The industry's most authoritative handbook on flow measurement provides a road map to the field of flow measurement. This best-seller discusses strategies for problem solving and puts the whole array of types of flowmeters at the reader's disposal. The text includes laminar flow elements, critical flowmeters, statistics for measurement, laboratory primary standards, and uncertainty in flow measurement. Emphasis is placed on the importance of accuracy in measurements and ways of ensuring accuracy and avoiding equipment damage through correct forecast of operating conditions, flowmeter selection, installation, calibration, and maintenance. Fundamental considerations such as mixed-phase flow, piping effects, and flow conditioning are examined at length. The problem of attaining a meaningful flow signal through linearization, compensation, and totalization is discussed. Join the thousands of engineers, technicians, managers, and salespeople that have found this reference text an invaluable resource.
TABLE OF CONTENTS Chapter 23 - Custody Transfer Measurement
Why should a separate chapter be written about custody transfer measurement
since flow measurement is flow measurement? This is true, except that, when
money is to be exchanged, achieving the best accuracy becomes important. The
desired limit on accuracy for flow measurement is that it must be 100% correct.
However, for reasons discussed elsewhere, no measurement is absolutely accurate,
but it is accurate to some limit.
For some flow measurements (such as control applications), a repeatable signal
may be more important than an accurate signal. Once a process is under control,
repeatability is of prime importance.
On the other hand, if money is going to change hands, accurate flow measurement
is required so that the parties to the transaction are treated fairly. The accuracy
limit for control signals may be several percent; for custody transfer it should
be tenths of a percent.
| An example of accuracy and the relationship of dollars to flow accuracycan be shown in a large crude-oil measuring facility (see Figure 23-1). This facilitywas delivering 12-million barrels of oil per day. Calculating on a basis of $30 per barrel, this meter station was handling $360 million of oil per day. A 0.1% error in flow represents $360,000 per day. |
| In the example, a 0.1% accuracy represents a significant quantity of dollars, but it is the limit of accuracy that can be obtained. Operational measurement may require no more than ±5%. Control measurement may be acceptable at ±2%. But custody transfer measurement converted to dollars is not ± anything, since accountants send out bills to the penny. For this reason, quantities for custody transfer are treated as absolute. The goal for this measurement is to reduce all inaccuracies to a minimum so that a measured quantity can be the agreed upon quantity for exchanging custody. | It is important in custody transfer metering to be constantly aware that flow measurement is dollars, and the perspective on the measurement changes accordingly. |
