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By Thom Hanna, PG
This column is the first of a two-part series where we will discuss the physical properties of polyvinyl chloride (PVC) casing. In the next column, we will use the physical properties to determine how to safely complete a well.
PVC is used commonly in small-diameter wells such as residential wells and those used for environmental monitoring. The use of plastic well casing has grown significantly since the s. Stronger materials and advances in extrusion technology have broadened the range of diameters and wall thicknesses available. There have been more large-diameter wells completed with PVC casing and stainless steel screens over the last 10 years as the benefit of corrosion-resistant casing and a screen that can be easily developed can make for a well that is a good value for the owner.
There are many advantages to using PVC, and for smaller-diameter wells, it is probably one of the more common construction materials for screen and casing. PVC is lightweight and easy to handle in the field and it has adequate tensile and collapse strengths for most shallow well applications. The relatively low cost of PVC also makes it attractive.
However, one of the greatest advantages of using PVC is its corrosion resistance. PVC is almost completely resistant to biological attack and cannot serve as a nutrient to microorganisms. Paleontologists in the future will find a thin sheet of plastic that will exist in the geologic record many millions of years into the future, indicating the current period of time man lived on earth. That is how corrosion-resistant plastics are!
The best source for technical information about PVC products is from the manufacturer. Joint strengths will vary as each manufacturer has a different design. Even in F-480 threads, there is enough room in the specs that different manufacturer threads can be slightly different—enough that not all F-480 pipe will fit other manufacturer F-480 threaded pipe and both will meet the ASTM specification.
One of my favorite sources for information on the pipe is found on the George Fisher website.
There are too many tables of data to include in this column, but the data is all available from manufacturers for pipe and fitting strengths.
Standardization of thermoplastic well casing is covered under ASTM Standard F-480 titled “Standard Specification for Thermoplastic Water Well Casing, Pipe and Couplings Made in Standard Dimension Ratios (SDR), SCH40, and SCH80.” This document specifies the minimum physical and chemical characteristics of PVC materials and defines the standard dimension ratio (SDR) as the outside diameter divided by the minimum wall thickness.
As the diameter increases, the wall thickness also increases. In this way the collapse resistance remains the same for all sizes of pipe built to a given strength standard. The exceptions to the standard dimension ratios occur in schedules such as Schedule 40 or Schedule 80, where the wall thickness and diameter have no distinct relationship and are built using the standards for steel.
The collapse strength of plastic casing is much less than that of steel casing. Design hydraulic collapse pressures for thermoplastic casings manufactured under ASTM F-480. PVC suppliers can provide collapse strengths for PVC casing.
Under ideal conditions, where well-compacted materials fully support the casing, the values for collapse resistance given in ASTM F-480 are reliable. The actual strength needed for any situation depends on the standard dimension ratio (Figure 1): wall thickness uniformity, roundness of the casing, rate of loading, and temperature of the casing when the loading is applied.
Although plastic casing can be set to depths of feet (305 m), industry professionals should use and follow good design practices for drilling wells that are deeper than 600 feet (183 m).
Other strengths applicable to plastic materials include impact resistance, toughness, and pipe stiffness. Long-term exposure to the sun’s ultraviolet rays can significantly reduce the impact strength of the material. There also are several issues unique to PVC that should be considered.
Very rarely is the tensile strength (Figure 2) of PVC casing exceeded in a water well construction application. The forces on the screen in casing will typically be from the weight of the materials and the filter packing material as it is being placed. You will need to consult the manufacturer’s specifications for the materials to look at the joint strength of the product you are using in a well.
Also, in water well applications when placing screen and casing in a fluid-filled hole, there is a buoyancy factor. This will also reduce the tensional load on the joints of the materials being used to complete the well.
Burst pressure is the internal pressure (Figure 3) that can be safely applied to the inside of the casing pipe or screen and is generally caused by fluid or air pressure forces. This pressure would tend to burst the casing pipe or screen if it exceeded the strength of the product during well development.
Care should be taken when using a tight-fitting surge block as vigorous use during development operations of a tight-fitting surge block could exceed the burst pressure limits of PVC during assembly.
Collapse pressure is the external pressure (Figure 4) that can be safely applied to the outside of the casing pipe and screen. This can be caused by fluid or solid material forces. This pressure would tend to collapse the casing pipe or screen if it exceeded the strength of the product.
Typically, the limiting factor in using PVC while casing is the collapse strength of the material. In addition, when using cement grouts, the heat of hydration can also reduce the strength of PVC. More on this later.
Head pressure exerted on a casing assembly by the water column under static conditions is equalized inside and outside. As the water level is lowered in the well, the assembly can be subjected to hydrostatic pressure.
The pressure differential the casing or screen experiences is one PSI for every 2.3 feet of head differential between the inside and outside water level because the water level in the casing typically reacts much faster than the water level outside the casing. Therefore, a water column differential of 231 feet would exert a pressure of 100 PSI at the base of the casing assembly.
Collapse pressure can also be produced with drilling fluid additives and cuttings during well construction. Additives and cuttings can increase the fluid density as much as 20% in unit weight, as the density of fresh water increases from 8.3 lbs/gal to about 10 lbs/gal when drilling mud is used. Cement slurries typically weigh 13 to 15 lbs/gal, which increases the fluid density by 55% to 88% over that of fresh water.
For more information, please visit Water Well UPVC Filter Pipe.
