Submersible Pump Motors and Well Casing Clearances
Irrigation Craft offers the following general recommendations for Submersible Pump Motor Sizes, Flow Rates, and Well Casing Sizes. At the bottom of this document are suggestions when there are problems.
Maximum Recommended Flow Rates:
4" Submersible Motor In 4" Sch-40 Well Casing Max Flow = 60gpm
6" Submersible Motor In 6" Sch-40 Well Casing Max Flow = 100gpm
When flow rates are greater than those values shown above, increase the well casing size by at least one nominal size. A 4" pump motor would then be installed into either a Sch-40 5" or 6" well casing, and a 6" pump motor would be installed into a Sch-40 8" well casing.
Why?
Highly turbulent conditions and excessive reduction in NPSHA , leading to shortened motor and pump life, reduced performance, efficiency losses.
See for yourself by examining the following scale drawings showing the clearances for these motors and various pipe sizes.
As bad as the conditions illustrated in those graphics are, the problem is actually worse. In a normal pipe flow there is only one boundary layer where turbulence and friction loss originate, the outside wall of the pipe. But the flow cross section of water flowing over a motor inside a pipe, is a donut shape, with two boundary layers, the outer pipe wall and the inner surface of the motor. Therefore, in addition to the small area available for flow, there is increased turbulence and friction loss even within that small area.
If the problem is truly unavoidable, the following suggestions are offered as assistance.
Notify all interested parties, in writing, of the problem.
Select pumps with low NPSHR and low Specific Speeds
Limit flow rates as much as is practically possible.
Provide extra submergence to increase NPSHA.
If during installation the pump and pipe become stuck, drop from a higher elevation.
Refuse the job.
We are just joshing on Suggestion 5, we got that from an experienced fellow who had been leaned on, one time too many, to jam 6 pounds of pump down a 5 pound hole.
