Head pressure loss in cylinders

[Tigé21v]

How does the diameter of a cylinder affect head pressure? Do you lose more flow if you are pushing water up a 6" cylinder than say a 3" cylinder?
TIA

 

[Wazzel]

For the same flow rate, the losses go down and the diameter increases.

 

[sleepydoc]

Not only that, they go down by 1/r^4, so a pipe that is twice as big has ⅛ the resistance.

Edit - That is only a rough guideline; resistance is dependent on the fluid, the interior surface, the flow rate and whether the flow is turbulent or laminar. Turbulent flow actually has higher resistance/head pressure than laminar flow, so the resistance may be even higher depending on your flow. To make things even more complicated, the pumps we use drop their flow in a nonlinear fashion in response to increasing head pressure, making the whole thing a rather nasty differential equation.

The best thing to do is to go to the head loss calculator on the RC homepage and try some calculations. If your pump is not listed, use some trial & error to pick a pump that has a similar flow to yours at the head loss of interest. Then try entering a pipe diameter slightly larger and smaller than your original and see the effect on flow.

 

[Tigé21v]

Doesn't the weight of the column of water come into play at some point?

 

[Gorgok]

Doesn't the weight of the column of water come into play at some point?

No.

Only the height of the water column matters. How wide the column is doesn't change anything as far as the pump sees (it only sees the column right over the opening of the outlet).

Having a pump plumbed into a 1/4" pipe or the bottom of a bucket will both have the same head from height at the same height. The 1/4" pipe will more than likely flow sooner over the edge (as the bucket takes longer to fill up) but once the bucket is full it will flow more water per hour over the edge than the pipe (because of high friction in the pipe). It will flow slower, visually, but at a higher rate... if that makes sense.

Shorter answer is bigger is better, but ridiculous is still ridiculous. Upsize stuff a size or two, going much more and the benefit isn't worth it.

 

[Tigé21v]

I get what you are saying.
But, to be ridiculous....
A maxijet 1200 plumbed directly under a 55gallon barrel could push water to the same height in the barrel as it could push water up a 2" cylinder plumbed in directly above it?

 

[Gorgok]

Yes. Even to the really ridiculous, it would have even less loss at the bottom of a pool pushing water up through the bottom than in the same height of 2" pipe. The difference in loss between the 2" pipe (already oversized for a 1200) the drum and the pool will be very slight, with the pool being the best in terms of flow if you could measure the difference...

2" pipe on a 1200 is already pretty silly though.

 

[Tigé21v]

It's ridiculous.:spin2:
Very interesting though. I would have thought the physical weight of the water would have an impact on the pump's performance.
So, I guess the inverse of the scenario would be that the water would drain with the same velocity from both the 2" cylinder and the 55 gallon drum filled to the same height?

 

[Fishmommy]

One of the few elegant things about hydrodynamics!

 

[Gorgok]

It's ridiculous.:spin2:
Very interesting though. I would have thought the physical weight of the water would have an impact on the pump's performance.
So, I guess the inverse of the scenario would be that the water would drain with the same velocity from both the 2" cylinder and the 55 gallon drum filled to the same height?

Yes, it would be the sameish until the height in the 2" pipe drops, then it slows. I say sameish because the friction in the pipe will slow it some more than in the drum.

 

[Wazzel]

It's ridiculous.:spin2:
Very interesting though. I would have thought the physical weight of the water would have an impact on the pump's performance.
So, I guess the inverse of the scenario would be that the water would drain with the same velocity from both the 2" cylinder and the 55 gallon drum filled to the same height?

It sort of does, but is based on the height of the water only. No matter the size of the body of water the pressure at depth is the same (with all conditions the same).

 

[sleepydoc]

Pressure is measured as force per unit area (eg pounds per square inch). A larger pipe has a larger total force, but it's also spread out over a larger area, so the pressure stays the same.

If you think about it, the water pressure at 3 feet deep is the same in the ocean as it is in a swimming pool.