submersible pump and piping corrosion 1

[Jazzerman]

I operate a well water source heat pump heating / cooling system. After running the system for 4.5 years, our submersible well pump quit and we just pulled it out of well. The pump casing (cast iron) is weakened and cracked in half when set on a trailer. There is a dime sized hole (looks like electrolysis or galvanic corrosion) which has been eaten in the pipe wall about 10' up the schedule 40 galvanized discharge pipe from the pump dischage. Where the well contractor wrapped the pump power cables to the pipe with tape, the pipe underneath is shiny. Where no tape was covering the pipe the galvanizing is gone and a significant layer of steel is eaten away. Looks like a black and silver candy cane. (see photo) There is bronze check valve in good condition. Right above the bronze check valve the steel pipe is significantly corroded away.
Some details on the well / setting / system:
650 foot well depth
320 foot casing depth
345 foot pump setting depth
pump is Gould model VS
drop pipe is 3" sch 40 galvanized steel
we run the pump motor on an a VFD
we operate the pump continuously at a steady speed, VFD is used to obtain desired flow
The pump was set about 20 feet below the bottom of the well casing. The pump and all piping below the bottom of the casing are covered in a thick black sludge. The piping which was above the bottom of the casing is much cleaner, no black sludge.
What is the corrosion method at work here, and what is the best way to stop it?
I saw some similiar symptoms and discussions in this forum in 2003 "??submersible well pump erosion" but it didn't finish with definitive diagnosis / fixes.

Thanks

 

[bimr]

The black crud is most likely iron sulfide.

Sulfate reducing bacteria are widely found in soils and water formed deposits that have anaerobic conditions. They are also implicated in the corrosion of many different types of metals. Sulfate reducing bacteria gain energy by converting sulfate to sulfide. They utilize dissolved hydrogen (H2) or organic acids in this process. They can promote corrosion through cathodic depolarization, which involves utilizing dissolved hydrogen evolved at the cathode. The sulfate produced during this process may also be corrosive.

However, these microorganisms often live in a biofilm, which can contain a range of chemical environments that do not occur in the bulk water chemistry. These environments can favor electrochemical corrosive processes, which may not occur in the rest of the ground water. Within these biofilms a range of conditions promotive of corrosion may occur. Bacteria can convert dissolved organic carbon in the ground water into fermentation products such as organic acids. These acids can be corrosive. Any sulfate reducing bacteria in the biofilm may depolarize the cathode by utilizing dissolved hydrogen. The biofilm can trap dissolved ions such as chloride within it. The irregular thickness and conditions within the biofilm may allow concentration cells to form.

Most often identified by its “rotten egg” odor, hydrogen sulfide is generated by the anaerobic decomposition of sulfate reducing bacteria. As the microbe community continues to live and develop within the nodule, the by-products eventually lower the pH to acidic levels (sulfuric acid), which in turn increases the corrosive conditions within the underlying crevice on the metallic surface (i.e., well casing and/or screen). Interestingly, the acidic conditions actually promote the growth and development of other acid-producing bacteria whose own acid by-products further reduce the pH to even lower levels.

https://books.google.com/books?id=6...sulfate reducing bacteria roscoe moss&f=false

Recommendations:

1. Periodically chlorinate the well to control the microrganisms
2. Use plastic or stainless materials in the well.

 

[Jazzerman]

