A well casing is a steel pipe that is used to seal and support the sides of a drilled water well. The diameter of a residential water well casing is 4", 5", 6" or less-often, 8" in circumference. Much larger steel casings are used in high capacity community water wells, commercial or agricultural wells, and in some other water wells.
While soil and well designs vary as ground, soil, rock and water availability also vary in different areas, generally, after the well has been drilled the casing is driven into the drilled opening in the earth to a dept that (usually) inserts the bottom of the casing in bedrock. The drilled well depth continues downwards into bedrock, through which water flows into the well.
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The well casing is shown in cross section, represented by the two vertical red lines in our drawing at left. But of course actually the well casing is a round steel pipe. The water piping that picks up water from the well near its bottom is routed up the center of the well casing until it reaches that exit opening in the casing side.
There the piping (and the water it carries) exits the well casing through a pitless adapter (a fitting that seals the hole in the casing), continuing on to bring water to the building the well is serving.
Question: Would an eight inch well as opposed to a six inch well deliver more water pressure?
Would an eight inch well as opposed to a six inch well deliver more water pressure? Are plastic or metal pipes better for a residential well?
Reply: no. but you might want a larger diameter well casing for other reasons we'll give here
THANK YOU for asking a great question: why would we want to drill a larger-diameter well bore or put in an 8" casing rather than a 6" well casing?
Summary: Larger casing diameter = more static head reserve
In short, a larger diameter well casing will not quite give you more water pressure, but it might give you more water while saving your pump.
Summary: Using plastic well pipe casings, benefits, risks
Some sources, no surprise the manufacturers of plastic or PVC well casings describe the advantages of PVC rather than the traditional steel well casing materials. In my amateur OPINION, the title of one of the best documents I found on this also hints at where there could be an achilles heel in plastic well casings: the risk of breakage or hydraulic collapse from the pressure of surrounding earth, a problem that will be more-likely to occur if the installer chooses the wrong material.
Improper installation of PVC well casing can also lead to a later collapse. Certainteed (2013) points out that the heat generated by curing portland cement used as well bore grout around the plastic pipe can generate heat that causes the plastic to become more prone to breakage. The company recommends using bentonite clay instead of portland cement for well bore grout around PVC casings.
PVC offers many familiar advantages to the well driller, including excellent corrosion resistance and ease of assembly. However, unlike conventional PVC pipe, the primary loading on well casing is external pressure rather than internal pressure. Because of this distinction, understanding the effect of external pressure on PVC well casing is vitally important, as is use of this information in the well casing selection process.
Kurt, Carl E., Roy C. Johnson, and James C. Warman. Engineering Performance of Thermoplastic Water Well Casings. Water Resources Research Institute, Auburn University, 1980.
Manual on the Selection and Installation of Thermoplastic Water Well Casing,
National Water Well Association and The Plastic Pipe Institute,
Worthington, Ohio October, 1980. This manual is available in many libraries.
Parker, Louise V. "Suggested guidelines for the use of PTFE, PVC and stainless steel in samplers and well casings." In Current Practices in Ground Water and Vadose Zone Investigations. ASTM International, 1992.
This paper reviews the literature on PTFE, PVC and stainless steel well casings and samplers and develops a series of suggested guidelines for use of these materials in sampling programs. Three factors are considered: the resistance of these materials to chemical reaction and thus degradation by the environment, the effect these materials have on the integrity of ground water samples, and the physical strength of these materials.
Specifically, factors involved in the corrosion of stainless steel and the ability of aqueous solutions of organics to degrade PVC are discussed. Also considered are sorption and leaching of organic and metal species by these materials. The differences in the behavior of flexible vs. rigid PVC are emphasized.
Now I'll carry on with the reasons to go to a larger diameter well casing. Or not.
Definitions of Water Quantity, Water Pressure, Water Flow
Well water quantity limit - how much water can you get out of a well
Water pressure limit - what is the highest water pressure in psi that a pump can deliver
Water flow rate - how many gallons per minute can you get out of a system or will you see at a faucet
I admit that normal people refer to the force of water exiting their shower head or sink faucet as "water pressure" and it's true that at higher system pressure more water will be delivered through a pipe. But a lager diameter water pipe system could also give more gallons of water flow at the same fixture at the same actual water pressure!
Provided we are not changing the water piping size in the building itself, the well or water pump type, location, horsepower and other features determine both the water pressure and water flow rate - as it is taking water out of the well and sending it to the building.
By the lift height - how high the pump has to lift the water to get it to you since lifting higher reduces the flow's output GPM. Lift height is also discussed in the pump capacities article above.
By restrictions in the water piping system: pipe diameter, lengths, number of bends or elbows, even to a degree by the actual material of the pipe and its internal friction
By the well's in-flow rate: this is the upper limit of the water system once any reserve of water in pipes or casing or tank has been exhausted: the rate at which water flows INTO the well while you're taking water out.
A pump that has a GPM rate (GPM-PF) that's taking water out of the well that is above than the rate at which water from the earth flows into the well (GPM-WF) can deliver water to the building at GPM-PF only long as it is
drawing water out of the well bore itself - the static head or volume of water that is in the well bore when the well is at rest and fully recovered.
But if GPM-PF is higher than GPM-WF, once the pump has drawn water down to where the pump's intake opening is in the well, one of two things happens:
1. The pump starts to suck air, or runs dry, or burns up -
2. A pump limiting device built into the water system right at the pump or at other controls that may be elsewhere will actually begin to operate to slow the pump's output to protect it from running dry and being ruined.
