API 650 Appendix V - 8.1.2

[chrismoore2]

All,
Can someone help me with the units defining the design external pressure? I am using Mathcad, and plugging in directly (Psi = 2.5, HTS = 1.97m, D = 1.97m, tmin = 6mm, E = 200GPa) I get an anwer of .236Pa. However, I noticed in the nomenclature that E is defined in MPa and tmin in mm. Which if you just plug and chug the numbers without units conversion you get 85.187 Pa (I assume Pa, the equation just says "SI units"). Does anyone know what the 15203 value is?

Just an FYI, I am trying to size a vacuume relief valve for a 2.5 kPa draw on my tank.

Thanks!

 

[JStephen]

I don't know about that particular equation, but it is common in API-650 to have equations that are not dimensionally consistent- the definitions of the terms should define the units to be used (ie, somewhere, it should say that "P is external pressure in MPa (psi)" or whatever the units actually used are.

 

[fegenbush]

Both Ps and Pe are defined in V.3.1 as having the units of kPa (lb/ft^2). Do not assume consistent units in API-650, as JStephen said.

 

[chrismoore2]

Thanks guys.

Just an FYI for anyone who runs accross this. For the equations that call out D/tmin; divide by 1000 to be dimensionally accurate. In equation V8.1.2, also divide the top variable "E" by 1000000 to move it from GPa to MPa. Numbers will come up the same as if you did them non-dimensionally.

 

[chrismoore2]

Edit : In equation V8.1.2, also divide the top variable "E" by 1000 to move it from GPa to MPa and keep the answer in kPa. Numbers will come up the same as if you did them non-dimensionally.