Cavitational pitting or galvanic action 1

[seakro]

What comes first... We have a set of shipboard pumps which are in my opinion showing pitting similar to cavitation on the inlet vanes of the impellors. Some camps claim that due to galvanic action or electrolysis has caused this wear by first knocking off the oxide layer then accelerating wear....(indicative of small scalloping marks on the impellors, gray in color) The kicker is in my thinking is if there was galvanic corrosion going on in the system we would see it in many other areas not on the ni al bronze impelors where solely cavitation is expected. Gage lines and small pipe nipples of inferior material would be the first to rot. If have some good photos if anyone cares to view.

Any help would be appreciated

Keith

 

[PUMPDESIGNER]

I am into this.
I think this needs to be done more, study the failure, obtain the data, design the correct solution first time.

Can you post the photos on a webite quickly to look at or download? For some reason the managers here hate to see email addresses popping up on these threads.

I am really wanting to see those photos.
Hey, can you post high resolution too?

There are some studies (I think I have them in a format I can share), where cavitation is known to accelerate or more properly exacerbate corrosion due to chemicals, I will start checking to see if galvanic action can be affected by cavitation also.

What is the Specific Speed and Suction Specific Speed for the pumps?
NPSHr, RPM also.

PUMPDESIGNER

 

[seakro]

Pump Designer


Several impellor photos are here.

http://compulounge.com/keith/categories.php?cat_id=1

Seakro

 

[PUMPDESIGNER]

I do not think those photos are hard to diagnose.
Suction Side Cavitation.
I cannot rule out galvanic corrosion as a minor contributor but:
1 The localized nature of the damage is where suction cavitation is expected.
2 Galvanic processes work evenly across the whole impeller but the damage exhibited is highly localized.
3 As you stated, other metals are more vulnerable.

The cavities are being created BEFORE the impeller, then when they strike the impeller they are collapsing and eating the impeller away. If that be true, then you have a severe cavitation problem that probably can be diagnosed easily.

Run pumps at maximum flow, should be noisy with cavitation sound.

Reduce flow rate until cavitation noise dissappears.
If the pump becomes quiet at lower flow rates you have suction cavitation.

Do the NPSH calculations for NPSHa and then obtain if you can the pump manufacturer's NPSHr, that should reveal the problem.

Try this link to read about cavitation.

http://www.irrigationcraft.com/cavitation.htm

PUMPDESIGNER

 

[PUMPDESIGNER]

I forgot to say that if galvanic action is occurring in a minor way, it may be happening only because cavitation is eating away the protective oxide layer constantly.

PUMPDESIGNER

 

[PUMPDESIGNER]

I posted a photograph of the same wear pattern and appearance due to cavitation at this link.

http://www.irrigationcraft.com/special_downloads.htm

PUMPDESIGNER

 

[seakro]

Thanks for your comments....

This damage is occuring on a certain class of vessels of which all of the sea water pumps were showing these signs of wear... as a lay marine engineer I felt it was solely a cavitatioal process however others contributed a galvanic corrosion going on.... one of the reasons for this thought was because the vessels have rubber lined piping systems and they first thought this created the rapid wear....This damage is less than a year or two old.


I just cannot shake my argument that any electrolytic process would also effect other areas and surfaces of the pump. We have istalled "anode" steel spool pieces on the suction side to test, as well as adding bonding straps on the flange joints. For some reason this slows down the wear... could it be an attempt to continue and redevelope an oxide layer? The vessels also have a Cathelco system which uses a copper anode to emitt cu ions to control growth, I believe.. along with this is a mild steel anode to maintain an oxide layer...... now some camps also say if the anode is too far away the ions will attach to other elements instead... comments?

Taking another position... in order to have that cavitation we need air entrainment in the sea water. correct? Now we get into hull design, appurtenenances, causing air to develope improper venting of sea chests, etc etc...

Any additional comments would be appreciated.

Keith

 

[PUMPDESIGNER]

I cannot comment authoritatively on galvanic processes.

It is however my peronsal opinion that there is little or no galvanic component in the damage I see in the photographs.

If you read that webpage I showed the link for above, which is:

http://www.irrigationcraft.com/cavitation.htm

you will see that air entrainment has nothing to do with cavitation, they are not related. Entrained Gas bubbles cannot damage pumps unless they are large enough or sufficient in quantity to expose seal faces.

Bubbles arising from the cavitation process can destroy anything, huge difference between the two types of bubbles.

