• In 1903, after studying recorded friction loss measurements produced by dozens of experimenters, Allen Hazen and Gardner Williams published an empirical formula now known as the Hazen-Williams friction loss formula.

• Until the early 1970s using this friction loss formula was tedious, requiring the use of logarithms and a slide rule.

• Hydraulic calculations were first introduced into NFPA 13 Standard for the Installation of Sprinkler Systems in the 1966 edition.

• In 1972 the concept of sizing system piping and water supplies based on density and area of expected sprinkler operation was introduced.

• Between 1966 and 1978 the standard was revised four times to include successively expanded hydraulic design criteria such as area/density curves for different hazard severities.

• The advent of electronic calculators and personal computers made application of the Hazen-Williams formula routine and as a result, hydraulically designed systems eventually became the norm.

• The significant digits of a number are those digits that

carry meaning contributing to its precision (1.4136 vs 1.4).

• Significant digits in an answer to a calculation depends on significant digits in the data.

• Common mistake: reporting more digits in answer than justified by digits in data.

• Sprinkler system design: based on testing/measuring water supplies.

• Pitot tube in stream of water discharging from a fire hydrant.

• Read velocity pressure from gauge: – Hold pitot in correct position

– Needle “bounces”

– Wiping water off the face of the gauge (and out of your eyes).

• Resulting data will have a significant margin of error.

• Unfortunately many take that test data as gospel (instead of an approximation).

• Calculation of the sprinkler piping network

– Hazen-Williams formula is empirical

– H-W has certain limitations: not applicable to turbulent water flow.

– There are more accurate fluid flow formulas that account for turbulence & the variation and viscosities over a range of temperatures.

– NFPA 13: density & viscosity of water do not significantly change over the range of temperature where water is used for fire protection & effect of turbulence is extremely

minor.

• Good news: successful performance of sprinkler systems designed with H-W

• Bad news: designers utilize calculators/computers & report required flows and pressures of two (or more) decimal places!

• Sprinkler Hydraulics, by Wass: Ignor everything to the right of decimal point.

• Suggestion: – Round demand pressures/flows up to the next whole

number

– Round supply pressures/flows down to the next whole number.

• Unknowns concerning sprinkler system hydraulics including:

– Accuracy of the water supply test data

– Changes (degradation) in the water supply over time

– Corrosion of internal piping surfaces over time

– Building configuration changes that may be detrimental to successful application of sprinkler spray

– Human error

• AHJs often require a safety factor.

• Often a delta between required pressure and available pressure.

• Minimum fixed difference, or % of total available pressure, or some combination thereof.

• Arbitrary safety factor irrespective of water supply curve slope may not actually provide much “safety.”

• Should safety factor: pressure or flow?

• System flow & pressure are interrelated, safety factor should be the length of the line between the sprinkler system demand point & the point where the demand curve intersects the supply curve.

• Should safety factor: pressure or flow?

• System flow & pressure are interrelated, safety factor should be the length of the line between the sprinkler system demand point & the point where the demand curve intersects the supply curve.

• Typically calculations are performed ignoring velocity pressures.

• Water traveling thru pipe has kinetic and potential energy.

Pt = Pn + Pv

– Where : » Pt = total energy

» Pn = total potential energy component

» Pv = total kinetic energy component

Pn Pv

Pn = Pv – Pv

Considering impact of velocity pressures, the final system demand

flow and pressure will be lower

• Flow from 1st sprinkler is know.

• Flow from successive sprinklers must be estimated.

• Add flow from sprinkler #1 to estimate for #2 to calculate velocity pressure

Pv = (0.001123) (Q)2

(D)4

– Where : » Q = Flow (Sprinkler #1 + estimate for #2), gpm

» D = Diameter of pipe supplying second sprinkler, inches

» 0.001123 = conversion factor to yield PSI

• Fire sprinkler systems: Enviable track record since 1874

• “Built in” safety factors including: – Initial densities are higher

– Calculations started with design density at end sprinkler

– Hydraulically most remote areas are calculated

– Calculations developed on rectangular pattern

– Friction coefficient (wet-pipe) average higher than calculated C=120

– Hose stream demand: available to sprinklers in the early stages of fire

average density is 0.21 gpm/sq.ft.

• Calculations account for water used by fire department for manually suppression (“hose stream allowance”).

• Typically shown on a hydraulic graph as a line equal to the allowance extending horizontally from the maximum sprinkler demand.

• Problem: hose streams not flowing at the maximum pressure demand of the sprinklers.

• In reality fire department is “taking this amount of water away” from the available supply and the sprinkler system is “left” with a degraded water supply curve.

• Concept developed & promoted by Mike Thompson, P.E. (HydroAide)

• Rate of water application per unit area at the floor level. – Office space would typically be 0.10 gpm/sq.ft. – Retail space would typically be 0.20 gpm/sq.ft.

• Fire protection professional should not only know what NFPA 13 requires for various hazards but they should have a “feel” for the numbers if they are to truly understand how these systems can/will perform.

• Participating in an actual sprinkler discharge demonstration or experiment is best to truly understand these designs.

• Scenario: – Ordinary Hazard, Group 2 sprinkler system – Room that measures 10 feet wide x 10 feet long x 8 feet high – 0.20 gpm/sq.ft. over the entire room’s floor area – Assume the room is water tight – 10 minutes of discharge the room would contain 200 gallon of water. That would be

3.2 inches deep across the entire room and weigh 1,670 pounds.

QUESTIONS?