Increasing pump life; Reducing Wear 3

[kapo84]

I am an intern and currently trying to determine a way to decrease the number of rebuilds we have to perform on our pumps annually. I work at a polymetallic mine. I haven't had much experience with pumps before but have learned a lot since I've been here. I have done a lot of calculations on our pump system and have come to a wall. We pump a high density slurry through 4 Warman 6/4 DAH pumps. We get anywhere 100-500 hrs of use out of them before we have to replace the liners/throat bushing/impeller. There are a number of variables that affect the system (tons/hour, particle size, etc...) which can complicate things but I know there is away to fix the problem. We put a man on the moon right?
I'll start by asking what main factors contribute to pump wear? Do I simply look at the characteristics of the slurry coming in or do I focus on the manner in which the pumps are being operated?
Any feedback will be helpful

Thanks in advance

 

[kapo84]

Thanks again for all of your posts. We are currently making a few minor adjustments to our setup. We have installed a magnet on our Ball mill which should decrease the amount of damaging debris we have flowing through the system. We have also installed metal liners on our pumps that handle the larger particles. Other ideas we are considering range from changing to a larger impeller (16 inch instead of a 14.4 in), or going to a reduced eye impeller (to help decrease recirculation on our pumps that have lower flow), and/or changing the piping in our system.
We have a the pump reps out here lately and they are plugging numbers into their programs to try and help out. We'll see what they come up with.

I have posted a few more pictures of the most recent pump failure. It ran 684 hours (which is a little better than its average). It pumps the smaller sized particles @ ~850 GPM which is closer to its BEP than the other pumps we have. If you have a chance can anyone tell me what they think about the wear marks? It looks like recirc damage and a bit of cavitation. The mechanic decided to change out the liners when he saw slurry pouring out through those holes in the throatbushing. Also by the volute there seems to be a lot of wear, almost like the slurry is missing the exit point by a few inches. The impeller seems to have fairly normal wear.

Link:

http://kapo84.myphotoalbum.com/

Thanks again for all of your comments.

~Kapo

 

[Artisi]

The impeller diameter change looks to be a wise move- it will reduce the pump speed a little which in turn should reduces the wear rate. It will beinteresting to see what comes from the change to metal liners - are you going to use hi-chrome iron?

 

[kapo84]

We are currently using Hi-Chrome Iron. Now as far as a larger impeller is concerned, will I simply just be increasing the pressure in side the casing? We obviously hope to decrease wear by slowing down the pump seed but one of my mechanics suggested that by going to a larger impeller we will significantly increase the pressure in the housing. Is it a choice of simply picking your poison?

Thanks Again

~Kapo

 

[kapo84]

I believe we are using the Hi-Chrome Iron. Do you suppose by increasing impeller diameter will not only slow the pump down but increase pressure inside the casing thus increasing wear on the liners? Is a matter of picking your poison? Also, has anyone had experience with GIW(Georgia Iron Works) pumps?

Thanks again

~Kapo

 

[kapo84]

Sorry for the duplicate...

 

[Artisi]

If you fit a larger diameter impeller and you wish to maintain the same flow and discharge head - you will need to reduce the pump speed.

For impeller diameter increase

flow will increase at the ratio of the diameter change
head will increase at the ration of the diameter change (squared) and power at the cube of the diameter change

For speed reduction
flow will decrease at the ration of the speed change, head at the square of the speed change and power at the cube of the change.

So it follows, increase the diameter and reduce the speed to give the pump duty which in turn reduces the wear rate rate

 

[HEC]

I have to agree with Artsi and Pumpguykc in dealing with slurries speed is the key to wear rates, reduce the speed and life will go up. Generally this means pumps need to be sized 1 - 2 frame sizes larger, leads to increased project/capital/initial installation cost. Due to this the initial installation will be a smaller sized pump running at a higher speed. As polymet slurries can be very different depending upon the location, rule of thumb is used for the initial size. Actual running then provides the real situation. Feed this information back to the design and project team when the correct specific velocities for the slurry have been found.
What is the wear life in your pipelines like?
Regards

Mark Hutton

 

[kapo84]

Thanks again for the responses.

