What caused this?
What caused this?
(OP)
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
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





RE: What caused this?
RE: What caused this?
Mike Halloran
Pembroke Pines, FL, USA
RE: What caused this?
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?
RE: What caused this?
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
RE: What caused this?
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
RE: What caused this?
All I have to say is...
I think there's something in the water.
Regards,
SNORGY.
RE: What caused this?
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.
RE: What caused this?
What's your time frame for analysis time, purchasing and shipping time, replacement time, and re-replacement later?
RE: What caused this?
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
RE: What caused this?
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.
RE: What caused this?
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.
RE: What caused this?
The pump I included the picture of is not very old, I believe still under warranty, if my memory is working properly.
RE: What caused this?
Steve
RE: What caused this?
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.
RE: What caused this?
Regards,
SNORGY.
RE: What caused this?
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.
RE: What caused this?
RE: What caused this?
Steve
RE: What caused this?
http://www
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!
http://www.ap-dynamics.ab.ca/
RE: What caused this?
I think there may be something in the water causing a general corrosion mechanism.
RE: What caused this?
Sounds like you were only able to reach 85% of shut off because the check valve was forced open. You were probably further out then 800 GPM as well, but had this been cavitation damage, you'd experience a long history of vibration and system issues with this pump, which doesn't sound like the case.
The problem with capacity is likely assoc. with the worn impeller.
Did you know that 76.4% of all statistics are made up...