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RhoV2 criteria for Line Sizing

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causeandeffect

Petroleum
Apr 24, 2006
8
Dear members,
Has anyone heard of or use the criteria RhoV2 (fluid density multiplied by fluid velocity squared- has SI unit of kg/m2s ) for line sizing of gas or two/multiphase lines. I understand the criteria is to limit vibration in pipes. Does anyone know any internationally recognised code or standards that address this criteria?
cheers
 
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The general approach for pipelines I was aware of was that the optimum economic diameter is the one at which the sum of operating costs, interest and depreciation is a minimum.

[Δ]Pf [≈] L/50 Dh [ρ]V2/2 for gases with Re in the range 100,000 to 500,000.

BTW, your units aren't OK, they should be: kg/ms2, which means units of force per unit area, Pa or N/m2.

 
The mass flux is commonly used for applications where velocity or pressure drop criteria are insufficient. The conservation equations can be easily rearranged in terms of the mass flux. This is the form that is commonly employed when working with compressible/multiphase fluids. The mass flux remains constant throughout the length of the pipeline, whereas the velocity changes as a function of the pressure/density. Also, where choked flow may be a concern, there are equations and charts readily available identifying the choked mass flux. The mass flux limit that your line sizing is based on could be the choked mass flux or simply an 'industry proven' good practice. Either way, there is probably good reason why the limit was specified.

As far as standards go, I know that HEI uses mass flux sizing criteria for nozzles operating at saturation conditions.

I2I
 
Just realized that you have the velocity squared, is this correct? If so, it's really a limit on the kinetic energy, but the same argument applies.

Also, this is an established practice for control valve selection and could be used for line sizing as well.

I2I
 
The line sizing criteria for gas phase piping is usually based on allowable pressure drop , usually expressed as a % of inlet pressure. A 5% or 10% loss for a large gas piping system might be used. Knowing the system's total (fL/d ) can be used to yield the allowable pipe size, mass flow, velocity, etc to meet the desing % pressure loss. A final check is to ensure the outlet gas velocity is less tha ( 15% ) soundspeed to tensure low noise and vibration.

For incompressible liquids you could use 0.5 rho V^2, sometimes called "velocity head", but again that relates to overall pressure loss of the pipig system, if you know the total (fL/d ) and allowable pressure drop. Velocity limits are checked to ensure erosion is not an issue; 20 fps max for water in carbon steel pipe. Since liquid piping DP is a trade off with pump sizing and pump power, there is often an economic analysis done to see if there is an optimum pipe size/ pump size for minimum present value of pump power + investment cost. Traditional "economic pipe size" for water was 10 fps, based on this economic critieria but using 1950 cost for electric power.

2 phase flow has other issues, such as flow regime, erosion-corrosion, flow stability , etc.
 
No.

And rho v^2 has no consideration for length of line, which usually becomes one of the most important variables ...eventually.


"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
 

In the May 2008 issue of Hydrocarbon Processing is an article titled The effect of kinetic energy change on flow in gas pipelines by F.A. Demneh that can find your interest.
 
causeandeffect, the rho.v^2 criteria is commonly used to estimate the maximum or erosion velocity.

A typical maximum velocity (ft/s) = 100 to 150 / sqrt(density lb/ft3)

I consider this the maximum velocity for which a new pipe (in non-corrosive service) should be designed.
 
Rho-v^2 'at-caution' limit of 200000 kg/ms2 is recommended in NORSOK P-001 for gas relief & blowdown piping. Note that the dynamic load imposed at a change in direction is proportional to Rho-v^2.D^2


 
One company I worked for a while with used rhov2 as a secondary criteria for their flare headers. In kg/ms2 they had a limit of 100,000 which could be pushed to 150,000 if you analyzed the flare header for loads. They would not go over that limit. You also had to ensure obviously that allowable backpressures were also acceptable.

If you play around with it, it works out to the same as the API 14E erosional velocity limit with a fairly high C coefficient. Let me know if you want me to dig into my files and find out what C I backcalculated it was equal to.
 
Here's a link to a number of studies related to API RP14E that you might find interesting.

Be sure to check out the other interesting resources at the MMS website which is mostly related to offshore oil and gas industry.
 
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