1.Filling a magnet with liquid nitrogen can be a dangerous procedure. 2.The following are general instructions and guidelines that are offered to supplement (not replace) any existing documentation Varian magnet owners already have. 3.Specific magnet configurations may require additional procedures/precautions be taken. 4.If any liquid nitrogen filling procedure (Varian documentation) for your specific magnet configuration does not agree with these instructions, follow the directions written for your specific magnet configuration. The pages that follow describe safety precautions and procedures involved in the liquid nitrogen filling of Varian NMR magnets. The descriptions include: 1.Liquid nitrogen "handling" precautions. 2.A look at the cryo components in a Varian NMR magnet. 3.The liquid nitrogen filling procedure. Caution - Liquid Nitrogen "Handling" Nitrogen is an asphyxiant. Release of nitrogen or liquid nitrogen in an enclosed area with poor ventilation can lead to possible suffocation. Liquid nitrogen is very cold! The temperature of liquid nitrogen is -320° Fahrenheit (-196° C, 77 K) and will cause severe burns to the skin. If burns occur, treat the injury just like a heat burn, and consult a physician immediately. Handling Precautions* All personnel working with liquid nitrogen must wear all the following safety equipment: 1.Safety glasses or face shield 2.Insulated gloves and 3.Protective clothing (long sleeves, pants). *Consult your Material Safety Data Sheets (MSDs) and safety officer for more information on working with liquid nitrogen. A Tour of the Cryo Components in a Varian Magnet Magnet coil - Located in the center of your magnet cryostat. It is enclosed in a container filled with liquid helium (red). To prevent the helium from rapidly boiling away, a can filled with liquid nitrogen (blue) surrounds the helium container. The nitrogen is a thermal barrier between the outside shell and the helium can. Stacks - Some Varian magnet is equipped with three identical stacks that lead directly into the nitrogen can. Two are supplied with latex vent hoses of equal length; the third is fitted with a safety vent cap. If your magnet is equipped with N2 heat exchangers, replace them with rubber latex tubing before starting the fill procedure. Note the hoses are pointing away from the magnet. How Often Should the Nitrogen Can be Filled? Consult your magnet manual to find the nitrogen hold time for your magnet. This is the maximum safe hold time for your individual magnet. Rough guidelines for frequency of filling The Magnet Dept.at Varian Instruments recommends that you fill your magnet with liquid nitrogen once a week. This precaution will insure that your nitrogen is always kept at a safe level. We also recommend that you do your nitrogen fill at the end of the week to insure that the magnet is full over the weekend. About Low Pressure Dewars Varian strongly recommends that you NOT use high pressure liquid nitrogen transport Dewars to fill the magnet's nitrogen can! Low pressure Dewars have only two valves. One is labeled "vent" the other is labeled "liquid" . They also have a safety valve set for 22 psi or less. Transferring liquid nitrogen at high pressures (>22 psi) could cause your magnet's nitrogen can to rupture.
How to Recognize a High Pressure Liquid Nitrogen Transport Dewar High pressure transport Dewars have an additional valve labeled "gas" and a pressure building valve that is equipped with a safety valve set for over 22 psi. High pressure transport Dewars can only be used if precautions have been taken to insure that pressure in the Dewar cannot exceed 22 psi. Local supplier will often place the wrong pressure relief valve on the system allowing the pressure to build to 50 psi or more. Nitrogen Filling Procedure The following is a step by step description on the proper method for filling your NMR magnet with liquid nitrogen. The procedures must be performed completely, in the order described, or damage to the nitrogen can/vacuum vessel or individuals filling the magnet could result. 1. Attach rubber latex hose to liquid valve on non magnetic LOW PRESSURE (less than 22 psi)*transport Dewar. Do not use Tygon tubing! Tygon tubing will shatter when exposed to liquid nitrogen. _________ *If you are in doubt as to whether you have a low pressure or high pressure transport Dewar, review the previous paragraph before connecting the hose to the transport Dewar. Use 1/2 inch diameter(ID) rubber latex tubing. Vent hoses need to be directed away from the Dewar to prevent freezing of o-rings in the base plate and top flange. If the o-rings freeze or partially freeze, your magnet cryostat will lose vacuum. This will result in a high rate of cryogen boil off or even a possible quench. 