Return pump plumbing
- Thread starter svreef
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splitting_lanes
Supporting Member
Sure, you can cut the small barb off if you’re using tubing that fits the large barb.
Coral reefer
Past President
Yes and yes.
You don’t have to cut off the smaller part burn it will cut down flow a little if you leave it.
You don’t have to cut off the smaller part burn it will cut down flow a little if you leave it.
splitting_lanes
Supporting Member
I had to buy an adapter to convert from metric to standard. It was a hassle to figure out, but I wound up replacing the barbed fitting that attaches to the bottom of the tank with a fitting I got at Neptune aquatics. HTH
Yes, it’s to allow use with different tubing sizes. I wouldn’t (and I don’t on mine) bother cutting the smaller part off, though you can. It is a common misconception that having a short narrower part will significantly restrict flow, but like Mike said it will be minimal. The increased resistance to flow due to a narrower part is proportional to the length of the narrowing, which in this case is very short compared to all the tubing, so basically negligible.
Coral reefer
Past President
It’s fluid dynamics physics- Poiseuille’s law. Resistance of a tube (or part of a tube) to flowing liquid is proportional to viscosity and length, inversely proportional to radius to the fourth power:
And resistance overall is additive, so you add the resistance for each part of the pipe for the total resistance.
So when radius decreases by half, resistance for that part goes up a lot- 16 fold, but only over that length.
Example: Say you have 1 inch of half-diameter pipe, followed by 99 inches of regular diameter pipe. Compared with 100 inches of regular diameter (forget the units, just relative):
All regular diameter: R ~ 100
1” of half-diameter: R ~ 115 (99+16)
So it’s 15% more resistance, which isn’t really negligible, but but it’s not as much as what a lot of people imagine. Also, we aren’t dealing with straight horizontal pipes; we also have bends in the pipe adding more resistance, and most importantly gravity. These combine to give the head pressure we are pushing against. But viscosity n of saltwater is very low, so the overall amount of resistance to flow in a horizontal pipe is a lot less of a factor than gravity when we are pumping up. So we are talking about a <15% increase in the relatively small fraction of head pressure that is due to resistance.
And resistance overall is additive, so you add the resistance for each part of the pipe for the total resistance.
So when radius decreases by half, resistance for that part goes up a lot- 16 fold, but only over that length.
Example: Say you have 1 inch of half-diameter pipe, followed by 99 inches of regular diameter pipe. Compared with 100 inches of regular diameter (forget the units, just relative):
All regular diameter: R ~ 100
1” of half-diameter: R ~ 115 (99+16)
So it’s 15% more resistance, which isn’t really negligible, but but it’s not as much as what a lot of people imagine. Also, we aren’t dealing with straight horizontal pipes; we also have bends in the pipe adding more resistance, and most importantly gravity. These combine to give the head pressure we are pushing against. But viscosity n of saltwater is very low, so the overall amount of resistance to flow in a horizontal pipe is a lot less of a factor than gravity when we are pumping up. So we are talking about a <15% increase in the relatively small fraction of head pressure that is due to resistance.
Coral reefer
Past President
I bet the pic is 3/4” vs 1/2” barb.
Thanks for the formula and explanation. I had it in my head that it would be like the weakest part of a link and be the limiting factor on flow and slow everything down more than it likely does being that short of a bottleneck (apparently).
Thanks for the formula and explanation. I had it in my head that it would be like the weakest part of a link and be the limiting factor on flow and slow everything down more than it likely does being that short of a bottleneck (apparently).
rygh
Guest
Unfortunately, Poiseuille’s law only applies well to laminar flow where the length is substantially larger than the cross section.
Think of that law working well when the dominant loss is surface friction.
This case is WAY harder to calculate.
You would have to check out equations on flow through nozzles and orifices I think.
... it has been too long since college for me though.
I tend to agree that it is probably no big deal to leave that small section in.
But it is so trivial to remove, and it does matter at least a little bit, so why not?
Water flow through a properly sized return pipe is laminar flow where the length is significantly greater than diameter, so this IS the best way to look at it. That’s why I described this law among the many other much more minor factors that contribute to the situation.
The whole point I’m trying to make is that the weakest link/bottleneck way of looking at it isn’t correct, even though it makes intuitive sense to most people.
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Me too
The focal decrease in diameter does not cause it to stop being laminar flow. For this to happen, it would need to transition to turbulent flow, which does generate much more resistance but only happens at much greater degrees of narrowing. For the range of narrowing we are considering, the flow would just increase velocity over the narrower part but remain laminar.
The other way to look at this that might make more sense is how in general we don’t worry about the inside diameter being smaller in barbed fittings (even without various sizes). The inside of the fitting is significantly smaller diameter than the inner diameter of the tubing, but it doesn’t cause any noticeable problems, just a very slight increase in overall resistance. Similarly, we don’t worry about nozzles which are intentionally narrowing to increase velocity, but also slightly increase resistance.
rygh
Guest
Me too
The focal decrease in diameter does not cause it to stop being laminar flow. For this to happen, it would need to transition to turbulent flow, which does generate much more resistance but only happens at much greater degrees of narrowing. For the range of narrowing we are considering, the flow would just increase velocity over the narrower part but remain laminar.
The other way to look at this that might make more sense is how in general we don’t worry about the inside diameter being smaller in barbed fittings (even without various sizes). The inside of the fitting is significantly smaller diameter than the inner diameter of the tubing, but it doesn’t cause any noticeable problems, just a very slight increase in overall resistance. Similarly, we don’t worry about nozzles which are intentionally narrowing to increase velocity, but also slightly increase resistance.
The thing is, a small orifice definitely CAN be a limiting factor on flow.
Largely because the resistance becomes about viscosity, not friction.
At certain orifice sizes and pressures, the flow due to the smaller opening becomes totally limited.
In others situations, it can be safely ignored.
But in this situation... I simply don't know.
Someone would have to do the exact calculations or measure it to be sure.
Until then, I am reluctant to dismiss it.
Flagg37
Guest
I don’t know nearly as much as you two but what about the direction of flow? Does it flow through the fitting and out the barbed end or the opposite direction? I would think if it flows into the barbed fitting that it would “catch” on the smaller part and slow things down more than if the diameter of the pipe smoothly decreased.
Well, that was a lovely detour.
It turn out that 1” tubing fits over this barb but is not as tight as I would like. Some suggested zip ties. I’m sure traditional metal hose clamps are out of the question since this end will be in the water.
Sent from my iPhone using Tapatalk
It turn out that 1” tubing fits over this barb but is not as tight as I would like. Some suggested zip ties. I’m sure traditional metal hose clamps are out of the question since this end will be in the water.
Sent from my iPhone using Tapatalk
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