Part 1. Physical attributes of casing.
By Thom Hanna, PG, and Mike Mehmert
This is part one of a two-part series on well casing. It is written with another good friend of mine, Mike Mehmert, who authored the portions of Groundwater & Wells, Third Edition, on casing.
We’re going to discuss physical attributes of well casing in this column. There are many types of casing that are used for water well construction, and we’ll touch on some corrosion aspects of well casing, but we will focus more on its strength and physical characteristics.
Every well consists of an intake portion and a cased portion. In general, the function of the cased interval is to maintain the wellbore to the depth where water can be extracted. The casing can prevent aquifer contamination from the surface or upper aquifers if properly grouted and upper aquifers are not in direct communication to the aquifer being targeted by the well.
Additionally, the upper casing can be the housing for the pump or a secure conduit for injection of water in managed aquifer recharge (MAR), solution mining, or environmental applications (Figure 1).
Final casing selection should be based on well type, service life, water quality, planned depth and diameter, drilling method, budget, and compliance with regulations.
Depending on the purpose of the well (potable supply, dewatering, non-potable supply, environmental, MAR) there are numerous options in diameters, materials, connections, types of seals, and local regulatory requirements to be evaluated.
This column will attempt to highlight key design points to be considered during the assessment of these options.
The term nominal size (NS) is used routinely in reference to the functional diameter of a pipe. Pipe charts typically list NS as Pipe Sizes (PS) for convenience.
Actual inside diameter (ID) and outside diameter (OD) will vary depending on size and wall thickness. Pipe sizes 12 inches and less are nominally the ID of the pipe. Pipe sizes 14 inches and larger are designated as the OD of the pipe.
For both steel and PVC, the actual OD of 12PS and smaller are slightly larger than the PS reference, while for sizes 14PS and larger, the actual OD equals the PS reference. For example, standard (STD) 6PS has OD of 6.625 inches while STD 16PS has OD of 16 inches.
Typical steel water well casing sizes (4PS–36PS) are available in “Schedules,” which reflect a wall thickness and corresponding weight per foot for each diameter. Most charts will show weight classes under Schedule: Sch Number and Extra Strong (ES).
Common pipe schedules for 12PS diameter and smaller are 5, 10, 20, 30, 40, and 80 where Sch40 typically refers to STD and Sch80 to ES.
For 12PS and larger, the wall thickness of STD and Sch40 are different and for sizes 14PS and larger, STD refers to a specific 0.375-inch wall thickness and ES typically references a wall thickness of 0.500 inch.
Steel Pipe Manufacturing
The manufacture of steel pipe is either by a seamless or welded process. There are two basic welding processes: electric resistance welding (ERW) using no filler metal, and submerged arc welding (SAW) where filler metal is incorporated.
The forming of welded steel pipe begins as flat metal strips of suitable width and thickness in plate or coil form. The steel strips are welded end to end into flat ribbons and are cold-formed into tubes for straight seam welding ERW pipe or spiral seam welding SWP.
ERW pipe typically is produced in diameters from 2.375 inches to 24 inches. Spiral-welded pipe passes the steel coil into a roller-forming cage at an angle; thus the weld-seam runs the length of each pipe in a continuous spiral (Figure 2) as opposed to a straight seam. The feed angle determines the diameter, so spiral mills offer broader diameter flexibility.
Seamless (no weld) pipe is made from solid steel rod (billet), heated to a plastic state, pierced, and hollowed into a tube over a mandrel under high pressure (Figure 3). The tube is then further processed in finishing mills to desired dimensions. Seamless pipe typically costs more than welded pipe. Manufacturing tolerances for weight, wall thickness, and length are published by most suppliers. For standard and line pipe, weight tolerance is plus or minus 5% and wall thickness tolerance is less than 12.5%.
Although the working pressure of welded tube is 20% less than that for a similar seamless tube, it is not typically important in water well applications. Working pressure is not the determining factor for choosing seamless tube over welded pipe. The difference in potential impurities, which reduce the corrosion resistance of the finished pipe, is why seamless tube is typically specified.
PVC Pipe Manufacturing
PVC pipe starts out as a granular resin (pellets) which meets specific ASTM standards for the application intended. These pellets are fed continuously into an extrusion machine (basically a large heated, high-pressure screw pump) that melts the pellets into a thick molten slurry that is forced into dies, which extrudes a continuous pipe of a given diameter.
The molten pipe is cooled while moving and eventually cut to length and removed from the moving line for finishing. Manufacturing plants typically have multiple extrusion lines for different sizes and efficiency.
PVC pipe is sold as Class pipe, Standard Dimension Ratio (SDR) pipe, and Schedule (SCH) pipe, shown in Table 2.
Class ratings refer to internal working pressure and should not be confused with external collapse. All ratings for PVC assume an operating temperature of 73°F (23°C). As the temperature increases, the strength of PVC decreases. Table 3 shows the derating of PVC pipe due to temperature.
When operating at elevated temperatures, the actual values for collapse pressure, burst pressure, and tensile strength will be derated according to the chart.
SDR rating is defined as: SDR number = outside diameter/wall thickness.
In SDR pipe, as the diameter changes, the wall thickness changes, maintaining the same collapse. Common SDR ratings for well casing are 26, 21, and 17. SDR is helpful in the rating of water transmission pipe. No matter what the pipe diameter, the same SDR rating will have the same internal working pressure.
Class rating (CL) is defined as: CL = 4000 / (SDR – 1). The class rating refers to the internal working pressure.
Schedule pipe is like steel where, for a given schedule, each diameter has a specific wall thickness and collapse. For example, when comparing Schedule to SDR, the PSI collapse for 6PS, 8PS, and 12PS SDR17 pipe would be the same, while the collapse of 6PS, 8PS, and 12PS Sch40 would all be different.
The heat of hydration of Portland cement can cause short-term high-temperature spikes and possible well failures. In a borehole, the larger the borehole, the more heat of hydration is produced.
Careful evaluation of site conditions (borehole uniformity) and installation procedures (temporary filling casing during cementing) can avoid problems. It is not that difficult to have this situation with washout zones in the borehole that can be identified with a caliper log.
Casing for water wells also is constructed from various types of fiberglass-reinforced plastic materials, which are usually called fiberglass casing.
Fiberglass casing is resistant to most forms of corrosion, it is not conductive, and—for its weight—its strength is equivalent to that of steel. This type of casing has been used successfully for injection of highly corrosive waters. It also is used for water-supply wells, and in some areas of the world, for irrigation purposes.
Heat, however, can significantly reduce the collapse strength of fiberglass pipe, but for most water wells, this reduction in collapse resistance is minimal since the temperature of most groundwater is not elevated.
Fiberglass casing is somewhat permeable, and in formations where poor-quality water is cased off above potable water, some contamination of the water supply could occur. Well fittings such as centralizers, couplings, and surface fittings constructed of fiberglass are available for use with this type of casing.
Table 4 lists the standard properties of fiberglass well casing that has a wall thickness of 0.375-inch. Standard properties for other wall thicknesses can be obtained from casing manufacturers.
Part 2 of this casing series will discuss material strengths and variations that need to be accounted for in design and construction.
Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at email@example.com.
Mike Mehmert, is the former director of sales and marketing-well products (North and South America) for Johnson Screens. He was honored with a 2019 Life Member Award from the National Ground Water Association and was The Groundwater Foundation McEllhiney Lecturer in 2010. His professional career included consulting, contracting, and manufacturing.