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MAIN DRAIN ESTIMATOR
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BETA V0.2 © MEYERFIRE, LLC
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LIMITATIONS
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A hydrant or
forward-flow test is necessary for proper water supply evaluation. When not
possible to conduct large-volume flow test, a main drain test can estimate a
water supply, but is dependent on (1) fully-open valves, (2) drains free of
blockage, (3) relatively accurate c-factor, (4) unimpeded discharge, (5)
accurate gauge on the riser.
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GIVEN - DRAIN
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dDR
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drain opening size
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in
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C
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discharge coefficient of the opening
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lDR
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total length of drain pipe
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ft
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n90
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number of 90-degree elbows in drain route
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el(s)
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n45
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number of 45-degree elbows in drain route
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el(s)
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CDR
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c-factor for drain pipe
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O
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percent of drain pipe that is obstructed
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%
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schedule of drain pipe
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type of main drain valve
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GIVEN - RISER
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dRI
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riser diameter
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in
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CR
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c-factor for riser
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lRI
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length of pipe from riser gauge to main drain
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ft
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pRES
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residual pressure showing at the riser
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psi
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schedule of riser
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e
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elevation of riser gauge, relative to drain outlet
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ft
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BALANCED FLOW ESTIMATE (MEYERFIRE)
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Q
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estimated flow (MeyerFire)
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gpm
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WORKED SOLUTION: EQUIVALENT LENGTH
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lEQ,90
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equivalent length of 90-degree
elbows
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ft
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lEQ,45
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equivalent length of 45-degree
elbows
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ft
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lEQ,VALVE
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equivalent length of main drain
valve
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ft
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lEQ
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equivalent length of drain route
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ft
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lEQ = lDR + n90 (lEQ,90)
+ n45 (lEQ,45)
+ (lEQ,VALVE)
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dSCH40
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schedule 40 inside pipe diameter
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in
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dINT
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inside pipe diameter
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in
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dACT
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actual pipe inside diameter of
drain route, adjusted for pipe obstruction
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in
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dACT = dINT x (100-O)/100
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lEQ,ADJ
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adjusted equivalent length of
drain route
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ft
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lEQ,ADJ = lEQ x (dACT / dSCH40)^4.87
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dACT,RI
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actual riser internal pipe diameter
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in
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AACT,RI
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internal cross-sectional area of riser pipe
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in2
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A = π d2/4
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FLOW ESTIMATE ITERATION #1
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p1
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assumed initial pitot pressure
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psi
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FLOW ESTIMATE ITERATION #10
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p10
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pitot pressure iterated 10 times
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psi
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Q10
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resulting flow
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gpm
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Q10
= 29.84 c dSCH402 √p10
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pL10,DR
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pressure loss through drain pipe
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psi
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PL10,DR = (lEQ) x 4.85 x Q1.85 / (C1.85 x d4.87)
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pL10,RI
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pressure loss through riser
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psi
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PL10,RI = (lRI) x 4.85 x Q1.85 / (C1.85 x d4.87)
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pT,10
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total (dynamic) pressure at
riser gauge
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psi
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PT,10 = p10 + pL10,DR + pL10,RI - 0.433e
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vRI,10
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water velocity in riser (with
flow in gpm, A in2)
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ft/s
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vRI,10 = 0.3208 Q10 / A
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ρ
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density of water
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1.936
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slugs/ft3
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pRES,10
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resulting normal pressure at
riser gauge
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psi
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Bernoulli's Equation
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PRES,10 = PTOTAL - (1/144)(1/2)(ρv2)
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E10
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error in resulting normal
pressure at gauge
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psi
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E10 = pRES - pRES,10
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I
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iterations before balance achieved
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iterations
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FLOW ESTIMATE (MEYERFIRE)
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Q
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estimated flow (MeyerFire)
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gpm
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FLOW ESTIMATE (OTHER MODELS)
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Q
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estimated flow (Simplified ISO)
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gpm
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q = 53 √PRES
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Q
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estimated flow (INA Tables):
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for long discharge line
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gpm
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for short discharge line
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gpm
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Q
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estimated flow (Insurer model):
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gpm
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based on equivalent length
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COPYRIGHT ©MEYERFIRE,
LLC.
ALL RIGHTS RESERVED.
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