Venting Below the Liquid Level

DOE-HDBK-3010-94

3.0 Liquids; Aqueous Solutions

above the liquid level, whether or not the liquid can release gases or vapors upon

depressurization influences the drop formation mechanism. If the liquid has come to

equilibrium with the free volume atmosphere, liquids under pressure will generate drops by

bubble breakup at the surface from release of trapped/dissolved gases. For heated liquids,

drops are generated by bubble breakup at the surface of the liquid from vapor generation.

For liquids that have not come to equilibrium with a pressurized free volume atmosphere, the

bubbling action from release of absorbed/trapped gases can be minimal. Drop formation is

more accurately described as a function of shear stress on the liquid surface and stratified

two-phase flow. If the vent is greater than the Critical Freeboard Height above the surface

of the liquid, the flow pattern tends to be vertical and does not exert significant shear stress

across the liquid surface to create drops, and the relatively great distance from the point of

origin of the drops to the vent does not create conditions favorable to release. For vents that

are less than the Critical Freeboard Height from the liquid surface, drop formation is by

shear stress exerted due to the induced, high velocity flow parallel to the liquid surface.

While this release mechanism can result in significantly smaller release fractions than the

three mechanisms cited in the preceding paragraph, it requires data that is not practically

attainable in most cases. As a result, this mechanism, while discussed, will be represented

by the more conservative case of release of absorbed/trapped gases.

3.2.2.3.1 V en tin g B elow th e L iq u id L evel. If the container or pipe holding an

ambient-temperature liquid under pressure is breached, the liquid can escape in a variety of

ways. Breaches venting pressurized liquids can range from pinhole leaks in pipes (generating

a mist) to drips from very slow leaks to large jets of liquids that may gush from large holes.

The amount and aerodynamic size distribution of the spray generated are a function of the

size and characteristics of the breach, the upstream pressure, and the liquid characteristics

(e.g., viscosity, density, volatility).

For the purposes of airborne suspension, a conservative assumption would be the pressurized

release of the liquid via a very fine hole as occurs in a commercial spray nozzle. The size

distribution of some commercial spray nozzles as a function of orifice diameter and upstream

pressure were shown by Mishima, Schwendiman and Ayer (October 1978). The size

distribution of the liquid drops decreases with orifice diameter and increasing upstream

pressure. It is not anticipated that drops formed from breaches, cracks, leaks would generate

finer drop size distributions than equipment specifically designed for that purpose.

Therefore, the respirable fraction of the coarsest distribution generated by commercial spray

nozzles shown in Figure 3-4 is selected as the bounding ARF, 1E-4, with a RF of 1.0. For

other size fractions, the values can be inferred from the 0.128-inch (3.25-mm) diameter

spray nozzle values at 200 psig (1.38 MPag) upstream pressure.

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