110% pressure relief device setpoint per ASME BPV VIII UG-134(b)?

[paulsen2]

I have a sealed vessel with an MAWP of 300 psi. The only possible source of overpressure is heating (fire). I will have one rupture disk in the system. Can i set this rupture disk setpoint to 110% per UG-134(b)?

UG-134(b):
(b) For pressure relief devices permitted in
UG-125(c)(2) as protection against excessive pressure
caused by exposure to fire or other sources of external
heat, the device marked set pressure shall not exceed 110%
of the maximum allowable working pressure of the vessel.
If such a pressure relief device is used to meet the requirements
of both UG-125(c) and UG-125(c)(2), the device
marked set pressure shall not be over the maximum allowable
working pressure.

UG-125(c)
(c) Other than unfired steam boilers, when a pressure
relief device is provided, it shall prevent the pressure from
rising more than 10% or 3 psi (20 kPa), whichever is
greater, above the maximum allowable working pressure
except as permitted in (1) and (2) below and UG-127(d)(3).
(See UG-134 for pressure settings.)
(1) When multiple pressure relief devices are provided
and set in accordance with UG-134(a), they shall
prevent the pressure from rising more than 16% or 4 psi
(30 kPa), whichever is greater, above the maximum allowable
working pressure.
(2) When a pressure vessel can be exposed to fire or
other unexpected sources of external heat, the pressure
relief device(s) shall be capable of preventing the pressure
from rising more than 21% above the maximum allowable
working pressure. Supplemental pressure relief devices
shall be installed to protect against this source of excessive
pressure if the pressure relief devices used to satisfy the
capacity requirements of UG-125(c) and UG-125(c)(1)
have insufficient capacity to provide the required protection.
See Nonmandatory Appendix M, para. M-13 for cases
where the metal temperature due to fire or other sources
of external heat can cause vessel failure prior to reaching
the MAWP.

 

[EnergyMix]

How are you meeting the last sentence of UG-134(b)?

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[paulsen2]

I guess my thinking is that I do not have to meet UG-125(c) because of the exception "except as permitted in (1) and (2) below". Therefor because my relief device is excepted from UG-125(c) and meets UG-125(c)(2), i do not have to meet the stipulation for devices that meet both UG-125(c) and UG-125(c)(2).

Let me know if I am reading this wrong.