standard. If any of the meter data cannot be obtained, the metered volume may be questionable. If possible, compare the metered volumes with shore tank volumes.
• Temperature or density stratification may have an effect on floating roof correction calculations.
7.4 Floating Roof Tanks If a shore tank has a floating roof, it is important to verify that the roof is not in the critical zone at the start or finish of the movement. When filling a floating roof shore tank with product, there will be a time when one of the three conditions exist: 1. When the product level is not touching the roof and the roof is therefore resting on its legs. When this occurs, the tank is termed “out of floatation”. 2. When the product level initially causes the roof to begin to float but is not entirely in the floatation mode. This condition is where the product is entering the critical zone. 3. Where the product level is no longer in the critical zone and is fully floating the roof therefore terming the tank roof “in floatation”. The critical zone should be clearly marked on the shore tank calibration (strapping) charts. Accurate measurements of quantities within these marked areas are virtually impossible to determine.
• Product density (degrees API gravity / kg/m3) may be incorrect.
• The observed reference height should have been measured and may not be the same as stated on the tank capacity table (or information plate); however, the difference between the two measurements should be minimal. Any significant differences should be investigated and may be due to distortion or a build-up of sediment. • The datum plate height above the tank bottom may not allow for identification and quantification of material below the datum plate.
7.3 Static Shore Tank Measurements
Tank calibration tables should comply with recognized industry standards. In particular note and confirm the date of the most recent calibration.
This information should be requested from the facility.
• Unslotted standpipes may have been used for measurements.
When addressing suspected problems with shore tank measurements the following issues should be considered: • Differences between manual and automatic tank gauge (ATG) measurements. • Tanks may move and deform over time causing reference points to distort and capacity tables may become less accurate. • Inadequate settling time before gauging may result in inaccurate liquid measurements, particularly with regard to water determination. • Tank bottom flexing may affect measurements especially if tank level has changed considerably from open to close.
• Sediment build up in the tank or standpipe may affect free water determination, measurement accuracy, and/or tank critical zone positions. Also, the presence of sediment on the tank bottom may complicate free water determination as the horizontal water/oil interface may not extend across the whole tank. • Tank shell thermal expansion corrections may not have been correctly applied.
Measurements taken in the critical zone are not acceptable for custody transfer.
• Measurements taken within the coned bottom critical zone of a tank should be avoided.
In some locations, tanks may also be strapped for both high and low leg positions. Verify that correct capacity tables were used to obtain volumes.
Terminal balances and stock reconciliation may be needed to investigate any potential cargo diversions.
• Valve leakage.
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