Flow in Gas Pipeline
Flow in Gas Pipeline
(OP)
Hello,
I have a quick question that I cant seem to get an answer to. The velocity for gas in a pipe is calculated taking into account temperature and pressure:
v=14.7349(Qb/ds)(Pb/Tb)(ZT/P)
I understand this to be the velocity of a gas particle on a streamline travelling from point a to point b.
My question is, what is Flow/Area for gas in a pipeline? It is a velocity, but what is this velocity? Is it a gas particle on a streamline as well? For the same system, the two equations give two very different velocities. Thanks!
I have a quick question that I cant seem to get an answer to. The velocity for gas in a pipe is calculated taking into account temperature and pressure:
v=14.7349(Qb/ds)(Pb/Tb)(ZT/P)
I understand this to be the velocity of a gas particle on a streamline travelling from point a to point b.
My question is, what is Flow/Area for gas in a pipeline? It is a velocity, but what is this velocity? Is it a gas particle on a streamline as well? For the same system, the two equations give two very different velocities. Thanks!





RE: Flow in Gas Pipeline
Can't work out what the 14.73 is, but assume this is some sort of conversion factor from whatever the units are on the RHS to a different set on the LHS.
I would just work out the actual volumetric flowrate at whatever pressure and temp you are running at is as a single line first to be clearer what you are doing and then divide by the internal surface area to get your velocity
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Flow in Gas Pipeline
My apologies, I made a mistake in the formula. It is:
v=14.7349(Qb/D2) * (Pb/Tb) * (ZT/P)
v=velocity (m/s)
Qb=flow rate (m3/hr) at standard conditions
Pb= base pressure kPa
Tb=base temperature K (273 + degrees celcius)
P=pressure kPa
T=average gas flowing temperature K(273 + degrees celcius)
Z=gas compressibility factor at the flowing temperature
The 14.7349 is just a factor for the conversion into SI units, it is dimensionless.
The formula comes from the book "Gas Pipeline Hydraulics" by E. Shashi Menon
Using modelling software, the flow is solved for, and so is the velocity in a pipeline. Yet when I take the flow that the model give me, and divide it by the cross sectional area of the pipeline, I get a much faster velocity then the actual velocity of the gas molecules, as predicted by the above equation.
My question then remains, what is the difference between these two velocities? They are substantially different.
RE: Flow in Gas Pipeline
Somewhere, hopefully, you have the manual for your modelling software. What equation is it using? Otherwise what results are you getting (include units please) from your modeling program and what is your cross-sectional area? Actual numbers might help.
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RE: Flow in Gas Pipeline
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Flow in Gas Pipeline
Nothing wrong there with the conversion factor. Dingelde has just missed that in the formulation reported in the book of E. Shashi Menon (SI) the flow rate is expressed in m3/day and not in m3/hr. In fact 14.7349*24 = 353,6376.
RE: Flow in Gas Pipeline
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Flow in Gas Pipeline
The velocity that is calculated that way is "bulk velocity" (the equation is a crutch, all you are doing is converting from SCM to ACM, you do that with a ratio of density, and then dividing the ACM by flow area [pi/4 is buried somewhere in the constant]). We all know that there is a velocity profile across the pipe that ranges from zero at the pipe wall to maximum at some point within the pipe (hopefully fairly close to the center). Plus in turbulent flow there are velocity vectors of randomly varying magnitude in every possible direction.
Volume flow rate over area does not give you that. It "resolves" that chaos into a plug the size of the pipe moving at a "bulk velocity". The number is slower than max velocity and faster than zero, but it is not necessarily the average of all of the velocity vectors. It is a convenient number to use in other calculations and it is as close to "right" as you are going to get in most real flows.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Flow in Gas Pipeline
The way I see it, is the Q/A velocity is your average velocity of all the particles in the cross section. This velocity is actually much quicker than the velocity used to calculate the individual velocity of a gas particle. I think what it tells us is that while the plug of gas is flowing quite quickly, the individual gas molecules are traveling much slower.
At the heart of this is a way to determine the time it takes for a gas line purge. Tough to do, as really this is a non steady state problem, and we only have steady state software.
RE: Flow in Gas Pipeline
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Flow in Gas Pipeline
RE: Flow in Gas Pipeline
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Flow in Gas Pipeline
What I cannot understand is how one can get different values of velocity using the formula reported in the book of Shashi E. Menon (Gas hydraulics pipeline) or in general a formula used by a reliable software from that which comes from dividing the actual volumetric flow rate by the area of the relevant pipe cross section. I’m just curious to see a practical example, on figures, which shows such a discrepancy. I have run some numbers on my PC and got the same values.
