Helium Quench Pipe Systems
The 'quench' process is when liquid helium changes rapidly from a liquid state to a gaseous state requiring immediate venting to outside the building via the quench pipe.
For an MRI to function, the coils of the electromagnet need to be super-conductive which requires them to be immersed in liquid helium at minus 269° C (4° Kelvin). This assembly is the 'cryostat'.
To maintain that low temperature, an external 'cryocooler' will re-cycle any helium gas that has boiled off, as liquid helium. This is a normal event.
A quench event is when a rapid loss of helium occurs. This may be due to a failure in the cryostat or human intervention e.g. an emergency within the room. Once initiated, the sequence of events are (i) the temperature of the coils rise to a point whereby they loose their super-conductivity, (ii) this causes the coil resistance to rises, (iii) the current falls leading on to (iv) a collapse of the magnetic field.
The expansion ratio liquid helium to gas at 0° C, is 1 to 754 and the resultant rise in pressure within the cryostat causes large volumes of helium gas to be generated. Due to its low temperature, any gas leaking out will produce a rapidly developing fog. Ignoring any element of panic a quench event could have on the patient (noise being one factor), if the helium were to enter the MRI room, the possibility of asphyxiation or frostbite cannot be ruled out.
The quench pipe system design must allow for the impact of a sudden and violent imposition of cryogenic temperatures on pipework at an ambient temperature. This requires careful appraisal of the following critical factors that are all interrelated:
- Thermal shock and compensating for linear expansion / contraction
- Ensuring pipe supports etc will withstand the lateral forces present on quench
Ideally the pipe runs should have a compensating factor of between 4 and 5mm per metre run. Care also needs to be exercised to ensure accurate pre-positioning of the MRI connection pipe. Attempting to adjust matters post installation, is both time consuming and expensive and can impact on the overall project programme.
Thermal compensation pipe bellows
Pre-positioned pipe end to match MRI cryostat pipe
Typical pipe run with bellows
Evaluating material behaviour is critical as demonstrated by the insulation which has a key health and safety function. New insulation materials claiming low thermal conductivity frequently enter the market. However, for the type of cryogenic pipe work undertaken here, the critical combination is to be user friendly, easy to install and fire suppressed. In addition, the hygroscopic properties of the insulation are equally important. Whilst a quench event is infrequent, it can create a potential problem of moisture forming and freezing along the pipe's surface.