Conveyance: A Simplified Method for Sizing Pipe. Technical Bulletin

Transcription

1 Techical Bulleti Coveyace: A Simplified Method for Sizig Pipe Itroductio Oe commoly used equatio used i pipe flow calculatios is the Maig equatio as show i Equatios 1 ad 1(a). The formula ca be simplified so that the desig egieer ca easily determie of a variety of potetial solutios to the draiage problem. This documet demostrates the applicatio of this techique through use of coveyace factors. 2 / 3 1/ AR S Where: pipe capacity, (cfs) = Maig s, a term used to describe material roughess (uitless) A = cross-sectioal flow area of the pipe (ft 2 ) R = hydraulic radius (ft); 1/4 the diameter for full-flowig pipe coditios S = pipe slope (feet/foot) Or, i metric uits: 2 / 3 1/ 2 AR S Where: pipe capacity, m 3 /s = Maig s, a term used to describe material roughess (uitless) A = cross-sectioal flow-area of the pipe (m 2 ) R = hydraulic radius (m), 1/4 the diameter for full flowig pipe coditios S = pipe slope (m/m) Equatio 1 Equatio 1(a) Coveyace Factors For a specific kid of pipe flowig full, the pipe-related parameters, A, ad R become costats. They ca the be combied, as show i Equatio 2 or 2(a), to form the coveyace factor. Coveyace ca also be thought of as the flow carryig ability of the pipe; it is idepedet of siterelated coditios. For coveiece, coveyace factors have already bee calculated usig the Maig values show i Table 1; these values are provided i Tables 2 ad 2(a) /. AR Or, i metric uits: 2 / 3 AR Equatio 2 Equatio 2(a)

2 Substitutig k ito Maig s formula yields Equatio 3 which ca be used to determie the capacity of a specific pipe istalled at a kow grade. 1/ 2 ks Equatio 3 Note from Equatio 3 that coveyace has a direct relatioship to capacity. That is, if the slope is held costat, triplig coveyace will triple the capacity; halvig coveyace will halve the capacity. Simplified Pipe Sizig Techique Rearragig Equatio 3 yields Equatio 4, which will allow compariso of product optios. Q S 12 / Equatio 4 The required capacity that the pipe must carry, Q, is kow or ca be calculated based o the size ad permeability of the area beig draied, local raifall evets, stormwater regulatios, ad other criteria. The pipe slope, S, is a site-specific parameter ad is ofte dictated by the cotour of the draiage area. Oce Q ad S are kow, the coveyace factor k ca be determied. The coveyace factor calculated i Equatio 4 is the smallest allowable for the situatio; the selected pipe optio must have a coveyace factor of at least this value. Example Problem A parkig lot requires draiage at a rate of 3.25 cfs. The ruoff will be carried by a pipe istalled at a slope of 0.5%. Fid the pipe optios that will meet the eeds of this project. Solutio: Use Equatio 4 to determie the miimum required coveyace factor. Q S 12 / = 46.0 = 1/ 2 Refer to Table 1 to obtai Maig values for the pipe types of iterest. Take the Maig value iformatio from Table 1 to Table 2, the select a diameter with a coveyace factor closest to, but ot less tha, The most practical solutios are as follows: Type of Pipe Diameter Coveyace Factor Hacor Sure-Lok Pipe 12-ich 46.3 Hacor Heavy Duty or 15-ich 46.7 AASHTO Pipe Reiforced Cocrete Pipe 15-ich 64.6 Corrugated Steel Pipe 15-ich 70.0

3 Summary This techique usig coveyace factors allows oe to quickly develop differet pipe alteratives that will solve the same hydraulic problem. Oce the differet optios are established, they ca the be evaluated based o cover height limits, durability problems, product availability, cost issues, ad other project costraits to arrive at the best solutio(s). Table 1 Maig Diameter Maig s Roughess Coefficiets for Hacor Pipe Sure-Lok Pipe* all diameters AASHTO ad Heavy Duty Pipe ( ) ( mm) 10 (250mm) 8 (200mm) 3-6 (75 150mm) Roughess Coefficiets for Other Pipe Reiforced Cocrete Pipe all diameters (RCP) Corrugated Steel Pipe (CSP) 54 (1350mm) (1200mm) Maig values obtaied from 42 (1050mm) Hadbook of Steel Draiage & Highway Costructio Products ad 36 (900mm) are based o full-flow coditios, ad 30 (750mm) helical corrugatios 2 2/3 x ½ (other 24 (600mm) profile cofiguratios will have 18 (450mm) differet Maig values) 15 (375mm) (300mm) *Maig s coefficiet for Hacor Sure-Lok pipe determied at Utah State Uiversity Water Research Laboratory.

4 Table 2 Coveyace Factors (stadard uits) Dia Area (i) (ft 2 ) Dia Area (i) (ft 2 )

5 Table 2(a) Coveyace Factors (metric uits) 2/29/00 vld Dia. Area (mm) (m 2 ) Dia. Area (mm) (m 2 )