Rupture Disk Pressure/Temperature Setting
Rupture Disk Pressure/Temperature Setting
(OP)
The inspector in our plant refuses to allow rupture disks that have a pressure rating at ambient that is greater than MAWP. I say that the ASME only requires that the rupture disk pressure rating is less than MAWP at the temperature it is expected to rupture. Our vendor always stamps the disk with the specified burst pressure and temperature and the burst pressure at ambient conditions.
Example:
A vessel has an MAWP of 200 PSIG at 300 Deg F. The operating conditions are 150 PSIG at 250 Deg F. The rupture disk is specified to burst at 200 PSIG at 250 Deg F. When the disk is received from the vendor the disk will be stamped 197 PSIG (due to manufacturing tolerance) at 250 Deg F and at 215 PSIG at 72 Deg F. Our inspector will reject this disk because of the 215 PSIG rating at ambient.
I think our inspector is wrong, can anybody give me supporting evidence? Also, can anybody tell me why the vendor stamps with both the specified and ambient conditions. I know the vendor has to test at ambient but why stamp the disk?
Example:
A vessel has an MAWP of 200 PSIG at 300 Deg F. The operating conditions are 150 PSIG at 250 Deg F. The rupture disk is specified to burst at 200 PSIG at 250 Deg F. When the disk is received from the vendor the disk will be stamped 197 PSIG (due to manufacturing tolerance) at 250 Deg F and at 215 PSIG at 72 Deg F. Our inspector will reject this disk because of the 215 PSIG rating at ambient.
I think our inspector is wrong, can anybody give me supporting evidence? Also, can anybody tell me why the vendor stamps with both the specified and ambient conditions. I know the vendor has to test at ambient but why stamp the disk?





RE: Rupture Disk Pressure/Temperature Setting
To my mind, having the disk stamped at the desired relieving pressure at the expected temperature is no different than specifying a cold test pressure on a PSV (which will be higher than the equivalent 'hot' set pressure) in order to ensure when in operation at operating temperature, the PSV opens at the design set pressure.
UG-127, 3.c.3 seems to address what you are looking for and it states "The stamped bursting pressure of the rupture disk ... However, in no case shall the stamped bursting pressure of the rupture disk at the coincident temperature plus any pressure in the outlet piping exceed the maximum allowable working pressure of the vessel ...".
The biggest problem is assuring him that the coincident temperature at rated set pressure is realistic. Often, the location of a rupture disk can be such that it can see much lower temperatures than the vessel or if it is steam traced, estimating the temperature is a factor of the how much steam tracing is installed (eg # number of loops) AND the ambient temperature at the time of relief (which I just finished working through for one case and is not fun). In that case, if the disk temperature is lower than specified, the disk would fail at a higher than intended pressure.
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
Remember there are two kinds of tolerances built into the design of a rupture disk: One is the Manufacturing Range (varies with type of disk, cost, material and burst rating) AND the second is the burst TOLERANCE (+/-5%).
If the MAWP is 200 psig for your example and the Mfg Range is +6 psi and -4 psi; then the specified max STAMPED burst pressure should be = 200-6=194 psig and the min STAMPED burst = 194-4=190 psig. Thus you should order the disk to be STAMPED from 190 to 194 psig. The manufacturer may stamp the disk anywhere in this range (unless you request a tighter mfg. range - you can even get a 0% range sometimes but it costs a lot more). Once the disk is stamped, THEN the 5% code tolerance applies (if above 40 psig). So if the disk is stamped for example at 192 psig (between 190 and 194) then the code guarantees actual disk rupture between 180.5 and 201.6 psig. The other check is that the burst and oeprating margin be adequate; a tension disk requires a 70 % margin; while a reverse buckling type disk cna withstand up to a 90% margin. In your example, if you used a tension type disk, the maximum operating pressure must be limited to 0.7 x min burst pressure = .7x190= 133 psig (I prefer including the 5% code tolerance which is equal to 0.7x190x.95=126.35 psig).
Disk burst pressure is not determined like relief valve set points are (many people make this mistake).
Apologize if you alrady knew all this; but it may help others.
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
Remember there are two kinds of tolerances built into the design of a rupture disk: One is the Manufacturing Range (varies with type of disk, cost, material and burst rating) AND the second is the burst TOLERANCE (+/-5%).
