Calculating Flow Through A Pipe

When operators want to figure out how much water is flowing through a pipe, how do they figure out what their flow rate is?  If you use the formula that is on the formula sheet that we give to every operator taking a class, it will be Q = A x V.

In using the Q=A x V formula, we must make sure the units are correct before we can plug them into our formula.  Q stands for flow in cuft/sec, A is area in sq/ft and V is velocity in ft/sec.

To put this equation to the test, try this problem:

An operator wants to figure the flow of an 8-inch pipe that has a length of 5,000 ft. The operator has also already determined the velocity to be 2.1 ft/s . What is the flow rate through this pipe?

The last sentence will tell us what formula we need. The problem asks for flow rate, so we have to use Q=A x V. Now we write out the formula and plug in the numbers we have been given, converting if we need to get the correct units. In this problem, there is no conversion, but we do have to solve for the area of an 8-inch pipe. We use the formula .785 x D’ x D’ when solving this problem, and you must make sure that the units are correct. Diameter in our area formula is needed in feet, but we have it in inches. So to convert feet to inches, we must divide our inches by 12 to have the correct units.

Q=A x V

Q= (.785 x (8/12) x (8/12)) x 2.1 fps

Q= (.785 x .66 x .66) x 2.1 fps

Q= 0.34 sq/ft x 2.1 fps

Q=0.71 cuft/sec

To check your answers, Click Here.

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