Pressure in a straight pipe vs a curved pipe?

[Silverlight]

Hi everyone. My dad was debating with his friends over something, and he asked me for my opinion too. I've been on these forums before so I thought I'd ask here.

So let's say you have two pipes. Both have the same cross-sectional area, same fluid inside, and they both extend to the same height at the top. However, one is straight and one curves around a bunch like a snake. If we stick a pressure gauge at the bottom of both, will the readings be the same?

It's been years since I've taken physics, but I remember the formula for pressure in this case was like density*gravity*height. In that case, wouldn't it be the same for both pipes unless you count the whole length of the curved one? Even though his friends argued that way, it just doesn't seem right. I'm guessing there's some more complicated stuff involved besides just that simple equation.

 

[tkyoung75]

It will be and you have the correct formula. Provided that the pipe is sealed and full and there is no flow, the friction will not play a part. When you have flow, that is when friction plays a part. That is why sometimes when you turn a tap on it starts off at high pressure and then decreases because there is too much friction in your line.
Edit: also, when you close a tap it can take a time to build pressure again, but when its at equilibrium, it will show the same dead head pressure.

 

[CWatters]

Consider what would happen if the pressure was different. You could connect the two pipes together at the bottom and the increased pressure in the curved pipe would push water up the straight one. If you also connected them at the top the water would flow round and round. You could insert a turbine and extract some energy. In short you could violate conservation of energy. That's not possible so the pressure must be the same.

 

[tomdaily]

I think there is a G force added to the equation when the pipe is bent? What do you think?

 

[DaveC426913]

There's definitely a drop in throughput due to curved pipes. You want as few elbows as practical, and what ones you do use, should be as large a radius as practical.

This sure plays into how I connect up my pool filter and heater.

 

[hmmm27]

The same as if you compare the pressure at the bottom of a glass of water, with the pressure at the bottom of a kiddie's swimming pool filled to the same height as that in the glass.

Or standing in the middle of a kiddie pool, with walking out a few feet into the ocean surf.

 

[Chestermiller]

I think there is a G force added to the equation when the pipe is bent? What do you think?

Are you aware that this thread has not been visited for almost 5 years, and the OP hasn't been to Physics Forums since 2016?

 

[DaveC426913]

D'oh! Got sucked in!

 

[physicsInPools]

Wait - so I am doing some "POOL" network research - and was thinking about flow and trying to remember my physics and decided to google TODAY (funny I stumble on this today after you all did yesterday)

My question is - are slightly curved pipes (made by heating and bending) better than L or T junctions? Or are there other reasons to "curve" them (other than to get over each other) as opposed to using junctions? (like maybe it's cheaper using heat and less likely to break than installing a junction??)Thanks

(--- someone that writes stuff for TV and is looking for some great real answers---)

 

[DaveC426913]

Anything that causes friction in a pipe will reduce the flow.

Sharp bends are bad.
All those glued joints are a problem, too, but can't really be avoided.

So, as far as practical:
- minimize bends
- maximize radii of bends
- minimize unnecessary joints

Sure, long, straight pipes, bent into shape, would reduce friction - but probably not practical, so not necessarily advisable as general advice.

 

[physicsInPools]

https://blog.intheswim.com/swimming-pool-feet-of-head-calculations/