Tech Topics: Drop Pipe Load Capacity Critical for Deep Wells
Several years ago, I wrote an article about drop pipe: the pipe that connects to a submersible pump and carries the well water to the well head. In that article, I focused on plastic drop pipe, both rigid PVC and semi-flexible HDPE (poly pipe), describing the pros and cons of each and the load bearing capacity of the various sizes.
The importance of the load bearing capacity of drop pipe is directly proportional to the depth of a well. The deeper the pump setting, the more important it is to pay attention to the pipe manufacturer’s recommendations as to the maximum depth allowable for the size and schedule of the drop pipe, the maximum horsepower allowable, and the type of couplings and check valves to use. Table 1 from JM Eagle shows the maximum allowable pump setting depth for schedules 80 and 120 PVC drop pipe. The other PVC drop pipe manufacturers publish similar load tables.
Since that earlier article on plastic drop pipe was published, I’ve had several calls asking for max load information on steel drop pipe. But, before opening that can of worms, let’s consider the components that make up the load carried by drop pipe. Table 2 shows these items. The first three rows are the pump setting, the motor horsepower and the pump cable size. In these arbitrary examples, all pumps are 4-inch diameter, 15-gpm submersibles with three-wire motors running on 230 volts. The “weight of water” numbers assume that the static water level is at the well head. The weights of the couplings, check valves, torque arrestors and centering devices are not considered in this table.
So what does Table 2 tell us? We know from Table 3 that a 1-inch bronze check valve can safely carry a load of 1,700 pounds. This tells us that you can use bronze check valves to a depth of 900 feet in our scenario with plastic drop pipe. But Table 1 says that the depth limit for schedule 120 PVC drop pipe is 540 feet. So, you know that bronze check valves are strong enough for any PVC drop pipe application.
Table 5 shows that Flomatic’s stainless steel 1-inch check valve maximum recommended load is 2,600 pounds, so you can use it with steel drop pipe in up to 600-foot settings. Table 4 shows that their ductile iron 1-inch check valves have a maximum recommended load limit of 3,500 pounds, so you could use them in settings up to 800 feet.
What about settings deeper than 800 feet? One option is to use a hybrid piping system where you use PVC on the bottom of the string and steel drop pipe on the top. For instance, in a 1,000-foot steel example, if schedule 120 PVC is substituted for the bottom half of steel drop pipe, the total weight goes from 4,132 pounds to 2,337 pounds, which means you could use 1-inch ductile check (or their stainless steel check valve).
Another option would be to use a 1-inch diameter ductile iron check valve, which has a maximum recommended load of 4,200 pounds with reducing bushings to cover the 4,132 total load of a 1,000-foot setting.
What about the maximum depth limit for steel drop pipe? None of the steel pipe manufacturers are willing to stick their necks out to say. We know that the steel that schedule 40 drop pipe is made from has a tensile strength rating of 48,000 psi and a yield strength rating of 30,000 psi. The net cross sectional area of 1-inch schedule 40 steel pipe is .688 square inches. Multiplying that by 30,000 puts the yield strength of a straight section of schedule 40 pipe at 20,640 pounds.
However the weakest link is the threaded portion. The amount of wall loss due to threading of 1-inch schedule 40 steel pipe is 60.4 percent according to one report (http://www.corrview.com/piping-bulletins/technical-bulletin-p-01). The loss percentage is greater in smaller diameter pipe and less in larger pipe. Taking this loss into consideration, the static yield strength of a threaded schedule 40 section of pipe would be 8,173 pounds (20,640 times .396), which is more than twice as much as is needed for the 1,200 foot setting in Table 2.
The big unknown is metal fatigue cracking. The design and fabrication of the threads creates stress points at the root of the threads, and vibration from the pump and motor can cause a crack to develop at the first thread above the coupling, leading to premature failure. Fortunately, the metal is much thicker at this location, giving some additional margin of safety.
And finally, the steel pipe manufacturers refer to the couplings that come attached to the ends of the pipe thread “protectors,” instead of couplings. Throw these away and use banded steel couplings in any but the shallowest of settings.
Years of experience have shown that schedule 40 steel drop pipe stands the test of time in deep-set applications. I’d still recommend installing a braided stainless steel safety cable from the well head to the pump, just to be safe.
Next month, we will focus on how to select and install submersible check valves.
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