API MPMS Chapter 19.6.2:2019

This standard provides guidance for estimating emissions that result from forced ventilation of storage tanks such as may occur while removing the liquid heel (free-standing stock liquid) and cleaning the remaining deposits of stock liquid mixed with residue and water (sludge) from the bottoms of aboveground storage tanks.
The emissions addressed in this report are those that leave the tank during forced ventilation. This standard does not address:
the fate of vapors after they have left the tank (other than accounting for the efficiency of a control device, as discussed in Section 8);
the fate of sludge after it has left the tank (or emissions that may occur during sludge treatment or disposal); or
emissions that may be expelled by the vacuum pump of a vacuum truck or suction pump, if such devices are used in the tank cleaning process.
In other words, this report addresses the estimation of the mass of volatile organic compounds that leave the tank as vapor during forced ventilation of the tank, such as may occur during the tank cleaning process. It does not address emissions that may result from the handling of liquids or sludge after such materials have been removed from the tank.
This standard is not a guide for entering and cleaning storage tanks. Such procedures are addressed in API Standard 2015 and API Recommended Practice 2016.