Interesting Equation 1

[zdas04]

I was looking through a company piping standard today and found a maximum allowable velocity of:

v_max=380 ft/sec * SG * T^0.5 / P^0.1

Temperature in Rankine, Pressure in psia, and SG relative to air.

The equation results in pretty reasonable numbers (higher pressures or lower temperatures allow lower velocities which makes sense if it is trying to limit momentum), but I can't figure out why it works. Does anyone have a feel for where this empirical equation might have come from?

David

 

[zdas04]

pmover,
The API equation is a constant divided by the square root of density (you didn't say what "rmix" is, but I'll assume it is density of the mixture). This "constant divided by the square root of density" equation is a liquid model which results in outrageous velocities for gas. For example at 30 psia and 60F, a 0.65 SG gas gives the following values:

Continuous = 314 ft/sec (0.237 Mach)
Intermittent = 393 ft/sec (0.296 Mach, this is the highest velocity where many researchers will use incompressible flow assumptions)
Erosional = 503 ft/sec (0.379 Mach)

No one would intentionally design pipeworks with those velocities.

The company in question has not done any original research in a couple of decades. The standards author that first included this equation got it from somewhere (granted, there is no assurance that somewhere along the line it wasn't modified via typo, which is why I started this thread to find out if anyone else had seen it so I could verify that it had been entered correctly).

David

 

[CRG]

David you are correct. API RP 14E erosional velocity doesn't look like it has much to do with the origin of your equation in your first post.

The equation from API RP 14E uses the erosional velocity equation in the context of two phase gas/liquid flow.

According to the "Handbook of Natural Gas Transmission and Processing," it states:"In most pipelines, the recommended value for the gas velocity in the transmission pipelines is normally 40 to 50% of the erosional velocity (Mohipour, 2002)"

Also it states that if there is much CO2, velocities should be limited to less than 50 ft/sec.

 

[jmw]

There is a blog on erosion over at cheresources


JMW

http://www.ViscoAnalyser.com