A typical MRI can hold over 1000 liters of liquid helium. If there is a quench, that 1000 liters of liquid helium becomes more than 8000 liters of helium gas and that gas needs to go somewhere. If obstructed, the pressure builds until there is a violent explosion. The American College of Radiology recommends that cryogen vent pathways be inspected yearly to prevent such a tragedy. In our years of performing inspections of cryogen vent pathways (quench pipes), we have seen crushed quench pipes as well as quench pipes clogged with debris or filled with water. The biggest problem that we see, is water trapped in the elbow of the quench pipe. In the event of a quench, the cryogenic temperatures hitting the water will instantly turn it to ice at the and block the egress of the rapidly expanding gas. The result is disastrous, destroying the MRI, the room that it is in and anyone around it!
Quench Pipe inspections are typically done on a yearly basis. You can hire a professional company to do this, or have your own maintenance department perform the inspection. The inspection should begin in the scan room:
- Remove the ceiling tiles around the vent and inspect the pipe insulation/vapor barrier, pipe connections and where the vent penetrates the RF shield. Are the bolts tight? Are the hangers secure? Are all the clamps tight? Is the vapor barrier or insulation soggy?
- The drain plug should be removed from the bottom of the quench valve elbow and drained of any water. Look for any condensation or water damage on the vent pipe, insulation or ceiling tiles. This could indicate a leak in the flashing on the roof.
- Is the vent of the proper diameter to maintain an adequate pressure drop? The longer the run and the more bends, the greater the diameter must be. Where the vent is vertical on the roof, a 90 degree horizontal discharge will not sufficiently protect from severe wind-driven rain as it only takes a wind speed of 20 to 25 MPH to drive rain at greater than a 45 degree angle. The screen covering the exit of the vent must be inspected for any holes larger than the recommended screening size to prevent the ingress of birds, small animals and debris.
- A professional will have a remote camera which travels along the inside of the quench pipe taking high resolution videos of the pipe as it goes through. The videos are then analyzed for any defects, debris or liquids. A full report is provided to the customer outlining the deficiencies of the system and the corrective actions that are required to bring the cryogen vent pathway back into conformance. Pictures taken from within the vent are also provided to highlight the deficiencies.
- Additionally, signs must be posted within the vent exit area to warn first responders, workers or pedestrians of the dangers of a quench. Anyone within a 25 foot radius of the vent at the time of a quench is subject to sudden cryogenic temperatures which can result in frost bite, asphyxiation or injury caused by flying debris.
- Don’t ever assume that this important piece of safety equipment is functioning correctly. Have it checked annually by a trained professional or disaster may be just out of sight!
About the author: John Aff is a production engineer at Independence Cryogenic Engineering LLC in Little Egg Harbor, NJ. He has been working in the field of cryogenics for 12 years, designing and building cold heads and helium compressors. He previously spent two decades as a biomedical engineer.