Tank filling-bottom or top-rotary pump 1

[cement001]

From the following website,

http://www.mcnallyinstitute.com/12-html/12-03.html

"If I wanted to fill a tank with a centrifugal pump I would fill the tank from the top because that is the only way I could keep a constant head on the system and keep the pump close to its best efficiency point. If I were using a rotary pump, I would fill the tank from the bottom because the pump would be using less power during the filling process (power is foot pounds or head times capacity)"

Question: Is above statement about rotary pumps is correct? If yes, could someone care to explain.

Thanks,

 

[micalbrch]

I think what the author wanted to say is that the pressure constantly increases with increasing fluid level in the tank when you fill the tank from the bottom . So the power consumption of the rotary pump will be less when the tank is empty and will be higher when the tank is nearly filled because the pressure increases during the filling process and so does the power requirement. That will be of course the same for a centrifugal pump but as it's curve depends upon the head it is more efficient to operate it with constant head.

 

[cement001]

Thanks micalbrch. What if we keep constant head for rotary pump ( top filling) wouldn't it take less power throughout the filling process?

 

[micalbrch]

Only if it's efficiency is better.

 

[LittleInch]

This also depends on how much the head increase from empty to full is compared to the friction losses in the pipe work, e.g. if the tank is 4 m from empty to full, but the friction lossess inthe pipework at the filling rate was 16m, then the difference is relatively small and a centrifugal pump would be within an efficient operating range regardless of tank level. A friction loss of 4m would reslt in an inefficient range from a centrifugal so as ever, it all depends on the system design and you shouldn't take a general guidleline like that to be an exact instruction.

It also ignores any beneficial impact from a dip pipe in a syphon effect to avoid spraying product into a tank from a high point inlet.

Also if the flow rate was variable, then the centrifugal pump would probably fill it faster as it would flow more product at the start than the finish whereas a rotary pump would be more or less the same flow regardless of tank level.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mec5216]

micalbrch's absolutely right. Theoretically, if you choose to fill the tank by the its bottom, it will consume less power than if it was filled by the top.
As we know, the operation point is at the intersection of the pump curve with the system curve. Varying the level of the tank might or might not change substantially the system curve. If the operation point with the tank completely filled is very close to the operation point when the tank is completely empty, then there isnt much difference filling it by the top or by the bottom. But it will depend on both system curve and the pump curve. You will have to draw these curves to get to some conclusion.
When we talk about reciprocating pumps, all of that doesnt really matters. So I would recommend you to fill the tank by the bottom, so it would spend less.

cheers