PVC Flow characteristic question and example.

[World Citizen]

Hi,

I am planning a modification in my system and something is bothering me which I don't understand. It is a flow characteristic question and I have no serious knowledge in this skill.

Situation:
I want to combine two PVC tubes into one tube.
The flowrate in both tubes is not the same.
I want to maximize efficiëntie in combining the 2 flows. I want to lose pressure as little as possible.

Question:
Where do I put the junction.

The bothering part:
I am worrying about back pressure at the junction. Since the flowrate in the pipes is not the same, I want to reduce back pressure at the junction.

I have the feeling situation A is better. There is more mass in the slow flow pipe which makes it more resistent at the junction. I can't calculate this. That is beyond my skill.
Is there any difference in these situations? Does it even matter where to put the junction?



Pic: (theoretical situation)

Thanks!

 

[MnFish1]

Hi,

I am planning a modification in my system and something is bothering me which I don't understand. It is a flow characteristic question and I have no serious knowledge in this skill.

Situation:
I want to combine two PVC tubes into one tube.
The flowrate in both tubes is not the same.
I want to maximize efficiëntie in combining the 2 flows. I want to lose pressure as little as possible.

Question:
Where do I put the junction.

The bothering part:
I am worrying about back pressure at the junction. Since the flowrate in the pipes is not the same, I want to reduce back pressure at the junction.

I have the feeling situation A is better. There is more mass in the slow flow pipe which makes it more resistent at the junction. I can't calculate this. That is beyond my skill.
Is there any difference in these situations? Does it even matter where to put the junction?



Pic: (theoretical situation)

Thanks!

OK - so - it may be a language issue - but - can you give an estimate as to the flow in each of the 2 pipes going into 1 - and second - what is the reason you're asking. Normally - the single pipe would have a wider diameter to accept the flow easily from both sides - are both being pumped (physically) - if so, there will be a problem - if its gravity - not as much

 

[World Citizen]

OK - so - it may be a language issue - but - can you give an estimate as to the flow in each of the 2 pipes going into 1 - and second - what is the reason you're asking. Normally - the single pipe would have a wider diameter to accept the flow easily from both sides - are both being pumped (physically) - if so, there will be a problem - if its gravity - not as much

Hi

The situation is changing over time. This is why I want to prepare and build this section correctly now.

I am combining 2 pumps together and have the option to place the Y junction at any of those places.

Would you place it like A, far away from the pump?
Or
Would you place it like B, close to the pump?

Thanks.

 

[KStatefan]

The longer the single pipe is the greater loss from friction since the velocity will be greater in the single pipe. The friction loss from your fittings should be the same at either A or B

Lots of variables

 

[Randy Holmes-Farley]

A is a little bit better than B because the loses in the straight double section are a little lower than the straight single section.(assuming all pipes are equal diameter)

 

[DanP-SD]

If your space and any connections downstream from your images allow, your best bet will be to use a wye connection that has a larger output (the single end) than the two inputs. You can calculate how much larger by looking up the max flow rates of each diameter. Figure out the total flow going into the fitting (basically output of the pump minus head pressure, which you can estimate by measuring the length of pipe and number of elbows). This link has a handy chart with flow rates at various pressures: https://resources.hy-techroof.com/blog/how-much-water-can-flow-through-a-pipe

Based on the total estimated volume you can determine how much to neck up to avoid back pressure. It probably won’t be much. For example, if the lines coming in are 3/4”, at average pressure per the linked chart, the pipe will handle up to 1410 gph whereas a 1” out would handle 2200 gph. So even if both of the 3/4s are maxed out, a 1” out would likely be sufficient to minimize or eliminate back pressure.

If you’re unsure of your calculations, I’d neck up just one pipe size (ie 3/4 to 1, 1 to 1-1/4, etc). If you neck up too much, you’ll create negative pressure at the point right after where it widens. This is how venturis work. That introduces other issues like cavitation that can be more problematic than a little back pressure.

You should also be aware of some other issues with two pumps converging into one line. If a pump fails or you turn one off, the water from the other pump will run back through the line of the stalled pump. Depending on your layout, it may be all or most of your flow. You could install check valves to prevent this but check valves notoriously fail. If you have an option to plumb each pump with its own line you might be happier in the long run.