How fast can I empty a 5500 gallon tank into another tank?

Question

I need to empty a 5500 gallon tank into another tank that sits 20 feet higher if I use a 6" hose and the pump is capable of 517cfm. The media is septage.

How fast can this be done? And what are your calculations for figuring this out.

Answer 1

5500 gal is 735 cubic ft

so 735 cubic ft / 517cfm = 1.4 minutes

edit: so the 517 cfm number is meaningless?

Answer 2

Is this an academic or a practical question?  The reason I ask is that it will take, in theory, 1.4 minutes, but will likely be a bit longer than 1.4 minutes in reality.  Figuring out the exact time is a little trivial...

Friction won't be an issue over 6ft of pipe, unless your pipe has an extremely small diameter, which I doubt it does.  In pipe flow, we consider a "friction factor," such as the Darcy-Weisbach friction factor.  As long as your pump head is greater than the change in total head from the originating tank to the new tank, then you will be OK.  For all practical purposes, this change in head is 20ft.  It's actually a bit more than 20ft, but negligible.

Check the specifications of your pump to see what the pump head is.  If it's significantly more than 20ft, then you won't have an issue.

I hope this makes it a little bit more clear...let me know if you have more questions.

EDIT: Answer Dude brought up a good point about pressure within the pump.  What he is referring to is called cavitation...depending on the type of pump, cavitation can cause serious damage to the pump, as it's essentially like little explosions of vapor within the pump.

Answer 3

One has to wonder where you got the 517cfm figure,  from your additional information, it sounds like that may be the flow at zero or some relatively low value of head. The amount that the flow drops with increase in head varies with pump design. Usually the pump manufacturer will have a chart or graph of pump performance, it is easier to read a chart that to calculate the pump performance, especially without details of the pump design..

You know you have 20 feet of gravity head, and "Answer Dude" has given you a value for friction loss,  though it sounds a little low to me, I wouldn't be surprised if you'd have 20ft head loss in 20ft of hose  the velocity will be near 2600fpm at 517cfm in a 6in hose, which seems fairly high to me. (note you have not said how long the hose is, it must be at least 20 feet, but it will probably be longer)

If you have a chart for the pump performance, find the flow for, say, 50ft or so of head and then use the calculation Bill used. That should get you a ball park figure for the time.

More information about the viscosity of the liquid and the nature of the hose would be needed for more precise results.

Of course, this may require some iteration, since the flow will probably be less that the 517cfm you started with, the friction loss will be less than that estimated for 517cfm, and with a lower friction loss, you can go back to the chart for the flow rate, but an approximation is probably the best you are going to get with out actual experimentation.

Someone else has mentioned cavitation, if you have much hose on the inlet side of the pump you are likely to have problems.

Answer 4

517cfm is approximately 3870gpm

A 6in hose will have a pressure drop in excess of 10 psi per 100 ft, or >23 ft of H2O. The discharge pressure of the pump will have to be > 43 ft of H2O.

An 8in hose will still have a high pressure drop approaching 8 psi per 100 ft

A 10in is a better fit with a pressure drop of about 3psi/100 ft

Now pressure drop is proportional to length. Keep in mind that length is equivalent length, taking into account fittings (elbows, tees) and valves.

My reference is the Crane Technical Paper No. 410.

With a high enough pump discharge pressure you can get the liquid through the hose.

As the other responders have said... it will only take a few minutes to empty the tank, even if you cannot obtain the full 3800 gpm.

One thing you will need to make sure of is that the tank you are pumping out of is adequately vented to avoid pulling vacuum on the tank. Also need to make sure that the tank you are pumping into is capable of venting the displaced gas fast enough to avoid building up pressure above the pressure rating of the tank.

From an industrial perspective, I do not recommend pumping out the tank at this rate without doing an entire system design review.

Answer 5

Why don't you just do it already?  What's the difference if it is 1.5 or 2 minutes?