From: IN%"tschudin@speck.niddk.nih.gov" "rolf tschudin" 4-MAR-1994 09:
45:59.
65
Dan:
Beware of those sensors. Years ago we had an NMR in a small
room previously occupied by a computer system, with a smoke dedector
on the ceiling. When we had quenches, and sometimes even when filling
the magnet, the alarm went off resulting in about 3 fire engines racing
across the reservation, sirens blaring. Needless to say, we disabled
it after a few of these calls. My answer would be:
1. Normal boiloff rates are so low that we never worried.
2. Quenches occur only during energizing, when all the people
nearby know what to do, or on weekends when there is
nobody around.
Regards
Rolf
From: IN%"mainzv@aries.scs.uiuc.edu" 4-MAR-1994 10:22:34.17
> The questions are: 1) What do YOU do for oxygen deprivation indiocation
?
> 2) Do you have a permanent vent to the outside
> for helium and nitrogen and how do you
> do it? 3) What are the fire safety precautions you use?
1) Our instruments are in two rooms that interconnect, one is
20x30' and the other is 40x30'. The air turnover in the room is so large
that oxygen deprivation is not a problem. (The room had hoods at one time,
and the air handling system is still running.) Another system on campus
is in a 20x20 room, and does have a circulation problem. It has an
oxygen sensor installed that gives a room alarm to warn people in the room.
For the room being built for the new 600/500, the powers-that-be are
making noises about an oxygen sensing system. I don't have any objections
to one in the room - it just seems that everyone has gotten by without
one (and no reported asphyxiation deaths) for thirty years.
2) In neither my rooms nor the other system described above does
the nitrogen/helium vent to the outside. There actually would be a benefit
to having the helium vent outside the lab (it wouldn't infiltrate the
vacuum
tubes in our solids power supply and it wouldn't be around to break the
vacuum on the dewar), but we don't bother. One would think you wouldn't
have
to vent to the outside _and_ have an oxygen sensor.
3) We have a smoke alarm in every room in the building that rings
in the local fire house. As far as firemen coming into the room during
an actual emergency - no experience, no recommendations. Presumably, the
damage to the magnets would pale beside the havoc water and powder fire
extinguishers would wreak on the systems.
Vera Mainz
mainzv@aries.scs.uiuc.edu
From: IN%"Garber@chem.chem.scarolina.edu" 4-MAR-1994 10:27:47.50
I have a 300, 400, and a 500 in a fairly spacious area (the two
closest magnets are 19 ft from each other diagonaly across one side of
the room). I have fans over each magnet to vent excess helium into
the plenum (above the false ceiling) during fills or during quenches
but these fans must be manually turned on. I have no oxygen sensors
in the room. I feel the asfixiation hazard is overrated unless one
places a system in an extremely confined area with no ventilation.
In our building, the area above the false ceiling is a plenum return
for the hvac system. Large air handlers draw the air in, dilute it
with air form adjacent areas, heat or cool it, and then duct it back
to vents in the ceiling. A quench rarely involves rapid release of
much nitrogen, and even cold helium gas tends to rise rapidly to the
ceiling where it is sucked into the hvac returns.
I have a total of three exits (one through my office and
therefore not available during the off hours). All instruments are
wired through a main breaker box located just inside the main
entrance so any one or all can be shut down from there. I have
halon fire extiquishers on the wall by each exit. I have installed
emergeny battery operated lighting around the perimeter of the area so
one can see fairly well even when the power is off. The magnets are
roped off and I have the area around them marked with 2" yellow/black
warning tape on the floor at the 5 gauss line. The usual warning
signs are posted on the doors. I have met with reps from the fire
dept and explained and demonstrated the hazards associated with the
magnets. They took notes and are supposed to have them on file with
all the other information about specific hazards in our building.
- Ron
From: IN%"cgregory@uiuc.edu" 4-MAR-1994 12:51:11.33
Dan:
We have a VXR-500 in a similar room (by itself) with very low air flow
(about 1 change per hour). Calculation showed that a quench with loss of
full helium tank would displace more than half of the air in the room,
taking 15 minutes or more to clear. It quenched once during ramping and
that was about right. There is no external vent for cryogens. We use dry
N2 for VT work, another possible hazard.
I have a portable personal oxygen monitor from Neotronics, POBox 370
Gainesville, GA, 30501 (800) 535-0606. It was about the cheapest available
three years ago ($700 ?). It requires a replacement sensor about every
12-18 months ($150), and a new 9v battery monthly.
It is straightforward to vent the normal boiloff from Oxford magnets to the
outside, but the Oxford quench vents are not really set up for this, unless
they have changed. Larger magnets (e.g. 30cm bore horizontal, or clinical
MRI systems) usually have a provision for a quench vent to take everything
outside.
Larger magnets usually have an emergency quench heater with a battery to
discharge the magnet in case of a personnel accident.
We have a Magnex 4.7T 31 cm magnet, as well as the 500, and have handled
the personnel safety issue by 1) keeping all external (hallway) entrances
locked (Only our staff and the building supervisor have keys). 2) Inviting
the campus fire department in for brief training sessions (handouts and
demonstrations about magnet and cryogen safety). 3) plastic chains around
the magnets. 4) Lots of signs.
From: "Fred Morin" <MORIN@OMC.Lan.McGill.CA>
Organization: McGill University - Chemistry
Hello.
In one of the NMR labs here we had O2 detectors installed when the
instruments arrived. In several other rooms with instruments we
have not done the same thing, but there is a much greater turn-over
of air in them and it was not deemed necessary to put them in (maybe
we should ?). The detectors come from GasTech Inc and have tested
out fine. Chemical cartridges need replacing (refurbishing) every
couple of years.
We don't vent He or N2 out of the lab.
All our magnets have had a removable fence installed around them. The
justification for this is that it keeps people (visitors and
janitorial staff) and objects (dewars, cylinders etc) at a safe
distance. It didn't occur to us that they could also serve the
purpose of keeping firemen from getting stuck to them!
Cheers,
Fred Morin
****
Again, Thanks for the rapid response and valuable input. This is a GREAT
resource.
Dan Bearden
NMFS- Charleston Lab