Flexible Impeller

[dimockmatt]

I have a customer destroying nitrile impellers almost every week. We draw fluid from a roughly 10' diameter tank through a 1" line into the 3/4 port on the pump. The tank is within 20 feet of the pump. Any ideas on the cause and hopefully a solution?

 

[Unotec]

Cavities due to too much sugar?

Seriously, you are giving us no information on the pump, rpm's, fluid, solids, anything... Throw us a bone please!

Give some info and you might get help.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[dimockmatt]

The pump motor is 1.5HP 1750rpm, it is on a VFD and maintains a level in our process vessel, typically the pump runs 1750rpm to fill (15-20 min)then runs at roughly 900rpm(8-20 hours). It is a Jabsco 30520. The customer is processing industrial wastewater normally coolants. There could be suspended solids but the pump is designed to handle applications like that. We have 20 mesh strainers after the pump and the customer claims to only find parts of the impeller in there. I will say the failures do not appear to be chemical in nature, both the pump mfg and our chem-e agree with that.

 

[Unotec]

What is the water temperature? What is the NPSHr and NPSHa?
Could it be cavitation?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[dimockmatt]

It is a self priming pump so there is no head requirement, the temp is 75°F. It has a 10ft dry lift and can run dry for up to 30sec, the line is set up to provide flooded suction. I do suspect cavitation, my guess is as the customer's feed tank fills with sediment it chokes the suction side of the pump and it starves for water, the site is in Brazil so I cannot confirm the piping to our system. Does anyone know a way of verifying cavitation? We asked for a 4-20mA pressure transmitter to monitor suction pressure but they have not installed one.

 

[Unotec]

With the little you have mentioned and not knowing your piping system I'll say that the problem is cavitation.
So as long as there is a guarantee of flooded suction, the problem lies on some flow obstruction I guess.
As a side comment, I once had a centrifugal pump impeller destroyed couple times due to cavitation (much larger that what you have). Took us about a week to find the issue. Bugs!!! Bugs falling into the tank were sucked into the lines and a particular type of beetle always plugged it. It had a pretty hard shell.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[wimple]

quote "We draw fluid from a roughly 10' diameter tank through a 1" line into the 3/4 port on the pump"
have you a filter upstream of the pump?

What is the type of impeller of the pump, is it a submersible waste water pump?

for waste water fluids, you have typically three or four type of impellers (closed with 3 or 4 vanes, open, open similar to screw for example).

depending on the content in fiber, sand, solid content etc you need to go with the right impeller type and metallurgy.

 

[MikeHalloran]

How about converting the suction pipe to a hose and a float to keep it out of the sediment?



Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

If the pump has a 10 foot dry lift, it is not a centrifugal. This is a flexible vane pump which is positive displacement. This is the configuration commonly used as the internal water pump in outboard boat motors. It is still possible to cavitate a pump of this type. But, I am more suspicious of high differential pressure. At high DP, the impeller will deform and allow the fluid to recirculate back within the pump. I have very few pumps of this type in my facility, so I don't know what sort of failures this might cause. There is some good information on the web site for the manufacturer. I suggest you go there.

Johnny Pellin

 

[dimockmatt]

It is a positive displacement pump with a nitrile flexible impeller. We have a shutdown in the program when pressure on the discharge reaches 20psi, the impeller is rated for 25psi. Ideally the water should be filtered before it reaches the feed tank. A filter in the line to our system could cause blockage. Again, does anyone know an alternate way to diagnose cavitation other than a suction pressure reading? I am looking for a way to convince our customer they are causing cavitation but they are in South America so we do not want to travel there just to point that out to them.

 

[hydtools]

Have them send you several failed impellers. Take pictures and post them here. Some folks here have enough experience that they can tell from the failure evidence what caused the failure. Take good closeup pictures so the failure detail can be seen. Cavitation failure looks different from heat failure, from erosion failure, etc. Pictures of the cavity the impeller runs in would be helpful, too.

Ted

 

[dimockmatt]

Here is a close up of the failed impeller, I do not have a picture of the pump in use.

 

[dimockmatt]

To answer Mike H, the solids are suspended in solution so a hose would not change the outcome.

 

[JJPellin]

I do not believe caviation is the problem. This material should not be sensitive to damage from cavitation. I would expect cavitation damage to eat into the end plates on either end of the impeller. From the picture, it looks like one vane is broken up and gone while the one next to it is intact and in pretty good condition. I suspect material incompatibility. Nitrile is going to be sensitive to Benzene, caustic or amine. Viton would be better for low concentrations of Benzene, but stilt can’t handle caustic or amine. Kalrez or something comparable would be good for all of these contaminants. I don’t know what other materials are offered. If you have a new impeller and a failed impeller to compare, I would measure web thickness to check for signs of swelling. Then I would cut out samples of new and used material to test for degradation.

This impeller would probably not respond well if it was spun backwards. Is there any chance that this could be happening?


Johnny Pellin

 

[dimockmatt]

Our chem-e has tested samples and feels that nitrile is compatible with the fluid, viton would be more inert but the mfg tells me it would fail more quickly due to pressure or cavitation. There is always the possibility that they have introduced something harmful since the sample was taken so I will try to get a new one. The pump is direct driven by a VFD driven motor so the direction should not change, do you think there is another way the flow could be reversed?

 

[hydtools]

Some blisters and shredding on the bulb section look like heat damage. Run dry for too long a time.

Ted

 

[Artisi]

A pressure reading at the inlet would be a good start - I realise this has been requested and I would push for it, you need to eliminate each problem step by step.


I would agree with Johnny Pellin - any chance of the pump running backwards at any stage and rubber is very tolerant to cavitation damage?