Storage tank emissions: standing, working, and flash losses

Two routine losses

AP-42 Section 7.1 estimates routine losses from a fixed-roof tank in two parts. Standing loss comes from the vapor space breathing as temperature and pressure change through the day. Working loss comes from filling, as rising liquid pushes vapor out of the tank.

Working loss depends on turnovers

Turnovers compare a year's throughput with the tank's working volume. A tank that turns over often has less time to saturate its vapor space between fills, so AP-42 reduces the working loss with a turnover factor above a set number of turnovers. The reduction doesn't apply to vapor-balanced tanks or tanks where flashing occurs.

Crude oil gets a product factor

The working loss product factor depends on the liquid stored.

Flashing is a separate estimate

When liquid from a pressurized separator enters a tank at lower pressure, dissolved gas comes out of solution. The standing and working loss equations don't capture this, and Section 7.1 points to other methods rather than settling on one. Some state agencies publish guidance on the methods they accept; check the state hub and the agency before choosing.

Where flashing occurs, a standing and working loss calculation alone isn't the whole estimate. OOOOb, for example, asks for a method that accounts for flashing, working, and breathing losses when a tank battery's potential is calculated.

Controls apply to all three losses

If vapors go to a control device, apply its efficiency to flashing as well as standing and working losses. When the result feeds potential to emit, use only the efficiency an enforceable condition supports.

Check your setup against EPA's example

Section 7.1 includes worked examples. Before trusting a spreadsheet or tool, run EPA's inputs through it and compare. The AirPermitHub Fixed-Roof Tank Losses calculator is tested against the working loss example quoted here.

What to record

A tank estimate someone else can check records:

• Tank dimensions, roof type, and shell and roof color and condition
• Liquid stored, with vapor pressure and molecular weight and their source
• Annual throughput and its basis: metered, projected, or permitted
• The meteorological data used, and where it came from
• For flashing: the method, the sample and its conditions, and separator pressure and temperature
• Any control device and the basis for its efficiency
• The AP-42 section and revision date used