×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bernouli Equation

Bernouli Equation

Bernouli Equation

(OP)
I am designing a liquid outlet line on a separator operating at 350 psig.  Immidiately after the separator is a level control valve with a Cv of 33.4 (see attached sketch).

I have an elevation rise of 20 ft and 600 ft of 2" piping.  I am flowing light weight crude (SG=0.8).

My question is, if I know the pressure upstream of the level control valve and the Cv, can I use the Bernouli equation to calculate the velocity (in turn flow rate) of the fuid downstream of the valve?

Thanks in advance.

RE: Bernouli Equation

Why does everyone blame Bernoulli for everything?  There is no place in the equation for a Cv (or any flow resistance at all).

Look in Crane 410 for the nozzle flow equation.  That SG is about 35 API which can be pretty fluffy so you are going to get some noise in the data (the nozzle equations are based on incompressible fluids), but you should be OK.


David

RE: Bernouli Equation

Engpes,

Modified  Bernoulli applies as follows which is the classical incompressible flow equation in engineering.

V1^2/2g+p1/rho +z1= V2^2/2g+p2/rho+z2+losses
 
V1 pipe velocity, ft/sec
V2 velocity of  fluid rising in tank, usually neglected
z1 height of pipe at 1
z2 height of top tank surface

losses are :
1) (Q/Cv)^2 for control valve
Q is in gpm so you have to convert
Q=V1*60*pipe area/231
pipe area in in^2

2)fL/D*V1^2/2g for pipe length L
f friction factor obtained from Reynolds number

3)contraction loss fluid entering tank
K*v^2/2g
K approximately 1

RE: Bernouli Equation

(OP)
Desertfox I dont know the flow rate and am trying to determine this.

I think I agree with zekeman in using (Q/C)^2 to determine the valve losses and get flow.

RE: Bernouli Equation

Hi engpes

I think you need to re-read my post.
To obtain the flowrate downstream of the valve, you require the flowrate UPSTREAM of the valve if yu want to use Bernoulli.

How are you going to use (Q/C)^2 unless you know Q?

Q = gallons per minute = flow rate

desertfox

RE: Bernouli Equation

"Q = gallons per minute = flow rate"

Q=V1*60*pipe area/231
pipe area in in^2

from above post.

 

RE: Bernouli Equation

Hi engpes

So all we need according to Zekeman's post above is V1 but this is part of what you're trying to find I beleive?

desertfox

RE: Bernouli Equation

(OP)
I will look at it closer, but we should be able to substitute v1xA for Q and have 1 equation and 1 unknown, V1?

Being that it is incompressible I should be able to get flow from V1?

Thanks

RE: Bernouli Equation

I think DesertFox is correct, you need some information on the state of the upstream condition.

In doing orifice meters for example, the pressure drop gives me a measure of stream velocity. The accurate calibration of the orifice thereby leads me to mass flow. Continuity or the lack of mass lost gives me downstream velocity given the size of pipe and schedule.

But the computation of stream velocity is based on Bernoulli and incompressible flow, corrected for nozzle efficiency. I knew the upstream and downstream pressures, the orifice diameter and density of the working medium.

This is what I interpret DesertFox to be saying, which I believe is the correct approach to your computation.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

RE: Bernouli Equation

"I think DesertFox is correct, you need some information on the state of the upstream condition."

Look at the OP diagram. He gives the upstream pressure

p1=350psi
 
and

v= v1 is constant in the 2"pipe.

p2= atmospheric pressure

v2 is almost zero but by continuity you could get it.

So we are left with the single equation in v1.

Where is the problem?

This is a 2nd year engineering fluids problem for steady flow.


 

RE: Bernouli Equation

(OP)
I agree.

RE: Bernouli Equation

hi

Okay, so can you post a solution for the flow rate down stream with the information given in the original question.

regards

desertfox

RE: Bernouli Equation

agreed ... what is the upstream total pressure ? (static + dynamic)

RE: Bernouli Equation

"agreed ... what is the upstream total pressure ? (static + dynamic) "

look at my comment (6 Nov 11 11:10)and the OP sketch.

  

RE: Bernouli Equation

Calculate a guess flow rate Q' assuming a guess downstream pressure P2  (after the control valve) and using the following formula:

Q'= Cv*SQRT(deltaP/SG)

Check Q' with D'Arcy Weisbach equation (include also entrance effect and differential height).
 

RE: Bernouli Equation

so the total pressure in the system is 20' head ?

RE: Bernouli Equation

No,

P1/rho +v1^2/2g at station 1 since p1 is the static pressure.

would you please spend some time to read the sketch and note that the flow is from left to right and then maybe you might see it.
 

RE: Bernouli Equation

thx zeke, i did look at the picture and thought the only place in the circuit that we knew what the total pressure was was at the free surface and following that at the base of the reservoir.

IMHO, it doesn't matter that the flow is left to right, bernie equates total pressure at two places on the circuit, allowing for interchange between static and dynamic pressure (and losses).

RE: Bernouli Equation

"IMHO, it doesn't matter that the flow is left to right, bernie equates total pressure at two places on the circuit, allowing for interchange between static and dynamic pressure (and losses)"

If you did, you would wind up solving an equation where V^2 = a negative number. Try it! The reason is nonlinearity.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources