IN FILL, VENT OR SAFETY:
It is possible to collect air or other contaminants in the
fill, vent or safety lines between the room temperature ports
on the SRM and the helium reservoir. As will be discussed
in more detail in section 6 it is absolutely critical that
the plumbing to the safety line be undisturbed since this
line is the final pressure release if the fill and vent lines
become blocked. Since the fill and vent lines must be opened
for helium transfers these are the most likely lines to plug.
Any air in these lines will be cryopumped down the line to
a location where the temperature is below the freezing point
of the contaminant involved. If a sufficient quantity of contaminant
is cryopumped it will form a solid bridge across the bore
of the line and seal it from gas flow. This can, of course
be a very serious problem and great caution must be exercised
at all times to prevent contaminants from entering the system.
The fill and safety lines are thermally grounded at the inner
vapor cooled shield which operates at 80 kelvin without the
cryocooler and at 16 kelvin with the cryocooler. The vent
line is thermally grounded at both vapor cooled shields. The
outer shield is at 120 kelvin without the cryocooler and at
80 kelvin with the cryocooler. Therefore, when the cryocooler
is running, air will cryopump down any of the three lines
to the inner vapor cooled shield thermal ground and could
block the line at this location. Without the cryocooler the
blockage would form at the top of the helium reservoir.
The most common problems we have experienced are plugs forming
in the fill and vent lines with the cryocooler operating.
The following discussion addresses the identification of this
type of blockage and the techniques used to clear them.
4.1 Air plug
in the fill line:
A solid air blockage in the fill line will become apparent
during a liquid helium transfer. If the line is completely
blocked then there will not be gas flow out of the line when
the stopper is removed to insert the transfer line. In this
event the blockage must be cleared before helium can be added
to the system. First, connect the transfer adapter to the
fill port being very careful to flush the adapter with helium
gas and to prevent any additional air from entering this port
and seal the small side of the adapter with a rubber stopper.
Check very carefully to make certain that the adapter is tightly
clamped to the port. Now pressurize the fill adapter and port
to 6 psig with pure helium gas. Hold this pressure constant
and turn on the fill-safety heater by connecting the control
monitor cable to the 15 pin connector on the SRM top plate
with box 3 and activating the shield heater switch. This heater
is a 500 ohm resistor with 30 volts giving a power of about
2 watts at the thermal connection between the fill - safety
lines and the inner vapor cooled shield. This power will heat
the 16 kelvin portion of the fill - safety lines to 70 kelvin
in 5 to 10 minutes. During this heating the air plug should
melt and be blown into the helium reservoir by the 6 psig
helium gas supply. The air will freeze to the reservoir walls
and will not cause additional problems unless the SRM is warmed
up to above 70 kelvin. If the heater does not melt the plug
then the plug may be at the entrance to the helium reservoir.
Connect box #2 to the magnetometer and plug in the control
electronics. Turn on the SHIELD heater switch and this will
apply heat to the fill, vent and safety lines where they enter
the reservoir top. Keep the pressure applied to the fill port
and the plug should clear.
If the above 2 steps do not clear the plug turn the heaters
off, and leave the 6 psig gas pressure applied to the fill
port and turn off the cryocooler. The inner vapor cooled shield
will slowly warm to about 80 kelvin in 12 to 16 hours and
during this warming the plug should clear as described above.
If the plug still does not clear the next step will be to
alternately pump on and pressurize the fill line as described
below in section 4.4.
4.2 Air in the vent line:
Air can be drawn into the vent line during a transfer if the
vent valve is not closed quickly enough during a fill when
back drafting occurs or at the conclusion of the transfer.
Also, if the vent popoff valve leaks and the system pressure
is allowed to reach atmospheric it is possible that air will
counter-flow around the popoff valve seal and collect in the
vent oscillation damper where it can be slowly cryopumped
into the vent line. In either case the plug will form at the
entrance to the inner vapor cooled shield. This thermal connection
is very massive and it is impractical to heat it to melt the
air plug with the cryocooler running. Therefore, the cryocooler
must be turned off to allow the shield and vent line to warm
enough to melt the plug. Follow the steps outlined in 4.3
or 4.4 below to clear the plug.
4.3 Pumping on the plugged line to
Connect a vacuum pump with liquid nitrogen trap onto the VENT
port and evacuate the line up to the solid air plug.
This pump and the SRM MUST be monitored continuously during
the entire warming process so that the pump can be turned
off and the lines sealed immediately after the plug melts.
The plug will normally clear in the vent line in about 12
hours. In the fill line with its heater this can clear in
4.4 Clearing a plug by alternately
pumping and pressurizing a line:
A second technique we have used to clear solid air plugs is
to connect a vacuum pump as described above with a tee to
a pure helium gas supply. The lines from the tee to the SRM
port should be as short as practical. A section of each line
on the input side of the tee should be made of vacuum type
rubber hose, or quick action lever valves should be used in
each line. Now by pumping out the line, then closing the line
to the pump and opening the line to the gas supply set at
10 psig you will alternately remove the helium gas above the
plug and replace this gas with room temperature helium. This
pumping and pressurizing should be done as quickly as about
four seconds per cycle. It will take up to one hour of these
cycles to clear a solid plug. If a pressure-vacuum gauge is
used in the pumping-pressurizing manifold you will be able
to tell when the plug is starting to clear because the vacuum
will not decrease as much on each successive cycle. When the
plug clears flow helium gas through the vent and out the fill
port for 20 to 30 seconds to insure that the plug is completely
removed. Now pressurize the SRM to one psig then close all
ports and let the venting occur through the normal vent damper
and popoff valve.
4.5 Venting the SRM through the fill
port when the vent port is plugged:
There may be times when the vent port is plugged and it is
advisable to operate the SRM without taking the time and risks
to clear the vent as described above. It is possible to continue
operation by venting the helium boil off gas through the fill
port. Connect a vacuum needle valve into the fill port plumbing.
This connection must be done so that air is not drawn into
the fill port. This is extremely important since plugging
of the fill port will leave only the safety port clear and
it will then be mandatory to warm the SRM until both fill
and vent are clear, venting the gas out the safety. If the
safety line became plugged the system would not have a way
for the helium gas to escape and the internal pressure would
rise until the lines spontaneously clear due to the internal
pressure and the increased temperature and/or until the reservoir
and its plumbing rupture explosively. This situation must
be avoided at all cost.