Design for Maintenance
Improved MaterialsCorrosion- and deterioration-resistant materials slow the deterioration of well components and limit recurrence of preventable problems, making the success of maintenance actions more likely. Specific to well equipment, polyvinyl chloride (PVC) casing, for example, is corrosion-resistant and suitable for most applications. Alternative metal casings are available where plastic or fiberglass casings are not suitable. Notable product developments include the widespread availability of all-stainless-steel and stainless-and-plastic pumps; high-quality rigid plastic pump discharge (drop) pipe with twist-on, twist-off connections; and flexible discharge hose (specifically designed for well pump use) composed of reliable, high-strength, corrosion-resistant material that permits easy pump service. Relatively smooth pump interior surfaces and corrosion resistance are showing increasing intervals between pump service events.
Pump motor and discharge-end product lines can seem to have a remarkable sameness in a competitive market. On the other hand, pumps may be marketed for "environmental duty," which may not be superior to other products for aggressive ground water pumping applications. Some considerations:
Pump End SelectionA material designation of "stainless steel" includes a range of corrosion-resisting alloys. Some do well in anaerobic environments typical of high-organic-carbon water and some do not. The alloy should be selected to be compatible with the service environment.
Welding and stamping alter the corrosion-resisting characteristics of stainless steel alloys so that the manufactured product may not match the resistance of the unaltered alloy. In some cases, a cast stainless bowl selection may be superior.
While versatile, stainless steel may not suit every situation. In some high-chloride, biocorrosive environments, only high-silicon bronze or plastics may provide suitable service life. At high temperature or high radiological activity, some plastics degrade at unacceptable rates. In addition to bowl and impeller materials, selections of bearing materials and designs are factors in selection.
Higher efficiency pump ends are recommended. Pump impeller-bowl designs and numbers of stages should be matched to the operating head conditions.
In general, submersible models are more versatile but characteristically provide less wire-to-water efficiency than many lineshaft turbine models. Lineshaft installations offer the advantages of having the motor at the surface, where it is accessible, heavy motors for very large pumps are not suspended downhole, and motors are less expensive to repair. Disadvantages of lineshaft installations include:
- The need for a lineshaft and its associated bearings, which require lubrication and are vulnerable to wear, especially in aggressive, biofouling water.
- The need to use steel column pipe, which is subject to rapid corrosion.
- Restricted access at the surface for drawdown measurement and other access to the well casing.
- Greater skill is needed in lineshaft pump repair, and wells must be very straight and plumb.
- Surface-mounted motors must be protected from weather and heated or cooled as needed.
Equipment FeaturesAs exact matches to conditions and ideals may not be possible, pump choice may be a balance of features. In general, the highest efficiency pump models should be used. Exceptions occur where service is so severe that short operating lifespans can make more expensive, tunable pumps not cost-effective to operate. In these cases (particularly where efficiency differences are minor), low-priced but serviceable pumps that can be discarded and replaced or cleaned may be the better option.
Automated water-level and flow information facilitates data analysis and planning. Devices exist to provide real time water-level and flow measurements without personnel being onsite. Systems originally developed for process treatment can be adapted for well fields, permitting rapid, easy and continuous monitoring of well and pump hydraulic performance, and even physical/chemical changes. Pump controllers help to maintain regular current flow of the proper characteristics and phase to pump motors, thereby prolonging motor life and shielding motors from line surges. Therefore all pump motors should be equipped with automatic controllers.
One technology that has developed in recent years is the refinement of the controlled-inflow pump tailpipe referred to as a suction flow control device (SFCD). These simple devices are perforated pump intake pipes. The perforations are made in a pattern that forces flow to enter the well in a more cylindrical fashion, instead of an upward-faced cone pattern typical of pumped screened wells in which almost all flow enters through the top 10 percent to 15 percent of the screen when the pump is above the screen.
A maintenance-friendly wellhead setup is important to minimize the difficulty of performing maintenance. Issues include meeting limits to avoid confined space designation, making the well seal secure but removable, and discharge head and instrument connections easy to detach.
Wellhead Chemical TreatmentA hydrant should be installed between the well pitless discharge and the well house flow meter-valve assembly for discharge to waste during treatment. Several suitable self-draining hydrant styles approved for potable water distribution are available on the market. During the well treatment process, a hose may be run from the blow-off hydrant to containment and treatment.
Chemical feed pumps can be used to meter chemical mixtures into wells. The manufacturer should be consulted about the chemical compatibility of diaphragm and housing of the liquid end. Also, the suitability of hose installed for short-term, periodic service feeding pH 2 solutions should be double-checked.
Systems have been developed to systematically redevelop with the pump in place, and designed to provide treatment chemicals to the screen where past pump-in-place designs were not effective. An example is a system in which a valved return flow pipe is installed to permit periodic or demand flushing of the well water column. These should be considered as maintenance treatment options.
Distribution ConcernsDistribution lines from wells also may develop deposits of iron oxides and biofilm. If oxidation and fouling in wells are kept to a minimum, lines are likely to remain relatively clean. However, line clogging is a very common problem in systems pumping contaminated ground water to treatment.
If the system head shows signs of increasing, a program of pigging and flushing can be instituted. Pigging is the process of running a soft plug with a rough, abrasive outside surface through the lines to remove deposits. Some system modification will be needed to accommodate the procedure, and it is recommended that planning for this option be part of well system design. Pigging requires:
- An upstream entry point for the pig (for example, at a well house).
- A means of providing water pressure to propel the pig (water pressure from a potable water system fire hydrant would suffice).
- An outlet collection point for wastewater and pig.
- Alternatives such as electrostatic dispersion of colloidal fouling components also have been suggested, and possibly have application.
Well array design recommendations:
- Have enough wells installed in a pumping or injection array to permit continued operation and plume control while wells are out of service (being treated or pumps replaced).
- Install a ring of treatment wells around pumping or injection wells subject to clogging. These can greatly improve treatment success in the near-well formation by providing a way to force treatment chemicals toward the pumping well screen from the outside and also to provide more access for agitation of the near-well formation.
- On sites with very deep wells, the procedures above may be quite expensive. In these cases, where both replacement and rehabilitation may be very expensive and difficult, designing and planning for a rigorous maintenance defense of the existing pumping wells are especially important.