both the opening and closing gauge of a tank.
If the difference between sending and receiving tanks is less than the agreed tolerance, the pipeline can be considered full. If the difference between sending and receiving tanks is greater than the agreed tolerance, the pipeline is considered slack and appropriate steps should be taken to either fill the pipeline or account for the difference. In the absence of a tolerance agreed with the commercial parties, the tolerance shall be the total volume represented by twice the measurement range of the sending and receiving tanks (as stated in API MPMS Ch 3.1A/HM 4 and API MPMS Ch 3.1B): Basically, what the Chapter states is to consider the measurement precision/range on both the opening and closing gauge of all tanks. The stated measurement precision for gauging both shore and vessel tanks manually is 1/8”(3 mm). Therefore, when gauging manually take the sum of the volumes of 1/4” (6mm) from each tank involved in the transfer, which is 1/8”(3 mm) x 2, which then accounts for the measurement precision/range on both the opening and closing gauge of a tank. Where automatic tank gauges are used, the measurement precision/range on both the opening and closing gauge of a tank is 3/16”(4 mm), and hence, take the sum of the volumes of 3/8”(8mm) from each tank involved in the transfer, which then accounts for the measurement precision/range on
This process considers a gauge of one eighth inch to be the tolerance level. If product in the tank used for the line pack decreases by less than this, the line can be considered full. If product in the tank decreases by more than one eighth inch, the line pack should be repeated. If product in the tank used for the second line pack subsequently decreases by less than one eighth inch, it is considered that the first line pack condensed the vapors in the line back into a liquid, filled the void and the line is now full. The void space in a line will be filled with vapor and / or air. Compressing the vapor will turn it back into a liquid; and, while line packing cannot remove air trapped in a pipeline, it can compress it to a very small amount. Line packing, when properly performed can be very effective. Some terminals are reluctant to perform line packing for fear of over pressuring the line which could rupture it, resulting in a release of product. In addition to being effective, this method can be used both before and after the transfer.
5.5 General Procedures To recap what MPMS Chapter 17.6 requires regarding determining the fullness of the pipeline used in custody transfer: • Agree upon the method to use, both before and after transfer. • Establish the agreed tolerance, if a displacement method is used. • Agree on how possible voids in the pipeline will be accounted for. 5.6 Conclusion Pipeline fullness verification has many variables, and it is important that we keep our clients appraised of them. Not only is it necessary to agree on the method that will be used we should also agree, prior to the transfer, on what actions will be taken if the pipeline is found to be slack. The standard does not address the issue of adjusting the out-turn because of slack lines.
5.4 Agreed Tolerance 12
If one of the two-line displacement methods is going to be used, the first action to be taken is to establish an agreed tolerance with the commercial parties involved. This is an agreed upon volume of acceptable difference:
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