What I'd suggest would be to ...
Published by Lyle Mariam
What I'd suggest would be to use the length of the pipe and determine the pressure loss at both pipe diameters, remember to adjust the friction factor for the velocity and diameter change. Then take the differential pressure between the two pipe diameters and back calculate the BHP to flow the fluid using the delta P as the TDH. Assume a pump efficiency of 70% (or whatever the actual pump is) and a 94% efficient motor to convert the BHP to KWH. Then multiply the KWH by the electric cost per KWH. That will give you the differential cost to operate the line.
BHP = GPM * TDH /( 3960 * EffPump * EffMotor) (English Units)
KWH = BHP * 0.746
The additional cost to pump the liquid with a smaller diameter pipe will tell you if it is more economical. Naturally, you'll have to factor in the increased cost of larger pipe and any increased installation costs but this method will give you a good idea of which is more economical.
If you think of this as what's the cost to pump the fluid at the differential pressure between the two pressure losses is the amount that you're going to save using the larger diameter pipe.
Keep in mind that recommended pipe size is a generality and may not fit your application.
P.S. An easy way to calculate velocity of water (English units): v = 0.408 * GPM / Dia^2
v in Ft/Sec, Q in Gal/Min, Dia in inches
2 Comments
Lyle has given the methodology and David Russel the additional parameters to be factored.
Published by Ahmed Soorma, General Manager at Soorma & Sons
Historically, the specification on water velocity has been primarily to prevent settling of solids in gravity flow situations. The recommended minimum velocity for sewers is around 2' second which is about 0.6 m/ second. Lower velocities don't make sense unless one is looking at overall demand.
You might want to consider the "shower flush syndrome" when one is scalded or frozen with a blast of off temperature water when the system demand hogs all the hot or cold water, as when you are in the shower and the dish washer kicks in and demands hot water, or alternatively when the toilet is flushed when you are showering.
It's a question of head losses and demands. Frankly, I'd rather use larger pipe sizes to reduce the fluctuations due to the instataneous demands.
Published by David Russell, President at Global Environmental Operations, Inc