Causes of pump casing damage??

[SFwatereng]

The casings on two of our pumps are suffering from damage.

These two pumps are horiztonal split case, double suction with end discharge. They were manufactured by Worthington-Dresser in Germany in 1991. They are about 1000 HP, with Ductile Iron casings and bronze impellers.

The pumps are used with both raw and treated water.

Any ideas on the CAUSE? Has anybody else had similar problems with Worthington Pumps bought at the same time (1991)?

 

[SFwatereng]

Click below for the pictures.

Picture 1

Picture 2

Picture 3

 

[checman]

sfwatereng
What is the PH of the water? The bronze impeller looks ok in the picture, is it?

Regards checman

 

[SFwatereng]

Yes, the impeller looks fine, so I am not suspecting cavitation, atleast on that portion.

As for the PH, when we are using the pumps for treated water, the PH can get as high as 8.8 to 9.

It's been reported that there is sometimes sand in the water. Could sand or silt have caused this damage?

 

[Artisi]

To me it looks like a classic case of corrosion erosion and if there are any fines in the water this will only accelerate the damage by a significant amount.

If you look closely at the photo's you can see corrosion in the stuffing boxes and under the wear ring locations, there is no flow in these areas so no erosion has taken place.

My suggestion is all bronze pumps for this application.

Naresuan University
Phitsanulok
Thailand

 

[stopsunset]

I agree....it appears to be corrosion/errosion.You may also want to consider a protective coating such as Belzona.

http://www.belzona.com

 

[mucour]

I also agree it is likely to be caused by corrosion. On close examination it appears that the pump could have been on standby as a spare pump for the corrosion to have taken significant effect. Could confirm if this pump is used as a spare pump or standby pump? It could also be that the pump works on intermittent mode.

 

[RGRAJANI]

It is difficult to judge thickness loss but it is not that bad, after 14 years of operation.

There can be various reasons for this corrosion : Gas entrainment in water, cavitation, operation away from BEP.

Damage is less evident on impeller because of it's superior metallurgy.

 

[SFwatereng]

Thanks for all of the responses so far.

Yes, the pump is on intermittent mode and is not constantly used.

What are some of the causes of the erosion corrosion? Possibly the sand?

Artisi: You mentioned "fines" in the water? What do you mean by "fines"?

 

[SFwatereng]

Also wanted to mention that at the same location where the above ductile iron pump is used, we have a second pump that has a cast iron casing. The cast iron pump is not showing nearly as much corrosion as the ductile iron pump is.

There should not be much difference in corrosion advantage between cast and ductile iron. Plus, at the same time the pump was manufactured (1991), many companies started to have their casings casted in other countries, such as China and India.

Could it be that there was a problem during the manufacturing of the ductile iron casing during that specific time?

 

[Artisi]

"fines" are fine entrained solids in the water, ie., sand as you have described in your postings.

Naresuan University
Phitsanulok
Thailand

 

[mucour]

Corrosion is a chemical reaction causing loss of metal while erosion results from mostly abrasives (like sand) in the liquid. Under high velocity this creates a situation that can be likened to sand blasting. The result is also loss of metal over time.

If the pump is on intermittent mode as suggested and the liquid is not treated with chemicals that prevents/reduces corrosion you cannot escape from having corrosion over time unless your material selected is a Corrosion Resistant Alloy.

 

[SFwatereng]

Agreed. But what is still puzzling is that the Cast Iron and other Ductile Iron pumps are not showing the same degree of corrosion.

Which leds us to suspect that it not only could be a combination of the sand/silt, but also a manufacturing/casting defect with this particular ductile iron pump?

 

[Artisi]

" mucour (Mechanical) 30 Nov 05 1:42
Corrosion is a chemical reaction causing loss of metal while erosion results from mostly abrasives (like sand) in the liquid. Under high velocity this creates a situation that can be likened to sand blasting. The result is also loss of metal over time."

In a situation where corosion is a problem, any entrained fines (solids) can very quickly cause major damage as the corosion is always a lot less abrasive resistant than the parent metal. In extreme conditions a pump unit can be totally destroyed in very short order.



Naresuan University
Phitsanulok
Thailand

 

[Artisi]

As for the suitability of or problems with the D.I. I think you should talk with a metallurgist and / or have the material tested.

Naresuan University
Phitsanulok
Thailand

 

[arunmrao]

CI casings are more poular due to lower cost and lesser propensity to corrosion. While DI casings are expensive and not so corrosion resistant. One explanation could be higher P content in CI .

Your case seems to be a case of erosion corrosion,where the sand particles which are extremely abrasive have accelerated the wear.

 

[SFwatereng]

Thanks for the replies so far everyone.

I don't think it's a problem with all ductile iron casings. I think it might be a problem specifically with this one pump. The metallurgist test is a good suggestion.