Upgraded material for pump impellers in sea water application 1

[Mech5656]

Hello,

We have a pump used in sea water that needs repair and the impellers are washed out and pitted. Impellers are made from Bronze 630 and we are thinking to upgrade them to 316 SST. Is that a good upgrade? Do you recommend 316 SST instead of bronze in sea water?

I have attached picture of impeller condition after removing from sea water. Unfortunately, we don’t know how old these impellers are because of no documentation left by previous owner.

 

[TugboatEng]

You really should look at fixing the root cause of the problem. Your pump is either operating too far to the right on the pump curve or has insufficient suction head. This damage is caused by cavitation erosion. Something as simple as throttling the overboard could be all you need to do to prevent this kind of damage. With that said, stainless steel is more resistant to this type of damage but is more susceptible to corrosion from the salt water. There are also rubber coatings available to help prevent damage from cavitation.

 

[LittleInch]

Bronze is better.

Like tug says, fix the problem first not the pump material.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

1. 316L is not a suitable alloy for handling seawater.
2. This pump is being operated wrong. That isn't 'typical' corrosion or erosion damage.

While a stronger material might last longer it would only be making the operational problems.

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P.E. Metallurgy, consulting work welcomed

 

[Artisi]

Certainly cavitation erosion with possibly some erosion from entrained material, as already pointed out - bronze is the better choice of material.
How many hours / days / weeks years in operation?.
Suggest you post detail on installation, hydraulic duty - flow / head, inlet conditions, discharge detail, etc - or get a qualified pump engineer to assess the operation.
The material failure (cracks) on the blades etc are interesting - why, from the massive forces being applied from the cavitation implosions? Any suggestions?

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.)

 

[TugboatEng]

The cracks are caused by the thinning of the material. This one textbook for a pump installed in a system that requires an orifice plate but doesn't have it.

 

[Artisi]

Tugboat. Yes, I would agree on one hand, on the other probably disagree somewhat. Not that it really matters but I would suggest the cracks are caused by constant flexing of the blades from high loading from the implosive cavitation forces.
And agree completely regarding the need to either rectify the operating conditions or reduce flow throughput.
Interesting to see what comes back from the OP.

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.)

 

[itsmoked]

I bet that pump shakes like a constipated dog.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

This is obviously a main sea water cooling pump on a ship or platform. They get spec'd generously. It's a central pump that pushes water wide open through a bunch of heat exchangers and then goes overboard. Commissioning is done to prove it provides sufficient flow to keep the machinery cool and it does until the vessel is out of warranty. A flow meter should be used to determine an orifice plate that will provide optimum life but this is never done. MSW pumps live a hard life.

Keith, your comment is inappropriate because this problem is serious and now I'm laughing at it.

P.S. those aren't cracks. Cavitation likes to localize itself and will form grooves like this.

 

[Artisi]

And probably makes as much noise as a constipated dog.

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.)

 

[TugboatEng]

I should ask my friend the name of the semi-sub platform he was working on. It might be the same one. The impellers went so out of balance they were breaking shafts. It was incomprehensible that throttling the pump could possibly prevent the failures. They decided to eat the costs instead.

 

[MJCronin]

Mech5656

First, recognize that the term "Seawater" can mean several different corrosive environments. The Dead Sea has FAR MORE CHLORIDES than the Baltic, which has the salinity of a light beer.... Can you be more specific about your operating conditions ?

Second, understand that part of your problem may be that the pump is operating far away from it's design point. Cavitation and erosion may be augmented by its operating point and other factors. Now is the time to perform an evaluation of the pump design and NPSH suitability.

Third, Of course there are no records left by the previous owner !!! .... The primary function of a new group of maintenance people when taking charge of a new facility is to discard ALL PAPER RECORDS ! By doing this, they can never be asked to research the operating history o r parts for any equipment because there are no records. If anything breaks, call in the engineering consultant !!!

Fourth, recognize that you have a whole spectrum of impellor and shaft materials to select from. Some are cheap and others extra expensive ! How much do you have to spend and what would be a reasonable schedule for replacement of shafts/impellers on these pumps ???

The most common pump metallurgies for seawater services generally fall into one of the following category types:

• AISI type 316L fabrications or ACI type CF3M castings with type 316L, Nitronic 50 or type 2205 duplex stainless shafting
• Ni-resist or Ductile Ni-resist casings with ACI type CF3M impellers and type 316L, Nitronic 50 or type 2205 duplex stainless shafting
• Aluminum-bronze fabrications or castings with nickel-aluminum bronze or ACI type CF3M impellers, and AISI type 316, Nitronic 50 or Monel® K500 shafting
• Standard duplex stainless fabrications or castings with duplex alloy 2205 shafting
• Super-duplex stainless with type 2507 super-duplex shafting
• Super-austenic stainless with type 2507super-duplex shafting

(Excerpt from Stephen Morrow paper)

Fifth:.... For many years, this TAMU publication has been the seminal paper on seawater pumps.

https://turbolab.tamu.edu/wp-content/uploads/2018/08/Tutorial-05.pdf

Take more pictures (including close-ups of the impellor damage) and post them. How is the pump shaft condition ??

Do you know who manufactured the pump ???.... Have you talked with them ??? Have you been damaging pump seals and/or pump bearings also

https://www.eng-tips.com/viewthread.cfm?qid=410183

Keep us informed....... and good luck

MJCronin
Sr. Process Engineer

 

[IDreamChaserI]

I wonder if you ever consider machinable glass ceramic, great material to anti corrosion.

 

[Artisi]

Fix the installation / hydraulics and the problem will disappear.

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.)

 

[Nutzman]

From experience, Wear on the inlet vanes of an impeller indicate the problem is on the suction side piping. As mentioned above identify the system/piping problem and the impeller will last. Is there a bend just before the pump inlet, or is the suction pipe a size or 2 smaller than the pump inlet............ or both?
Considering the product is seawater, I would have considered galvanic reaction. But typically that would show itself between the pump casing wear ring on the impeller wear ring. Based on the photo, that is cavitation. The next component to go, will be the pump shaft, right behind the impeller, cyclic fatigue.

 

[Rocket2M]

316 probably will be more exposed to stress corrosion cracking in seawater. Bronze or nickel aluminium bronze is better

 

[Mech5656]

Thank you everyone for your responses. You amount of knowledge and experience on this topic is just amazing.

MJCronin:
The shaft TIRs are good (0.0015" or less) but it has rub marks from bearings (Thordon) so we are going to replace the shaft.
Pump OEM is Goulds and they recommend Ni-Al Bronze (C63000/C95500)
Pump bearings were not in bad condition but were oversized by 0.005".

Most probably, I will go with Ni-Al Bronze impellers and then I will talk to operations about performing the pump per OEM recommendations.

 

[TugboatEng]

Have you considered an orifice plate in the system to bring the operating point back on to the pump curve do this doesn't happen again?

 

[EdStainless]

Or can you get the actual head and flow that this was running and select a different model pump that would deliver that at BEP?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Mech5656]

Hello Dear Engineers,

I was able to find old documentations showing that these impellers were replaced in 2009 (therefore these lasted about 12 years), which is not bad.

These impellers are made from Ni-AL Bronze.