4. PLUGS 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 clear:

            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 several minutes.

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.