Tuesday, January 10, 2012

Diesel Fuel Tank Size for Fire Pumps

Guidance for the sizing diesel fuel tanks is quite straight forward due to the prescriptive requirements of the code.  Just take your engine HP x 1.10 and the result in gallons is the minimum required diesel fuel storage tank size.  The exact code reference from NFPA 20 (2010 edition) is provided below:
11.4.2* Fuel Supply Tank and Capacity.* Fuel supply tank(s) shall have a capacity at least
equal to 1 gal per hp (5.07 L per kW), plus 5 percent volume
for expansion and 5 percent volume for sump.
A.11.4.2 The quantity 1 gal per hp (5.07 L per kW) is equivalent
to 1 pint per hp (0.634 L per kW) per hour for 8 hours.
Where prompt replenishment of fuel supply is unlikely, a reserve
supply should be provided along with facilities for transfer
to the main tanks.
How the committee arrived at these simplified guidelines is as follows.  First, lets look at the conditions for when we expect the fire pump to run:

  • Quarterly Refilling of the Fuel Tank (approx 12-weeks)
  • A weekly test run for 30 Minutes
  • A minimum run time of 2-hour (or 4-hours during a fire depending upon your needs)
Multiply this out and you get basically 8-hours of continuous run time depending upon your run time during a fire.  Take the NFPA 20 appendix guidance of 1 pint/hr/HP (0.125 gallons/hr/HP) x 8 hours and you get 1 Gallon per horse-power.

Lets compare this to the actual published data for a specific diesel engine.  Take the smallest diesel engine Cummins makes a CFP5E-F10 which produces 95HP at 1760 RPM.  The published fuel rate is 4.9 Gal/hr (18.5 L/hr).  4.9 Gallons/hr x 8 hours x 1.10 (sump/expansion) = 43 gallons minimum.  If we use NFPA 20 guidance we would get 95 HP x 1 Gal/HP x 1.10 = 104.5 gallons minimum.  As you can see the for this specific example NFPA 20 is much more conservative.

The other item you need to verify is that the fuel tank complies with UL 142  as required by NFPA 20 (2010 edition) paragraph  Fuel tank sizes are limited to 1320 gallons and the standard sizes available are as follows:

Nominal Tank Sizes (Gallons) Usable Volume (Gallons)
119 105
187 165
300 270
359 320
572 515
849 766
1100 993