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Low Flow in Pipes- posted in Ankur's blog


Pressure Profile In Pipe Flow

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#1 Halflight


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Posted 28 August 2022 - 10:18 AM


I'm confused to understand Bernoulli's equation. That equation means when we reduce line size, it makes pressure lower and velocity faster.

I thought that equation is right; it can explain cavitation in pump suction or control valve suction.

However, did it have the same meaning that expanding line size makes pressure higher and velocity slower?

We usually expand line size after the pump discharge line. Could we control discharge pressure by expanding line size? Did it make the pump's head higher?


In my workspace(engineering company), I never heard about considering pressure by change of pipe size. We only consider friction loss and the relationship between elevation and pressure. Not velocity.


For last question, I determined the design pressure of discharge line to shut-off pressure of pump until now(and as I known it's a general method). Should I determine the design pressure higher if the pressure is raised by the expanding line size?

#2 Bobby Strain

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Posted 28 August 2022 - 11:15 AM




#3 Halflight


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Posted 29 August 2022 - 10:03 AM

Could someone explain to me why it doesn't make pressure higher?

We can see cavitation at vena contracta and it was occurred by velocity change. It's awkward to me.

#4 latexman


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Posted 29 August 2022 - 10:52 AM   Best Answer

Bernoulli's equation is a mechanical energy balance.


v2/2 + gz + P/rho = constant     Notice - no friction loss.


At a pipe reducer, velocity increases/pressure decreases due to Bernoulli and pressure decreases due to friction.  At a pipe expander, velocity decreases/pressure increases due to Bernoulli and pressure decreases due to friction.


You would benefit greatly by doing some practical calculational examples using real pump and fluid flow information.  The pressure changes caused by going up or down one pipe size is usually not that great.  There are exceptions sometimes though, thus you should look at the examples using your conditions to see the effects.


In general, I have found, since a pipe reducer has a somewhat larger pressure decrease caused by two factors, Bernoulli and friction, most engineers account for pipe reducers.  Conversely, since a pipe expander has a pressure increase caused by Bernoulli and a pressure decrease caused by friction, they kinda cancel out, and a lot of engineers do not account for expanders.  Again, it depends.  Work some real examples.


My second thought is, your pipe design pressure should be greater than the maximum operating pressure by a safety factor that is big enough so you don't have to worry about small pressure changes.  And, once again, some calculations by you will guide you on setting a safe design pressure.  Plus, if you consider the greatest pressure / dead head pressure from a pump, there is no flow, so Bernoulli and friction changes are zero.


I've never seen anyone worry about this after they look at some real numbers.  Numbers are hard!

#5 Halflight


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Posted 30 August 2022 - 09:24 AM

Dear Latexman,

thanks for help. I tried to find out real cases about pumps and I knew those are small enough not to consider.

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