6. HAZARDS OF SYSTEM EXPLOSION :
The liquid helium reservoir of the SRM system is in danger
of a high pressure rupture whenever there is a loss of vacuum
in the superinsulation region if the reservoir is at helium
temperatures. If the fill, vent and safety lines have become
plugged at the same time the danger then becomes a certainty.
The purpose of the safety line is to provide a venting path
that is never opened during normal system operation and therefore
should never become plugged. This points out the critical
purpose of the safety line and the requirement that it must
not be disturbed without specific instructions from 2G. We
have never experienced a rupture of one of our 2G systems
so it is difficult to predict the extent of the damage that
would occur, but it is clear that if a rupture seems likely
then the region within at least 100 feet of the SRM must be
evacuated to protect life and property.
Figures 1-1 through 1-4 give
a cross section layout of the 760-R SRM and show the relative
locations of the reservoir, vapor cooled shields and the superinsulation
volume. The 760 and 755 systems have the same basic layout.
The fill, vent and safety lines are 0.25 inch OD stainless
steel tubes that pass through the superinsulation region from
the helium reservoir to the outside of the SRM. The long length
(up to 180 inches for the vent line and 80 inches for the
fill and safety), and the small diameter are very important
in achieving the very low helium loss rates. The same length
and diameter makes it very easy for a solid air plug to block
the line. In the following section we will discuss the most
likely ways that a hazardous condition can occur in the SRM,
and the time that one might have to resolve the problem before
any danger occurs.
The time that may be required to reach rupture conditions
depends on the circumstances of the system failure. In the
following two paragraphs we will discuss the two most likely
6-1. Catastrophic vacuum failure:
The loss of dewar vacuum is the least likely but the most
hazardous situation. If the vacuum is lost due to a major
accident such as fire in the laboratory, an object falling
on the SRM and rupturing the outer vacuum jacket, or failure
of the sample access tube, then the pressure will build up
in the reservoir to the rupture point in a few minutes so
the only choice is to evacuate the area immediately.
6-2. Slow vacuum failure:
If the vacuum loss occurs due to a leak in a vacuum seal or
rupture of one of the plumbing or electrical fittings, then
the pressure build up in the reservoir will be much slower
and can probably be released fast enough through the vent,
safety and fill ports. As soon as it is realized that a vacuum
failure has occurred the operator must open the vent valve.
Rapid flow of very cold helium gas will exit these three ports
so great care must be used to prevent any contact with the
cold gas. Most of the helium gas will be vented in about 10
minutes and after this time the volume of cold gas remaining
in the reservoir should not produce enough pressure in further
warming to rupture the reservoir walls. Continue to let the
reservoir vent until the flow almost stops and/or the reservoir
has warmed to room temperature (0.4 volts on the reservoir
and shield diodes). It will take from 30 minutes to more than
24 hours for the reservoir to reach room temperature. As soon
as possible contact 2G for further advice.
6-3. Plugging of the plumbing lines without
The liquid helium reservoir is connected to the top flange
of the SRM with three 1/4 inch OD stainless steel tubes that
are within the superinsulation of the instrument. If the room
temperature end of any of these tubes is left open to the
atmosphere without a strong exit flow of helium gas, then
air will be cryopumped into the line and will quickly (in
a matter of a few seconds to a minute) solidify in the line
somewhere between the outer and inner vapor cooled shields.
This is a distance of up to 80 inches down in the lines. A
solid air plug of this nature will support very high pressures,
well beyond that required to rupture the helium reservoir!
If any indication is observed
that the lines may be plugged PLEASE contact 2G immediately.
We have discussed in section 4 several ways to clear a plug
in the fill and vent lines The danger occurs if all three
lines become plugged at the same time. This should not occur
since the safety line should NEVER be opened, BUT IF FOR
WHATEVER REASON ALL THREE LINES DO BECOME PLUGGED THE SYSTEM
IS IN DANGER OF RUPTURING. If the cryocooler is left on
and is operating properly, then the liquid helium will slowly
evaporate and the reservoir pressure will rise to equilibrium
with the cryocooler at about 20 Kelvin. The pressure will
be about 40 psig at this point and the reservoir walls will
support this pressure with no difficulty. It will take many
days for the pressure to rise to even 10 psig so it should
be possible, following the procedures given in section 4,
to clear at least one line before a danger point is reached.
Please contact 2G as
soon as any evidence exists to show that the lines may be