How to read tables 2-32 in API 526?

[aaliwa]

I am referring to API 526, 5th edition, June 2002
In table 16 for example, Chrome Molybdenum Steel: there are two rows both of which are 8T10 with 300x150 rating. why is it repeated?
It seems that the ratings (valve, bellows and flanges) are different between the two rows. shouldn't they are be the same?
IN other words, when I say a valve has 300# rating inlet and 150# rating outlet, shouldn't I have the same values for pressure ratings at different temperatures?
Thanks

 

[The Obturator]

aaliwa. Safety Relief Valves flange ratings can get confusing in API 526 and from manufacturer to manufacturer. One important point. On larger orifice sizes, such as the "T" you mention, the actual pressure rating is not achievable on the inlet (and outlet) as would be expected, as for a specified rating such as 300#. The reason here is the limitation of available spring design. When API 526 was put together many years ago, it was between certain manufacturers at the time. The result was based on limitations and what was serving the industry then (since then, very high pressures for large orifice sizes can be accommodated with pilot operated SRV designs or special spring designs by some vendors).

The example you quote actually has different pressure/temperature limits on both inlet and outlet for the 2 300# inlet T orifices'. Bellows designs also limited the outlet available rating.

Generally, lighter rated components (spring, bellows etc.), are used for the lower pressure section and may well impact the vendor economically.

API 526 is a user standard. Not all manufacturers may meet all listed size, rating, actual pressure rating configurations. The idea of this standard is that specified valve types are supposedly interchangeable when specified to API 526.

As far as I recall, API added these higher pressure, albeit with the same rated flange, but to be distinct against the earlier availability. Some, but not all, suppliers could meet it.

In practice, the manufacturers body and bonnet designs may well be the same for all 300#. It is API's way of separating the listed availability for conformity reasons.

Per ISO, only the term Safety Valve is used regardless of design.

 

[aaliwa]

Thanks a lot for your comprehensive and helpful reply. To confirm my understanding, it is not enough to specify flanges rating. but we need to confirm pressure and temperature ratings for the valve as well.
EDIT : or can I specify ASME B16.34 ratings as the relevant design standard for flange ratings instead? It seems that ASME B16.34 is more stringent in this aspect than API 526 (for example the values in parentheses in API 526 are less than those in ASME).

 

[The Obturator]

Yes, you need to confirm the flange rating for the VALVE capability, as per APR 526. If you have already settled on a manufacturer, I would suggest you use their SRV flange pressure/temperature ratings for the selected orifice size, from the curves in the corresponding catalogue. Not every manufacturer is equal. These curves are also generally incorporated into the calculation program selection process for APR 56 listed materials.

Per ISO, only the term Safety Valve is used regardless of application or design.