2. Vent air from hose by opening the valve slightly. This step prevents air in the hose from entering the magnet and causing an ice blockage. 3. While the hose is venting, remove the vent cap from the magnet and attach the hose. A rubber band can be used on the end of the hose to insure a tight fit. 4. Slowly open the transport Dewar's liquid valve 3 degrees from the closed position or about 1 full turn of the valve. The liquid nitrogen should just start to flow. During nitrogen fill, pressure created by normal boil off in a transport Dewar forces liquid nitrogen through the hose and into the magnet. NEVER LEAVE A MAGNET UNATTENDED WHILE FILL IS IN PROGRESS!! 5. Monitor the Filling of the Magnet The hose will quickly freeze as flow is started. Do not move the transport Dewar or tubing will break. A flashlight can be shown through the rubber latex tubing, to monitor flow. Visually check to insure that vapor is freely flowing out of both vent stack hoses. Look for equal sized vapor trails from the vent hoses. A small or non existent vapor trail indicates that an ice block has formed. Vent stacks can become clogged with ice if air enters the nitrogen can. Moisture in the air freezes as it comes in contact with the cold nitrogen gas. To remove an ice blockage 1.Reduce the flow of nitrogen via the Dewar valve to a slight trickle. This will keep positive pressure in the nitrogen can. 2.Remove the rubber latex tubing from the clogged stack. DO NOT LOOK DOWN THE STACK WHILE DOING THIS PROCEDURE, because the dislodged ice will blow out of the stack. 3.Use a thumper tube to dislodge the ice block by quickly poking it down inside the stack. 4.Immediately replace rubber latex tubing and resume fill. 5.When liquid begins to flow out of the vent hoses, the magnet's nitrogen can is full. Immediately shut off the transport Dewar valve to stop the nitrogen flow. The nitrogen can is filled when excess liquid nitrogen flows from the ends of the latex hoses. A crackling sound can be heard just before overflow. 6. When the Magnet is Filled 1.Allow the hose to thaw or use a blow dryer to speed the process. 2.Once the hose is thawed, remove it, measure the depth, and quickly replace the vent cap. The magnet should never be left open for more than a few seconds. Quick closure prevents air from entering the magnet and causing an ice blockage. 3.Record nitrogen fill into log book or chart, show the initial level and final level. 7. Filling Procedure has been completed. PROBLEMS OR WHY IS NITROGEN FILL DIFFICULT TO PERFORM ? 1.NITROGEN REFILL PROCEDURE WAS NOT FOLLOWED PROPERLY Please consult the nitrogen refill procedure described in the magnet service page. If difficulties continue please contact the Varian Magnet Department. 2.NITROGEN SUPPLY Dewar IS EMPTY Gages of many nitrogen supply Dewars do not register liquid levels correctly. Open the liquid valve of the supply Dewar and make sure that liquid nitrogen is flowing out. 3.NITROGEN TRANSFER LINE DOES NOT WORK Make sure that the nitrogen transfer line is connected to the liquid valve of the supply Dewar. Inspect the transfer line for any type of blockage. 4.PRESSURE IN THE LIQUID NITROGEN SUPPLY IS NOT SET PROPERLY Verify what is the normal operating pressure of the supply Dewar. Do not use liquid nitrogen supply Dewars which are set at pressures higher than 25 PSI. It is possible to damage the cryostat's nitrogen vessel if higher pressure is used during nitrogen refilling. If the pressure during liquid nitrogen refilling is too low add external pressure using nitrogen gas. However, do not exceed 25 PSI. 5.ICE BLOCK IS PRESENT IN THE NITROGEN VESSEL Use a thin non magnetic rod to confirm that the nitrogen vessel neck tubes are free of ice blockages. Do not use extreme force to break through the ice blocks. Blow warm helium gas down the nitrogen neck tubes to eliminate the ice blockages. Confirm that all three nitrogen neck tubes are free of any obstructions. WHY ISN'T ANY ICE FORMED ON THE NITROGEN VESSEL NECK TUBES ? 