Mulm Suspension Speeds

[zapatero]

Hello Folks, I'm new here, first post...

For perspective, I am trying to design a combination tropical water garden indoor/outdoor pond/aquarium.

That sounds pretty daunting by itself, but please don't let that derail the discussion -- my present question, (I think, hope) is pretty focused:

Imagine a gravity-fed bottom drain from a tank, that is connected to a "J" shaped pipe. The top of the J will be at the tank's water level, and will in fact control the tank's water level. (Water will flow into the tank from an input pipe, and the J pipe will be the outflow.)

I realize that there are many factors involved, (including pipe diameter and height), but my question is: Is there any general rule-of-thumb for flow velocities (i.e., in the J pipe) that will keep tank mulm sufficiently suspended such that it will be expelled from the top of the J pipe, and not simply settle IN the pipe? (Please ignore the complications of leaves, sticks, plant matter, string algae, etc. -- I am only talking about fish feces and similarly sized particles.)

Thank you very much,
~~~zapatero

 

[zapatero]

I guess that I haven't stirred up very much interest with my question...

But I have attempted to create a drawing to show what it is that I'm talking about. Please bear with me though, because I have never attempted to send an attachment before, so I don't know how, or if, it will come through. (Maybe someone can tell me -- because I assume that I'm not permitted to look at it yet since I have made less than five posts.)

Thank you very much,
~~~z

 

[Alien]

In my 2000 l tank I have an overflow like this, which is directly linkt to the sewers. I cannot even guess the flow velocity, but the diameter of the tube is about cm (little bit more than 2 inches). If I simply take out the tube, leaving only 5cm of tube on the bottom, the tank is empty within about 3 minutes. I only did this once after building and first time filling (waterthight check).

Sorry for my bad spelling and missing of some special vocabulary, but english is not my native language.

For exact calculations, You have to contact a specialist.

Greatings
Thomas

 

[zapatero]

Hello Thomas,

Thank you for your reply! And your English is just fine!

The data that you have given me is really for a different problem than I am trying to solve -- since flow rate is determined by the pressure head, which in your case would be the depth of your tank; but for me the pressure head will only be a few centimeters at most. But you probably already know that, and have given me the data that you have available.

Anyway, what you have given me might offer us an interesting calculation:

I would calculate the cross-sectional area of your pipe, (if 2.25 inches) as: Pi*(2.25/2)^2 = 3.98 sq.in. = .02761 sq.ft.

And the average flow rate from your tank as: 2000 liters * .03531 cu.ft. per liter / 3 minutes = 23.54 cu.ft./min

Then, 23.54 cu.ft./min / .02761 sq.ft. = 852.6 ft/min = 14.21 ft/sec, average velocity, which is roughly three times as fast as a person walks. That seems pretty reasonable to me. Obviously it would be higher when your tank is full, and lower as it nears empty.

But for darn sure those speeds would keep mulm suspended!! However, what I really need now is someone to tell me would be the lowest speed necessary to do so...

Anyone...(?)

Best regards,
~~~z