SIZING of EMERGENCY – PRESSURE RELIEF VALVES
The following information is taken directly from the N. B. F. U. No. 30 Standard of the National Board of Fire Underwriters for the Storage, Handling and Use of Flammable and Combustible Liquids.
1010. This standard applies specifically to flammable and combustible liquids with a flash point below 200° F. and are accordingly exempt from this standard. Such liquids, however, involve some degree of hazard, which may be controlled by application of certain provisions of this standard, with appropriate modifications. Attention is directed to the fact that liquids of flash point higher than 200° F. may assume the characteristics of lower flash point liquids when heated, and under such conditions it shall be appropriate to apply the provisions of this standard to liquids with flash point above 200° F.
2140. Emergency Relief Venting for Fire Exposure for Aboveground Tanks.
2141. Every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure fires.
2142. In a vertical tank the construction referred to in Paragraph 2141 may take the form of a floating roof, lifter roof, a week roof-to-shell seam, or other approved pressure relieving construction. The weak roof-to-shell seam shall be constructed to fail preferential to any other seam.
2143. Where emergency venting is not provided in accordance with Paragraph 2142, the total capacity of both normal and emergency venting devices shall not be less than that derived from Table 6, except as provided in Paragraphs 2144 or 2145. See Appendix A.
|Sq. Ft.||CFH||Sq. Ft.||CFH||Sq. Ft.||CFH|
|90||94,800||700||428,000||2,800 and over||742,000|
NOTE: Interpolate for intermediate values. The wetted area of the tank shall be calculated on the basis of: 55 per cent of the total exposed area of a sphere or spheroid, 75 per cent of the total exposed area of a horizontal tank, and first 30 feet abovegrade of the exposed shell area of a vertical tank.
2144. The total emergency relief vent capacity for any specific liquid may be determined by the following formula:
Cubic feet of free air per hour = V x ( 1337 / ( L x Square Root of M ))
where V= cubic feet of free air per hour from Table 6, L = latent heat of vaporization of specific liquid in Btu per lb., and M = Molecular weight of specific liquid.
2145. The required air flow rate of paragraphs 2143 or 2144 may be multiplied by the appropriate factor listed in the following schedule when protection is provided as indicated. Only one factor May be used for any one tank.
0.5 for drainage in accordance with Paragraph 2162 for tanks over 200 square feet of wetted area.
0.3 for approved water spray.
0.3 for approved insulation.
0.15 for approved water spray with approved insulation.
2146. The outlet of all vents and vent drains on tank equipped with emergency venting to permit pressures exceeding 2.5 psig shall be arraigned to discharge in such a way as to prevent localized overheating of any part of the tank, in the event vapors from such vents are ignited.