1.ICE PLUGS ARE PRESENT IN THE NECK TUBES The nitrogen boil off can stop while performing a liquid helium fill. Air can be drawn into the nitrogen neck tubes and freeze.The ice plugs can be removed with a rod or by using warm helium gas. Always make sure that the nitrogen vessel neck tubes are closed off during a liquid helium fill. 2.NITROGEN VESSEL IS EMPTY When the nitrogen vessel becomes empty the helium boil-off will increase. Refill the nitrogen vessel as soon as possible with liquid nitrogen. The nitrogen vessel must have liquid nitrogen in it at all times to insure a proper helium boil-off. The nitrogen vessel should be filled weekly. 3.HOSES ON THE NITROGEN VESSEL NECK TUBES ARE BLOCKED Inspect the hoses on the nitrogen vessel neck tubes for blockages. Make sure the hoses were unplugged after the liquid helium fill. Generic Instructions for Helium Fills on NMR Magnets Read This Before Proceeding Further! 1.Filling a magnet with liquid helium can be a dangerous procedure. 2.The following are general instructions and guidelines that are offered to supplement (not replace) any existing documentation Varian magnet owners already have. 3.Specific magnet configurations may require additional procedures/precautions be taken. 4.If any liquid helium filling procedure (Varian documentation) for your specific magnet configuration does not agree with these instructions, follow the directions written for your specific magnet configuration. The pages that follow describe safety precautions and procedures involved in the liquid helium filling of Varian NMR magnets. The descriptions include: 1.Liquid helium "handling" precautions. 2.The liquid helium filling procedure. Helium is an asphyxiant. Release of helium or liquid helium in an enclosed area with poor ventilation can lead to possible suffocation. Liquid helium is very cold!! The temperature of liquid helium is -452 degrees below zero Fahrenheit (-269 ° C, 4.2 K) and will cause severe burns to the skin. If burns occur, treat the injury just like a heat burn, and consult a physician immediately. Handling Precautions* All personnel working with liquid helium should wear all the following safety equipment: 1.Safety glasses or face shield 2.Insulated gloves and 3.Protective clothing (long sleeves, pants). ________ *Consult your Material Safety Data Sheets (MSDs) and the safety officer for more information on working with helium. WARNING: Please read all instructions before transferring any liquid helium. 1.1 PREPARE THE HELIUM TRANSPORT Dewar. 1.Slowly release the pressure in the helium transport Dewar by opening the "vent valve". Do not allow a white flame to form. (Liquid helium is being wasted). 2.Determine the helium volume using a "thumper tube". "Thumper Tube"-- The "thumper tube" is a long tube with a round cup fitted at one end. When the tube is lowered into liquid helium, a vibration is felt by resting a finger on the end of the cup. 1.Lower the thumper tube to the bottom of the helium transport Dewar and mark the thumper tube shaft in relation to the top of the Dewar valve. 2.Raise the tube until a dramatic change in frequency occurs. Mark the tube at this point. 3.The distance between the two marks can be translated from inches to liters using the chart that is mounted on the side of the transport Dewar. Note: A piece of thin latex can be stretched over the top of the thumper tube to enhance the feeling of the vibration or moisture on your finger may serve the same purpose. The rapid change in frequency can also be seen by watching the latex flutter on the cup. 4.Record the volume of helium measured. 1.2 - PREPARE MAGNET FOR HELIUM FILL The magnet Dewar should never be open to the atmosphere for more than 3 to 4 seconds! 1.Loosen, but do not remove, the helium check valve clamp, mounted on the back of the helium manifold. 2.Remove the black plug and cap assembly from the helium fill port. Quickly place the black plug back into the fill port funnel. Note: Make sure that no extension tube is on the end of the transfer line that will be inserted into the magnet! 3.Place the red-rubber collar, brass washer, and o-ring onto the short leg of the helium transfer line. 1.3 - INSERT TRANSFER LINE INTO TRANSPORT Dewar Gloves must be worn at this point. 1.Close the "safety valve" on the helium transport Dewar and open the "top valve". Slowly insert the transfer line into transport Dewar. A chugging sound should be heard at the other end of the transfer line. When the line reaches the liquid level, there will be a change in the sound of the chugging. A cloud of white vapor will appear, then a little white flame. The flame size should not exceed 4 inches. 