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Variable Flow Pump Question...

Variable Flow Pump Question...

Variable Flow Pump Question...

(OP)
As I slowly get back into the engineering field after having been away from it for about 6 yrs or so, I've found a problem that while seems simple enough, the solution simply escapes me (too much rust built up on the brain I think).

I have a pump that will be used to pump relatively small amounts of iron ore slurry into a dewatering cyclone.  The pump will be used under two drastically differant flow rates however.  During one stage the pump will be flowing 419GPM of slurry at 25% concentration up 59 ft and horizontally about 100ft to a dewatering cyclone requiring 200 ft head....while that's basic enough, it will also be used to pump 210 GPM of slurry at 25% concentration to the same cyclone.  I am using a 3" pipe for the 419GPM flow, my concern is that if I use the same pipe for the lower flow, I will sand out the pipe due to the lower flow rate.  Short of running a smaller diameter pipe along side the larger pipe and using as valve to control which pipeline to use, is there a simpler solution, or would you recommend just slowing down the pump to match the lower flow rate and use the same system?

Thanks for your help and expertise.
David  

RE: Variable Flow Pump Question...

Hello, David
I think that there must be some limits for slurries velocity in pipes, I mean, on one side, producing erosion in elbows or fitting as well as a very high frictional loss  or, on the other,  leading to the build up of scale in the pipe.
These limits are hard to establish as there are many variables involved in the game, perhaps some practical rules of thumb exist for your process. In view of these boundaries, you could define if one or two pipes are required.
Regards

RE: Variable Flow Pump Question...

The required velocity in the piping to maintain the slurry is dependent on the particle size and weight.  It is a balancing of drag (size) and gravitational (weight) forces.  The vertical piping is where you will have the largest problem.  That type of turn down (almost two to one) will be difficult to handle in a single pipeline.  Your problem appears not to be so much the pump but how to properly size the piping.  Indeed, too high a velosity could result in erosion problems and not enough and the product will not move along the line.  I am not sure what can be accomplished by slowing the pump down.  Please elaborate on what you meant.  

Your first approach is to decide the minimum velocity needed to keep the solution flowing and then the maximum velocity to prevent erosion.  Once you establish those two bounds, size the piping accordingly for the maximum and minimum flow conditions, arrive at the pressure drop requirements and size the pump.  It may be that it can not be accomplished with a single pipe.        

RE: Variable Flow Pump Question...

My rule of thumb is to keep the velocity between 0.8 and 1.2 metres per second, I calculate yours at 0.82 m/s so should be ok. Of course the S.G. will affect this but without knowing exactly you may have to 'suck it and see' :)
Good luck

RE: Variable Flow Pump Question...

My suggestion will be put two small pipelines instead of one large and one small. When you run at 210 gpm, use one of the pipeline, if 419 gpm then use both. You can also connect flushing water to individual pipelines and flush any deposit out when any of them is idled.

RE: Variable Flow Pump Question...

(OP)
A quick thank-you for all your insight and suggestions, sorry it took so long to send this back.  I ended up using basically the same idea 'bhwtam' had suggested with a control valve to open and close the second pipeline.  

Thanks again for your suggestions and insight
David

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