What caused this?

[SteveWag]

I was recently asked to investigate a loss of capacity problem with a drinking water pumping facility. I found two split case pumps with a shop drawing capacity of 800 GPM @ 214 feet each. Only one pump can run at a time (electrical interlock). Both pumps have a flooded suction with water service about 5 feet above the impeller, from a tank about 4 feet away. The curve indicates a required NPSH of 26 feet at 800 GPM. The compound suction gauge has always indicated a positive pressure. Tests indicated the pump (only one was operable) was only capable of reaching 85% of shut-off. I ordered a new rotating assembly. While I was there, I noticed the Operator had disabled the check valve (propped it open) in order to get some more flow from the pump. I cautioned him that a power failure would cerate havoc due to reverse flow.

Sure enough we had a failure last weekend. The pump now would not run and was torn down.

The below link is what I found yesterday:

http://www.gswagner.com/RSPumps/RS14.jpg

I assume the broken impeller was due to the high-speed anti rotation of the pump as the 1,000,000 gallons of stored water emptied through the pump. The pump was operating quietly when I was doing my tests, no vibration, rotation of the motor was correct.

Now that the assembly was out of the pump I was able to inspect the impeller:

http://www.gswagner.com/RSPumps/RS1A.JPG

What caused this?

The shaft sleeves are Bronze same as the impeller:

http://www.gswagner.com/RSPumps/RS3.jpg

The sleeve (next to the spring extending to the impeller) shows no sign of damage.

Nor do either wear ring:

http://www.gswagner.com/RSPumps/RS4A.JPG

Any comments?
Thanks
Steve

 

[JoeySoap]

Cavitation.

 

[MikeHalloran]

Have the motor inspected.

Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

An excellent example of cavitation. The question now is why?

It is possible that the pump was running further out on its curve than than 800gpm, therefore requiring more NPSH than available even though the inlet pressure gauge was positive. Any records of its output when first installed?

 

[JJPellin]

I respectfully disagree. I see damage on all wetted surfaces. There are areas where cavitation should never occur. This looks like general corrosion damage.

In some water systems there are chemical additives or treatments to the equipment. Do they ever use acid or caustic to clean or treat?

I suspect that even if the wear rings and sleeves are listed as Bronze, it is a different grade. The cast material of the impeller is being attacked more than the machined rings. Ask the manufacturer for the material specifications on these parts and I believe you will find that they are different.

What is the condition of the water? Demineralized water is very corrosive in some situations.


Johnny Pellin

 

[rmw]

Who is drinking all that water? At a gallon per day per person (and that is a lot of water consumption for most folks) that is enough water for 384,000 people per 8 hr shift.

Could it be that the pump has that much capacity, but about 380,000 of the people aren't drinking any water on any given shift and the pump is alternately boiling the water and pumping at other times?

rmw

 

[SNORGY]

To research this, I watched a re-run of Jaws.

All I have to say is...

I think there's something in the water.

Regards,

SNORGY.

 

[SNORGY]

OK...I apologize - that wasn't especially helpful.

I see what looks like pitting. I don't know what the PREN number is for bronze. Are the impellers cast?

I would be looking for anything that might promote pitting in yellow metals. I must admit I have never seen anything like this.

Regards,

SNORGY.

 

[racookpe1978]

Are you going to try a stainless steel impeller for a few months, then find out what the actual flow will be when there really is a pump in the system? 8<)

What's your time frame for analysis time, purchasing and shipping time, replacement time, and re-replacement later?

 

[JJPellin]

Just to test my point about materials, I pulled up the Bill-of-Material for one of our water pumps that has bronze impeller and wear rings. Our pump is a little bit larger than yours. It is rated for 20,000 gpm in cooling water service.

The data sheet lists bronze impeller and bronze wear rings. But the BOM spells out the difference:

Impeller: Cast 88-8-4 ASTM S B584 AL 903
Impeller Wear Rings: Alum Bronze ASTM S B148 A954 HT
Case Wear Rings: 85-5-5-5 ASTM S B584 AL 836

Not all bronze is created equal.


Johnny Pellin

 

[TenPenny]

Is that a bronze impeller? I have seen similar, bronze dezincification.

You probably need to change to a different bronze material. I'm pretty sure there's a google-able tech note on that from Peerless or Aurora floating around the 'web somewhere.

