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API RP 521 - Corrected Hydrotest Pressure - confusing

API RP 521 - Corrected Hydrotest Pressure - confusing

API RP 521 - Corrected Hydrotest Pressure - confusing

Hello all,

While studying API RP 521 6th Edition (Pressure-relieving and Depressurizing Systems) I was confused by wording in Appendix C.7 Corrected Hydrotest Pressure Examples. I rewrote a paragraph in a way that makes more sense to me. After reading some thoughts by other users of this forum on this same Appendix, I wanted to share my interpretation on this forum in case it helps someone's understanding. If I'm in the wrong to post this publicly, let me know.

Specifically, I was confused by the second major paragraph of Page 222 discussing why a certain method is used to calculate the Corrected Hydrotest Pressure. The confusing paragraph starts and ends as such (most omitted since I imagine API doesn't like paragraphs of their standards posted publicly):

Quote (API RP 521 (6th Edition) section C.7.1 (page 222))

When evaluating an overpressure scenario, it is a common and generally ... at the temperature at which the hydrotest was conducted. In other words:

My clarification is bolded in the quote below:

Quote (baltakatei@gmail.com on 2016-02-03T00:56Z to API RP 521 (6th Edition) section C.7.1 (page 222))

When evaluating an overpressure scenario, it is a common and generally conservative practice to use the design temperature to determine the material stress value when the overpressure occurs For the example above, the vessel would ideally be warmed to its design temperature (650 °F) and hydrotested at its MAWP times the pressure design code hydrotest factor (H_f = 1.3 for ASME VIII Div 1 (2008)). After the vessel passes the hydrotest this pressure could serve as a limit above or below which certain practices could be advised (such as the use of Administrative Controls as described in 4.2.2).
Unfortunately, vessel manufacturers rarely hydrotest vessels at their design temperatures. Instead, the hydrotest is often performed at ambient temperature and the hydrotest pressure is multiplied by a temperature-dependent material stress ratio in order to account for the general trend of material allowable stress values decreasing with higher temperatures.
Therefore, in this example, the temperature-dependent material stress ratio must be removed from the hydrotest pressure. This corrected hydrotest pressure would then be an estimation of a successful hydrotest pressure of the vessel at design temperature (650 °F). The pressure can be calculated so:

I'd like your thoughts.

RE: API RP 521 - Corrected Hydrotest Pressure - confusing

I also find that paragraph to be confusing. To me, it's far less confusing to just skip down to C.7.2. There you'll find the two alternate equations for calculating the corrected hydrotest pressure, and a clear explanation of the variables involved.

Engineers tend to think that the ASME hydrotest pressure is simply 1.3(MAWP) without recognizing that ASME includes another multiplier in that equation. That multiplier corrects for the fact that hydro testing is done at ambient temperature (using ambient water) rather than at the vessel design temperature. This is the point that explained in C.7.1. It's impractical, and unnecessary, to heat the water to the vessel design temperature, and maintain it there for the duration of the hydrotest. That point is overlooked in your re-write. So, I wouldn't say that it's "unfortunate" that vessel fabricators don't perform the hydo at the vessel design temperature. In reality, that's just not a practical thing to do, and it's not necessary because we can easily adjust the hydrotest pressure to account for this temperature difference. That's what the ASME hydrotest equation does.

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