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Velocity change through piping question

Velocity change through piping question

Velocity change through piping question

(OP)
Okay, this may be a really silly question, but it certainly has me confused.  I have been given a project to design a sprinkler system for dust suppression.  I've done all the steps to find my pressure losses due to elevation changes and friction, and I'm pretty certain my pump produces enough pressure to overcome them. What has me stumped is..I made an excel spreadsheet to calculate and keep track of these losses and to show how much pressure I will have at so many feet intervals throughout the run.  I have also included GPM loss through each sprinkler head, and from this, i created a column to show me how much GPM I have left after water passes through each sprinkler head.  So my GPM left decreases over the whole run of the piping system as you cross over the sprinkler heads...what I am confused about is the little saying that says as pressure decreases, flow increases...now I know this is true for diameter changes through pipe..BUT is this true for all piping?  Since my pressure is decreasing over the whole run due to friction losses and elevation changes, should my velocity be increasing due to these same things?  I would like to say the answer is no, but can't seem to verify it in my mind.  Someone HELP haha...tell me it's correct that my velocity will NOT increase as pressure decreases if my pipe size remains the same...

RE: Velocity change through piping question

With a centrifugal pump, as outlet pressure decreases, output flow increases.
Along piping with established flow, velocity does not increase with pressure decrease.

Ted

RE: Velocity change through piping question

alecia7,
The point you are missing is that everything you said is only true for a closed system.  If you add mass, you get a different continuity equation.  If you deduct mass (say through a sprinkler head?), then you get a new continuity equation.  

So if you have 10 spray nozzles and you've designed a perfect system to take exactly 1/10th of the water out each nozzle then the mass flow rate after the first nozzle is 90% of the mass flow rate out of the pump.  If the pipe is the same size then your velocity will something like 90% of the velocity upstream of the nozzle.  And so forth.

As to velocity always increasing when pressure decreases, you are confusing several concepts.  For a closed system, velocity equals mass flow rate divided by density divided by cross sectional area.  So if density decreases (say with a pressure drop in a compressible fluid) then velocity has to increase for a constant mass flow rate.  Your water is not a compressible fluid (density doesn't change significantly with a pressure drop) and you don't have a closed system.  The only thing causing significant changes in velocity is the removal of mass through the nozzles.

David

RE: Velocity change through piping question

(OP)
Okay THANK YOU! that makes complete sense...I had a strong feeling that the velocity would not increase, but I just needed the complete reasoning to solidify it.  Y'all have been a big help and I really appreciate it.

RE: Velocity change through piping question

Pressure changes occur in distributor piping as a result of both frictional and momentum effects.  I'm sure you understand and have accounted for the frictional pressure losses.  But have you included the momentum effects?  As the velocity in the main pipe decreases from the initial velocity to 0, the momentum effects cause the pressure to increase.  This momentum pressure rise is analogous but not equal to the pressure rise predicted by application of the Bernoulli Equation. It is given by:

(P2-P1)momentum = 1.2 ρ V12/(2 gc)
 

Good luck,
Latexman

RE: Velocity change through piping question

(OP)
Hi, Latexman,

thanks for the feedback..I do not believe I am familiar with the equation you listed.  I do see how it resembles bernoullis, but do not believe I have ever seen it.  I'd like to do some more research on the matter but can't seem to find anything on it through google. Do you perhaps have any websites or references in which I could learn more about the pressure rise due to momentum phenomenon?

RE: Velocity change through piping question

Assume incompressible, low velocity flow,
The equation you need to understand is,
Bernoulli's modified for frictional head loss,
P2/γ = P1/γ + Z1 - Z2 + (V1-V2)2/2/g - Hfriction
For low velocity systems, < 10 fps, the momentum term can be ignored, as it won't amount to more than a head of beans (about 1/2 psi).

From "BigInch's Extremely simple theory of everything."

RE: Velocity change through piping question

I got it out of a company design manual.  No reference given.

The Fluid and Particle Dynamics chapter in Perry's Handbook speaks to this in their Perforated-Pipe Distributors section.  Their equation looks different and they suggest using 1.0, not 1.2.

Also, Perry's recommends a factor of 2.0 velocity head pressure drop for converging manifold.  My design manual says 1.8.

I suspect Perry's is based on theory and my design manual had some empiricism thrown in, i.e. adjusted to fit some data.

In summary, momentum effects in a diverging manifold have 1.2 velocity head pressure rise and those in a converging manifold  have 1.8 velocity head pressure drop, based on the initial (diverging) or final (converging) velocity in the main header.

BigInch is right.  If you have a low velocity system, you may be able to ignore it.

Try Googling some of the keywords used in this post.  I found http://pubs.acs.org/doi/abs/10.1021/i260028a019 which looks promising.

Good luck,
Latexman

RE: Velocity change through piping question

If you have a high enough veocity to make a difference in head, you've usually got a lot more pressure to play with to, so relatively it still doesn't factor in much.  For pipeline work, it's low velocity, so we always wind up ignoring it.  Jet ski, ya, better keep tabs on it.

From "BigInch's Extremely simple theory of everything."

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