 

[TenPenny]

Here is a pic of the impeller I was involved in.

The solution is to go to a lower zinc (6%) version of our standard bronze. Also, the customer can look at a zincless bronze such as Ni Al or at 316 stainless. Of course these latter two options are more expensive but will provide much longer life.

 

[TenPenny]

Just as a note, it's a result of the chlorine reacting with the zinc. The one I was involved in was in a pumping station where there is additional chlorine injection into the water; since your application is also drinking water, I would suggest it's likely a similar issue.

The pump I included the picture of is not very old, I believe still under warranty, if my memory is working properly.

 

[SteveWag]

The water is from a Roaring Spring in Pennsylvania. Also using the spring is a bottled water company and a paper mill. The water is pumped from the spring to an 8,000-gallon concrete tank where chlorine is added. The pump(s) in question then transfer the water to a tank elevated about 210 feet. Almost all discharge head is static. The pump(s) ran at about 550-660 GPM about 5 hours a day. Lately, the system has gained some leaks and operating hours has gone to 10 hours per day. Typical Cent PA Municipal water system. The pumps are Aurora 413s, 4x5x10B with 8.1875-inch impellers. Chlorine is maintained at 0.3 to 0.4 mg.L. Records indicate the pumps produced about 660 GPM when new and are (were) capable of 550 when destroyed. New rotating assemblies were shipped yesterday. I have not seen the innards of the second pump. I probably can post pics of number two next week. There is and has been a flowmeter on the discharge since construction. I calculate NPSHR at the suction as being above 28 feet, however the Compound Gauge on the pump suction has never indicated a negative pressure.
Steve

 

[ash9144]

I agree with Snorgy and Jaws. Something in the water.

Did this pump sit idle for extended periods while other one ran? Looks like this impeller may have sat with water stagnant on the impeller allowing some corrosion possibly weak acid attack.

 

[SNORGY]

Assuming that TenPenny is correct and there might be chlorine reacting with the zinc, then if there is 0.4 mg/L chlorine (0.4 ppm?) per SteveWag's post, would that be enough? I am definitely not as knowledgeable as you folks in the Pump forum, but I wouldn't be alarmed at first glance at this level of chlorine. I would try a stainless steel impeller next.

Regards,

SNORGY.

 

[ccfowler]

I agree that corrosion is very much an issue, but I'm sure that cavitation is involved, too. Flooded suction is fine for keeping the pump full for start-up, but that does not assure sufficient NPSHa to avoid cavitation damage. Corrosion and cavitiation work so very effectively together to destroy a pump.

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.

 

[TenPenny]

Oddly enough, the pics I posted were of an Aurora pump as well. I don't know what chlorine level is in the water at the installation I saw.

 

[SteveWag]

I went to the site yesterday and they had cleaned the place up some and removed a broken down desk, revealing used two rotating assemblies lying on the floor. Nobody knows why they are there. One shows damage, as the lower bearing had failed and the shaft wear ring was almost destroyed. The other looks usable, both show no sign of pitting, either in the eye area or anywhere else. I talked to the former Aurora sales rep. He did not sell the replacement assemblies; the ones installed now, but did remember ordering low zinc impellers for the original install. Apparently the low-zinc units were replaced sometime with “standard?” units, don’t know when, nobody keeps records, and after some time, this was discovered. Again, maybe next week I’ll have some more photos.
Steve

 

[ColonelSanders83]

You may wish to review the thread below.

http://www.eng-tips.com/viewthread.cfm?qid=258967&page=1

If you are in Pensylvania, your stream is likely quite cold for a goood part of the year and is a sponge for oxygen and nitrogen amoung other gasses.

I assume when you calculated your NPSHa you made use of the vapor pressure for water from a steam table. This a common practice that yields an incorrect result for the application in question. Steam tables have data for pure Deaerated water which is not applicable to a stream with saturation levels of dissolved gasses. The effect is that cavitation begins much more easily and at a higher NPSH then you calculate.

The differance in vapor pressure between Pure Deaerated Water and saturated water is approximatley the average of the vapor pressure for pure Deaerated water and atmopheric pressure. This type of cavitation is not as damging as classical cavitation but will have long term damgeing effects dependant upon the cavitation resistance of the material exposed to the low pressure fluid.

Just my two cents worth

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

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