4 replies to this topic

[miloc]

Can someone explain me where the formula in the attached image is coming from (cerm 17.30 and 17.31 give me a different number)? also when the pressure at the end of the pipe is calculated, where the 0.433 is coming from?

thanks.

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[mrt406]

There are multiple forms of the Hazen-Williams equation, each for different units of measurement for each variable.

This particular form of the equation is for L = length in feet, Q = flow in cfs, and D = diameter in ft.

The 449 in the solution is a conversion from gpm to cfs (60 seconds per minute x 7.48 gallons per cubic foot).

The 0.433 is a conversion from feet of head to psi.


As an aside, this is a prime example of the type of problem solving needed on both the FE and PE exams. They will give you a relatively simple problem, but try to catch you on units. ALWAYS check your units.

[IlPadrino]

0.433 is the conversion factor between "feet of water" and psi (0.433 psi = 1 ft of water). You will find different equations for different units. The equation given is for Q in cfs, which is why there's another conversion factor included in the solution (1 cfs = 449 gpm), and D in ft (again, another conversion factor).

Please take this advice: ALWAYS write down units for the equations in your reference material and ALWAYS write down units in your solution.

[VTEnviro]

As an aside, this is a prime example of the type of problem solving needed on both the FE and PE exams. They will give you a relatively simple problem, but try to catch you on units. ALWAYS check your units.

Agreed. This is critical. Especially since there aren't any units shown in the answers. You need to read the problem carefully.

[Jacob_PE]

When I took the exam in October, I habitually wrote down the units given to me and the units requested. This helped me remember to convert.