API St 2560:2003(R2010) pdf download.Reconciliation of Liquid Pipeline Quantities
1 Introduction
1.1 In the ideal world every drop of liquid received into a pipeline system and every drop delivered out of the system, as well as all liquid inventory within the system, would be mea- sured and accounted for precisely, and a comparison of all receipts and all deliveriesÑadjusted for inventory changesÑ would be exactly the same. The system would never experi- ence a loss or a gain. Unfortunately, this ideal pipeline bal- ance seldom exists in the real world.
1.2 Most pipeline systems typically experience some degree of loss or gain over time. This represents the normal loss/gain performance for a system. From time to time, losses or gains greater than normal may occur for a variety of rea- sons. Excessive or unexplained loss/gain often leads to con- tention between participating parties, sometimes requiring monetary settlements to adjust for abnormal loss/gain. In such cases, it is necessary to be able to (1) identify abnormal loss/gain as quickly as possible, (2) determine the magnitude of abnormal loss/gain, and (3) institute corrective actions.
1.3 Sometimes losses or gains are real, and adjustments must be made to correct shipper batches and/or inventories. Most of the time, though, there are no real physical losses or gains. The loss/gain that occurs in day-to-day operation is usually small (a fraction of a percent) and is caused by small imperfections in a number of measurements in a system.
1.4 In a sense, loss/gain is a measure of the ability to mea- sure within a system. Loss/gain should be monitored for any given system at regular intervals to establish what is normal for that system and to identify any abnormal loss/gain so that corrective action can be taken.
6.3 CONTROL CHARTS
6.3.1 Good measurement can be assured by continuously monitoring measurement results to determine if systems, or equipment and procedures, are performing in predictable ways and are operating within acceptable limits. This may be done by the use of Control Charts.
6.3.2 Control charts display a collection of data over some period of time and include control limits shown as horizontal lines on the charts. Control limits help deÞne normal and abnormal system performance, and may indicate when some- thing in the system has changed and/or corrective action(s) may be required.

6.3.3 Control limits are often determined by historical per- formance of the system. In other cases the control limits are set on an established arbitrary value, e.g., contractual limits. Control charts are the most common method of ascertaining system loss/gain performance. Control charts display a col- lection of data over some period of time and include the con- trol limits. Control charts help to deÞne normal trends of a system and may indicate when something has changed. Typi- cal loss/gain charts as shown in Figure 1, indicate a systemÕs performance based on a percentage of throughputs over time. Typically, because accounting systems encompass a 30-day period, monthly evaluations of a system are commonly used to evaluate performance. Control charts may be prepared for any time span (e.g., weekly or daily) if adequate data are available.