??Electrolysis erosion of lineshaft deep well pump "iron" components

[swaneyg]

HI,
We have 9 deep well vertical turbine pumps near the local river that supply water to our process. Since the installation of pump #8 we've had reduced life of pump bowl components. At this time we also upgraded to VFD drive packages to reduce shock loading on pumps. We do not use VFD to control flow.

Upon pulling the last pump, the nonferrous items were in excelent condition but the cast iron bowls were practically gone and the stand pipe was also showing signs of erosion?? Thinking it was related to VFD's, we added grounding brushes to shaft to short any potential from motor from traveleing down shaft (long shot I agree).

To no avail. The last pump we installed is already showing increased vibration (3 months old). We've talked to local municiple water company technicians and they stated they had not seen this before. We had electrical design companies look at the drives and even reverted one to a typical line starter but it is also showing early signs of failure.

Any suggestions or comments are very welcome.

Thanks,
Glenn Swaney

 

[d23]

Glenn:

We have seen applications where electrolysis seems to be a problem with submergible pumps. In these cases we use a sacrificial anode on the end of the tubing string. The ones we have are about 3 feet long and have a 2” male thread on the end that can be screwed on the submerged end of the equipment. The anodes are only a band-aid that extends run life. They do not fix the problem. A magnesium anode is not very expensive to try as a test, but in the end you will need cathodic protection for your wells.

You may want to try using fiberglass tubing and rods. That will isolate the pump bowls a little better.

I would recommend that you look at something reasonably quick. If the bowls are destroyed then your well casing is being destroyed too.

I’m not sure where we get our anodes. If you contact Schlumberger (REDA PUMP Group) or Baker (Centrilift Pump Group) both companies offer them. I doubt if either company will tell you where they buy them, but they are “pass through” devices that don’t get marked up very much anyway.

The best fix is to hire a consulting company that specializes in electrolysis.

Good Luck!
David

 

[dadfap]

glenn,

d23 is correct, anodes are a quick fix.

Has there been any appreciable change in the water, I know this is perhaps a question that has already been considered?

Have your other pumps started to deteriorate or is the problem developing on the existing installations?

How deep are your applications?

dadfap

 

[dadfap]

glenn

you may want to consider two possibilities with these installations
1. Increase for some reason in the dissolved oxygen content of the river water
2. Increase in biological activity leaching the iron from these components. This may sound a little far fetched but i have seen the results of this in submersible applications and it is a problem recognised by NACE.

all the best

dadfap

 

[swaneyg]

Thanks, All,

I have worked at this plant for 25 years but just recently was asked to look into this problem. My primary job has been mechanical equipment design, hydraulic servo systems and standard hydraulic controls. Water wells will be new to me.

There have been water samples taken but I do not have the results back yet. I'll keep you posted.

Some of the data I have:
Pumps are vertical turbine five stage 122'-6" long and setting above ground level 17' for flood protection. Water lubricated spider bearings. Well is 109' deep according to report I have. 300 HP @1785 RPM.

First 7 wells had no problems as far as we know like this. They have been in since 1965-66. Not a lot of good records kept back then.

Well 8 was installed and commissioned in 1996 do to losing well 1 from a cave in, I'm told. Within one year pump had to be replaced for apparently the same reason as the problem now. It has been in and out for repairs ever since then on an annual or so basis.

Well 9 was brought on line in 2001 and pump is out now. I have some photos of this pump. I was not involved with removing it so photos are all I have. It appears that the destruction is in the high velocity area from the slip at the edge of the impeller. The bowls are cut completely in two in that area. The Superintendant tells me that you can take your finger and poke through ares of remaining cast iron bowl. I am awaiting an old bowl to look at for myself. From his description, it sounds like the iron is being pulled out by some means.

Wells 8 and 9 are about 600-700 feet from the original well field. Soil and boring reports are similar to the original wells. I didn't see anyting odd or greatly different.

Agian, thanks for the replies and I'll keep looking into this.

Glenn

 

[phex]

you stated that most corrosion is to be found at high fluid velocity areas of the pump. this could be a problem with solids in your fluid, i think. have you had the time to analyse the content of solids and what they are? i'd imagine that something like SiO2-crystals could do something like that...

just a guess, but perhaps it helps
chris

 

[d23]

Swaneyg:

Both dadfap and phex make excellent points that need to be considered. Oxygen will cause corrosion, solids in higher velocity areas will cause erosion and as you originally suspected electrolysis will lead to failure too. You have several good options:

1) Send the bowl back to the manufacture for an analysis.
2) Send the bowl to an independent metallurgist for analysis.
3) Get a fluid analysis for solids, ph etc…

When you install a new pump you could install an anode without spending much money. This may not fix your problem, but would eliminate one possible problem.

If you would please let us know what any analysis reveals.

 

[swaneyg]

We are sending the bowl out for analysis this week and I will post any viable information from it to this thread. Thanks everyone for your interest and support. I do not have the complete water analysis information yet. I will include it also with bowl information.

I do have several photos that I could e-mail in jpg format if anyone is intersted in seeing the damage.

Glenn

 

[BobPE]

sounds like cavitation from a missed pump design...give us more info if you can on the pumps and system pertaining to hydraulics.

BobPE

 

[swaneyg]

Sorry about the late come back on this but I've been asigned to another area and function and will not be following this problem any loger. I do have some results from an independant lab on the bowl errosion issue. It is "quoted" as follows:

"1. The Brinnel hardness of the bowl indicates that the hardness values are well below acceptable values of around 200 HB as per ASTM A 48 Class 30B.

2.The % of pearlite is very low with the majority of the matrix consisting of the soft ferrite phase with very large graphic flakes that is indicative of very low strength.

3. The corrosion has occurred due the the very soft nature of the material with the area between the graphite flakes and the matrix being the starting point. As seen by the SEM images and EDS analysis the corrosion are consists of the cast iron base material with Fe, Mn and Si and Mo being added for strength along with salt related elements picked up from the water.

4. A random check of Brinnel hardness on the bowls with a minimum acceptable value of 180 HB may be a future checkpoint prior to installation of pumps."

I can assume from this report that we need to ensure our rebuild shop and internal stores parts are being manufactured to the coorrect specifications. As I stated earlier, I will not be following this project anymore but thanks again for all the feedback I received from the group.

Glenn Swaney

 

[unclesyd]

Your mention of pure water and the condition of the pump, graphite flakes, upon inspection suggest that your pump is under going “Graphitization” of the CI. This is form of corrosion that can affect CI in relatively pure water. This can be a very slow or a relatively fast process depending on several variables.

Are you see “rust” in the water or on the components when you pull the pumps?

What is the CO2 content of the water?

 

[swaneyg]

Thanks,
I'll e-mail this to the party covering it now. I no longer work with this equipment.
Thanks to all. Glenn Swaney