feedwater pump failure modes ? 2

[cogener]

just getting started on a feedwater pump failure problem which "has been gong on for years" according to the onsite team, the problem description starts out like this: we are regularly replacing pumps due to "pinholes" in the housings and recirculation piping, we switched to a high purity (demineralization system a few years ago and have had problems since.
Question: does anyone have a recommendation on where to go to research water chemistry and it's impact on high pressure feedwater pumping systems?
my initial thoughts are that this is a flow accelerated corrosion situation but i am short on technical data to support the theory or offer alternative possibilities.
Thnaks in advance for any and all ideas and helpful comments.
Best Regards
co-gener

 

[someguy79]

What type of feedwater is it?
Nominal temperature & discharge pressure?

 

[JJPellin]

Demineralized water that has not been deaerated is very corrosive. If you removed the minerals but do not remove the oxygen, you have some of the most corrosive "pure" water possible. We no longer allow carbon steel in this service. All of our demin pumps are now CA6NM (11-14% chrome) as a minimum. What material is the pump case?

Johnny Pellin

 

[JJPellin]

In my experience, corrosion of carbon steel in boiler feed water or demin water is highly dependant on velocity. Damage can be localized. I have seen holes as large as 1" diameter cut through more than an inch of cast carbon steel between stages in a multi-stage, horizontal, split-case pump. We also require CA6NM for all of our big boiler feed water pumps. Pumps that had lasted for 30 years started having problems 10 or 15 years ago. It may have to do with changes to our demineralizer processes or the treating chemicals they use. We gave up trying to deal with the problem and just changed everything over to chrome.

Johnny Pellin

 

[cogener]

Thnaks! for the comments and insight,it is hard to beat many years of expeirience with similar problems. next step: gather field data, (presseres, temps, pump/pipe materials, chems & chem feed points)and sketch the layout. (deaerator operates at 50 psig sat. and pumps discharge at about 1000 psig).
the water is deaerated prior to enerting feedwater pumps, we have residual sulfite in boilers, so we should be good there, will check curves and min recirc rates to see if we have ultra high velocities @ recirc lines.
one other note, the recirc lines feed back into pump suctions and not back to the tank, not sure this is a problem but it is not what we usually see witht these set ups.
thnaks again for the feedback.

 

[pumpman50]

Yes, I agree with Johnny Pellin and this is a very old problem. On smaller feed pumps in small industrial plants, carbon steel of even cast iron seems to stand up to treated feedwater providing the Ph is not below 6.5 and velocity is low. Pumps can be bought cheaply but prove expensive if they have to be replaced with more exotic materials if corrosion takes place. Always worth giving the manufacturer a feed water sample and details of de-aeration before ordering or you could be in for an expensive shock a year or so later!

I would always recommend 11-14% chrome materials typically 13%chrome/4%nickel for impellers on larger high pressure feed pumps certainly above 100hp.Wearing parts i.e. impeller wear rings in hardened 17/4ph material running against 420 stainless steel casing rings with a good differential hardness between the 2 components to prevent galling.

I was told once by a chemist that demin. water has a memory and always looks for ways of extracting certain minerals out of the materials in close contact to it. Hence the pitting experienced.
Andy

 

[Artisi]

I don't know about demin. water having a memory, but it certainly tries its hardest to become mineralised again - hence the destruction of pump and allied materials.

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.)

 

[ccfowler]

Recirc lines feeding into the pump suction would surely produce turbulence that is always undesireable. The mere existence of the recirc connection at the pump suction may be producing needless turbulence even when there is no recirc flow. That would depend on the exact details of the pump and piping configuration, and it would be wise to pay some serious attention to this possible contribution to elevated turbulence.

Johnny Pellin has given you some superb advice.

Seemingly small variations in water chemistry can produce very large effects on corrosion in pumps. You may want to check on the variations in your water chemistry. It may be much less stable than you think.

I would want to pay particular attention to water chemistry variations in the flow from the demineralizer trains. There may be something in the configuration of your piping and tanks that may allow for "hide-out" problems that could provide slugs of "non-compliant" water chemistry.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[cogener]

Gentlemen. Thanks for your insight and ideas. As we work our way into this I think we have lots of things going on at once:
PH levels in the feedwater that range from 10 to 5, dead legs where chemicals "reside" until pumps are cycled into service, high turbulence in pumping and recirc lines, and probably a fair measure of flow accelerated corrosion. perhaps some chemical control issues as well. it seems we have the deaerators working well though.
can i forward a few pictures in an attempt to identify other possible causes?

 

[Artisi]

Please do.

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.)

 

[cogener]

recirc line errosion / fac picure.

 

[cogener]

casing corrosion / erosion / fac picture

 

[cogener]

old casing corrosion/erosion/fac

 

[cogener]

old casing failure pic

 

[Artisi]

What I see is erosion.

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.)

 

[vpl]

cogener

Are the last two files supposed to be e-mails or pictures?

Patricia Lougheed

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