MDMT Questions
MDMT Questions
(OP)
Hey everyone, had a question regarding MDMT. If I am understanding the Code correctly, MDMT is a value the vessel can operate at and still have a high toughness so that the material is in a ductile region when it is failed. Is that right?
That being said if I operate below MDMT, theoretically is the vessel still able to operate and pass all the other Code requirements, but now if my vessel fails I will fail in a brittle manner (not leak before break)? Is this right?
Finally, when calculating leak before break on thicker vessels with an assumed flaw, does this calculation really care about MDMT? Can you have a leak before break condition with a vessel that is operating below it's MDMT?
That being said if I operate below MDMT, theoretically is the vessel still able to operate and pass all the other Code requirements, but now if my vessel fails I will fail in a brittle manner (not leak before break)? Is this right?
Finally, when calculating leak before break on thicker vessels with an assumed flaw, does this calculation really care about MDMT? Can you have a leak before break condition with a vessel that is operating below it's MDMT?





RE: MDMT Questions
Code does not permit "pressurization" at a temperature below the MDMT for a vessel - period. That would be a flagrant violation of code requirements on it's own so I'm not sure there is much point to talking about whether you are otherwise in compliance with the code. Note: "pressurization" for a Div 1 vessel would be 35% of the MAWP.
Whether you are above or below MDMT may not affect whether or not a flaw progresses to failure, although you are probably be more likely to propagate a crack to failure in a brittle condition. However, there are many flaws that could result in a catastrophic failure in a brittle condition but which might only have resulted in leak in a ductile condition. Take a look at the linked picture of a vessel that failed in a brittle manner.
Since code is concerned with the metal wall temperature, there is some latitude to consider what the metal wall temperature is relative to transient process temperatures or ambient temperatures. Regarding process temperatures, you need to consider normal and abnormal operation and auto-refrigeration effects. Regarding ambient, you need to consider ambient in view of normal process conditions and determine a reasonable design minimum temperature; this should also include consideration of what would happen in a shutdown in cold winter conditions and how you might start the vessel up again without "pressurizing" it while below the MDMT.
RE: MDMT Questions
RE: MDMT Questions
'Leak before break' is the demonstration that a flaw will only grow in such a manner that, by the time it traverses the wall thickness, it is of sufficient size to be noticeable and still of insufficient size to result in a catastrophic failure. It's not a simple process and is outlined in BS 7910 annex F.
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
I stand corrected but I'm not sure that was the scenario proposed.
Note: In Canada, CSA B51 provides some additional impact testing requirements that would apply over and above the requirements (and exemptions) that might be presented in ASME VIII Div I so it is always worth reviewing those requirements as well (in Canada).
If I'm not mistaken, UCS-160 provides certain conditions under which the vessel can be operated at less than the MDMT.
Okay, you're right to the extent that those rules do allow some reduction below the MDMT provided that the operating stress ratio is less than 1.0 (e.g., you are operating below the MAWP). Below the pressurization level I mentioned (35% of MAWP) then there is a blanket exception.
RE: MDMT Questions
RE: MDMT Questions
You can have other design combinations of MDMT (and MAWP) that would typically involve a lower MDMT and MAWP that the code might also permit. In my opinion, when you know there are multiple such design cases, indicate more than one MDMT / MAWP combination on the nameplate so that this is clear to the end users that more than one possible operating scenario is okay. UG-20(b) discusses this option.
The other complicating factor is that some manufactures stamp the MDMT as the minimum temperature specified in the original PO or vessel data sheet. For example, this might be 0 C when based on the materials in use, this vessel might have a real MDMT of -29 C at the specified MAWP.
RE: MDMT Questions
the calculations say what it is really rated for without additional charpys.
RE: MDMT Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
thread292-259316: Impact test temperature for sub-size impact test specimen
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
Perhaps this is a bit more conservative than code since code does allow some temperature suppression with any pressure drop below MAWP, but I find this is relatively simple to explain and implement and due to the non-linear nature of the temperature decrease on blowdown, it greatly increases the minimum temperature you design to relative to designing for blowdown to atmospheric pressure.
On occasion, since blowdown is a transient condition, I have also allowed the EPC firm doing the work to complete detailed metal wall temperature analysis (dynamic over time) to predict the lowest metal wall temperature that would be expected rather than the process temperature predicted.
RE: MDMT Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
Where did that 35% of MAWP for "pressurization" come from? Just curious, I have never heard of a value for that concept.
RE: MDMT Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
RE: MDMT Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions
RE: MDMT Questions
No. If you review API 579 there is a broad definition of what envelop for CET - startup, shutdown, upset and standby conditions. If you have a leak and auto refrigeration occurs this does not fall under the above categories. If a leak develops an assessment is made to either operate or shutdown. However, this is a separate event, similar to a hazard analysis.
The bottom line is this, the Design Engineer has the ultimate responsibility to evaluate process scenarios AND atmospheric conditions that would result in the lowest metal temperature under the conditions I mentioned. One also has to balance the fact that you can design for -320 deg F and see what clients are going to pay for this.
RE: MDMT Questions
Determine the minimum temperature tmin, that the component may be subjected to during operation, use the lowest of the following:
1) The minimum design temperature
2) The minimum temperature as estimated by the process engineer, including upsets
3) If the vessel or pipe is filled with a pressurized liquid, the boiling point of the liquid at atmospheric
pressure. For example liquid ammonia has a boiling point of –33°C [–28°F] and propane has a boiling point of –40°C [–40°F].
This language is much broader. I do not personally believe that a loss of containment scenario needs to be addressed in a FFS or RBI, but I'm trying to come up with a reasonable argument to defend that opinion. The best I can come up with is to have bolt-up and pressure test procedures to minimze the chance of a flange leak and monitoring (leak detectors, video cameras and operator rounds) to minimze the leak duration, should one occur. I was wondering if anyone else had attempted to make a similar case.
RE: MDMT Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: MDMT Questions