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Gas velocity in straight pipe
- Thread starter martynast
- Start date
LittleInch
Petroleum
Yes there are many formulae, but most are interested in flow or pressure drop then you need to work out velocity based on pressure at the point you're interested in.
If pressure varies from one end of the pipe to the other by more than 10%, the change in density of the gas becomes significant and you need different methods than just assuming an average density.
you need to describe your system in much more detail to get the best formula.
There is no one simple formula that covers all varients that you can plug into a spreadsheet or program if that's what you're looking to do.
If you can explain more what you're trying to do then maybe we can come up with some ideas, but currently this is far too vague.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
If pressure varies from one end of the pipe to the other by more than 10%, the change in density of the gas becomes significant and you need different methods than just assuming an average density.
you need to describe your system in much more detail to get the best formula.
There is no one simple formula that covers all varients that you can plug into a spreadsheet or program if that's what you're looking to do.
If you can explain more what you're trying to do then maybe we can come up with some ideas, but currently this is far too vague.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
A number of important points:
[ul]
[li]There is no equation for velocity if you don't know volume flow rate.[/li]
[li]There is no closed form equation for volume flow rate at arbitrary conditions.[/li]
[li]You need to pick one of the empirical equations for volume flow rate (e.g., AGA, Panhandle, Weymouth, Isothermal, etc) that comes closest to your conditions.[/li]
[li]Most empirical equations will give you volume flow rate at standard conditions. This is a seed number for velocity, but not a determinant number.[/li]
[li]You have to convert volume flow rate at standard conditions to volume flow rate at actual conditions.[/li]
[li]Once you have volume flow rate at actual conditions then you can simply divide it by the cross sectional area of the pipe to get an average velocity at that point.[/li]
[li]Since velocity is zero at the pipe walls (i.e. the "no flow boundary"), you can make a number of assumptions and determine the maximum velocity which will always occur some distance from the pipe walls but not necessarily at the pipe centerline.[/li]
[/ul]
As LittleInch said, there are whole books written on the determination of fluid velocity, which is a key element in so many important calculations.
[bold]David Simpson, PE[/bold]
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
[ul]
[li]There is no equation for velocity if you don't know volume flow rate.[/li]
[li]There is no closed form equation for volume flow rate at arbitrary conditions.[/li]
[li]You need to pick one of the empirical equations for volume flow rate (e.g., AGA, Panhandle, Weymouth, Isothermal, etc) that comes closest to your conditions.[/li]
[li]Most empirical equations will give you volume flow rate at standard conditions. This is a seed number for velocity, but not a determinant number.[/li]
[li]You have to convert volume flow rate at standard conditions to volume flow rate at actual conditions.[/li]
[li]Once you have volume flow rate at actual conditions then you can simply divide it by the cross sectional area of the pipe to get an average velocity at that point.[/li]
[li]Since velocity is zero at the pipe walls (i.e. the "no flow boundary"), you can make a number of assumptions and determine the maximum velocity which will always occur some distance from the pipe walls but not necessarily at the pipe centerline.[/li]
[/ul]
As LittleInch said, there are whole books written on the determination of fluid velocity, which is a key element in so many important calculations.
[bold]David Simpson, PE[/bold]
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
georgeverghese
Chemical
Assuming P2 is not much lower than than P1, then gas velocity can be assumed to be nominally constant across the entire length. In this case, velocity is related to mass throughput by
v = m/(ρ•0.785•d^2), where m = mass rate in kg/hr, d = pipe id in metres, ρ= gas density in kg/m3, and v = m/sec. Here, ρ = MW•P/z•R•T, where P is gas pressure in kN/m2 abs, z= gas compressibility factor, R=universal gas const.=8.314kJ/kgmole/degK, T= gas temp in deg K = 273.2 + degC
To get m, have a read of Crane TP410, Fluid Flow or any other Uni text that deals with fluid flow. Include any fitting losses such as elbows, entrance and exits and any other losses from valves etc.
v = m/(ρ•0.785•d^2), where m = mass rate in kg/hr, d = pipe id in metres, ρ= gas density in kg/m3, and v = m/sec. Here, ρ = MW•P/z•R•T, where P is gas pressure in kN/m2 abs, z= gas compressibility factor, R=universal gas const.=8.314kJ/kgmole/degK, T= gas temp in deg K = 273.2 + degC
To get m, have a read of Crane TP410, Fluid Flow or any other Uni text that deals with fluid flow. Include any fitting losses such as elbows, entrance and exits and any other losses from valves etc.
georgeverghese,
Mass flow rate is constant in a line that does not have any additions or removal of mass. That is ALL you can say about rate data along a line. If you are trying to differentiate 100 m/s from 1,000 m/s or 10 m/s your approach is fine. If you are trying to calculate a non-dimensional parameter for similtude then it ain't close. Why would you even say such a thing? If pressure is constant along a line, then there would be no flow and the bulk velocity is zero. With real flow there is a pressure drop.
With a pressure drop there is a velocity increase. Sometimes the increase is significant. To tell someone to be concerned about fitting losses, but not velocity changes is kind of odd.
[bold]David Simpson, PE[/bold]
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
Mass flow rate is constant in a line that does not have any additions or removal of mass. That is ALL you can say about rate data along a line. If you are trying to differentiate 100 m/s from 1,000 m/s or 10 m/s your approach is fine. If you are trying to calculate a non-dimensional parameter for similtude then it ain't close. Why would you even say such a thing? If pressure is constant along a line, then there would be no flow and the bulk velocity is zero. With real flow there is a pressure drop.
With a pressure drop there is a velocity increase. Sometimes the increase is significant. To tell someone to be concerned about fitting losses, but not velocity changes is kind of odd.
[bold]David Simpson, PE[/bold]
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
racookpe1978
Nuclear
To get even more detailed, you need to determine "how" the fluid (gas, liquid, slurry, or whatever) is flowing because that begins the approximations and assumptions used in the above different methods: Turbulent is most common in gas flow, but laminar is possible. More likely in liquid or slurries.
In all cases, fluid velocity will vary in different ways across the dia of the pipe.
In all cases, fluid velocity will vary in different ways across the dia of the pipe.
georgeverghese
Chemical
Hi David, Which is why I said "nominally constant"; which may be interpreted as v2 is not more than 1.1*v1, or that rho-1 is not more than 1.1*rho-2.
@martynast, Run the calc based on this simplified assumption of constant velocity first to see if P1 is more than 1.1*P2. If it isn't, then you've got the answer. If it is, then use the more elaborate isothermal compressible flow equation in TP410 or your Uni text to get the mass flow.
@martynast, Run the calc based on this simplified assumption of constant velocity first to see if P1 is more than 1.1*P2. If it isn't, then you've got the answer. If it is, then use the more elaborate isothermal compressible flow equation in TP410 or your Uni text to get the mass flow.
LittleInch
Petroleum
Well the OP has logged in once after posting his only ever question so I'm not anticipating too much, if any, feed back...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
btrueblood
Mechanical
gv said:
For that equation to work as stated, mass flow needs to be in kg/sec; as written, the velocity will come out in m/hour.
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