PSV Fire app for iPhone and iPad

PSV Fire allows you input vessel data (diameter, T/T length, elevation from grade, etc...) to determine the heat duty into the vessel from an external fire. Then, in a separate tab, calculate the orifice area required based on inputs of Temperature, molecular weight, Z, and k (Cp/Cv). All units of measurement are in US units.

API 521 guidelines are used in the coding for the equations.

Entries can be done separately for horizontal or vertical vessels. For both, heads are assumed to be 2:1 elliptical heads. Partially filled vessels are fine, the program does calculate the wetted area of the cylinder and heads for partially filled vessels, both horizontally and vertically.

For vertical vessels, you can choose to include or exclude the top and bottom heads (e.g., exclude bottom if you have a concrete skirt). Program will not include the top head unless you specify liquid full conditions and turn on the switch to include the top head.

Here are some more details:

DISCLAIMER: This tool is intended to be used as a shortcut tool for checking values quickly and easily, but is not intended to be a design tool for sizing PSV’s for installation. Please use industry-approved programs for such a task.

NOTES:
The PSV fire calculations account for API 520/521 rules as much as an app this simple can. The equation for heat input is the typical “Q=21,000 * F * A^0.82” equation. This of course assumes adequate fire fighting techniques and good drainage. If you want to assume the 31,500 value in place of 21,000, then change the Environmental F factor to 1.5 instead of 1.

For iPad version only, you can include any boots that may be present.

The app accounts for the typical API limitation of anything above 25 ft. elevation above local grade is not included in the heat duty input area. For example, if you have a 30 ft. tall vertical vessel with bottom tangent 10 feet above grade, then only the bottom 15 ft (the amount of the vessel below the 25 ft. elevation mark) is included in the calculation. This is also true for boots on horizontal vessels.

In the iPad version only, you can also input a percentage factor for increased wetted surface area to account for nozzle projects on vessels and attached piping. Typical values for piping alone range from 0% to 20%. It is recommended to use 10% for piping for screening purposes. See app notes page for more details. Note especially that the percent adder is applied to the wetted area term and not directly to the duty term. For example, if you input 10% adder, then the equation for Q will now be “Q=21000 * F * (Ax1.1)^0.82”, which is NOT THE SAME as “Q=21000 * F * A^0.82 *1.1.

All calculations assume 100% vapor generation and do not account for: A) Whether vapor disengagement will be promoted within a liquid full system, or B) Entrainment to the PSV inlet flange. For details on this, please refer to API 521, as recent trends in industry generally allow for the assumption that vapor only will be relieved during fire so long as some common sense is applied to the analysis of the physical installation of the PSV and equipment its protecting.

The equation for orifice area required is A=W/(C Kd Kb P1) * (T Z / MW)^0.5 -- see app notes for full explanation of variables.