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Pipe ID (inside diameter) affects pressure loss in water systems and needs to be considered when specifying pipe.

 

It is common in pump sizing to look at a friction loss chart and calculate head loss due to friction when choosing the pipe size that you plan to use. This becomes an issue when the pipe is not specified or the correct chart for the pipe is not used.

 

For instance, here is a sample of 1” pipe ID:

Schedule 40 - 1.029"

Schedule 80 - 0.936"

Schedule 120 - 0.891"

SDR 21 - 1.169"

CTS - .875"

 

As seen above, 1” pipe can have a diameter of 1.169” or as small as .875. The ID is a crucial factor in calculating the charts, and as one can imagine, a big factor in head losses on a long run of pipe.

 

Deep well Pump

An example of this is a deep well pump on 1” pipe to produce 15 gallons per minute into a system.

If the pump is set on 1” schedule 80, the loss in 400 ft is 81 ft or 25 psi in pressure drop at the end of the pipe.

 

If the pump was set on schedule 120 pipe, the loss in the pipe is 103.4 ft or 44.8 psi.

 

If the system was designed with schedule 80 and installed with schedule 120 the system would be short by almost 20 psi when using 15 gallons per minute.

 

Water Treatment System

Often high-purity water treatment systems must be run through a continuous loop to maintain purity. The function of the pump is to overcome pipe losses and move a minimal amount of water and at a given pressure to various points of use. Often CTS (Copper Tube Size) tubing is specified.

 

Consider a loop that is 300 ft long running through a facility. Calculating head loss using a common 1” schedule 40 pipe loss chart for 10 gallons per minute flow would show 16.5 feet of pipe head loss, or 7.1 psi. If it is sized for the specified pipe, the CTS in this case, the pipe head loss is 40 ft or 17.3 psi. 10 psi in this case can be the difference in getting the required 10 gpm or a reduced flow. The system would not function properly.

 

There are many reasons that a different pipe may need to be specified. For instance, in the well application above, the system may have required high pressure that the schedule 120 could provide over the schedule 80. In the second case, perhaps the CTS is required by local codes.  Any number of reasons can require a change in piping specifications, but the system and pump must also be adjusted to compensate for the pressure changes due to the pipe ID.

 

Specifying pipe? Try our Friction Loss Calculator located on the right side of our Document Center page under Helpful Tools.

 

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