Hem Vs. Hne Impact Of Iso 4126 On Psv Sizing For Two Phases 2

[mdepeche]

I am evaluating the impact of HNE (ISO 4126) model over HEM (API Omega) for liquid and two-phase (vapor+liquid) flows,
as I understand the HNE model predicts critical flows in excess of 200% and more over HEM model and that can signicantly reduce the size of orifices under certain conditions,
how are engineering companies reacting to this (upcoming) standard ?
Should the decision (about the procedure to apply) left entirely on the hands of manufacturer ?
We are purchasing several PSV for vapor+liquid service and I have noticed that for some orifices the area calculated with ISO-4126 is about 50% of the area from API Omega.

 

[PaoloPemi]

the area where HNE predicts a critical flux higher than HEM is limited to small fractions (up to about 15%) of vapor mass quality, similar behaviour for subcooled fluids,
see the attached Excel page distributed with Prode Properties (prode.com),
according many experts values calculated with HNE are more accurate than HEM then I would accept that solution providing the manufacturer shows specific experience is this kind of applications.

 

[Latexman]

Basically, you are betting the fluid does not have enough time to reach equilibrium before it is through the relief system (or part of the relief system where the HNE assumption was made). Has anyone seen data supporting this? Since HNE is not a conservative assumption, I would want to see some convincing data before using HNE to size a relief device. IMO, HEM is inherently conservative, so it is an easy choice when said data is not available.

Good luck,
Latexman

 

[PaoloPemi]

In Europe there has been a lot of discussion related to HNE-DS model now proposed as standard,
honestly I am not entirely persuaded about HNE theory, I prefer to think about Non-Homogeneous flow (which in my opinion is more in agreement with the speeds of sound that I can calculate), but anyway the values predicted by HNE seem to fit better than HEM experimental data (see for example the report from Sozzi & Sutherland on critical flow of saturated and subcolled water)

 

[mdepeche]

thanks both for the comments!

Latexman,
yes compared with HEM the HNE-DS seems less conservative, supposing for example that vapor fraction is not constant the orifice may be undersized... to me this method seems much more critical with reference to process condition.

PaoloPemi,
yes I am aware that HNE gives values comparable to HEM for vapor fractions > 0.15 but I have a couple of valves discharging liquid propane (chiller condender) where the differences are in excess of 200%

 

[apetri]

mdepeche,
our policy for PSV sizing is to follow API standards,
however we can adopt different methods if the accuracy has been proven.