Page 224 - Petroleum and Gas Field Processing

8.2.2 Types of Storage Loss
In general, hydrocarbon losses in storage tanks are identified as follows:
Working losses
(a) Filling
(b) Emptying
Other losses
(a) Breathing
(b) Standing
(c) Boiling

Filling losses occur when vapors are expelled from a tank as it is
filled, no matter how the vapors are produced. This loss occurs when the
pressure inside the tank exceeds the relief-valve pressure. For API tanks,
the relief pressure is low and, therefore, filling losses can be relatively high.
Emptying losses are experienced by the vapors that are expelled from
a tank after the liquid is removed from it. Because vaporization lags
behind the expansion of the vapor space during withdrawal, the partial
pressure of a hydrocarbon vapor drops. Enough air enters during the
withdrawal to maintain the total pressure at the barometric value.
However, when vaporization into the new air reaches equilibrium, the
increase in the vapor volume will cause some vapor expansion.
Breathing losses occur when vapors are expelled from a tank under
one of the following conditions:
1. The thermal expansion of the existing vapors
2. An expansion caused by barometric pressure changes
3. An increase in the amount of vapors from added vaporization in
the absence of a liquid level change
Breathing losses take place in most types of tanks and occurs when the
tank’s limits of pressure or volume changes are exceeded.
The fixed-roof API type tanks used to store stock tank oil are
designed for only for a few inches of water pressure or vacuum and suffer
relatively large breathing losses.
Standing losses are losses of vapor which result from causes other
than breathing or a change in liquid level in tanks. Sources of standing
losses are vapor escape from hatches or other openings and from glands,
valves, and fittings.
Boiling losses occur when liquid boils in a tank and vapors are
expelled. In other words, the vapor pressure of the liquid exceeds the
surrounding pressure.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.