Tank recirculation question 1

[RUFUS2K]

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.

 

[BobPE]

LOL Antiice:

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

 

[TD2K]

Bob's right. Typically, even if the recirc lines are sized for maximum erosional velocity, the pressure drop is negligible because the runs are 'typically' short and the pump is going to be running near the end of its curve.

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).

 

[RUFUS2K]

Thanks everyone. That is what I had thought initially. Bob(or anyone really), can you elaborate on your comment "The pump then operates off its curve and ends up breaking"? This pump ran for about 30 minutes and then the electric motor overheated and shut down. If the application itself is not the problem, then I have to look elsewhere. The pump is an Oberdorfer 750B (1.5 x 1.5; 80 GPM @ 18 PSI, water) coupled to a Leeson 2HP single phase 115V motor. The motor that comes standard with this pump is 1.5HP. The system will run all day with the discharge valve half open, but when the valve is 100% open, the pump will eventually bog down and begin to surge repeatedly until the motor overheats or the circuit breaker trips.

 

[BobPE]

ANTIICE:

I guess i have an engineering fetish for pumps!!! LOL I figure that was the case with your pump simply because of the questions you were asking and some of the answers people gave to you in supprot of their design. !* psig is too much of a DESIGN discarge head, so you pupm is seeking a point other that that point that the designer calculated. This is causing the pump to run out to the right of its curve (too much flow becuase the disign head is not there). You motor is overheating because the dsigner did not design the motor to be non overloading over the entire pump curve. Instead he disigned the motor HP for his calculated pump operating point, which is wrong.

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

 

[saxon]

Antice, A simple and quick solution to your motor overload problem would be to install a restriction orifice in the pump discharge. This forces the pump back onto its curve and appropriate operating range and hence eliminating the motor over load and eliminating the use of the equipment block valve as a throttling flow control valve (eventually you will have problems with this valve because its not designed for throttle service). One note though, this isn't the most energy efficient method.

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

 

[BobPE]

Saxon:

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

 

[bhwtam]

I am a bit confused with this one:

"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.

 

[RUFUS2K]

bhwtam,

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.

 

[saxon]

Bob, If tank turn-over time is critical, then I agree, another approach would be needed. But, from what I've seen in industry it doesn't matter if the turn-over occurs in 1 hr., 2 hr. or 4 hr.. The issue is just getting the contents turned over on a regular basis.

saxon

 

[RUFUS2K]

I think that the solution that I am going to have to use is to place a restriction in the dicharge line. Each part of this system was built to the customers specification (suction and discharge line sizes, motor size, pump specs, etc). The customer wanted a pump rated at 80GPM @ 10ft of head. Upon further review, the pump is rated at 80GPM @ 44ft. This is what I get for not double checking the pump supplier's work. Since I can still get the desired flow rate from this pump by restricting the discharge, I think that will have to suffice.

 

[BobPE]

ANTIICE: The pump supplier gives you what you ask for......

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

 

[RUFUS2K]

BobPE:

????

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.

 

[BobPE]

ANTIICE:

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

 

[RUFUS2K]

BobPE:

Well said on all points.

Thanks again for your help.

 

[quark]

Antiice!

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

 

[mixsist]

Antiice,
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.