ASME B31.3 - Chapter IX - Pressure relief set value

[cex10k]

Dears,

I have sized a piping according to formulas of chapter IX "High pressure piping" (K304.1.2), the internal design gage pressure is 315 bar. Could I set the pressure relief valve at 330 bar (+4.7%)?

According to paragraph K322.6.3(b) I could. Or have I misinterpreted the text?

(b) System protection must include one relief device
set at or below the design pressure at the operating
temperature for the relieving condition, with no device
set to operate at a pressure greater than 105% of the
design pressure, except as provided in (c) below.

Thanks
Regards
Michele

 

[LittleInch]

I think what this refers to is where you have multiple pressure reliefs to cope with differing levels of relief flow. So you sometimes see a bank of two or more relief valves all set at different pressures.

what this section says is that one of those valves needs to be set at DP or lower with any of the other relief valves set up to 105% of the DP. Or this could mean a secondary item such as a bursting disc or bent pin type relief where small thermal expansion could be relieved using a small relief valve (set at or below the DP), but a catastrophic event is catered for using a high volume relief set at up 105%.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cex10k]

Thanks LittleInch.

Now I better understand the paragraph. I will use a small relief valve at DP and then the large ones at 105% of DP.
According to K322.6.3(a) the pressure during relief could rise up to 10%.

 

[Ehzin]

The intent of K322.6.3(b) is to provide an allowable set pressure over design for additional devices as part of a multiple relief valve setup. In your scenario, if you want to keep set pressure as high as acceptable to prevent premature lifting due to operating pressure being close to design you would put your set pressure at your design pressure (MAWP for some codes). A single device rated for the capacity would be set at 315 bar in your example for only one device needed. When calculating the rated capacity, you're allowed to perform the capacity calculations assuming a pressure 10% over this value. This is because in an overpressure scenario it's expected to temporarily exceed the set pressure up to 10%. If the relief scenario requires more capacity than what a single device can pass, you can install multiple relief devices in parallel. To prevent them all from lifting at the same time and "competing", say if you have just two, one of the relief valves can be set at 315 bar with the second relief device being set up to 330 bar. The allowable overpressure in this scenario when performing the capacity calculations can be up to 16% over the design pressure. This is because the 5% overpressure times 10% over is 16% (1.05 times 1.1 = 1.155).

The code still has the overpressure event start at or under the design it just gives certain allowances for the pressures to exceed the design considering both the nature of an overpressure scenario and the required overpressure of a relief device until it may experience full flow. I'm unsure if I clarified anything or just muddied the waters. If there's any questions or issues with anything stated above let me know and I can clarify if possible.

Edit: As a brief comment to your latest post, K322.6.3(a) allows the assumed pressure when performing capacity calculations to be up to 116% of set pressure or 16% over the base set pressure of 315 bar. So when performing calculations you can assume a pressure of 365 bar.

Thanks,
Ehzin