It would help me greatly if I knew the following:
Specific Speed - Ns
Suction Specific Speed - Nss
BEP
RPM
NPSHr at BEP
NPSHa if someone has calculated that, which for sure they should have to properly select the pumps in the first place.
What kind of pump is it? End Suction, Double Volute, Double Suction, single or multi-stage and how many stages.

If all you have is BEP flow and head, with RPM and NPSHr at BEP, I can do my own calculations for Ns and Nss.

PUMPDESIGNER

 

[BRIS]

Erosion corrosion is often confused with cavitation. Erosion corrosion occures where erosion (high velocity silt laden water) prevents an oxidisation layer forming and protecting the metal. It is indiacted by smooth horse shoe shaped marks. The horse shoe being in the diretion of the flow. Cavitation erosion is far more brutal creating pits and often rapid erosion. Cavitation will also knock off the oxidising layer and accelerate galvanic (erosion) corrosion.

 

[BRIS]

Your photographs do tend to show smooth polished surfaces indicatve of erosion corrosion rather than rough pitting indicating cavitation. But Pumpdesigner is the expert - give him the data and I am sure he will confirm whether or not you have a cavitation problem.

 

[ktm5]

seakro,
I am curious as to the layout of the physical system, ie.. are there elbows in the suction piping near the pump? Sometimes in a cramped engine room of a ship, space limitations create problems such as this.
rgds,
ktm5

 

[PUMPDESIGNER]

ktm5 is correct that an ell right on the pump intake connection, or close enough to it, can cause suction side recirculation cavitation. The pattern of wear on the impeller does not reflect that in my opinion, but who knows? More likely you are looking at insufficient NPSHa.

PUMPDESIGNER

 

[Artisi]

From my intial look at your pics, I agree with PUMPDESIGNER, suction cavitation, however I think there is some other factor involved in this damage because of the very smooth look of the damaged area's, although the smooth look could be because the al-bronze impellers are ductile and do not pit as do iron or steel impellers.

Question: Is the water "clean" or does it have any fine suspended solids in it as I am tempted to say that the problem look a little like cavitation coupled with minor erosion. There could also be some galvanic action taking place on the newly exposed virgin metal.

International College
Naresuan University
Phitsanulok
Thailand

 

[BRIS]

Al-bronze is particularly susceptible to erosion corrosion and I think the pictures are showing corrosion (smooth surfaces) with some cavitation at the edges – (there is obvious cavitation pitting at the edges). So I agree with Artisi - you probably have both.

Re your initial comment “was galvanic corrosion going on in the system we would see it in many other areas not on the ni al bronze impellors” This is not the case – galvanic erosion corrosion will only occur where the erosion is “knocking off the oxidising layer” as you put it.

Brian

 

[seakro]

Thank you once again for all your replies. I unfortunately have not had the time to give you other specifics such as the NPSH.

I will tell you though, that these ships operate in the Caribbean where usually always you have clear clean water. Of course the biggest headache is in port, depending on your draft will determine the amount of dirt/ slit shook up, but I dont believe this could do it as manuerving is such a short period of total running time.

As far as the suction piping goes each pump has close to a 12 inch branch off a main 48" line. Whether running 1 or 6 pumps at once (dependent on load) the suction pressure remains a positive .3 bar.

Any other suggestions or recommended procedures to try and come to a conclusion as to what is causing this?

Regards

 

[PUMPDESIGNER]

You probably have or can easily obtain the pump manufacturer's NPSHr values. That together with the flow rate for the pumps would answer some questions immediately.

PUMPDESIGNER

 

[seakro]

I am sorry for not getting the data for you to look at. I have to go at this gingerly.

I am beginning to lean on simply, impingement attack due to alot of bubbles in the sea water. The galvanic aspect of this wear simply does not connect, mainly because of no other evidence, for example the steel spool piece on the suction side not showing any loss of material. The wrong material being loss heck the impellor is ni /al /bronze, the casing is bronze.

I have not recieved reports of true cavitational problems or symptoms as well. The pumps are varible speed dependent on engine load.

Comments welcome. Thanks.

 

[Artisi]

Sorry cannot agree that air enrained in the water is causing this problem, see previous comments from PUMPDESIGNER, unless of course it is some type of localised corrosion damage resulting from the air / seawater / material.

To comment a bit further on air entrainment - purposely entraining air into the inlet side of a cavitating pump is one remedy to relieve some of the damage from suction cavitation, it is not a cause of this type of damage.

International College
Naresuan University
Phitsanulok
Thailand

 

[unclesyd]

Here is another source of some very good information concerning cavitation problems with pumps and the remedies for same.

http://www.mcnallyinstitute.com/01-html/1-3.html