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 the pump motor’s starting torque. They also center the pump in the well casing. Torque arrestors are mandatory when using PVC or poly drop pipe, and often used with steel drop pipe as well. A torque arrestor looks like an 18-inch long piece of 2-inch radiator hose with four longitudinal slits running most of its length. It is installed using stainless steel hose clamps, the ends of which 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 and 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.
Drop Pipe
Drop pipe is the term given to the pipe that runs down inside the well from the wellhead to the submersible pump. Drop pipe can be made from any material suitable for transporting the pumped fluid that can support its own weight, plus the weight of the pump and pump cable. There are three types of drop pipe commonly used in submersible systems – PVC, high-density polyethylene (HDPE) and galvanized steel.PVC and HDPE drop pipe can be used in most residential applications, and are preferred by many installers because they are lighter, easier to handle, and resist corrosion. The three most common types of plastic drop used today are threaded schedule 80 PVC, threaded schedule 120 PVC and high-density polyethylene (HDPE).
PVC Drop Pipe
Early attempts to use PVC pipe for drop pipe involved the use of schedule 40 PVC sprinkler pipe, either glued together, or by gluing on male and female adapters and screwing them together. It is cheaper than threaded schedule 80 and schedule 120 PVC drop pipe, but the potential strength problems well outweigh the savings.Threaded schedule 80 and schedule 120 PVC drop pipe come in 20-foot lengths, have 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 (at right) shows one manufacturer’s depth and horsepower limitations for the four most popular sizes of its threaded drop pipe. The depth numbers on this chart are for a 30/50 pressure switch, and are included for illustration purposes only – they 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 for settings deeper than 300 feet.
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. Galvanized couplings are OK if they are specifically designed for use with drop pipe, and 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 a lost pump in the bottom of a well. They are fine for light-duty, aboveground applications, but they 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 warranties 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 only are a little bit more expensive, the schedule 120 couplings probably are worth it, just for the peace of mind.
Poly drop pipe is more flexible than PVC, and it is supplied in coils from 100 feet to as much as 1,200 feet per coil. Most distributors stock coils in standard lengths like 100 feet, 200 feet, 300 feet, etc. Large coils – 1,000 feet to 1,200 feet in length – are offered so that distributors can cut to length for their customers. If you have a 120-foot setting, they will sell you a 120-foot length of pipe, instead of your having to buy a 200-foot coil and wasting 80 feet.
Poly drop pipe used in ground water applications can be either medium-density or high-density polyethylene, but high-density usually is preferred for its greater strength. 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 (ID), so that it can be spliced and connected with insert fittings. As the wall thickness increases, the ID stays the same, and the outside diameter (OD) increases.
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 manufacturers’ 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 (above) shows one manufacturer’s (Centennial Plastics) maximum depth settings for its 3408 poly pipe.
Poly drop pipe connections are 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.
Galvanized steel drop pipe still is preferred by many pump installers, but, like plastic, it has limitations. Difficulty in handling and rust and corrosion are the two main reasons for the increasing popularity of plastic, except in the deeper applications where the strength of steel is needed just to hold the weight of the drop pipe, pump cable, water and pump. Steel drop pipe is shipped with a coupling on one end, which is used to make the connections, except when a check valve is required (every 200 feet, as outlined last month), in which case a malleable iron check valve should be used. Many installers use no-lead brass or stainless steel check valves on galvanized steel drop pipe, but I don’t recommend it because these dissimilar materials have the potential to cause galvanic corrosion, which can attack the steel drop pipe, causing the threads to fail, allowing the pipe, wire and pump to drop to the bottom of the well.
Next month, we will continue the section on accessories with a look at well seals and pitless adapters. ’Til then ....
ND
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