Connecting pipes of different sizes

Hi everyone,

I apologize, this is probably going to seem like a really dumb question. I'm working as an intern in a developing country with rural water systems. I previously studied treatment, not infrastructure, and I'm having a conceptual brain fart with some pipe work.

I have a water storage tank that has two outlet valves connected to PVC pipes. One is 1.5" diameter and the other 0.5". It was proposed that we connect the two outlet pipes, since we've been having issues with the users on the receiving end of the smaller pipe not receiving water.

Note: This is actually more of a logistical problem: there isn't anyone in charge of opening the valves, so nearby users simply open the big valve and drain the tank while the other small valve remains closed. So really, organization and rules re: valve opening and water use are more key than engineering work here.

Regardless, my dumb question: If I connect a 1.5" pipe (via a Y or T connector) to a 1.5" pipe and a 0.5" pipe (with reductor), can I calculate the flow rate through the two pipes knowing the original flow (say, for example, 1 L/s)? How would I do that? Also, am I missing any other really obvious considerations here, like creating a bad pressure profile by doing something like this?

Hi and welcome.

If you have a 1.5" pipe as the header from the tank before your "Y" that will limit the max flow rate you can expect regardless of what pipes you have downstream of the "Y". You would need a 2" header at the tank to see any advantage fo extra pipes below the "Y" but it would be marginal. As the Head (and thus pressure at base) in the tanks drops, so too will the flow rate drop.

You could try using smaller diameter pipes to the people getting water and larger to the others who do not.

You could try and regulate that with a valve but I suspect people would tinker with the valve and mess up the balance. Far better to try and design your pipe runs so that flow rates equal out becuase of pipe diameters or lengths of total run.

Hi PinkertonD,

Thanks for the response!

To clarify - we have the 1.5" pipe heading to a group of houses, and the 0.5" pipe heading to another group of houses. What we would like to do is simply connect these two pipes to one tank exit (ie. the 1.5" one), which I figured we'd do by having a Y connection, starting as 1.5" and branching into 1.5" and 0.5". We don't have the money to replace all the piping, and thought it might be a decent solution to ensure that the one group of houses doesn't get neglected and everyone would receive water each time the valve is opened.

(Like I said before, it would be totally fine to just [I]open both exit valves every time[/I], but it's complicated by non-engineering factors like forgetfulness and neighbour feuds. Having them connected would circumvent that problem).

I'm just unsure of what a set-up like this would do to the individual flow rates, or if it would really disrupt the head (ie. such that the water wouldn't arrive at the final destinations).

[QUOTE=PinkertonD;5466]You could try and regulate that with a valve but I suspect people would tinker with the valve and mess up the balance.[/QUOTE]

Yeah, it's a definite problem, although we can potentially cover and lock it. By this, do you mean installing globe valves on both exits from the Y?

[QUOTE=Cheke;5467]do you mean installing globe valves on both exits from the Y?[/QUOTE]

That would do it as long as you used same or close to, diameter pipes to both outlet points all the way from the "Y". Having one 1.5" and one 0.5" pipes at or after the "Y" will ensure that the 0.5" never get much water until the 1.5" people shut off the valve at the end of their run.

Ideally have say, 1" or 1-1/4" pipe going from the "Y" to BOTH outlet points. With valves on both pipes at the "Y" you can then control the flow rates through both lines.

[ATTACH=CONFIG]546[/ATTACH]

Start with all four valves closed
Get a 5-gallon (or whatever) bucket and a stop watch.

Turn on Am full open
Turn on Bm full open
Nothing running as both outlets are still closed.

Turn Ao full open
Run water and time for 5-gal - let's say 2.0 minutes
Turn Ao closed

Turn Bo full open
Run water and time for 5-gal - let's say 2.6 minutes
Turn Bo closed

Average for an equitable time for both Ao and Bo = 2.3 minutes.

Make sure all four valves are closed

1: Turn Am about half open
2: Turn Ao open full and measure time to fill bucket.
3: Turn Ao closed
4: Adjust Am for 2.3 minutes and repeat if needed until Ao = 2.3 minutes.

Do the same for the B set up.

Lock both Am and Bm so they cannot be adjusted.
[I][COLOR=#0000cd][B]
OK, some flawed logic in the above, see the following post!!![/B][/COLOR][/I]

[I][COLOR=#0000ff]Post 2:00am next day, eureka waking moment... :)[/COLOR][/I]

From the example above, the max flow rate through the 1.5" pipe from the tank, the "Y" and shortest run to Ao is 2.5 Gallons Per Minute (GPM -- 5Gal / 2Min = 2.5GPM).

Make that the maximum flow available from the tank. If both people need to have water running at the same time then 2.5GPM must be shared. Soooooo

Close all four valves.
Open Ao fully
Adjust Am for 1.25GPM at Ao (5Gal / 1.25GPM = 4 minutes to fill the 5-gallon bucket)
Close Ao
Open Bo fully
Adjust Bm for 1.25GPM at Bo (5Gal / 1.25GPM = 4 minutes to fill the 5-gallon bucket)
Close Bo
Lock Am and Bm.

When only one side is drawing water the flow will be close to the original 2.5GPM and when both are drawing, they will get about the same.

[COLOR=#0000cd]I think I have it right now... :)[/COLOR]

Thanks a lot, PinkertonD! This is very helpful (especially after the update). Plus, it looks like we might have enough budget to fix the piping coming from the tank, so I'll take the suggestions into account.

Cheers!