A cable guard.
Submersible pump accessories are the plumbing and electrical components of a pumped water system that are particular to a submersible pump system vs. a jet pump system. For simplicity's sake, we will divide them into two categories - down-hole accessories and aboveground accessories. The down-hole accessories are check valves (which we talked about in the April article), torque arrestors, cable guards, drop pipe, pitless adapters, pump cable, splice kits, etc. We will cover the torque arrestors, cable guards and drop pipe this month, and the rest of the down-hole accessories in coming months.
Torque arrestors- A torque arrestor is a rubber or flexible PVC device that is mounted on the drop pipe a few feet above the pump to prevent the pump from rotating due to start-up torque. They also center the pump in the well casing. Torque arrestors are mandatory when using PVC or poly drop pipe, and often are used with steel drop pipe as well. They look like a 2-foot piece of 2-inch radiator hose with four longitudinal slits running most of their length. They are installed using stainless steel hose clamps, and the ends are pushed toward each other, forcing the center section to bulge out to make contact with the inside of the well casing.
Cable guards- These are inexpensive, molded-plastic or rubber devices that slip over or snap onto the drop pipe, centering it in the well casing to prevent the drop pipe from trapping the pump cable against the well casing, abrading it. They always should be used when the drop pipe and well casing are steel, since steel surfaces can become very abrasive as they rust and corrode with time. Many installers use them with plastic well casing and drop pipe as well because they make a nice, clean installation.
They should be installed every other joint or about every 40 feet. The pump cable should be stretched tight and taped to the drop pipe with electrical tape adjacent to both sides of each cable guard and at each coupling in-between the cable guards.
Table 1. Depth ratings for schedule 80 threaded PVC drop pipe.
Plastic drop pipe can be used in most residential applications and is preferred by many installers because it is lighter, easier to handle, and it resists corrosion. The two most common types of plastic drop used today are threaded schedule 80 PVC and high-density polyethylene (HDPE).
PVC drop pipe- Early attempts to utilize PVC pipe for drop pipe involved the use of schedule 40 PVC sprinkler pipe; it was either glued together, or employed by gluing on male and female adapters and screwing them together. It is cheaper than threaded schedule 80 PVC drop pipe, but the potential strength problems well outweigh the savings.
Threaded schedule 80 PVC comes in 20-foot lengths, has male pipe threads on both ends, and can be connected with couplings or threaded check valves. There are limitations to the amount of weight that can be supported by any drop pipe, and, as you might guess, plastic carries less weight than steel. Table 1 shows one manufacturer's depth and horsepower limitations for the four most popular sizes of its threaded schedule 80 PVC. The depth numbers on this chart are included for illustration purposes only and may be different from those for the pipe you are using. Ask your pipe supplier for a chart on its PVC drop pipe before deciding to use it past 300 feet to 400 feet in depth.
Drop pipe couplings- Most installers use either plastic or stainless steel drop pipe couplings with PVC drop pipe. Brass and galvanized couplings also are available. If choosing brass, make sure it is no-lead brass. I don't recommend galvanized couplings because they are not specifically designed for use with drop pipe and do not have the long, tapered entry that facilitates assembly.
Plastic drop pipe couplings must be the extruded and machined type, as opposed to the injection-molded type. Never use molded couplings to connect PVC drop pipe, unless you need practice fishing for a lost pump in the bottom of a well. They are fine for light-duty, aboveground applications, but do not hold up well under the stresses encountered in a drop pipe application. They crack easily and can fail, causing the drop pipe string and the pump to fall to the bottom of the well. In fact, most PVC drop pipe manufacturers void their pipe warranty if molded couplings are used.
The machined PVC couplings are available in schedule 80 and schedule 120. I have never seen a broken machined schedule 80 PVC coupling, but many installers go the slight extra expense of using schedule 120 couplings for the added safety margin. Since they are only a little bit more expensive, the schedule 120 couplings probably are worth it, just for the peace of mind.
Table 2. Maximum setting depths for submersible pumps on 3408 poly pipe at 60 degrees F.
Poly drop pipe used in ground water applications is made from high-density polyethylene, vs. medium-density, because of the extra strength of the high-density. It is known as Type 3408, referring to the ASTM designation for high-density polyethylene. It is designated by SIDR types like SIDR 19, 15, 11.5, 9 and 7. The SIDR stands for standard inside diameter ratio, and the number (19, 15, etc.) is a ratio of the diameters. The larger the ratio number, the thinner the wall of the pipe.
Poly pipe for drop pipe applications is dimensionally controlled on the inside diameter (I.D.) so that it can be spliced and connected with insert fittings. As the wall thickness increases, the I.D. stays the same and the outside diameter (O.D.) increases.
Type 3408 poly pipe comes in a number of different wall thicknesses and pressure ratings, from 80 psi to 200 psi. Choose the pressure rating adequate to handle the weight of the pipe and pump, and the pressure output of your pump. There is an adequate safety margin in the manufacturer's pressure ratings, so you can choose pipe with a pressure rating just large enough to do the job without having to worry about overstressing it. Table 2 shows the depth settings for 3408 poly pipe.
Poly drop pipe connectionsare made using insert fittings that are inserted into the inside of the drop pipe. Insert fittings are available in a number of configurations to connect the poly to itself and to threaded fittings. They are held in place with stainless steel hose clamps. In well applications, use two stainless steel clamps on each side of each fitting.
Fittings are made from galvanized steel, stainless steel, brass and plastic. If brass fittings are used, make sure they are no-lead brass. Plastic insert fittings are not recommended for connecting drop pipe for water well applications because of strength considerations.
PVC coupling
Steel drop pipe is connected just like threaded PVC, using stainless steel, no-lead brass or galvanized couplings, or stainless steel or no-lead brass check valves.
Next month, we will continue the section on down-hole accessories with a look at pump cable. 'Til then ... .
ND
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