Benefits of a Larger Diameter Well Casing or Well Bore
If for the same depth we drilled for and installed a larger diameter well bore and well casing, what do we get?
The larger diameter increases the volume of water in the well bore or the well's static head.
There I note that when the well is fully-recovered, that is, water has risen in the well as high as ground water will push it when no water is being taken out of the bore, then the volume of water in the well is called it's static head.
I found my numbers on the volume of water you'll get in a foot or meter of well casing for different casing diameters like a 6-inch or 8-inch casing by using our on-page search box of InspectApedia.com to look for “volume of water in a well casing”
one foot (12") of six inch well casing holds about 1.5 gallons of water
one foot (12") of eight inch well casing holds about 2.8 gallons of water
If you want to see how to calculate the volume for a well of any diameter and depth, that same article includes the formula at STATIC HEAD VOLUME CALCULATION
How Much More Water is in a Two-Inch Larger Diameter Well Casing?
That means that your 8-inch well casing gives you almost double the volume of water in the static head (actually 1.86 times as much as a 6 inch casing if my math is right).
So you will NOT get more PRESSURE from your fatter water well casing but your well will have a larger static head.
When Do We Want a Larger Diameter WelL Casing?
Whether or not that makes the slightest difference depends entirely on the total well depth and even more-so on the actual height of water in the well at rest - the true static head.
If your particular well were just twenty feet deep (chosen to make the math easy) and if the static head were ten feet, then you'd have a static head of (2.8 x 10) = 28 gallons instead of (1.5 x 10) 15 gallons.
Now suppose somebody actually installed that not-very-deep well.
Suppose the installer also put in a pump that has a capacity of GPM-PF just 5 gallons per minute GPM of output. And let’s pretend that the lift height nd piping and other restrictions were zero.
Then the pump in the 6-inch well could run for just 3 minutes (15 gallons / 5 GPM-WP) before it would depend entirely on the flow rate INTO the well - not so nice, right,
But suppose the well has a well inflow rate GPM-WF that is 10 gallons a minute.
A pump with a GPM-PF of 5 is sucking water out at 5 gallons a minute but the well admits water in to its static head at 10 GPM-WF - the well is always ahead and the pump can keep sending water to you.
For that case, an 8-inch well casing and the 28 gallons didn’t buy you anything because the well flow rate was already greater than the pumping rate.
What if the well flow rate is terrible, say 1 gallon per minute.
In our well with a 10 foot static head holding 15 gallons, during the 3 minutes we are sucking water out at 5 gpm and getting water in at 1 gpm so over 3 minutes we took in 3 gallons and sent out 5.
We have 2 gallons spare - that gets sent out in the next minute and the pump quickly falls behind the well's flow-rate.
The pump either runs dry and burns up or its output must be limited by a controller to that of the well: 1 gallon a minute - and that’s all you’ll ever see in the house after the first 3 minutes (approximately)
Big Well Casing to the Rescue
So what does the well driller do about this mess?
She drills a deeper well AND / OR she uses a larger well casing (perhaps both) so that she gives you a well with a huge static head, maybe a 100 foot static head holding 150 gallons of water, figuring that you’ll usually turn off the shower well before you’ve sucked out all of that 150.
THAT’s why a driller might want a bigger diameter well casing - you might get a bigger static head without having to drill deeper in nasty ground.
Well flow rate is explained in detail at a live link we give below WELL FLOW RATE
The total water quantity that can be gotten out of a well is explained at a live link given below for WELL QUANTITY TOTAL
Continue reading at STATIC HEAD, WELL DEFINITION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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Questions & answers on how to repair a leaky or damaged well casing, posted originally at this article are now found at WELL CASING LEAK REPAIR FAQ
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 J.E. Johns, TecWel; D.N. Cary, Seal-Tite International; J.C. Dethlefs, ConocoPhillips; B.C. Ellis, Seal-Tite International; and M.L. McConnell and G.L. Schwartz, ConocoPhillips, " Locating and Repairing Casing Leaks with Tubing in Place - Ultrasonic Logging and Pressure-Activated Sealant Methods", Society of Petroleum Engineers, Offshore Europe, 4-7 September 2007, Aberdeen, Scotland, U.K., Document 108195 (Conference Paper), 2007
 Link-Pipe, Inc., 27 West Beaver Creek Road - Unit #2 Richmond Hill, ON CANADA L4B 1M8, Link-Pipe manufactures "no-dig" pipe repair products for wells. Tel: 800-265-5696 or email: email@example.com or website: http://www.linkpipe.com/wells.htm
 Prentice Creel and Ronald J. Crook, Halliburton Energy Services, Inc., " Injectrol® and PermSeal Sealants Repair Leaks, Restore Integrity to Casings", Halliburton Service Center, 877-263-6071, web search 6/27/12, original source: http://www.halliburton.com/public/cem/contents/Papers_and_Articles/web/I_through_O/InjPerm.pdf [copy on file as Injectrol.pdf]
 W. B. Allen, Flowing Wells in Michigan, 1974 U. S. Geological Survey, Water
Information Series Report 2
 ASTM D 5299-92, Standard for Decommissioning of Ground Water Wells, Vadose Zone Monitoring Devices, Boreholes, and Other Devices for Environmental Activities, 1993, American Society for Testing Materials, West Conshohocken, Pennsylvania.
"Freezeproof Your House," Mike McClintock, Rodale's New Shelter, p. 30, October 1985 (approximate date)
"How to Winterize Your Pipes," Mike McClintock, Homeowners How-To Magazine, p. 59-62, Nov-Dec 1979.
Thanks to reader Dan Babb for discussing well piping leaks, July 2010
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