equalizing two storage tanks

[Custom2014]

Hi,

I have a 135' dia steel water tank that is inside a concrete basin. If something happened and somebody let the sludge lines open in the 135' tank, and that tank started to drain, I need to be able to take water from the outside tank to the inside tank. I need to be able to keep the two tank equalized. My inner tank will withstand up the 3.81" of water column. Does anybody have any ideas on how to make this possible.


thanks

danny

 

[rb1957]

i think it's a tank like a swimming pool ... open at the top.

Quando Omni Flunkus Moritati

 

[Custom2014]

yes it is

 

[JStephen]

I have seen flow data for Waterman flap valves, but it is based on discharging into atmosphere. Normally, a flap valve is used in applications where it has minimal effect on flow, and so flow data is not that readily available.

For emergency vents on petroleum tanks, they use a large opening with a lightweight cover that can lift under pressure, but restrained by long bolts. For that kind of a situation, you are allowed to calculate theoretical flow through the orifice and apply a coefficient of 0.5; that is you treat it like just a hole and then design for the half the flow capacity that gives you. That would seem to be a reasonable way to design with the flap valves. The differential pressure has to be enough to open the valve, of course. Use Bernoulli's equation between the two tanks, account for minor losses due to re-entrant flow and abrupt exit, etc.

Also keep in mind that flap valves are often used in applications where there can be no backflow anyway. It's possible they won't be overly effective at sealing in the reverse direction if that's an issue.

For external pressure on a steel cylinder, there is some variation in the factors of safety that are used, and you may be able to juggle your numbers some due to that.

 

[paddingtongreen]

I don't know what structural engineers are supposed to work out on fluid flow through a valve.

The problem is the lack of head. At maximum we have 3.81 inches of water, about 19 lbs/sqft! at the bottom of the pressure triangle, average pressure of about 9 lbs/sqft. That will not move a lot of water very quickly.

I have a difficult time with the idea that more than 4" differential will bring a tank down. If it were there for the whole height, maybe, but in with the limited height, the load will spread over a considerable distance above the top of the fluid.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[JStephen]

The differential between inner and outer pressures will be applied through the whole depth of the lower fluid, not just at the top. The design wind pressure for tanks is typically around 18 psf, although it doesn't control the design that often. But that pressure rating is a reasonable figure.

 

[LittleInch]

A simple cross section sketch with heights, normal liquid levels etc would help, but from what I've read so far you have a thin inner metal tank with normal flow from inner tank to outer "bund". You then have an upset condition where levels in the inner tank could be less than the outer tank and you are worried about collapse of the steel tank.

What is the normal liquid height difference between inner and outer?

What size pipe / valve does it normally flow through?
what is the normal liquid flow rate out from the inner tank?
What is the flow rate out from the sludge lines?

3.81" looks suspiciously like 10mbar to me. As said earlier - this is a very low differential pressure, both for tank collapse or to make it flow. Who gave you this limit?

So IMO, you can either
a) Strengthen your tank to resist greater differential pressure
b) Do some more structural calcs to see if this differential (3.81") acting on only part of the tank (we have no info on height of the relevant liquid levels) actually causes collapse or not
c) Make about 50% of the tank area at the base slots to allow flow in (again we have no idea about flow rate going out through the "sludge lines")

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ztengguy]

Structurally, Where do you need help in this "Structural engineering other technical topics" Unless I missed something, isnt this a mechanical engineering problem?

 

[Compositepro]

This thread makes no sense. 3.81" wc (inches water column) sounds like it could be a vacuum limitation on a closed tank of that size, but this is an open tank. That much water standing outside of the tank could not possibly damage it. 3.81 feet might be a problem. The simplest solutions to that issue would be to place braces inside the tank to support the walls, like an inside-out above ground pool.

 

[paddingtongreen]

The solution might be to place siphons over the inner wall so that the ends are near the bottom of the tanks. They would have to be primed. Don't know how to size them though.

JStephen, I wasn't thinking straight.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[dhengr]

Paddington:
I’m glad you finally mentioned siphons, or they got mentioned again. Take a look back at my 4FEB14, 15:26 post. The OP’er. was too busy tweeting, at that point in time, to actually read his own thread very carefully. I thought it would at least generate a comment or some discussion, but I guess not, until now, maybe. I figured, screw it, if they are too busy playing with themselves, and can’t take the time to read more than two sentences for content, maybe they don’t need that idea.

 

[JStephen]

I don't see how putting primed siphons over the wall would be any different from just putting an open hole in the way, free flow either way?

 

[rb1957]

that's what i was thinking about ... fill both tanks at the same time, have a direct fliud path between them ... poses the question , why two tanks ?

if you knew a max height for the inner tank you could provide a fluid path a little below this. But it sounds more like a relative height difference? that's why i thought of a float valve ... floats on both tanks, open a valve if difference greater than X(<3.8)

Quando Omni Flunkus Moritati

 

[ameyerrenke]

What is the specific gravity of the sludge?