4 replies to this topic

[JoeWong]

Dear all,

API released the API Std 520 Part 1, "Sizing, Selection and Installation of Pressure-relieving Devices in Refineries Part 1 - Sizing and Selection Eighth Edition, Dec. 2008, 2 months ago.

A few identified important points :

i) It became a STANDARD, instead RECOMMENDED PRACTICES
ii) Emphasis use of ideal gas specific heat ratio for vapor relief
iii) Use of Homogeneous Equilibrium Model (HEM) based on thermal and mechanical equilibrium for two phase relief. Nevertheless, the Leung Omega Two point method is still remained as a choice of method.
iv) Two phase Vapor Liquid Relief method presented in Annex B have not been validated by test (accoring to API) and there is no any recognized procedure for certifying the capacity of PRVs in two phase flow service.

If you aware of other, please add.

[gvdlans]

Thank you for this important information.

Did you notice that in the thread header you mentioned API Std 521, while in your post you mentioned API Std 520 Part I...

[JoeWong]

Thank you for this important information.

Did you notice that in the thread header you mentioned API Std 521, while in your post you mentioned API Std 520 Part I...

Ah...thanks.

[skearse]

One point Omega method is now gone.

I've used the HDI and 2 point Omega methods a couple of times so far and the nice thing about it is that they both give roughly the same flux for sizing. It's a nice way to sort of back check your calcs. Another nice thing about the HDI method is that it seems to reduce to the single phase equations if you have single phase flow. For example, if you took the HDI methodology and used a vapor fraction of 1.0, you'd end up with the same approximate sizing as if you had used a vapor sizing equation (and for a vapor fraction of 0.0 you endn up with the same as a liquid-sized valve).

For two phase sizing API states that the discharge coefficient is now more accurate, and this edition adds a viscosity correction factor.

[JoeWong]

Thanks for your inputs.