All this week we are reviewing the ** MOST **important water math concepts, in preparation for this Saturday’s Water Distribution Operator certification exam here in California. Today we look at number four in our

**: the**

*MOST Five*

*Velocity Equation.*This is probably the least used equation of our ** MOST Five**, especially for the D1 and D2 exams. We tell folks to expect zero or one problem using this equation at those levels. And if you do get a problem at these grades, you may be asked to find a

**, given a**

*Flow Rate**Velocity*and the cross-sectional

*Area*of a pipeline. Or you may be asked to find a

**, given the pipe**

*Velocity**Area*and the

*Flow Rate.*For either of these, use the Velocity Equation, Flow Rate = Velocity x Area, or

*Q = Av.*We are rarely given the ** Area **directly, and instead get the

*Diameter*of the pipeline. But that makes for a pretty easy first step: we just calculate the cross-sectional Area of the pipe using Area = 0.785 d² (or πr²).

The best ** Flow Rate** units to use with this formula are cubic feet per second (cfs). These units are consistent with the normal

*Velocity*units of feet per second and

*Area*units of square feet. If your Flow Rate is in gpm or MGD, make sure you convert to cfs first.

For the higher grades of Distribution exams, expect more problems using the Velocity Equation, and also expect one that requires you to find the ** Area **instead. However, that step may not be obvious because that won’t be the final answer requested.

The typical problem of this type will give you a *Flow Rate* and *Velocity*, and then ask for the ** Diameter **of the pipe. To find the

*Diameter*, you have to find the

*Area*first, using Q = Av. Then you can find the diameter using Area = 0.785 d². To get the

*Diameter*, divide the Area by 0.785, and then hit the (very shiny, due to such rare usage!) square root key on your calculator.

But you’re still not quite through. This calculation will give you the Diameter in *feet*, and you’ll almost certainly need to convert that into *inches* instead.

If only all the steps in this calculation were that easy!

Keep studying! Saturday is coming fast!