Philips F2000 1.5T: vacuum goes negative after quench
What happened
After a night quench the burst disc was replaced. With the cryo-compressor running, the insulating-vacuum reading started positive but, as reported, dropped sharply into negative within about an hour. The vacuum heater brought no improvement; with the cryo-compressor switched off, the reading crept down to zero and stopped there despite the heater still running.
What was checked
- The engineers checked helium pressure and flow on the cryogenic circuit, as reported in the case.
- They measured the pressure in the cryostat's insulating vacuum volume.
- They checked the burst disc area with a helium leak detector, collecting helium in a bag held over the disc to concentrate any leak; accumulation there pointed to the disc as the leak path.
- They compared the vacuum behaviour with the cryo-compressor on and off, to separate a compressor effect from a true leak.
Cause
In this case the engineers considered it most probable that the quench had damaged the sealing ring on the burst disc, leaving a leak path into the insulating vacuum. In a similar reported case, the same pattern of pressure loss after a liquid helium fill was traced to a burst sealing rubber that was only found with a helium sniffer. The outcome here was assessed as probable, not fully confirmed.
The cause is probable and was not fully confirmed.
What was done
The burst disc was replaced together with a new seal. Keeping the cryo-compressor off while the pressure was falling was judged the right action; the engineers noted, however, that shutting the cryo down gives little benefit in the first days once ice has already been drawn into the vacuum space, and that on Philips systems, as reported in the case, a mechanical protection cuts the cold head below roughly 10 mbar. Full recovery of the insulating vacuum is a procedure from the manufacturer's service documentation, done by authorised personnel.
Safety
After a quench, treat the magnet room as potentially helium-filled: confirm ventilation and oxygen monitoring before entering, and remember the residual magnetic field and cold, pressurised cryogenic components while working on the burst disc.
Lesson: A sharp negative turn in cryostat vacuum right after a quench or helium fill points first to the burst disc and its seal — confirm with a helium sniffer before blaming the compressor or the vacuum system.
Covered in the course: MRI Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.