I just got an assignment from my boss to calculate fire protection of vertical vessel. NFPA 15 (National Fire Protection Association 15) is a standard for water spray fixed system for fire protection.
As per NFPA 15, water spray refer to the use of water in a form of having a predetermined pattern, particle size, velocity, and density discharged from specially designed nozzle devices.
Standards for Vessels
The following standards from NFPA 15 are applicable to the protection of vessels.
7.4 Exposure Protection
7.4.1 General. A system for exposure protection shall operate as intended for anticipated duration of the exposure fire
22.214.171.124 Water spray shall be applied to vessel surfaces (including top and bottom surfaces of vertical vessels) at a net rate of not less than 0.25 gpm/ft2 [10.2 (L/min)/m2] of exposed surface
126.96.36.199 When rundown is contemplated, the distance between nozzles at different levels or elevations, protecting vertical or inclined surfaces, shall not exceed 12 ft (3.7 m) as measured along the surface
188.8.131.52 The horizontal distance between nozzles shall be such that spray patterns meet or overlap at the protected surface
184.108.40.206 Spherical or horizontal cylindrical surfaces below the vessel equator shall not be considered wettable from rundown
220.127.116.11 Where projections (manhole flanges, pipe flanges, support brackets, relief valves, etc) will obstruct water spray coverage, including rundown on vertical surfaces, additional nozzles shall be installed around the projections to maintain the wetting pattern that otherwise would be seriously interrupted
18.104.22.168 All uninsulated vessel skirts and any uninsulated steel saddles greater than 12 in (305 mm) high at the lowest point shall have water spray applied on one exposed (uninsulated) side, at a net rate of not less than 0.25 gpm/ft2 [10.2 (L/min)/m2]
As per NFPA 15, operating pressure at the hydraulically farthest nozzle shall not be allowed to drop below 50 psi (3.5 bar).
On this post, I try to make calculation excel of water sprayer for vertical vessel with cone roof. I hope you find the excel is useful.