NCPI CALCULATOR

FOR GRAVITY SEWER PARAMETERS

HOW TO USE CALCULATOR

This is a Gravity Flow Hydraulic Calculator with several scales to relate the various parameters used with Manning's Formula for Hydraulic Flow for the solution of sewer problems. With varying sets of known values, it is possible to determine required pipe diameters, slopes, capacities (in three different units-1000 gpm, cfs, and mgd) partial flow rates, velocities, or depth of flow.

EXAMPLES:

I. Given a flow rate of 11 cfs, "n" Factor of .013 and a slope of .28 feet per 100 feet, determine the required pipe size and velocity of flow.

SOLUTION: Set the Flow Rate indicator (wheel C) opposite 11 cfs (wheel B). Set "n" value of .013 opposite "n" indicator arrow. Read required pipe size opposite the .28 feet per 100 feet slope (bottom of wheel C) pipe size is 24", velocity is 3.8 fps flowing full. Read velocities at varying depth ratios by placing the arrow on (wheel D), opposite the original velocity (3.8 fps). Read partial flow rates in cfs directly opposite corresponding Liquid Depth to Pipe Diameter Ratios (upper window wheel C).

II. Given the same flow rate (11 cfs) and "n" Factor (.013), determine pipe diameter and velocity of flow at .15 slope.

SOLUTION: Set "n" value of .013 opposite "n" indicator. Set the Flow Rate indicator (wheel C) on 11 cfs (wheel B). Reading the slope at .15 feet per hundred gives a diameter between 24" and 27". Realign the .15 slope opposite the larger diameter (27") and reset the "n" indicator, if necessary. The new full flow rate (12 cfs) is seen on (wheel B) and the new velocity (3.0 fps) is now indicated in the lower window of (wheel C) opposite the pipe diameter (27"). Read the d/D ratio opposite 11 cfs to determine depth of flow (upper window wheel C) (.77 full). Return to the lower window on (wheel C), set (wheel D) arrow at 3.0 fps and read the velocity at .77 full (3.4 fps).

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