Custody Transfer LMS

Custody Transfer LMS

introduction

Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another. This includes the transferring of raw and refined petroleum between tanks and tankers; tankers and ships and other transactions. Custody transfer in fluid measurement is defined as a metering point (location) where the fluid is being measured for sale from one party to another. During custody transfer, accuracy is of great importance to both the company delivering the material and the eventual recipient, when transferring a material.

 The term "fiscal metering" is often interchanged with custody transfer, and refers to metering that is a point of a commercial transaction such as when a change in ownership takes place. Custody transfer takes place any time fluids are passed from the possession of one party to another.

Metering Methods

Custody transfer is one of the most important applications for flow measurement. Many flow measurement technologies are used for custody transfer applications; these include differential pressure (DP) flowmeters, turbine flowmeters, positive displacement flowmeters, Coriolis flowmeters and ultrasonic flowmeters.

Differential pressure flowmeters

Differential pressure (DP) flowmeters are used for the custody transfer of liquid and gas to measure the flow of liquid, gas, and steam. The DP flowmeter consist of a differential pressure transmitter and a primary element. The primary element places a constriction in a flow stream, while the DP transmitter measures the difference in pressure upstream and downstream of the constriction.

Liquid custody transfer

Custody transfer of liquid flow measurement follow guidelines set by the ISO. By industrial consensus, liquid flow measurement is defined as having an overall uncertainty of ±0.25% or better. The overall uncertainty is derived from an appropriate statistical combination of the component uncertainties in the measurement system.

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

1. Expected ambient temperature variation;
2. Maximum static line pressure;
3. Static line pressure and temperature variation;
4. Maximum allowable permanent pressure loss;
5. Flow turndown; and
6. Expected frequency of flow variation and/or pulsation.