Custody Transfer - Best Practices

Best Practices

In any custody transfer application, a true random uncertainty has an equal chance of favouring either party, the net impact should be zero to both parties, and measurement accuracy and repeatability should not be valued. Measurement accuracy and repeatability are of high value to most seller because many users install check meters The first step in designing any custody transfer system is to determine the mutual measurement performance expectations of the supplier and the user over the range of flow rates. This determination of mutual performance expectations should be made by individuals who have a clear understanding of all of the costs of measurement disputes caused by poor repeatability. The second step is to quantify the operating conditions which are not controllable. For a flow measurement, these can include:

• Expected ambient temperature variation;
• Maximum static line pressure;
• Static line pressure and temperature variation;
• Maximum allowable permanent pressure loss;
• Flow turndown; and
• Expected frequency of flow variation and/or pulsation.

The third and final step is to select hardware, installation and maintenance procedures which will ensure that the measurement provides the required installed performance under the expected (uncontrollable) operating conditions. For example, the user can:

• Select a static and/or differential pressure transmitter which has better or worse performance under the given real-world operating conditions.
• Calibrate the transmitter(s) frequently or infrequently.
• In the case of a DP flowmeter, size the primary element for a higher or lower differential pressure (higher DP's provide higher accuracy, at the expense of higher pressure loss).
• Select a flowmeter and pressure transmitter with faster or slower response.
• Use long or short interconnection (impulse) lines, or direct connect for fastest response.

While the first and second steps involve gathering data, the third step may require calculations and/or testing.