Processing Magazine

Reducing costs and improving accuracy in dairy custody transfer flow measurement

April 9, 2007

Thousands of dollars worth of unprocessed milk changes ownership daily, as haulers transfer milk in large tank trucks from dairy farmers to milk processing plants. While ensuring that all milk is accounted for during transfer is important for both seller and buyer, historically, milk processing plants had no custody transfer systems in place or used scales, which were expensive to maintain.

Custody transfer flowmeters, sometimes called fiscal meters or billing meters, may be somewhat less expensive to maintain, but are not maintenance-free. Positive displacement (PD) or turbine meters, the most widely used flowmeters for custody transfer applications in other industries, still require regular maintenance on meter bearings, gears, and other moving parts, and once maintenance is complete, the meters must be recalibrated and typically replaced within a year.

Coriolis meters have emerged as the choice for milk transfer applications because they provide outstanding accuracy and direct measurements of mass, density, and temperature. Coriolis mass flowmeters have few moving parts and thus provide a lower-maintenance alternative to PD meters. But conventional Coriolis meters are not suitable for measuring two-phase flow, a common condition when transferring milk. For example, tank truck transfer lines contain both air and milk at the beginning and ending of a delivery. This could result in substantial over- or under-billing.

The Foxboro Measurements and Instruments Division of Invensys Process Systems has developed a custody transfer solution that requires little or no maintenance and is certified nationally for accurate fiscal metering of milk.

FOXBORO SOLUTION

The Foxboro custody transfer solution includes a Coriolis flowmeter, custody transfer indicator, and slip printer. The heart of the system is the Foxboro CFT50 Coriolis transmitter, which employs patented digital technology that compensates for variations in the amount of gas or liquid in a flow. Developed collaboratively by Invensys and Oxford University in the United Kingdom, the CFT50 incorporates a digitally synthesized flowtube drive signal with patented advances in areas such as digital signal processing and operational steadiness of validated mass measurement. The microprocessor-based meter prevents erratic flowtube vibrations that cause measurement failures with liquid/gas flow. This provides accurate measurement of quantities from the beginning to the end of a batch custody transfer. The CFT50 flowmeter has no moving parts to wear out, no internal mechanisms to replace, and can go up to a year without calibration. In addition to the CFT50, the Foxboro custody transfer solution includes a transfer indicator with a printer interface. The CFT50 sends pulses to the indicator to

start the batch count. The custody transfer indicator then communicates with the printer to produce a ticket that accurately documents the volume of milk transferred. The indicator also features a stop/reset button to re-zero after each batch and to trigger a printout of the total.

The Foxboro custody transfer solution is certified nationally by The National Type Evaluation Program (NTEP) to have an accuracy of <0.2%. The NTEP controls fiscal metering in the U.S., and certifies custody transfer mass meters for compliance per the National Conference on Weights and Measures (NCWM). In addition, each individual installation must be certified and sealed by the local state inspector. The Foxboro custody transfer solution includes seal out hardware to allow a local inspector to verify and protect the CFT50 flowmeter from tampering or cheating. After calibration, the indicator’s frontbuttons are disabled to prevent accessing the instrument’s calibration menu after the device has been sealed. Also, the system is available with 3A Authorization for sanitary applications.

RESULTS

Dairy industry users of the Foxboro custody transfer system can expect rapid payback, usually within six months of implementation. This system is ideal for fast transfer of milk from tank trucks, where a high volume of air accompanies the liquid transferred from the bottom of the trucks. In addition to gaining additional revenue by not overpaying or underbilling, users can save thousands of dollars previously spent on maintaining scales, or the inefficiencies and poor performance of traditional custody transfer meters and conventional Coriolis flow measurement devices.

Based on its proven reliability and accuracy, the Foxboro custody transfer system has been certified for dairy applications by the Ohio Division of Weights and Measures, with approval pending in other states as well.