Potential casing assembly failure can occur if the hydrostatic pressure differential approaches the rated collapse pressure of the casing. In addition to the hydrostatic pressure differential, the force exerted against a PVC well assembly during the installation of a filter media into the outside annular space should be considered. Voids in the annulus caused by material bridging can lead to an imbalance in wall pressures.
This can cause casing failure.
Plastic casing deforms at relatively low temperatures (Table 1), and temperatures greater than ambient temperature reduce its working strength. When grouting, it is important to minimize the effects of the heat of hydration. Restricting the annular grout thickness to no more than 2 inches (51 mm) thick or filling the casing with water can minimize temperature effects.
During the grouting process, the curing of cement and grout generates sufficient heat to affect PVC strength and performance. When operating at elevated temperatures, the actual values for collapse pressure, burst pressure (working pressure), and tensile strength will need to be de-rated according to Table 1.
There are other times when a reduction in collapse of PVC due to heat needs to be considered. Submersible pump motors can become hot. If the pump is placed in a well without enough annular space or there is insufficient cooling around the motor, it is possible to collapse the casing.
As with any thin-walled casing or screen with low collapse strength, differential pressures can be created during well development. Tight-fitting swabs should be used with caution until there is good communication between the aquifer and the well.
The most common type of PVC pipe used is solvent weld, glued pipe. This connection relies on a glue (cement) to hold and seal the two pipe sections together; typically one end is belled or coupled (female end) (Figure 5) and the male end is glued into the belled end.
The primer and glue are actually dissolving the PVC surfaces when done correctly. At low temperatures, the solvents penetrate the PVC more slowly, so tests should be performed to assure proper bonding strength is achieved. ASTM Standard D describes the general gluing and assembly process.
Always read and follow assembly, safety, and ventilation instructions of the primer and glue (cement) manufacturer. In cooler temperatures, the installer needs to be aware that the solvent weld will take more time to set.
F-480 threads (Figure 6) are normally used for casing because they leave a joint that is smooth (flush on the inside and outside of the pipe). It is commonly used in the environmental industry because they can fit inside a hollow stem auger.
The thread relies on an O-ring to seal. Therefore, no glue or thread sealant is required. Do not use Teflon tape or any thread dope with this connection. It will allow the connection to unscrew easier and reduce the tensile (vertical pulling) force of the joint.
Prior to installation, the proper O-ring should be installed into the O-ring groove (after both are cleaned of any dirt). O-rings can be made of different materials. Viton is recommended for water well application.
With F-480 threads, Schedule 40 and Schedule 80 threads do not screw together. A Schedule 40 × Schedule 80 adapter is necessary. To assemble the threads, align the joint and turn with a clockwise motion. After hand tightening, torque casing with a strap wrench, applying approximately 10 lbs/ft per inch of outside diameter for Schedule 40 casing and 15 lbs/ft per inch of outside diameter for Schedule 80 casing. The use of standard pipe wrenches will damage and weaken PVC pipe. Use strap wrenches that do not leave indentations in the plastic whenever tightening PVC pipe or casing.
These connections use a plastic spline placed in a groove in both the male and female connector to hold the two lengths together (Figure 7). The seal for the connection is obtained by use of an O-ring.
Clean the O-ring and grooves. Place the O-ring in the groove. Lubricate the O-ring (not the spline groove). Insert the male pipe into the female pipe or Bell until it bottoms out in the female or Bell. Insert the spline into the spline opening in the female or Bell and feed into the groove.
When the spline has been inserted all the way around the groove, until it hits the incoming spline, it has been fully inserted.
Cut off any remaining spline. You may leave 2 to 6 inches for future removal of the spline later if desired.
The connections made using flush spline (Figure 8) are like those belled spline connections, but the pipe is flush end, meaning there is not an upset. To handle the pipe, a groove is cut below the female end that an elevator with a shoulder can clamp on to the pipe.
Flush connections have the advantage that the casing and screens can be placed in a smaller borehole, and without the upset of a coupling at the joint, it facilitates the installation of filter pack.
PVC has become a widely accepted material for well casing due to its distinct advantages, including corrosion resistance, ease of handling, and cost effectiveness. Its growing use in both small-diameter and larger-diameter wells demonstrates the material’s versatility and value, especially in environments where corrosion or biological fouling are concerns.
However, maximizing the benefits of PVC requires a thorough understanding of its limitations, particularly regarding mechanical strength, temperature sensitivity, and proper joint assembly.
PVC’s tensile, burst, and collapse strengths are generally sufficient for shallow and medium depth wells, but careful design considerations are essential, particularly in deeper applications or environments with significant pressure differentials. Factors such as temperature effects, grout heat, and high-density drilling fluids can significantly impact PVC performance, necessitating adjustments in well design and installation techniques.
The choice of joint type—whether solvent-welded, flush-threaded, or belled with spline—plays a critical role in the overall integrity of the well casing. Ensuring proper assembly and compatibility of connections is vital to avoid failures under operational stresses. Collaboration with manufacturers for technical guidance and specifications is recommended to ensure that the selected PVC components meet the specific
demands of the well environment.
As we move forward into the next installment of this series, which will come in the January issue of Water Well Journal, we will explore how to apply these insights into practical well design, focusing on the best practices for completing wells safely, and effectively using PVC casing. By combining an understanding of PVC’s physical properties with sound design principles, professionals can achieve reliable, long-lasting well installations that provide excellent value to well owners.
Sterrett, R.J. . Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.
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