(4) don1980 (Chemical)
20 May 11 16:29
The last time I looked there were 10 chapters to ISO 4126. each covers a different topic. Some chapters apply only to manufacturers while others, called "application standards", apply to relief designers. You can easily find the topics covered in each capter by going to the internet. You can also find some good articles in trade journals that discuss ISO 4126. I've found the journal 'Valve World' to be the most helpful.
That said, I haven't found a good document/publication that summarizes the differences between ASME/API rules & practices and those found in ISO 4126 (or other similar EU application standards). I've spent 4-5 years piecing together different bits of info obtained from different sources, and I finally feel that I have a pretty good understanding of the differences. The puzzle is further complicated by the fact that ISO 4126 isn't the only EU harmonized relief design standard. There are others, such as EN 764-7 which applies to pressure vessel protection (I particularly like that one). PED simply says that you have to use a "harmonized standard", as opposed to the old country-specific standards. All harmonized standards are fully acceptable in all EU countries. in other words, no member country can take exception to anything in the standard, nor can they mandate additional requirements on top of those listed in the harmonized standard.
Here's a short comparison summary:
1) Inlet & outlet loss rules are the same (3% and 10%). Whereas API 520 Pt2 allows an engineering analysis to justify inlet losses higher than 3%, the EU standards don't. For example, it's common practice in non-EU countries to allow up to 5.0% inlet loss for existing valves if the blowdown is 7% or more (maintaining a minimum of 2% margin between BD and inlet loss).
2) The accumulation limits are the same, but this is often misunderstood. That is, some will claim that fire cases are limited to 10% in EU countries. That's not true. While PED does say that accumulation is limited to 10%, it doesn't specifically say that that applies to fire cases. There was an official PED inquiry in 2002-2003 asking if this 10% limit applied to fire cases, and the answer was "no". Interestingly, they didn't specify a limit for fire cases. They just said that you're not limited to 10%. You can find this in the interpretation section posted on the PED website. I think it's number 5/12. So, you can practice 21% accumulation, just like we do in ASME jurisdictions.
3) The rules for rupture disks are different. They don't have separate values for "burst tolerance" and "manufacturing range". Instead, they use a single term (the name escapes me) that places all of the tolerance on the low side (below the MAWP). So, you have to fill out the specification data sheets a little differently.
4) For overpressure protection, ASME & API limit us to relief devices, HIPS, or UG-140 (safety by system design). The EU standards allow relief devices, HIPS, and couple of other high-integrity control system alternatives that were legacy practices in old European standards. What's new is that the EU harmonized standards now allow pilot operated PSVs. This wasn't an option in the past.
You also mentioned ISO 23251. This is the same as API 521. Just like ISO 28300 is the same as API 2000. API has broaded its reach by "co-branding" API 521 and API 2000 as international ISO standards. However, the contents those respective documents are exactly the same as the API documents.