2.Partially open the transport Dewar "vent valve" if the flame is too large. This may temporarily stop the transfer of liquid. (Pressure can always be restored to start the flow again.) 1.4 - INSERT TRANSFER LINE INTO MAGNET Warning: It is very important that liquid helium is not blasted into the magnet at the start of a fill. This may cause a quench! 1.Frost may form near the tip of the transfer line. Remove this frost with a rag before inserting the transfer line. 2.Quickly remove the black plug from the helium fill port and slowly insert the helium transfer line. 3.Immediately remove the helium check valve.If the line hits the bottom of the helium stack, raise it up 2 to 3 inches. There should be a white vapor exhausting out of the back of the manifold. 4.Slowly push the transfer line into the transport Dewar until it reaches the bottom. Pull the transfer line up about 1/2 inch if it should reach the bottom. Note: Liquid helium will not transfer if the line is resting on the bottom of the transport Dewar. It is possible to clog the transfer line with slush that may have collected on the bottom of the transport Dewar. 1.5 - PRESSURIZING THE HELIUM TRANSPORT Dewar 1.Once the white vapor from the helium manifold reduces in size, connect a supply of helium gas to the "vent valve" of the transport Dewar. The transferring pressure should not exceed 2 Psi. 2.Adjust the pressure so that there is NO more than 10 inches(25 cm) of white vapor exhausting (in the horizontal plane) from the helium manifold. 1.6 - MAINTAINING THE HELIUM TRANSFER 1.The nitrogen can neck tubes should be temporarily sealed after the transfer has begun so that air is not drawn into the neck tubes. 2.Let the transfer continue until the exhausting white vapor intensifies. A white flame will become large and liquid air will start to drip off the manifold. The helium can is now full. Warning: The same indications can appear if the transport Dewar is empty or the transferring pressure is set too high. Always check the magnet's helium level sensor to confirm helium level. The helium can should have a positive pressure at all times so that NO air infiltration occurs. 1.7 REMOVING THE TRANSFER LINE FROM THE MAGNET 1.Gradually release the gas pressure from the transport Dewar's "vent valve". This will stop the transfer of liquid helium into the magnet instantly. 2.Immediately replace the helium check valve onto the manifold. The clamp may not close all the way at first. This is okay for now, just make sure the clamp is tightened after the frozen manifold condensation has begun to thaw. Warning: Gloves will be needed at this point. 3.Remove the transfer line end from the magnet only! (Do not remove the transfer line from transport Dewar at this point.) Do this in one steady upward stroke. Avoid letting liquid air drip off the end of the transfer line into the helium fill port. 4.Quickly place the black plug into the fill port. 5.Remove the black collar, brass washer, and o-ring from the transfer line and place back onto the black plug. Install the plug assembly back onto the helium fill port. Note: Remember that the magnet should not be open to the atmosphere for more than a few seconds. A positive flow from the helium can should exist at all times. 1.8 REMOVE TRANSFER LINE FROM TRANSPORT Dewar 1.Remove the transfer line from the helium transport Dewar. 2.Close the "top valve" and the "vent valve" on the Dewar. 3.Open the "Safety Valve". Warning: The transport Dewar's "safety valve" should remain open at all times except during the actual helium transferring process. 1.9 - MAGNET FOLLOW UP 1.After the helium neck tubes have warmed, check to see that the helium and nitrogen boil off have returned. If not, check the clamps and o-rings. 2.Open the sealed off nitrogen can neck tubes. 3.Thump the transport Dewar to confirm the volume of liquid helium that was transferred into the magnet. Note: Never go back into the magnet until the helium neck tubes have warmed and a boil-off is indicated on the helium flow gauge. It is possible to suck air into the magnet if the neck tubes have not warmed up. Cryo pumping of air to the inside walls of the helium neck tubes will occur. 4.Record the Helium and Nitrogen levels and Helium boil off readings into a log book. 5.The He fill procedure is completed. WHY IS THE HELIUM BOIL-OFF HIGHER THAN NORMAL ? 