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.
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
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
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
I think you might need some information about the flow rate upstream of the valve before you can use Bernoulli.
ht
desertfox
RE: Bernouli Equation
I think I agree with zekeman in using (Q/C)^2 to determine the valve losses and get flow.
RE: Bernouli Equation
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=V1*60*pipe area/231
pipe area in in^2
from above post.
RE: Bernouli Equation
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
Being that it is incompressible I should be able to get flow from V1?
Thanks
RE: Bernouli Equation
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
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
RE: Bernouli Equation
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
RE: Bernouli Equation
look at my comment (6 Nov 11 11:10)and the OP sketch.
RE: Bernouli Equation
Q'= Cv*SQRT(deltaP/SG)
Check Q' with D'Arcy Weisbach equation (include also entrance effect and differential height).
RE: Bernouli Equation
RE: Bernouli Equation
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
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
If you did, you would wind up solving an equation where V^2 = a negative number. Try it! The reason is nonlinearity.