Fire Pump Sizing and Calculations

When starting a fire pump design, the most important item to consider is the water supply. supply. If you are utilizing the city water supply as the primary source for the pump, you need to make sure that an accurate city water test is used. Some good rules to follow are: • Make sure the water test is less than a year old. • Make sure the water test is performed as close to the tap point as possible. • Make sure the test is taken during the time of highest water use for the area. In a cold climate, testing during the summer may be a better choice, since residents are watering their lawns and commercial properties are using more water for cooling. dditionally, in highly residential areas, peak morning use typically occurs early in the day, between !:"# and $ a.m., so that may be the best time for the test. nother consideration consideration regarding the water test is this: %id the city flow enough water to meet &'# percent of the fire pump design point( )his will ensure that the municipal supply will pro*ide enough *olume to meet the system demand. If not, re+uest a new test using more hydrants or plot a water supply cur*e see -igure &. If you are using a pri*ate water supply, lake, or ground/le*el storage tank, you must remember that you are not permitted to use a suction lift with a fire pump. )hus, if the water supply is located below the suction inlet to the fire pump, you may ha*e to utilize a *ertical turbine fire pump in lieu of the other types of pumps a*ailable. )he last 0ob parameter you need in order to size the fire pump is the re+uired flow for the systems the pump will be ser*ing sprinklers, standpipes, or other. -or standpipe systems, this flow is related to the type and size of the structure the pump is protecting. In any case, the demand of the system will dictate the pressure and flow re+uired. Calculating standpipe system pressure )wo )wo types of structure calculations for pressure are a*ailable. 1ne is for high/rise structures buildings greater than 2' feet in height, measured from the lowest le*el of fire department *ehicle access to the floor of the highest occupiable story and one is for non/high/rise structures. )his is a factor because any high rise re+uires a pressure of &## pounds per s+uare inch psi at the top of the structure while flowing the rated gallons per minute gpm of the fire pump. )his discussion concentrates on high rises because the pressure calculations for most non/high/rise buildings are determined through the use of software specifically designed for fire sprinkler hydraulic calculations. )hese programs are used by sprinkler contractors contractors to keep their pipes as small as possible, which controls the cost of the 0ob. When calculating the water pressure for a high rise, it is a good idea to use a calculation sheet such as the one shown in -igure 3. If you use a calculation sheet, a few *ariables need to be filled in:

• 4ressure drop in the backflow pre*ention de*ice and water meter • -riction loss in the most remote standpipe when flowing '## gpm • 5le*ation change -or the e6ample in -igure 3, the parameters are: • 7uilding height: 3&3 feet • 8ity water pressure: 9' psi static "' psi residual • ;e+uired flow rate: &,3'# gpm ssume that the pump is &,3'# gpm for this e6ample. s you can see, the calculated re+uired pump psi is &owe*er, the friction loss and pipe sizing become an issue when doing this calculation, which is why most contractors and sprinkler designers use software.  note about pressure 1ne thing that some engineers forget is that the pump will discharge at a much higher pressure at churn no flow than at the design point. 4er ?-4 3# 3#&#: Standard for the Installation of Stationary 4umps for -ire 4rotection, fire pumps are allowed to ha*e a 9# percent rise in pressure from rated flow to churn. )his is almost ne*er the case, but different pumps and speeds affect the churn pressure, so you should always look at a cur*e to determine the shutoff pressure. )he reason for re*iewing this is typically to understand what the ma6imum pressure no/flow churn will be in the system to determine whether high/pressure fittings are needed. 8alculating pump gpm )o calculate pump gpm, two sizing methods are a*ailable, the standpipe method and the sprinkler area calculation. In a f ully sprinklered structure with standpipes, ?-4 &9 3#&#: Standard for the Installation of Standpipes and >ose Systems says that the first standpipe re+uires '## gpm and each additional standpipe re+uires 3'# gpm, up to a ma6imum of &,### gpm. -or e6ample, a building with two standpipes would re+uire a 2'#/gpm pump '## gpm for the first standpipe and 3'# for the second, and a building with fi*e standpipes would re+uire a &,###/gpm pump because that is the ma6imum allowed by ?-4 &9. ?ote that the local code or the insurance carrier may re+uire more than the ma6imum allowed by ?-4 &9. rea calculations are more difficult. @ou need to know the sprinkler hazard classifications of the building and its contents to determine the design density, and the s+uare footage area of operation of each hazard must be calculated. )he fi*e types of hazard classifications from ?-4 &" 3#&#: Standard for the Installation of Sprinkler Systems are:

