Chemical & Process Technology

The hydraulic-expansion equations given in 5.14.3 may be used to calculate the initial liquid relieving rate in a liquid-filled system when the liquid is still below its boiling point. However, this rate is valid for a very limited time, after which vapour generation becomes the determining contributor in the sizing of the pressure-relief device.

There is an interim time period between the liquid-expansion and the boiling-vapour relief during which it is necessary to relieve the mixtures of both phases simultaneously, either as flashing, bubble, slug, froth or mist flow until sufficient vapour space is available inside the vessel for phase separation. With the exception of foamy fluids, reactive systems and narrow-flow passages (such as vessel jackets), this mixed-phase condition is usually neglected during sizing and selecting of the pressure-relief device. The aforementioned exceptions are discussed further in 5.15.3.4. Experience as well as recent work in this area [53], [54], [55], [56] has shown that the time required to heat a typical system from the relief-device set pressure to the relieving conditions allows for the relief of any two-phase flow prior to reaching the relieving conditions. As such, full disengagement of the vapour is realized at the relieving conditions and the assumption of vapour-only venting is appropriate for relief device sizing.

Experience has shown there is minimal impact on the discharge system for the two-phase transition period. However, the user may consider the impact of transient two-phase flow on the design of the downstream systems.

If a pressure-relief device is located below the liquid level of a vessel exposed to fire conditions, the pressure relief device should be able to pass a volume of fluid equivalent to the volume of vapour generated by the fire.

Determination of the appropriate state of the fluid can be complicated. A typical conservative assumption is to use bubble point liquid.


[53] H. G. FISHER and H. S. FORREST, "Protection of Storage Tanks from Two-Phase Flow Due to Fire Exposure", Process Safety Progress, Volume 14, No. 3, July 1995, pp. 183-199

[54] H. S. FORREST, "Emergency Relief System Design for Fire Exposure with Consideration of Multiphase Flow", 1995 International Symposium on Runaway Reactions and Pressure Relief Design, ISBN 0-8169-0676-9, Published by American Institute of Chemical Engineers, pp. 604-630

[55] H. MOTT and B. SPARROW, "A Simple Relief Valve Rating/Sizing Method for Fire Relief from Liquid-filled Vessels", presentation to API RP-521 Subcommittee, September 30, 2002

[56] L. L. SIMPSON, "Fire Exposure of Liquid-Filled Vessels", paper D29-190-1 presented at the 29th DIERS Users Group Meeting, Las Vegas, NV, April 29-May 1, 2002

A 2-inch diameter relief device (nozzle) was rapidly opened on a tank that was 95% filled with 550 gallons of city water at approximately 15O0C and under its own vapor pressure of about 58.5 psig. Approximately 28% of the tank contents vented by two-phase flow (See below image).

The experiment was repeated, except that 1000 ppm of a liquid household detergent were added. Approximately 96% of the tank contents vented by two-phase flow (See below image).

• For non-foamy liquid, two phase relief via PSV will occur in liquid filled vessel until liquid level drop below a a level where vapor liquid disengagement occur. Liquid swelling in the vessel is main factor contributor to two phase relief.
• For foamy liquid, two phase relief would occur in liquid filled vessel until almost all the liquid in the vessel is empty.

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