I am thinking on paper here.... We have the pumps running off of VFDs. The VFDs are controlled by the mill box levels and the desired flow rate of the system. By increasing the impeller diameter we will be able to pump the same amount of slurry at a lower speed, thus reducing wear on our pump. This should especially be beneficial for those pumps that handle a large particulate.

We haven't had problems with wear on our pipelines. We are using a goodall hose because of how well it handles abrasion.

 

[kapo84]

Just as a little FYI; Our steel liners gave us 866 hours of life. A little over double our average at a little under double the cost. Not exactly what we were hoping for. I have been in contact with a pump manufacture and the larger impeller(25") seems to be the next step.

 

[kapo84]

One other question:
I am trying to determine which pump would run better in our system. The first is a 4X6 which has a peak BEP @ 70%. Due to our low flow we would only be able to run the pump at around 45%. The advantage to this pump is it is the same model as others we have so therefore we wouldn't have to worry about stocking unique parts just for this pump.
Option #2 is a smaller 3X2 (21" impeller) pump which has a peak BEP @ 45%. We would run it in the 35-38% range.
Which is better?
I know it is always best to run a pump as close as possible to its BEP but will it more advantageous for us to stick with a more uniform pump setup?

Thanks again.

 

[Artisi]

What are the relative operating speeds of the 2 units.
There is a lot to be said for standardisation.
The 6/4 is more efficient at 45% than the 3x2 which is 35/38%.
In the end it comes down to an engineering decision based on your maintenace philosophy and the running costs.

 

[eminentfoundrypro]

Very interesting answers you got here, the bushings that you currently use are they carbide and if so what kind, also high chrome ( Ni-Hard ) cast irons are only as good as the foundry producing them - w. other words the size of the microstructure components - if it is oversized ( poor foundry practice ) the wear resistance is also low...I would suggest using dissimilar materials for the impeller/diffuser assembly if any and/or casing if you are using open face impellers...Hard facing is an option of course however is a costly option, I do agree w. one of the responders that rubberized coatings will probably fail in this environment - I would suggest focusing towards picking up a proper alloy for the cast components instead, maybe one with VA heat treating capabilities - another important factor is the corrossion component of the slurry, what exactly is the corrossive agent and in what concentration !!??

 

[kapo84]

In order to determine what effect this impeller diameter change(and motor change) will have on our electricity costs per year I have done the following calculations:
(Do these numbers look right according to the affinity laws?)
New Impeller Diameter: 25"
Old Impeller Diameter: 14.4"
New Motor HP,KW: 75,(56.25)=60(?)
Old Motor HP,KW: 100, 75
Old Motor RPM: 1125
New Motor RPM75*1125/100)=487.7
Old VFD Kw draw: 57-60 kw/h

Impeller Ratio= 25/14.4= 1.736
New Flow= 937 gpm*1.736= 1626.6 gpm

Since we want to maintain the same flow as we do now we can multiply results by .576 as a correction factor. (1626.6*.576=937 gpm)

So, in order to get the new Kw/h value we...
(57-60 kw/h)*(.576^3,correction factor)= 10.89 kw/h.

By increasing impeller diameter and decreasing motor size (as recommended by manufacture) we will save 30-50 kw/h.

Can you verify this?

Thanks again!!

Kapo

PS: these are the formulas I used:

http://www.mcnallyinstitute.com/02-html/2-01.html

 

[Artisi]

kapo84
Can you post a copy of the pump curve at the link you used previously (

http://kapo84.myphotoalbum.com/)

and advise exactly what the pump duty needs to be and the existing and suggested impeller diameters. We can then calculate the new pump speed etc. for you.