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Project Planning - Hydraulic Design

Compared to cast iron, clay and vitreous pipe systems, stainless steel pipes have a considerably smoother bore (Manning Coefficient: 0.011) and in general, are less susceptible to internal scaling.

In some instances, low roughness coefficients (ks) are not generally a true reflection of the long-term hydraulic performance of the installed system.Roughness coefficients of 0.6mm should be used for rainwater/storm drainage and 1.5mm for soil/foul drainage.

Flow Tables

Two sets of flow tables are presented within this design guide.

Table 1 is for pipes installed with level (or nearly level) gradients where the steady,uniform flow equations are not applicable.The data therefore has been generated from ACO’s hydraulic design program’Hydro’ that is based on the equations of spatially-varied flow.

Table 2 is for pipes installed with varying gradients. The data is based on the Colebrook-White equation using an appropriate roughness coefficient for stainless steel.

When draining storm or foul water, it is inevitable that sediment deposits will occur within the drainage system.

Sediment and scale deposits will reduce the flow rate through any pipe system and it is recommended that an allowance is made for this within the design and planning phase.

Flow rates based on a spatially-varied flow formula for steady non-uniform flow. Manning Coefficient 0.011

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Table 2a. For rainwater/storm drainage applications

Flow rates based on a Colebrook-White formula. Roughness coefficient ks =0.6mm

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Table 2b. For soil/foul water drainage applications

Flow rates based on a Colebrook-White formula. Roughness coefficient ks =1.5mm

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The flow rates shown above assume an unrestricted discharge from the pipe. For installations without an unrestricted discharge, the flow rate will be affected by the downstream throttle. For shallow gradients, the Colebrook-White formula underestimates flow rates (because when gradient tends towards zero%, velocity also tends to zero). For level (or nearly level) installations (slope < 1%), spatially varied flow tables should be used; refer to Table 1.

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