Bimr, thanks for the detailed reply, sorry I have not replied sooner. We have the "rotten egg" odor, a strong sulphurous smell in the discharge water. We have had it a long time. About 2.5 years ago we did a chlorine bleach shock to the well to try to get rid of the smell. It never really went away. We were unable to set up a recirculating flush, simply introduced the chlorine at the top of the well and ran the submersible pump sending the water to drain. We did water sampling before and after looking for bacteria and didn't get bacteria counts high enough to indicate a problem. You did mention in your reply that the range of chemical environments contibuting to the corrosion may not show in the bulk water chemistry - this I take to mean supply water samples taken from the aboveground discharge pipe of our pump may not show the sufate reducing bacteria even if it is alive and fluorishing.
We pulled the pump February 5th and found all the corrosion described in my original post. We ran a camera down the well Feb 13th, to inspect the casing condition. Looks like a small patch of corrosion in the steel casing at 295 feet, but otherwise didn't see anything else of note. Bottom edge of the casing was at 319' where it was expected.
Your comment about corrosion through cathodic depolarization - it did look like there was metal removed from certain areas, an then actually a buildup of more material (albeit a black crud) further on up the pipe - maybe the corrosion cell spanned that section of pipe, removing from the middle where the hole was and redepositing it up higher.
Recently week of Feb 16 we have done some water sampling with a bladder pump, I believe at 200, 250, 300 and 350 feet to check water quality, and for samples to send off for bacteria culture anlaysis. The ph of the water tested has been 7.6. We don't get bacteria growth results back for two weeks. Would you think this kind of testing will show the problem if that is what happened? Ideally we might have scraped black crud samples right from the area of worst corrosion, but since the pump and pipe has been sitting in the open air now the bacteria may have died.
Is there any chance this is caused from damaged motor leads? I'm thinking not, but I would like to share our motor and wiring insulation test results for comment: When the pump was 1st put down the well we used a megger and measured at the well head 8 to 10 mega ohms from each of the 3 phase motor leads to the ground wire. We did the same test at the well head before we pulled the pump two weeks ago. The resistance from each lead to ground wire was 0.4 mega ohm. The pump manual says for readings in the well of 0.2 to 2.0 mega ohm the insulation of the leads or winding has been damaged but will still operate. I'm thinking this is not the problem as if we actually had current leaking somewhere we'd have a full on short. 0.4 mega ohm is still a very high resistance.

 

[bimr]

You have hydrogen sulfide in the well water. It is a naturally occurring substance. It comes out of solution as the pH drops. The existence of hydrogen sulfide in sulfur-bearing waters is pH dependent.

Hydrogen sulfide (H2S) is slightly soluble in water and acts as a weak acid. At pH 7, hydrogen sulfide is approximately 50 percent of the total dissolved sulfides; at pH 5, it is practically 100 percent of the total; at pH 9, it is nearly all hydrosulfide ion.

http://soiltesting.tamu.edu/publications/L-5312.pdf

The rotten egg smell just indicates that the hydrogen sulfide is present.

The corrosion problem is caused by the microorganism byproducts, not the hydrogen sulfide. The problem is that the anaerobic bacteria in the well are reducing sulfate (SO4) to sulfuric acid and causing the corrosion.

The chlorine will oxidize the hydrogen sulfide in the well, but hydrogen sulfide will come back as additional well water flows into the well. What you are trying to do with the chlorine is to sterilize the well by killing the bacteria. It is very difficult to completely sterilize the well. So, I think you will end up having to sterilize the well every year or so.

Note that there are many types of microorganisms present that you bacteria testing will not show. The bacteria test is an indicator test, not an actual analysis or count of microorganisms.

I don't think that the corrosion materials produced by the bacteria will have any affect on stainless or plastic, but will definitely severely corrode carbon steel. I would not expect the wires to be damaged.

 

[Artisi]

there is a good range of SS pumps from Grundfos - worth checking.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Jazzerman]

I will look into stainless pump from grundfoss, good idea. And we will coat or use stainless pipe on the 1st stick connected to the pump. The corrosion problem only appeared below the bottom or our (steel) well casing on the pump and portion of pipe below the bottom of casing. This would be because this is where new water is flowing into the open hole below the casing. There would have been essentially no flow of water within the casing with the pump inlet set 28 feet below bottom of casing. If we were to re-install the pump inlet shallower, a few feet up from the bottom of the casing, do you expect we would see corrosion of the steel casing because then we would then have fresh sulfate rich water now flowing past the casing?
Maybe I will install the new pump (stainless) and coated or stainless pipe still below the bottom of casing so that there is no flow in the casing and all will be protected. We were a little concerned with the hole in the side of the pipe had eroded the open well hole, but our camera inspection showed the hole in good condition. And now that we will fix the corrosion problem we won't develop a new hole.

 

[bimr]

Would think that the flow of water is causing erosion of piping material that have been attacked by the corrosive byproducts of the bacteria. If you clean off the corrosion, you also expose new areas to corrosion.

The water flowing into the well is probably not corrosive, particularly since the well is anaerobic.

 

[weldstan]

Do check out the use of HDPE pipe as well. Many new water wells at similar depth used HDPE pipe when I lived in California and needed to drill new wells or change out pumps.

 

[SteveWag]

Most of my work is in central PA, much of it in the soft coal areas. I have encountered this situation many times. SS pump and plastic drop pipe solves the problem every time. 3-inch could be HDPE or better, grove-lock w/ set-screws PCV with SS adapters (CERTA-LOK® DROP PIPE).
Steve