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Calculating Water Pressure Loss

Calculate Water Pressure Loss

Years ago, engineers developed tables to quickly calculate water pressure loss. They're often referred to as hydraulic friction loss tables. Studies produced constants (numbers which you use to help in the calculation) which make the math fairly easy.

What Do You Need to Know to Calculate Pressure Loss?

You need to know several things:

  • size of pipe
  • flow in gallons per minute (GPM)
  • the distance which the water flows

Do Fittings Affect the Actual Distance?

Yes, fittings increase the actual distance. If you have lots of bends and tee's, 100 feet of pipe might really be 130 feet or so.

Flow is the thing that stumps most people. Flow rates can also be found on tables. Here are some standard flow rates for residential fixtures:

  • toilet - 2.5 GPM
  • kitchen sink - 1.25 GPM (H or C - times 2 if both handles are wide open)
  • washing machine - 1.6 GPM (H or C - times 2 if H & C on at the same time)
  • shower 2.5 GPM (both hot and cold on at the same time)

What are the Constants?

Now for our constants. I have listed several to show the relationship as the gallons per minute of flow increases.

Listed are 1/2 and 3/4 inch copper pipe Type M. Type M is the standard grade of copper tubing used in residential construction.

What are the Pressure-Loss Constants for Small Pipe?

The constants listed are pressure loss per hundred feet of pipe run.

 

1/2 inch pipe: 1 GPM = 2
2 GPM = 7.22
3 GPM = 15.3
4 GPM = 26.0
5 GPM = 39.2
6 GPM = 55
3/4 inch pipe: 1 GPM = 0.354
2 GPM = 1.28
3 GPM = 2.71
4 GPM = 4.60
5 GPM = 6.94
6 GPM = 9.75

 

Notice the difference in the constants for different-sized pipe, but yet the same GPM flow! It's HUGE. See why 3/4 is better?

Now, here is how you do a pressure loss calculation.

By the way, we need one other constant and it is a number that relates to pounds per square inch. It's really pretty complicated, so I would appreciate it if you would just trust me on this one. The number we need to remember is 0.4335. Okay, if you don't trust me and want to know more about the secret constant of 0.4335, click or tap here.

Can You Do A Sample Pressure-Loss Calculation?

Yes, let's calculate what will happen to the pressure if we flush a toilet while a shower is running in a bathroom 100 feet away from where the water enters the house. We are using 1/2 inch pipe.

Toilet flow = 2.5 GPM Shower Flow = 2.5 GPM Total flow = 5 GPM

Constant from table multiplied by 0.4335 = Pressure loss in PSI
39.2 X .4335 = 16.99 PSI

If we substitute 3/4 inch pipe, here is the calculation:
6.94 X .4335 = 3.00 PSI

Quite a difference!

Column B372

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