Fixed Water Spray Demand Calculations For Fire Protection Of Tanks / Vessels

Of late I have developed quite a bit of interest in HSE engineering and specifically related to Loss Prevention protection and I have been going through a lot of literature related to Loss Prevention and specifically related to fire protection.

When we talk of fire protection, some of the most comprehensive guidelines and standards developed for fire protection in the chemical process industry can be attributed to the "National Fire Protection Association" better known by its acronym NFPA. Today's blog entry makes an attempt to provide calculations for fixed water spray for fire protection of tanks and vessels based on their specific geometry or shape and as per the guidelines provided in NFPA 15 - "Standard for Water Spray Fixed Systems for Fire Protection"

Let us begin with what NFPA 15 has to say for "Vessels"

Section 7.4.2.1: 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.

Although the above statement does use the terminology exposed surface, for the sake of calculating the design spray water demand it is normal to consider the entire surface area of the vessel falling in a fire zone. In fact, the total spray water demand for one fire zone will be the sum of the spray water requirement for all the vessels and equipment in that fire zone. It is however possible, that when calculating peak demand for spray water for the entire plant or unit all the fire zones may not be considered simultaneously to require spray water. This is again based on the policy of the company for fire protection and can vary from place to place. This may also be dictated by the statutory requirments applicable for fire protection at the location of the plant or unit.

Let us go to the calculation part for spray water for tanks / vessels based on the geometry of the vessel.

Surface Area Calculations for Tanks / Vessels:

Cylindrical with conical roof and flat bottom:

SA_cone = (π*D*L) + (π*(D/2)*S)

Cylindrical with 2:1 Ellipsoidal Dished Ends:

SA_2:1 = (π*D*L) + (2*1.084*D²)

Cylindrical with 100-6% F&D Heads:

SA_F&D = (π*D*L) + (2*0.9286*D²)

Cylindrical with Hemispherical Heads:

SA_Hemi-head = π*D*(L +D)

Sphere:

SA_sphere = π*D²

where:

D = Diameter of the vessel (cylinder, sphere), ft (m)
L = Length of the cylinder (tan-to-tan), ft (m)
S = Slant height of cone, ft (m)

Once the surface area of the vessel is calculated, multiply it with 0.25 gpm if you are calculating your spray water demand per square feet (USC units) or with 10.2 L/min if calculating in square meters (Metric units).

Some companies require to add a margin on the calculated flow due to hydraulic imbalances in the spray water network and the ineffectiveness of run-down water on sprayed surfaces. Typically the margin used is 10% over the calculated flow.

For other equipment such as pumps and compressors NFPA 15 recommends a different spray water rate and states that:

7.3.2 Pumps, Compressors, and Related Equipment: Pumps or other devices that handle flammable liquids or gases shall have the shafts, seals, and othe critical parts enveloped by directed water spray at a net rate of not less than 0.50 gpm / ft² [20.4 (L/min) / m²] of projected surface area of the equipment.

The difficult part for pumps and compressors is to estimate the surface area unlike tanks / vessels whose surface areas have been calculated as mentioned above. I would request a Loss Prevention specialist to provide some insight in calculating surface areas for these equipment.

For the vessels / tanks part, I have already prepared an excel workbook where you can select the vessel geometry from a drop-down list and it will automatically calculate the surface area for the selected vessel geometry and thus the spray water requirement.

Hope all of you enjoy this blog entry and I look forward to you comments and observations.

Regards,
Ankur.