overflow from chamber to tank

[Andrew88]

Hello,

I have a task where the chamber and tank are connected together by a pipe that conveys fluid based on gravity principle. The pipe inlet will be open and in a form of a weir. The outlet into the tank submerged.

What is the maximum liquid level I can reach in that tank so that the inlet is not submerged (top of pipe)? Not sure how to crack on with headloss calcs if only one end is submerged.

If I were to treat this as an open channel flow then the gradient of about 0.8% for this pipe size would give approximately 100 l/s which I want to achieve but this approach must be incorrect as the end is submerged.

Would it be conservative if I were to assume that I have a pressurized system and calculate level from this approach? I need to be sure that the water level wouldn't rise above crown of inlet pipe while keeping the water level in the tank as high as possible (with given pipeline parameters.

Thanks for tips in advance.

Jed

 

[LittleInch]

You're over thinking this.

Max liquid level in the tank so that the inlet is not submerged is the level of the inlet. If your sketch is accurate in terms of level the tank would overflow first

Your worst case head loss scenario is an open channel flow along your pipe. How long your pipe is before you reach the end point is irrelevant, son long as you have a few metres of pipe as the gradient is the same so the velocity will increase to max very quickly and then stay there.

The pipe full scenario will occur at the water level of the tank. Now that the pipe is full compared to ??% in your open channel flow the pressure drop will be much lower so yes the pipe full scenario will commence a little higher than the tank level but not much.

Just because the end is submerged changes nothing further up the pipe above the water level in the tank.

If you like do a calculation for a tank water level below the pipe entry into the tank.

Now do one assuming the water elvel in the pipe is say 50% of the height of the inlet to the entry into the tank. For a full pipe use your flow of 100l/s and calculate head loss in the 50% of the pipe which is now totally full. See what this compared to the level in the pipe. Probably pretty small.

however unless I'm mistaken a 75m long pipe with only a 0.8% gradient is a fall of 600mm. This only gives you a MAX of 300mm of tank height before your inlet becomes submerged assuming your inlet is "always submerged". That's very small. Please check your design / numbers.

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[Andrew88]

Thank you a lot for the reply LittleInch

If I consider a bit different parameters where my pipe inlet is at level 10.4m and entrance IL to tank at 7.0m I get a static head difference of 3.4m. Now 50% of this is equal to 1.7m - my pipe would be full in the middle of the run. The flow velocity for this pipe size and 100 l/s would be about 1.4m/s and headloss of about 0.2m. For the whole pipe length it would give 0.4m, which means that level in the upstream chamber will be approximately 0.4m higher than the level in the tank.

Is it right or did I misunderstand?

 

[LittleInch]

You almost got there.

Those dimesnions are a bit more like it.

Ok, making the assumption here that as you describe this is an weir from a chamber so the inflow is somehow controlled to your 100 l/sec.

In your question the key is how high can the tank be.

From your calculation above at 100 l/sec if the pipe was full for virtually the whole length then the flow resistance = 400mm.

I am a little surprised at the velocity though and if it is going this fast along a pipe when it is completely full, how fast is it going in your open channel/pipe flow? I guess your gradient is now a lot more than 0.8%??

Therefore in your worst case if the liquid level in the tank is 400mm below your weir, the water will just disappear without impacting the liquid level in the chamber. I would add a margin of say 200mm to this to allow the weir to function without impact from the connecting pipe.

At all other (lower) levels of the tank your pipe will operate in a mixture of partly full and partly open channel. The only thing to note here is that the pipe must fall continuously with no high or low points, i.e. a negative or at worst flat gradient at all times. Otherwise you could get air locks and surging etc which won't be good.

~it would be good to get some decent sketches of this and some calculations to see if you've made an error somewhere. I've done a quick check and I think to get your open pipe to work you're in super critical flow. Basically your pipe looks too small to me. 1.2% looks like the critical slope.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Size the pipe for pressure flow.

The weir will control the height of water in the tank. Size the weir length so that you have "x" height of flow over the weir. If the weir length is too small, the height x will be too large. The height x is something that you select.

The down pipe in the intake tank will transition from gravity flow to pressure flow. Oversize this down pipe one pipe diameter so that venting (burping) of air from the pressure pipe is not an issue