RE: Flow in Gas Pipeline
Starting pressure (source node)P1 = 500 kPa (gauge)
End pressure (flare stack outlet) P2= 0 kPa (gauge)
Pipeline inner diameter D1 = 137.74 mm
Pipeline length L1 = 900 m
60 mm PE Ball Valve V1 constant = 25 m3/hr-kPa
Purge Hose inner diameter D2 = 25.732 mm
Purge Hose length L2 = 10.0 m
Flare Stack inner diamter D3 = 49.34 mm
Flare Stack length L3 = 2.0 m
Friction Factor Equation - Colebrook-White
Base Temperature = 15 degrees celcius
Specific Gravity = 0.6103
Viscosity = 0.01144 cp
Friction Factor = 0.015
Roughness = 0.0015
From the attached picture, this is how I have it set up in the model. I have two known pressures, 500 kPa and 0 kPa. From that, the model can calculate the flow. Based on the diameters and legths, I get a flow of 1,784.273 standard m3/hr. The resultant upstream/downstream velocity for Pipe 1 is 5.6 m/s & 5.6 m/s. For Pipe 2 162.2 m/s & 838.9 m/s. For Pipe 3 228.2 m/s & 255.8 m/s.
Dont get hung up on the valve. This is to represent the riser coming out of the bell hole, which has a valve that can be throttled to ensure that the flare continues at a nice even pace.
The criteria for purging are:
1. Minimum pipeline velocity of 0.8 m/s to ensure displacement purge
2. Maximum flare veloicty of 150 m/s to ensure noise control
3. Minimum flare velocity of 5 m/s to ensure flashback will not occur.
4. Specify the amount of time it will take before a flare is seen
So, the question is, for 1 & 4, is it the "resultant velocity" that I use, or the standard flow over area that I use?
RE: Flow in Gas Pipeline
RE: Flow in Gas Pipeline
All the velocities are "bulk velocity" I don't know what the heck "resultant velocity" would even be.
One of the posters above said that the q/A velocity would be average. I can think of a half dozen ways to calculate "average" and they are all valid and each give a different number. The only number that makes all the other equations work is q/A. Any other valid way of calculating an average give you wrong answers when you use that velocity for anything (such as calculating acceleration or force).
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Flow in Gas Pipeline
Assuming for a second that I can get velocity, does it make sense to use that velocity to predict the time required to purge? From the time they release the squeezer to the time that the flare begins is all unsteady state. The pressure in the pipe is building as the gas propogates throughout the pipe, displacing the air. I thought maybe using the flow or velocity the model gives would help to approximate the time to see a flare from squeezer release.
My theory is that using the flow or velocity from the model will give you a much quicker time than actual. As soon as the squeezer is released, the initial pressure in the pipe would be near zero. The gas would propogate, displacing the air. I think that using a starting pressure of 500 kPa (system pressure) is unrealistic, and should maybe reduced to around 30 kPa to account for this effect.
RE: Flow in Gas Pipeline
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow in Gas Pipeline
The reason for standard flow rate to act like mass flow rate is because the flowing gas is converted to imaginary conditions at every point and this imaginary temperature and pressure is the same at every point. You cannot use imaginary conditions to calculate a physical property like velocity. You have to convert to actual. v(actual)=q(std)*ρ(std)/ρ(actual)/A. If you skip the density over density step you get nonsense from imaginary land.
Yes, I use bulk velocity to estimate when gas will arrive at the end of the pipe.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Flow in Gas Pipeline
dingelde, in order to get an estimate of the time required to see flare, I would use an average value of the actual velocities for each segment (v1+v2)/2, being v1 the actual velocity at the beginning of a segment and v2 the actual velocity at the end of the same segment.
RE: Flow in Gas Pipeline
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Flow in Gas Pipeline
zdas04/ione thanks for your thoughts. I wouldnt have been too concerned with it if it wasnt for the fact that the two velocities (and therefore time) are so different. Using the standard flow rate gives a velocity about 6X faster than the calculated "actual" velocity for pipe 1 in the example I gave above. I may consider using the slower velocity to ensure displacement is occuring. The time estimate is not as big of a deal, I will run some field tests and see what I get. Thanks!
RE: Flow in Gas Pipeline
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Flow in Gas Pipeline
RE: Flow in Gas Pipeline
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat