4 ms·

I dont understand what the fault is - quenching is bad but happens. Blocking the quench path sounds bad - is it saying that something is causing it to be block

I dont understand what the fault is - quenching is bad but happens. Blocking the quench path sounds bad - is it saying that something is causing it to be blocked or is the machine supposed to do something else if hte quench path is blocked (what can it do??)

EA-3167 3y ago
The rapid conversion from liquid or solid to gas, and the means to (briefly) contain that... is a bomb. The reason you don't block the emergency relief is that you turn a quench from an emergency involving asphyxiation, to something resembling a detonation.
- trebligdivad 3y ago
  Right, but that's true of all MRI's - so what's different in this one?
  - refulgentis 3y ago
    "During a quench, which is not common, a large amount of helium evaporates and is vented outside the building through a venting system," the recall explains. "If an unknown blockage is present in the venting system and the pressure exceeds design limits, the structural integrity of the system could be compromised."
    - op00to 3y ago
      This is a risk of any mri machine that uses helium to cool a magnet. Why are only these machines being recalled when if you block the exhaust of any mri scanner it will blow up?
      - jabiko 3y ago
        This is just speculation, but maybe there is a secondary rupture disk that vents the helium inside the room when the primary path (the exhaust to the outside) is blocked. And maybe that secondary rupture disk is too strong, so another random part of the helium containment fails, blowing up the machine and possibly injuring the patient. But please note again that this is pure speculation.
        op00to 3y ago
        Thank you - this makes a lot of sense as. :)
    - s1artibartfast 3y ago
      That doesn't explain what is different. I assume that is a feature common to all machines.
      - refulgentis 3y ago
        It's not. Thats why it's being recalled. God, HN. Answers right in the article. Numerous people reach to turn it into a mystery.
        s1artibartfast 3y ago
        The problem is you dont know what you are talking about and dont even understand the questions people are asking. Every MRI machine quenches and vents. The question is what is different, in detail, that heightens the risk with this machine. your response is like saying "the engine wont work" as a complete explanation for why a type of car has more automotive problems than another. Then, when a mechanic is curious about the nature of the failure, you call them stupid, saying "I already told you, i said it doesnt work!" I certainly hope you are not in charge of solving problems in any professional capacity.
        refulgentis 3y ago
        > Every MRI machine quenches and vents. The question is what is different, in detail, that heightens the risk with this machine. The design can't safely accommodate the level of energy that ends up in the machine if its clogged. In the spirit of analogies, it's like asking why a 10 oz glass overflows with 200 degree water and a 10 oz glass doesn't overflow with 100 degree water. It exceeded it's capacity. There's nothing more complicated to it, you could call in an MRI technician, get the PDF manual, talk to several doctors, disassemble it and put it back together, and learn no more than that. You got extraordinarily rude and personal, I hope you don't do that often on HN, that was an outlier in my 13 years here. I hope all is well and it's an exception.
    - Piezoid 3y ago
      Why not add a pressure relief valve on the quench path with a very loud whistle? That should be enough to take care of such rare and compounded failures. What does recall means in this context? De-energizing the superconductor and shipping it back? Seems like a waste and a planning nightmare.
      - userbinator 3y ago
        I suspect this recall is precisely because someone figured that the relief path wouldn't work.
      - fbdab103 3y ago
        I do not work with MRIs, but I work next to the guys who run the NMRs (which is the ~same technology). It is my understanding that all of these super cooled magnets are designed for the eventuality of an emergency quench. Which means the machine has a direct path to evacuate the gas, and it should be piped into the building's HVAC so that if a quench does happen, the people in the area do not suffocate because of lack of oxygen. A surprising amount of maintenance can occur while the magnets are cold and energized. My armchair-uniformed-guess is that they can replace the not-always-working relief path without venting.
      - azalemeth 3y ago
        A bursting disc is commonly used -- the diameter of the quench pipe is typically around 20 - 30 cm. The gas flow rates are insane; a PRV would fail and likely still not reduce the pressure inside quickly enough. Remember, cryostats are like Russian dolls suspended on torsion wire. You want the mass of the metal inside to be as low as possible because it forms cold bridges to the outside world and increases the boil-off rate. Quenches should not happen once the magnet leaves the factory, but until that point it's not uncommon for a machine to have several "training" quenches as the (typically NbSn or NbTi) superconducting wire effectively anneals in place. A fixable giant hole in the top (with a graphite, insulating series of bursting discs) is the approach usually taken.
  - azalemeth 3y ago
    It's a certification requirement that the magnet can quench with a patient inside and the patient is not affected – the cryogens are supposed to vent outside the room, or, in the even that does not happen, at least safely within the room. A blocked quench pipe and a stainless steel (typically A4 stainless) cryostat becomes a bomb: cryogens expand in volume by about a factor of ~700 when boiling and so the volume they occupy at 1 bar and 300 K is, well, far far larger than the volume of the room the scanner is in. This is the sort of thing that is a (remote) risk to life. I've only ever seen one (preclinical, not-for-human-use) magnet unexpectedly quench; copying and pasting a paragraph from my PhD thesis: *The overpressure created by the rapid boiling of the large volume of liquid helium surrounding the superconducting magnet was sufficient to burst the bursting disc (as designed), which allowed the venting of helium gas from the inner cryostat to the emergency quench pipe (the bursting disc was temporarily replaced by clingfilm in this photograph). Additionally, it was also sufficient to rip and blow off the quench pipe, break two windows, damage the floor, and relocate several light items within the magnet room (which, presumably, was not as designed)*
crest 3y ago
The Helium used for cooling expands a lot going from liquid to gas during a quench (iirc by a factor of ~1800). The pressure built-up from the phase change has to be relieved *quickly* before the machine turns into a crude pipe bomb. Oh and you can't get enough Oxygen from the air if most of it has been replaced with leaked Helium.

EA-3167 3y ago

The rapid conversion from liquid or solid to gas, and the means to (briefly) contain that... is a bomb. The reason you don't block the emergency relief is that you turn a quench from an emergency involving asphyxiation, to something resembling a detonation.

trebligdivad 3y ago
Right, but that's true of all MRI's - so what's different in this one?
- refulgentis 3y ago
  "During a quench, which is not common, a large amount of helium evaporates and is vented outside the building through a venting system," the recall explains. "If an unknown blockage is present in the venting system and the pressure exceeds design limits, the structural integrity of the system could be compromised."
  - op00to 3y ago
    This is a risk of any mri machine that uses helium to cool a magnet. Why are only these machines being recalled when if you block the exhaust of any mri scanner it will blow up?
    - jabiko 3y ago
      This is just speculation, but maybe there is a secondary rupture disk that vents the helium inside the room when the primary path (the exhaust to the outside) is blocked. And maybe that secondary rupture disk is too strong, so another random part of the helium containment fails, blowing up the machine and possibly injuring the patient. But please note again that this is pure speculation.
      - op00to 3y ago
        Thank you - this makes a lot of sense as. :)
  - s1artibartfast 3y ago
    That doesn't explain what is different. I assume that is a feature common to all machines.
    - refulgentis 3y ago
      It's not. Thats why it's being recalled. God, HN. Answers right in the article. Numerous people reach to turn it into a mystery.
      - s1artibartfast 3y ago
        The problem is you dont know what you are talking about and dont even understand the questions people are asking. Every MRI machine quenches and vents. The question is what is different, in detail, that heightens the risk with this machine. your response is like saying "the engine wont work" as a complete explanation for why a type of car has more automotive problems than another. Then, when a mechanic is curious about the nature of the failure, you call them stupid, saying "I already told you, i said it doesnt work!" I certainly hope you are not in charge of solving problems in any professional capacity.
        refulgentis 3y ago
        > Every MRI machine quenches and vents. The question is what is different, in detail, that heightens the risk with this machine. The design can't safely accommodate the level of energy that ends up in the machine if its clogged. In the spirit of analogies, it's like asking why a 10 oz glass overflows with 200 degree water and a 10 oz glass doesn't overflow with 100 degree water. It exceeded it's capacity. There's nothing more complicated to it, you could call in an MRI technician, get the PDF manual, talk to several doctors, disassemble it and put it back together, and learn no more than that. You got extraordinarily rude and personal, I hope you don't do that often on HN, that was an outlier in my 13 years here. I hope all is well and it's an exception.
  - Piezoid 3y ago
    Why not add a pressure relief valve on the quench path with a very loud whistle? That should be enough to take care of such rare and compounded failures. What does recall means in this context? De-energizing the superconductor and shipping it back? Seems like a waste and a planning nightmare.
    - userbinator 3y ago
      I suspect this recall is precisely because someone figured that the relief path wouldn't work.
    - fbdab103 3y ago
      I do not work with MRIs, but I work next to the guys who run the NMRs (which is the ~same technology). It is my understanding that all of these super cooled magnets are designed for the eventuality of an emergency quench. Which means the machine has a direct path to evacuate the gas, and it should be piped into the building's HVAC so that if a quench does happen, the people in the area do not suffocate because of lack of oxygen. A surprising amount of maintenance can occur while the magnets are cold and energized. My armchair-uniformed-guess is that they can replace the not-always-working relief path without venting.
    - azalemeth 3y ago
      A bursting disc is commonly used -- the diameter of the quench pipe is typically around 20 - 30 cm. The gas flow rates are insane; a PRV would fail and likely still not reduce the pressure inside quickly enough. Remember, cryostats are like Russian dolls suspended on torsion wire. You want the mass of the metal inside to be as low as possible because it forms cold bridges to the outside world and increases the boil-off rate. Quenches should not happen once the magnet leaves the factory, but until that point it's not uncommon for a machine to have several "training" quenches as the (typically NbSn or NbTi) superconducting wire effectively anneals in place. A fixable giant hole in the top (with a graphite, insulating series of bursting discs) is the approach usually taken.
- azalemeth 3y ago
  It's a certification requirement that the magnet can quench with a patient inside and the patient is not affected – the cryogens are supposed to vent outside the room, or, in the even that does not happen, at least safely within the room. A blocked quench pipe and a stainless steel (typically A4 stainless) cryostat becomes a bomb: cryogens expand in volume by about a factor of ~700 when boiling and so the volume they occupy at 1 bar and 300 K is, well, far far larger than the volume of the room the scanner is in. This is the sort of thing that is a (remote) risk to life. I've only ever seen one (preclinical, not-for-human-use) magnet unexpectedly quench; copying and pasting a paragraph from my PhD thesis: *The overpressure created by the rapid boiling of the large volume of liquid helium surrounding the superconducting magnet was sufficient to burst the bursting disc (as designed), which allowed the venting of helium gas from the inner cryostat to the emergency quench pipe (the bursting disc was temporarily replaced by clingfilm in this photograph). Additionally, it was also sufficient to rip and blow off the quench pipe, break two windows, damage the floor, and relocate several light items within the magnet room (which, presumably, was not as designed)*

crest 3y ago

The Helium used for cooling expands a lot going from liquid to gas during a quench (iirc by a factor of ~1800). The pressure built-up from the phase change has to be relieved *quickly* before the machine turns into a crude pipe bomb. Oh and you can't get enough Oxygen from the air if most of it has been replaced with leaked Helium.