If the MAWP is 200 psig for your example and the Mfg Range is +6 psi and -4 psi; then the specified max STAMPED burst pressure should be = 200-6=194 psig and the min STAMPED burst = 194-4=190 psig. Thus you should order the disk to be STAMPED from 190 to 194 psig. The manufacturer may stamp the disk anywhere in this range (unless you request a tighter mfg. range - you can even get a 0% range sometimes but it costs a lot more). Once the disk is stamped, THEN the 5% code tolerance applies (if above 40 psig). So if the disk is stamped for example at 192 psig (between 190 and 194) then the code guarantees actual disk rupture between 180.5 and 201.6 psig. The other check is that the burst and oeprating margin be adequate; a tension disk requires a 70 % margin; while a reverse buckling type disk cna withstand up to a 90% margin. In your example, if you used a tension type disk, the maximum operating pressure must be limited to 0.7 x min burst pressure = .7x190= 133 psig (I prefer including the 5% code tolerance which is equal to 0.7x190x.95=126.35 psig).
Disk burst pressure is not determined like relief valve set points are (many people make this mistake).
Apologize if you already knew all this; but it may help others.
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
If the disk is at top of tank and fire is beneath tank, the designer should have the disk rated to burst at the actual disk temperature (ambient in your example). The disk actual burst temperature must be specified. If I did not feel I knew with certainty what this was; then I would use ambient temperature. That's common sense. The example you state is an example of an invalid disk specification.
The code says what I've stated:
UG-127(a)(1)(a): "Every rupture disk shall have a stamped bursting pressure within a manufacturing range at a specified disk temperature (note 44),....."
Note44: "The specified disk temperature supplied to the rupture disk Manufacturer shall be the expected temperature of the disk when an emergency condition exists and the disk is expected to rupture."
UG-127 (a)(1)(b) goes on to say how many tests are done at the elevated temperatures and how many are done at room temperature; and how to interpolate between these temperatures to rate the disk for its actual stamped burst pressure at its coincident burst temperature.
The burst pressure at ambient that is included on the disk namplate is a requirement and is provided to state the base on which interpolation is made for coincident burst pressure and temperature.
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
Lets try this again, I may have been too quick to respond and not in the best of moods. Sorry!
I agree with your ambient temperature position IF it is the actual temperature of the disk at relief conditions. But then it is an incorect disk specification (the elevated temperature should not have been specified in the first place) and we should not even be having this discussion.
But if it is a fire basis for overpressure even as you stated (here is where I've been re-thinking the problem); then the vessel internals will have to come up to relief conditions at its equilibirum vapor pressure and temperature - thus it would relieve at its elevated temperature. That being the case, all that TD2K and I have discussed is true and the inspector should not disallow the disk because of the ambient equivalent pressure rating being greataer than the MAWP of the vesel at its elevated temperature.
After all, even the vessel has a higher rating at ambient conditions versus its elevated relieving condition.
The more you learn, the less you are certain of.
RE: Rupture Disk Pressure/Temperature Setting
I do not plan on responding any further to this issue which has been beat to death and probably no longer being looked at by the originator.
Think twice before responding once.
RE: Rupture Disk Pressure/Temperature Setting
RE: Rupture Disk Pressure/Temperature Setting
Thanks for input. I'll say this, I have actually measured disk temperature much less than vessel operating temperature. So it is possible and not even unusual. In my case the operating temperature was over 300 Deg F, but the disk temperature only SEVERAL FEET AWAY from the hot vessel was cool enough to TOUCH without discomfort! So I can envision a rupture disk spec rated to burst at a temperature between atmospheric and the elevated relief temperature. I look for this all the time. I don't have a problem with that. But once you investigate and state what that temperature for the disk is at the required burst pressure - THEN you should not be held by the inspector to the requirement that burst rating be less than the MAWP at the atmospheric temperature that the disk vendor includes. So I still agree with you in principle - it all depends how detailed the engineer is nad how much information he has regarding the installation. In some cases where little is known, I might even agree with RPG; but its still a position of preference, NOT A CODE REQUIREMENT. RPG claims that it is but the code cannot be interpreted that way.
The more you learn, the less you are certain of.