1.WEATHER FLUCTUATES The weather plays a major role in how fast or how slow liquid helium boils off. Low atmospheric pressure will elevate the helium boil-off. High atmospheric pressure will reduce the helium boil-off. This is normal. A helium gas flow meter is the best way to monitor the effects of weather. 2.NITROGEN VESSEL IS DRY Check to see if there are ice formations on the two ports that exhaust boil off from the nitrogen vessel. If there is no ice present, use the helium thumper tube to check if there is liquid nitrogen (LN2) in the nitrogen vessel. Heating the tip of the thumper tube before inserting it into the bottom of the can , will give a positive reading if it comes in contact with liquid nitrogen. If no LN2 is present, add some. 3.VACUUM HAS DETERIORATED Check to see if the sample is cold or iced in the shim stacks. If this condition is present remove the shim stacks. If ice formation occurs in the entire room temperature bore tube, the vacuum within the magnet cryostat is probably deteriorated. Call Varian Magnet Department immediately! 4.MAIN PERSISTENT SWITCH HAS OPENED and the MAGNET is DE-ENERGIZING The superconducting main persistent switch could accidentally open and energy starts to be deposited in the electrical short connection across the switch (shorting plug). The magnet current will start dropping and therefore the magnetic field strength will start decreasing. The helium boil-off will increase significantly because of the heat which is dissipated by the resistive short connection. The first sign is that "lock" signal is lost and no NMR signal could be obtained since the magnetic field could have dropped out of the probe range. Ice will begin to form on the tops of the helium neck tubes due to a very high boil-off. The ice that is found at the base of the N2 neck tubes will begin to melt. Helium consumption which could be read through the level sensor will jump from 1 - 2 % a day up to 2 - 3% an hour. Liquid helium must be added to prevent the magnet from quenching. If the helium can runs dry before the magnet is completely de-energized, the magnet will probably quench and the superconductive coil could be damaged since there will be no liquid helium left to transfer some of the heat from the coil. This is a very serious problem. Call Varian Magnet Department immediately. 5.RADIATION BAFFLES ARE NOT INSTALLED Check to see if the radiation baffles are installed in the cryostat. The baffles are an important component of the cryostat and are specially designed to reduce the heat losses for improved cryogenic performance. 6.INTERNAL MISALIGNMENTS OCCURRED If a ferromagnetic object is located too close to the magnet it could be pulled towards the magnet bore by the magnetic attraction force. If the object is larger than the bore it will heat the bottom flange and will cover the magnet bore. The hit could cause internal misalignments of the various cryostat subassemblies. This could lead to thermal shorts which would compromise magnet cryogenic performance. Call Varian Magnet Department immediately if a ferromagnetic object got stuck to the cryostat. DO NOT try to remove the object by if the magnet is still energized. Damage to the internal components may occur. 7.HELIUM CHECK VALVE DOES NOT WORK PROPERLY A malfunctioning check valve could produce pressure oscillation in the helium vessel. This oscillation causes the liquid helium to boil off at an accelerated rate. Check the ball on the helium flow gage to see if it is floating steady or if it is bouncing. A bouncing ball is an indicator of oscillation in the helium vessel. 8.ICE (FROZEN AIR) IS PRESENT IN THE HELIUM VESSEL Ice in the helium vessel can cause a thermal short between the solenoid components and the helium can or helium can supporting neck tubes. The frozen air needs to be carefully removed. A helium thumper tube can be used to determine if ice is present. Visual inspection down the helium neck tubes can also be performed to confirm ice formation. Contact the Varian Magnet Department before attempting to investigate the helium vessel for ice blocks. There is a risk of more air infiltration if the inspection is not done properly. 