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: flow velocity in a pipe
: flow velocity in a pipe -- stupidstudent2009	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 03/29/2009, 21:28:39 Author Profile eMail author Edit

Your problem is that the Bernouli equation is valid for ideal flow in the absence of viscosity and the RHS of that equation is NOT zero but the total head of the system times rho at any point in the system. What you are after, (after dividing by rho) is the LHS minus the loss of head from the starting point to any point downstream, that loss being for a simple pipe f*L/D*V^2/2g L= length of pipe. Get any decent book on incompressible fluid flow for an explanation.

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: : flow velocity in a pipe
: : flow velocity in a pipe -- zekeman	Post Reply	Top of thread	Engineering Forum

Posted by: stupidstudent2009 ® 03/30/2009, 00:38:44 Author Profile eMail author Edit

Zekeman, I appreciate your reply. That makes sense to me. However, I have one more question. Is the equation I used in my original thread correct and if it is, do I also need to divide by h (head), since that is on both sides of the equation...the as rho? Thanks again.

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: : : flow velocity in a pipe
: : : flow velocity in a pipe -- stupidstudent2009	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 03/30/2009, 07:55:26 Author Profile eMail author Edit

No, your equation is incorrect on 2 counts. The RHS is not zero, even for an ideal flow which yours is NOT. And second ,since it has friction you must subtract the friction loss, the term I show involving f*L/D... Basically, you take your LHS at the starting datum , subtract this term and equate that to the downstream point P2+rho*V2^2/2+h2 (a distance of L from that point) you are looking at. I strongly urge you to get a book on this to get a better grip.

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: : : : flow velocity in a pipe
: : : : flow velocity in a pipe -- zekeman	Post Reply	Top of thread	Engineering Forum

Posted by: stupidstudent2009 ® 03/30/2009, 18:34:29 Author Profile eMail author Edit

Zekeman, now I am thoroughly confused and will pick up a book this weekend. I actually got to see the pump that is driving this system and it gives a mass flow rate of 455 m^3/hr. Knowing the flow rate and pipe diameter, I can calculate the velocity at the beginning of the system. Since I am given a mass flow rate, I will take the sum of all friction losses (elbows and friction loss coefficients for the spool pipe) along with Bernoulli's equation P1+(rho*V1^2/2)+rho*g*h1=P2+(rho*V2^2/2)+rho*g*h2 to solve for V2 which I can then use to solve for mass flow rate (Q). My thoughts are to use the pressure losses to calculate P2 by subtracting P2 from P1. Which leads me to my question, this is a horizontal piping system so there is no change in elevation and rho and "g" are the same value on both sides so, can I drop these three terms on both sides of the equation and what do I do with P1? How do I figure out the value of P1 at the beginning of the system; is it "0"? Thanks and regards, Modified by stupidstudent2009 at Mon, Mar 30, 2009, 19:27:15

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: : : : : flow velocity in a pipe -- stupidstudent2009	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 03/31/2009, 18:22:46 Author Profile eMail author Edit

You're getting a little better but get that book and do some reading. Good question about P1. You get it by matching the total loss, f*L/d*V^2/2g to the pump curves which plot P vs gpm f is the friction factor (obtained from Reynolds number) and L is the total equivalent length of piping in question. In practice there are engineering monographs that plot gpm vs pressure drop for various diameters,so you can zero in on an answer fairly quickly.You are usually not that interested in P1 per se but only the gpm.

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