Changing pipe diameter


Changing the diameter of the piping in a pumping system P057

Business has not been that good and production has decided that maybe they can save costs if they increase the piping size so that a smaller and more economical motor can be used and the possibility of cavitation can be reduced. What exactly is going to happen to the horsepower requirement for this new installation? Will the additional cost of converting to larger piping be justified by the lower operating costs? What kind of a head reduction are we going to get when we move from a 3-inch to a 4-inch piping system?

We have been pumping 300 gpm through the present system. That’s not going to change.

Horsepower is measured in foot pounds, with one horsepower equal to 33,000 foot pounds. Since fluid has weight we can calculate how many pounds per minute we are pumping by finding out how much a gallon of our fluid weighs. After you have done that, multiply the gallons per minute you are pumping by 8.33 (the weight of a gallon of water) and then multiply that result by the specific gravity of your fluid and you will have the pounds per minute number you are looking for. Multiply that number by the total head the pump is producing and you have foot pounds per minute.

Here is an example:

You are pumping 300 gallons per minute of a fluid with a specific gravity of 1.3. The combined heads in the system total 80 feet. Of that total 20 feet is friction head loss in the piping

300 gpm x 8.33 x 1.3 sg. x 80 ft = 259,896.00 foot pounds/minute.

Divide this number by 33,000 and you get 7.88 horsepower.

This is called the water horsepower or the horsepower out of the pump. If the pump were 100% efficient, all you would need would be a 7.88 horsepower motor to drive the pump and it would do the job, but motors and pumps are not 100 % efficient and remember efficiency is determined by where you are on the pump curve, so you have to make some final calculations to determine the size and operating cost of the motor you choose for this application.

Let’s calculate our new friction loss. The rule says: The friction loss is inversely proportional to the fifth power of the pipe diameter ratio. The formula looks like this:

We were pumping 300 gallons per minute through a 3-inch line with a 20-foot head loss. We want to replace the 3-inch piping with a 4-inch diameter system

= 5 foot loss

Five foot instead of 20 feet. The engineer in charge, now has the information he needs to decide if he wants to go the expense of putting in a new piping system