Tank recirculation question
Tank recirculation question
(OP)
This is probably a very simple problem which I have made difficult by mulling it over in my head too many times.
Here is the situation: I have a 5000 gallon "silo" type storage tank which contains 12lb/gal calcium chloride solution. It is necessary, from time to time, to recirculate this product to keep the disolved solids in suspension. We have always done this by drawing liquid out from the bottom and then pumping it back into the tank via a fitting located 90 degrees to the suction fitting. The pump is a centrifugal type.
So here's my question: From the pumps point of view, is this a difficult operation? I have been told that yes, it is difficult because the pump is working against the total head pressure of the tank, and I have also been told that since the same head pressure is on the suction side as well, that the two sort of cancel each other. As I said before, I have run myself in circles on this one until I can't think about it anymore. I could use some help.
Here is the situation: I have a 5000 gallon "silo" type storage tank which contains 12lb/gal calcium chloride solution. It is necessary, from time to time, to recirculate this product to keep the disolved solids in suspension. We have always done this by drawing liquid out from the bottom and then pumping it back into the tank via a fitting located 90 degrees to the suction fitting. The pump is a centrifugal type.
So here's my question: From the pumps point of view, is this a difficult operation? I have been told that yes, it is difficult because the pump is working against the total head pressure of the tank, and I have also been told that since the same head pressure is on the suction side as well, that the two sort of cancel each other. As I said before, I have run myself in circles on this one until I can't think about it anymore. I could use some help.





RE: Tank recirculation question
That's a good question and it does make a lot of people think!!!! No its not a tough job for that pump. The pump only has to overcome friction loss of the suction and discharge pipe strings. Most of the time, I have seen people get confused and size the pump thinking it was a big job by mistake. The pump then operates off its curve and ends up breaking.
BobPE
RE: Tank recirculation question
The height of the liquid doesn't make a difference between a higher outlet pressure also increases the suction pressure and the pump only cares about the 'differential' pressure (for a centrifugal).
At the risk of being blasphemous, "what the outlet height taken away, the outlet height giveth back" (on the suction side).
RE: Tank recirculation question
RE: Tank recirculation question
I guess i have an engineering fetish for pumps!!!
Get someone else to look at your system, I hate to sound like a poster child for engineers, but make sure that person knows pumps and is a registered professional engineer in your state. If you do get a PE, then if they design the system wrong you have some recourse to be compensated for his error.
BobPE
RE: Tank recirculation question
Another approach would be to utilize the pump affinity laws to vary the flow and head of the pump and reducing the H.P. requirments that are causing the overload and subsequeent shut down. This would entail the changing of motor rpm or changing the impeller diameter or both.
Good Luck!
saxon
RE: Tank recirculation question
I agree with your approaches but I was taking that the flow in ANTIICE system is the critical factor since he is mixing a tank and adding chemicals. I think he sould probably consider a new pump, it really appears to be that far off in from the original calculations.
I just wanted to clarify my last post in that you could probably fix this pump, but at what cost to the system.
BobPE
RE: Tank recirculation question
"We have always done this by drawing liquid out from the bottom and then pumping it back into the tank via a fitting located 90 degrees to the suction fitting."
Do you mean the pump discharge is connected back to the pump suction line? If yes, then surely you have a problem.
RE: Tank recirculation question
No, the pump discharge is connected to the tank. To visualize, viewing from the top of the tank, there is a fitting at 12 o-clock and 9 o-clock.
RE: Tank recirculation question
saxon
RE: Tank recirculation question
RE: Tank recirculation question
I don't mind answering questions here, but I prefer them to be from engineers like thr forum rules outline. I was not lead to believe you were an equipment supplier from your original post. I would think the correct thing to do is to give the customer what they wanted in the first place, that is what an engineer would do and this is why I am so concerned.
BobPE
RE: Tank recirculation question
????
I'm a little confused now. I guess I didn't see in the rules where it says that an Engineer employed by an OEM can't post a question.
This forum is a valuable resource and you have been a great help in identifying the problem that I was experiencing.
I would like to continue to use this forum as a resource. However if you can direct me to the rules that say that I should not do so...then I won't.
Thank you for your help.
RE: Tank recirculation question
Sorry for rambling, I guess the problem I have is that I didn't like your answer. If someone asked for something, then as an engineer we sould give it to them. If we make a mistake we should clearly point it out to them and recommend fixes with advantages and drawbacks.
As of your last post, you clarified the fact that you are an engineer. As engineers that is what we do. I also realize that you are an OEM guy and the stresses in that industry are great since it's primarily sales based.
Being an engineer then I don't want you to shy away from the forum by any means, just asking you to think how an engineer would think with no influence from their prespective workplace.
BobPE
RE: Tank recirculation question
Well said on all points.
Thanks again for your help.
RE: Tank recirculation question
For your sort of problem I don't advice any engineering superhuman (others say a consultant) to attend your problem or some hightech gadgets to be placed in the system.
As already Bob pointed out the height of water (or solution) in the tank will help your pump. Now as your pump is designed considering total height of the tank as static head (and not in actual case) pump gives out more discharge. (i.e it operates to the right of the design point thus drawing more power)
Always control the discharge valve (there is a possible chance of separation of particles here) or else you can change the impeller (lower dia.) to fit the system.
Regards,
Repetition is the foundation of technology
RE: Tank recirculation question
If I was projecting this system, I would carefully think about adopting an agitator, or mixing system to the tank. Instead of forcing the pump recycling the solution you can avoid the separation of the solids and it is possible to keep the solution in permanent homogeny through a mixer causing fluid motion, working a liquid-solid type of mixing.