• Aight hazard: Aow +uantity of combustibles with low heat release e.g., churches, hospitals, museums • 1rdinary hazard &: Moderate +uantity of combustibles with moderate heat release and eight/foot stockpiles e.g., mechanical rooms, restaurant kitchens, laundry facilities • 1rdinary hazard 3: Moderate +uantity of combustibles with moderate heat release and &3/foot stockpiles e.g., stages, large library stack rooms, repair garages • 56tra hazard &: >igh +uantity of combustibles with high heat release and no flammable or combustible li+uids e.g., aircraft hangers, saw mills • 56tra hazard 3: >igh +uantity of combustibles with high heat release and flammable and combustible li+uids e.g., plastics processing, flammable li+uids spraying ;efer to ?-4 &" for a more thorough definition of the classifications. 1nce the hazards ha*e been determined, you ne6t take the most remote &,'##/s+uare/foot area of sprinkler operation and multiply it by the density found in ?-4 &" -igure &&.3.".&.' see -igure ". )hen you must add the inside and outside hose stream demand to the area calculation. )his information can be found in ?-4 &" )able &&.3.".&.& see )able &. >ose stream demand is the amount of water that must be added to the sprinkler system hydraulic calculation to fill the hoses as well as ensure enough supply to operate the sprinklers. Inside hoses are generally &/ to &B/inch standpipe hoses that may be connected to the sprinkler system for initial fire attack. -or e6ample, if you ha*e a 9#,###/s+uare/foot building that is all ordinary group &, the calculation would be &,'## 6 #.&' density C 33' D 3'# hose demand C 92' gpm total for the fire pump. If the structure has multiple hazards, the hazard with the highest gpm calculation dictates the pump size. Make sure you touch base with the insurance carrier for a particular pro0ect, as they may re+uire higher s+uare footage or density re+uirements, depending on the 0ob. Selecting the pump 1nce you ha*e calculated the gpm and psi re+uirements for the pump, you need to determine the type of pump that works best for the 0ob. )he three most widely used pumps are horizontal split case, inline and *ertical turbine. >orizontal split case pumps are also called double/suction fire pumps, because the water pathways direct water to both sides of the impeller. )hey are the most common type of fire pump on the market, partly because of the ratings a*ailable in this style of pump, typically 3'# through ',### gpm. )his was the first type of pump used for fire protection systems. Inline fire pumps offer se*eral benefits: • )heir size and design offer space sa*ings. • )hey offer the ability to increase the ratings allowed by ?-4 3# from a ma6imum of 9$$ gpm, to

2'# gpm, to todayEs unlimited rating. )he largest currently a*ailable is &,'## gpm. • )hey offer a low cost of installation because they donEt re+uire a base plate that needs grouting. Fertical turbine pumps are used in situations where the water supply is below the suction flange of the fire pump, because ?-4 3# re+uires a positi*e suction pressure to a fire pump. )he other item that needs to be determined is the type of dri*e: diesel or electric. 1nce that is determined, you can find the appropriate pump model and horsepower in a manufacturerEs catalog. I donEt recommend using pump cur*es to select fire pumps, as e*ery selection must be GA appro*ed, which might lead to picking the wrong horsepower for a particular selection. 1ne other note on fire pump selection is that selecting pumps that ha*e a higher rpm is not necessarily a misstep, because fire pumps only run once a week for a limited amount of time, so the length of life will be about the same for a &,2'# rpm pump as for a ",'## rpm pump. Power supply If a generator is going to be used as a secondary power supply, the fire pump will re+uire a transfer switch, which must be dedicated to the fire pump.  typical design would be to use a combination controller and transfer switch in a cabinet to a*oid the need for additional re+uirements laid out in ?-4 3#.  reduced/*oltage start also should be considered when connecting to a generator to potentially reduce the size of the generator. )his is true e*en for normal power considerations, as large/horsepower fire pumps with across/the/line starting put significant strain on power systems. )he two most commonly used by designers are solid state soft start and wye/delta closed transition. )hese two ha*e the best starting characteristics of the appro*ed options on the market. Code issues -ollowing are some code re+uirements for fire pumps that you should factor into pump selection and system design. • >orizontal elbows or tees upstream of a fire pump must be &# pipe diameters from the suction flange on a split/case fire pump. • 4umps must maintain a positi*e suction pressure at the suction flange. • 5lectrical feeds to fire pump controllers must ha*e a two/hour fire rating. • -ire pumps canEt be used as pressure/maintenance pumps. • Fariable/speed pumps are allowed by the code. • -ire pumps need to be installed in a 3/hour rated room. Avoiding trouble )o a*oid problems during the design and installation process, you should always do your homework and consult with the authority ha*ing 0urisdiction and insurance representati*e before you begin.

Some 0urisdictions ha*e special re+uirements for fire pumps. -or instance, ?ew @ork 8ity re+uires a manual round rotor fire pump with e*ery automatic fire pump, and the 1hio 54 re+uires suction control *al*es on e*ery fire pump to pre*ent going below 3# psi in the main. Insurance companies also may ha*e uni+ue re+uirements that go abo*e and beyond the code. -M Hlobal, for e6ample, re+uires diesel fuel tanks to be double wall and ha*e a spill basin, and e*ery pump room must ha*e a low pump room temperature alarm. nowing these location/specific and uni+ue issues before starting your design will help the process go more smoothly and be more successful.