Tank Size Calculations

[Matador]

I need to design a tank for ~ 3000 lpm (792 usgpm)outflow capacity.

The pumps will remove water continiously and makeup water will be added to the tanks as required to maintain level.

Does anyone know how big the tank should be so I have sufficient retention time. Where to find the info would also be great.


Thanks,

 

[Drazen]

Let us clarify:
You design outfeed pumps for capacity you want, not a tank.
It means you must select pumps according to outflow.

For tank sizing you need to know: how much accumulation (expressed in time units).

Why you need that accumulation if you have continious outflow? There could be three reasons:
1. startup reasons - to avoid waiting until tank is filled to at least minimal level which will allow outfeed pumps to start.
2. safety reason - to enable that system will work for some time even in the case of infeed system filure
3. energy saving or supply problems (you want to fill the tank when electricity cost is lower - overnight or similar - or supply is limited to certain periods of day: in this case you will need really big tank)

Calculation could be:
Size = (outfeed flow rate - infeed flow rate) x acumulation time reserve needed + (calculated ammount necessary to have minimal level in tank to avoid dry running of outfeed pump)

If your infeed flow is bigger than outfeed (that can be designer's decision in case No. 3), than you can combine how long you want to switch off infeed pump (in regular intervals) - that will be accumulation time needed and you simply use formula

Size = oufeed flow x accumulation time needed.

But I presume you will need to use first mentioned little formula.

If you have some special demands, please tell me to discuss it further.

Regards

 

[Matador]

In my case I want to clean debris from pulp by pumping it through centricleaners (long cylinderical cones). The pulp enters tangentially which forces the debris & dirt to the sides & out the bottom while the lighter material gets accepted out the top.

Anyway, I will purchase a pump with a 3000 lpm capacity which will draw the water it needs from the new tank. The pulp stock enters the pump suction between the supply tank and the pump so no agitation is required. This is standard practise in the industry.

The tank in question will remain a constant level because the makeup water flow will be controlled from the DCS.

My concern, is how big doea the tank have to be for level control to be effective. I've heard of numbers like 1 minute retention time for good process control but I can't any reference material to confirm that number.

Obviously, I would like the tank to have as small a footprint as possible. The floor is designed to accept 400#/ft which equates to 6 ft of liquid. Therefore I would design for a normal operating level of 4-5 feet.

To conclude, the pump will draw about 792 usgpm (3,000 lpm) from a tank which has adequate make up water supply. There will be a DP cell installed to control the level via the DCS. The question becomes how much retention time do I need? That will determine the footprint.

That's how I intended to proceed.

Your assistance is appreciated.

 

[Drazen]

I would like to help you, but still don't understand completely. Until I don't know the core of this I feel helpless
1. You clearly explained the purpose of the water flow but I still don't see the reason for supply tank. Can you relate it to alternatives I mentioned in previous message?
Why you need retention if you have constant need for 3000 lpm?
2. You use ejection mixing nozzle for conveying pulp by water as I can understand. That even enhances idea of constant flow needed. Whatever level of pulp stock you have I see no need to change water flow (pulp is really not my specialty) in normal situations.
3. Why do you need to control water level in supply tank?? Of course, this is related to previous questions. If you have that need for any reason, you should have idea of acceptable level variation (there must be some range for any control system!). Knowing that, you're coming to that simple formulas again. (I should be marketing agent for simple formulas!) You must know infeed flow to calculate needed retention (although I don't know what is DCS). Let's go on!