9.HELIUM TRANSFER LINE DOES NOT WORK PROPERLY It is possible to perform a helium transfer and not transfer liquid helium into the helium vessel. Warning: it is possible to cause a quench if transferring helium gas into the helium vessel. The vacuum in the helium transfer line can become soft so that the liquid helium is vaporizing before it reaches the magnet cryostat. A helium transfer line can also become partially blocked when the end of the line is allowed to touch the bottom of the supply Dewar. There may be frozen air at the bottom of the supply Dewar. This can allow helium gas to flow through the transfer line but not liquid. The plume exhausting from the end of the helium manifold may look normal while transferring, but the helium can is not being filled. It is possible to boil off the liquid helium that is already in the cryostat by introducing the relatively warm helium gas. It may take a significant amount of time for the helium boil off to return to normal if helium gas was transferred into the can. It is also possible for the superconducting persistent switch to open if helium gas was transferred into the helium vessel. The magnet will start to de-energize by depositing energy into the shorting plug across the persistent switch. The helium boil-off will increase dramatically. Contact Varian Magnet Department immediately if the magnet is de-energizing. WHY DOES THE FLOWMETER INDICATE ALMOST NO HELIUM BOIL-OFF ? 1.Helium neck tubes are blocked with ice This is a dangerous condition because the pressure in the helium vessel is increasing and can not be released because of the ice blocks. Check helium neck tubes for ice using a thumper tube. Do not allow air to enter into the helium can during the neck tube inspection. Contact the Varian Magnet Department immediately if ice blocks are found. 2.HELIUM MANIFOLD CLAMPS ARE LOOSE Check all of the helium manifold clamps for tightness. They are supposed to be perfect tightened after filling with liquid helium. 3.O-RINGS HAVE CRACKS O-rings can crack with time by being frozen during liquid helium transfers. Inspect the o-rings on the manifold insuring that no air enters the helium can during the process. Squeeze and partially collapse the o-ring to see if any cracks on the surface appear. Make sure o-rings are properly greased to insure long life. 4.HELIUM FILL PORT ASSEMBLY IS NOT INSTALLED PROPERLY Verify that the fill port assembly is installed properly. The parts are fitted together in the following order: stopper, black knurled threaded nut, brass washer and o-ring. Check to see if the fill port assembly is properly tightened. Make sure that the energizing side port plug is sealing properly. 5.ATMOSPHERIC PRESSURE IS HIGH High atmospheric pressure could reduce the helium boil off if the magnet cryostat is not equipped with an electronic atmospheric pressure device. The boil-off rate could be even lower than the minimum value which could be measured by the flowmeter. 6.HELIUM CHECK VALVE DOES NOT OPEN If the helium check valve does not open, there should not be a significant back pressure on the helium vessel if the check valve is removed for a very short time. Blow through the check valve to see if it opens easily. Replace the check valve if does not open properly. NEVER SUBSTITUTE THE STYLE OF CHECK VALVE USED ON THE CRYOSTAT! The check valves supplied with the cryostat are designed to maintain the helium pressure at a certain level. An improper check valve can cause permanent damage to the cryostat. 7.PRESSURE RELIEF VALVES DO NOT SEAL PROPERLY It is possible for the helium manifold pressure relief balls to become unseated during a liquid helium transfer or after a magnet quench. If the helium check valve is not immediately removed, when the liquid transfer line in inserted into the cryostat, the pressure in the helium can increases and causes the pressure relief valves to open. After the helium manifold has warmed, press down on the pressure relief balls to seat them. Make sure that the balls are not loose in the valve assembly. The pressure relief valve o-rings may have to be replaced if the ball does not seal properly. 8.CARBON BURSTING DISC IS DAMAGED If the magnet has quenched, visually inspect the carbon bursting disk located on the helium manifold. Make sure to cover the opening in the helium manifold immediately after removing the carbon disk pressure relief assembly. Hold the carbon disc up to a light source and inspect for cracks. Replace the disk if necessary, and reinstall the assembly onto the helium manifold. 9.HELIUM FILL PORT TUBE ON MANIFOLD IS CRACKED Inspect the base of the fill port tube where it meets to right hand top stack assembly. Look for cracks if the fill port tube has been bent. Inspect all helium manifold components for cracks. 