re not already an expert on the subject, there’s a great Control Engineering magazine article, Custody Transfer: Flowmeter as Cash Register. It’s written by Emerson’s Emerie (a.k.a. Butch) Dupuis and Gerard Hwang, both whom I’ve known and worked with for many years.

Custody transfer defines the point at which ownership changes hands for the product being measured. The authors describe its critical role:

Refineries, chemical plants, pharmaceutical companies, and a host of other industries have to measure raw materials and finished products accurately, because they pay for what comes in and get paid for what goes out.

When these products are liquids and gases flowing through pipes, the flowmeter is the measurement tool. Accuracy and repeatability are critical. The authors describe a large custody transfer system measuring $6 million (USD) per day with 0.25% inaccuracy cause a $15,000 error per day. That’s not exactly pocket change!

Many flow measurement technologies have been used custody transfer applications. These include differential pressure (DP) measurement across an orifice plate, turbine meters, positive displacement meters, Coriolis flow and density measurement, and ultrasonic flow measurement. The authors highlight the growing use of and preference for ultrasonic and Coriolis meters in the past decade.

Ultrasonic measurement is described:

Ultrasonic meters provide volumetric flow rate. They typically use the transit-time method, where sounds waves transmitted in the direction of fluid flow travel faster than those traveling upstream. The transit time difference is proportional to fluid velocity… Ultrasonic flow meters have negligible pressure drop, have high turndown capability, and can handle a wide range of applications. Crude oil production, transportation, and processing are typical applications for this technology… Spool-piece ultrasonic meters are commonly available in 2 to 24 in. pipe sizes.

Coriolis measurement is described:

Coriolis flowmeters provide direct mass flow measurement, with high accuracy and repeatability over wide turndown ratios. They maintain those qualities even when fluid conditions such as density, viscosity, and composition change frequently… Coriolis meters are used on lines from less than 1 to 12 in.

Custody transfer systems are more than just flowmeters and are highly engineered for the intended application. The authors note the key components in these metering systems:

  • Multiple meters/meter runs;
  • Flow computers;
  • Quality systems (gas chromatographs for to measure energy content of natural gas and sampling systems for liquid);
  • Calibration using in-place or mobile provers for liquid, or master-meter for liquid or gas; and
  • Supporting automation.

Various types of proving systems are used to calibrate liquid meters in-situ (not having to be removed from the process.) In an earlier post, Proving Flow Measurement Accuracy for Hydrocarbon Applications, I showed pictures of a mobile prover in action.

You’ll want to read the article for its guidance on how to improve accuracy through meter tube alignment, distance from the control valve, header meter run placement and sizing, flow computer placement, and site design. If you receive the print edition of Control Engineering, there are also some instructive diagrams including a P&ID drawing.

In an email to me about this post, Butch highlighted the critical role of maintenance in custody transfer systems. He wrote:

System components require proper maintenance, verification, and occasional calibration in order to continue to ensure that same high level of accuracy over time. Even the most accurate metering system could be rendered inaccurate by a failed component, internal damage caused by foreign material in the flowing stream, build-up of solids on meter system component internals, calibration drift over time, or a number of other factors. Periodic maintenance on custody metering systems is critical to maximize overall measurement accuracy and uptime.

The frequency of scheduled maintenance (system inspection and calibration checks) on custody metering systems is often mandated in custody transfer contract agreements. Most operators of custody metering systems have established routine maintenance practices, but we still find many who do not or others who have adopted less-than-optimal practices. A number of factors will affect the best possible maintenance practices for a custody metering system including: contract requirements, equipment manufacturer recommendations, practical experience, and the amount of financial risk to which improperly maintained measurement equipment might expose the operator. So, the bottom line is, custody metering is special and it requires special attention as well.

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