A plumbing/physics question

[Flight1]

Hi everyone, I have a physics/plumbing question for you. I hope you can help.

If I have two tanks of water (A and B) both connecting to a single drainage pipe, with tank A being higher (at greater altitude) than tank B, and I open the valves (simple valves, not check or non-return) of both tanks at the same time, what will happen?

1. Will tank A completely empty before tank B empties at all because the water from A is at higher pressure?

2. Will some of the water from tank B ‘compete’ with the water from A and therefore slow down the rate of flow from A?

3. If the volume of water from B was much greater than that of A would that make a difference, or will the pessure from A always be greater simply because it is higher?

4. Would the water from A create a syphon effect on the water in tank B and so draw it out?

4b. If it did this, surely it would then slow down the rate of flow from A?

//www.diynot.com/network/Flight1/albums/


Thanks for your help

 

[bernardgreen]

The prime rule is that the water finds its own level. Connect two tanks below their water lines with a pipe and water will flow until the levels in the two tanks are the same height above ground.

The full answer depends on the diameters and lengths of pipes involved.

Take the case that the pipes from A and B to the junction are 22mm and the drain from there is 10mm then the water from A will flow into B more easily and therefore faster than water can flow out of the drain.

If the pipes from A and B are 10 mm and the common drain is 22 mm then while the pressure from A will affect the flow from B the effect will be in-significant and both will drain as fast as they can through the 10 mm pipes.

 

[Flight1]

Thanks for the reply Bernard,

Ok, let's say the common drain pipe is 22mm as is the connecting pipe from tank A. However the connecting pipe from B to the common is 15mm.

What might be the effects of that?

Edit: Lets say the connecting pipes are 1 metre, the distance between A and B is 3 metres and from where B connects to the common to discharge is 2 metres.

Hope that's clear. Thanks again

 

[Norcon]

If the drain doesn't take it quick enough the lower tank could fill and over flow.
Thats assuming both discharge valves are opened at the same time.

 

[Flight1]

Thanks Norcon (and sorry for not getting back sooner),

Could you (or anyone else) confirm that the water from both tanks 'compete' for space in the common pipe.

I had thought that because the water from tank A is at higher pressure it would have sole use of the common pipe until empty, and then tank B would empty.

(Although as i thought about it i began to have my doubts, hence the reason i'm here)

Here's another question:

if two toilets (one above the other) are connected to a single soil pipe and there is a blockage in the common soil pipe just below were the lower toilet connects to the common. If the upstairs toilet is flushed, do the contents flow out of the downstairs toilet?

 

[Onetap]

Bolivia?

I vaguely recall doing problems like this on a HND course.

What happens depends entirely on the sizes of the pipes and fittings and the head available. Both tanks could empty, A could empty before B, or A could empty into B.

With no flow, the pressure at any point in the pipe depends only on the head. Once flow starts, there are pressure losses due to friction, and changes in velocity. From A, there is an inlet pressure loss (due to pressure being converted into the velocity head), then there are frictional pressure losses (depending on the pipe size, flow rate, pipe internal roughness, etc) for the pipe length and fittings.

At the tee, connecting to B, the pressure will be the head minus the pressure losses. If the pressure is greater than the head from B, water flows to B. If less, water flows from B.

I think you could work out what happens but you'd need all the pipe sizes.

PS Looked it up. Incompressible flow through branching pipes: the three-reservoir problem. Involves hydraulic gradients or resistance coefficients. I could do those problems in college, I've never had any application for it since. You'd need a fluid mechanics text book.

Google 'the three reservoirs problem'.

 

[mcmoby69]

apparently according to the other trades, there are only two things plumbers need to know.

water flows down hill and sh&t stinks.

dont get me wrong, i do a lot of plumbing not just gas work, it was summat else that reminded me of that saying.

 

[Flight1]

Bolivia?

Bolivia!

Really appreciate the help Onetap, you're the man

it was summat else that reminded me of that saying.

Been sniffing your hands again Moby?

 

[simcon]

This is a problem I studied at college while learnign fluid mechanics. If you know the parameters of the system (heights, pipe diameters, roughneses), then you can use this spreadsheet to find the flowrate (and flow directions)

http://excelcalculations.blogspot.com/2011/05/three-reservoir-problem.html

By the way, I wrote the spreadsheet. If you have any questions, then let me know