10.HELIUM FLOW METER TUBING IS LEAKING Inspect the helium flow meter tubing for cracks. Make sure that the tubing is firmly seated on the helium check valve and is properly connected to the flowmeter. Replace any deteriorated tubing. WHEN DOES THE CRYOSTAT NEED TO BE REFILLED WITH LIQUID HELIUM ? 1.Before the minimum level indicated in the magnet manual is reached Refer to the magnet manual to identify the minimum level of the liquid helium when the cryostat must be performed. The magnet manual should also specify the helium volume needed to fill the cryostat from its minimum refill level to 100%. DO NOT allow the liquid helium level to fall below the recommended minimum refill level or permanent damage to the coil may occur. It is always preferable to fill the helium can above the recommended minimum refill level. This allows a safety buffer if there are any complications while transferring liquid helium into the helium vessel. Many customers who only have one cryostat will find it advantageous to plan the helium refill for the volume of the liquid helium supply Dewar to be purchased. A 30-liter helium supply Dewar is large enough to fill some of the smaller cryostats which hold approximately 52 liters of liquid helium. The helium vessel will be refilled at a higher level than the minimum recommended by using this type of supply Dewar. However, this procedure will avoid any liquid helium waste. It is recommended that no helium refill should be performed below 30% on most of the cryostats until a few refills have been already done and the operator has become familiar with the helium transfer process. WHY IS HELIUM REFILL DIFFICULT TO PERFORM ? 1.Helium refill procedure was not followed properly Please consult the helium refill procedure described in the magnet service manual. If difficulties continue contact the Varian Magnet Department. 2.ICE IS PRESENT IN THE HELIUM NECK TUBES Ice can be collected in the helium vessel neck tubes if the helium manifold is left open to air for any long periods of time. The manifold should never be left open for more than a few seconds except while inserting the transfer line and venting helium gas during a fill. If ice is present in one or more of the helium vessel neck tubes the liquid helium transferred during refilling will be send back into the manifold. A large amount of condensation will usually be seen dripping from one or the other side of the manifold. Contact the Varian Magnet Department for proper procedures to remove ice blocks. The helium can must be able to vent to atmosphere at all times. 3.HELIUM TRANSFER LINE IS DEFECTIVE If the helium transfer line is frosting up on the outside it is probably defective featuring a soft vacuum. Do not continue to use the transfer line in this case. Helium refill can be also difficult due to any internal ice blockages which could form within the transfer line. To check the transfer line for ice blockages stop the helium transfer, remove transfer line from the magnet cryostat, and cap up the cryostat. Start flowing liquid helium. There should be a white plume of gas coming out and then a white flame. Condensation will drip from the end of the transfer line if liquid helium is flowing. If liquid helium is not flowing blow warm helium gas through the transfer line to eliminate internal ice blockages. Repeat the operation until liquid helium flow is achieved. 4.PRESSURE IN THE LIQUID HELIUM SUPPLY DEWAR IS NOT SET PROPERLY The pressure in the liquid helium supply Dewar can be too low or to high for proper refilling. Make sure that the pressure is high enough to provide a constant helium flow but make also sure that this pressure does not exceed 2 PSI. Increasing the pressure during refilling over 2 PSI will cause liquid helium to come out into the manifold, thus wasting helium. This could be determined by noticing liquid air dripping from the cryostat's manifold. 5.LIQUID HELIUM SUPPLY Dewar IS EMPTY The liquid helium supply Dewar may be empty and helium gas could be transferred into the magnet's helium vessel. The plume exhausting from the cryostat may not look so different than when performing a refill using a low pressure. The supply Dewar may also have ice on its bottom and it is possible to plug the helium transfer line if its end comes in contact with the ice. Check for liquid helium in the supply Dewar using a thumper tube.
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