API RP 2556:1993 (R2013)

Incrustation is defined, for the purpose of this recommended practice, as any material that adheres to the internal vertical sidewall surfaces of a tank when the tank is otherwise empty. Incrustation has the same effect on tank capacity as deadwood (anything that displaces liquid inside a tank) and should be treated as such as long as it remains in the tank. The problem of deducting the volume of liquid displaced by incrustation is complicated by two of incrustation's basic and typical characteristics: First, incrustation is difficult to measure, and second, its thickness is usually variable. Some oils present no incrustation problem, but many others do, usually on a field-wide basis. The error in measurement from any one tank may be slight, but the accumulated error from an entire field or from any one tank over a period of time could be substantial. The error always has the effect of indicating too large a tank capacity; therefore, a receiving carrier cannot be expected to continually absorb the effect of these errors. The method selected to correct the error should depend upon the desired approach to accuracy of measurement. The tables given in this recommended practice (see Section 4) show the percent of error of measurement caused by varying thicknesses of uniform incrustation in tanks of various sizes. These tables may be used as a guide by the tank owner and the carrier to negotiate an allowance for incrustation. If it is established that incrustation is causing a substantial loss to a carrier or to any other receiver using affected tank gauges as the basis for custody transfer measurement, it is the responsibility of the tank owner to provide a tank that will permit an accurate measurement or to agree to a reasonable adjustment.