Common signs of damage on a pressurized Dewar include rust or corrosion on any of the parts, dents on the external surfaces, tubing and valves that are bent and should be straight, missing pressure relief valves, a burst rupture disk, or a pressure gauge that reads greater than ambient pressure when the cylinder is empty. Never fill a Dewar which shows any sign of damage. It is also important to watch the Dewar once it is filled for any sign of abnormal operation. When withdrawing large flow rates of gas for prolonged periods of time, some ice formation is normal around the gas valve and fittings and sometimes on the sides of the Dewar. Some ice may also build up on the pressure relief valve if the Dewar is venting frequently. However, any ice buildup that cannot be explained through normal operation of the Dewar may be a sign of a defective Dewar. If any signs of damage are observed, or the Dewar behaves abnormally in any way, take the Dewar out of service immediately and contact the manufacturer of the Dewar or the vendor that supplied it. Only individuals who are trained, qualified and experienced in the repair of this type of vessel should ever attempt to repair a damaged Dewar.
In many cases, a vendor will deliver a Dewar pre-filled, and pick up the empty one. The vendor is then responsible for inspecting the Dewars before filling and ensuring safe operation. However, it is still important to watch for any unusual signs during operation of the Dewar, such as abnormal ice build-up. If anything seems abnormal, call the vendor immediately to have them pick up the Dewar, and be sure to inform the vendor that you suspect a defect.
Troubleshooting a Problem Pressurized Dewar:
| Pressure in Dewar is too low | · If gas is being withdrawn, the flow rate may be too high for the Dewar. · If gas is not being withdrawn, check that all valves are completely closed and that the Dewar is not empty. · Check that the pressure relief valve is not venting. If the Dewar is venting below its marked operating pressure, call the manufacturer or vendor. · If the Dewar is not venting, try opening the pressure builder (if present). · If opening the pressure builder does not help after a few hours, check that all valves are completely closed. · If pressure is still low with all valves completely closed, the Dewar may be leaking – call the manufacturer or vendor. |
| Dewar vents too frequently | · Check that the pressure builder is fully closed. · If liquid loss rate is normal, the withdrawal rate of gas may be too low to keep up with normal vaporization. This is a common situation. · If liquid loss rate is abnormally fast, is ice forming anywhere? · Ice on the pressure relief valve can lock the pressure relief valve in the open position – call the manufacturer or vendor for instructions to de-ice the pressure relief valve. · Ice forming on the sides of the Dewar while it is venting too frequently indicates loss of vacuum jacket integrity – call the manufacturer or vendor and explain the situation. |
| Ice on valves of Dewar | · For valves that are in use, ice buildup is normal and particularly high gas withdrawal rates may cause the entire top of the Dewar to ice over. · If the valves are all fully closed but forming ice, a valve may be leaking – call the manufacturer or vendor. |
| Ice on sides of Dewar | · For particularly high gas withdrawal rates ice on the sides of the Dewar is normal, and the ice may persist for a day or two. · Ice is normal for a day or two after filling. · If gas withdrawal rate is low or no gas has been withdrawn recently, the Dewar was not recently filled, and especially if the liquid loss rate is abnormally fast, this indicates a loss of vacuum jacket integrity – call the manufacturer or vendor. |
| Liquid dripping from any of the valves | · Do not touch the liquid. It may be leaking cryogenic liquid. · Check that the dripping valve is completely closed. · If it continues to drip, put on cryogenic liquid gloves and carefully catch a drop or two in a Dewar flask or a paper cup, and check to see if it stays wet. · If it remains wet, it is just condensation dripping. · If it evaporates almost immediately, the Dewar is leaking cryogenic liquid. Call the manufacturer or vendor immediately, take the Dewar out of service, and warn others of the risk of contact with cryogenic liquid until the Dewar is removed. |
| Pressure above maximum, but not venting | · Vent the Dewar manually with the vent valve to keep the pressure below the pressure relief valve setting. Call the manufacturer or vendor immediately.The most likely cause is that the regulator on the Dewar is out of adjustment or defective – if you have another regulator attached to the gas service valve you can check to see if they agree, but the Dewar should still be taken out of service immediately and either returned to the vendor or else repaired by the manufacturer or a trained, qualified and experienced individual. |
Troubleshooting a Problem Pressurized Dewar:
| Pressure in Dewar is too low | · If gas is being withdrawn, the flow rate may be too high for the Dewar. · If gas is not being withdrawn, check that all valves are completely closed and that the Dewar is not empty. · Check that the pressure relief valve is not venting. If the Dewar is venting below its marked operating pressure, call the manufacturer or vendor. · If the Dewar is not venting, try opening the pressure builder (if present). · If opening the pressure builder does not help after a few hours, check that all valves are completely closed. · If pressure is still low with all valves completely closed, the Dewar may be leaking – call the manufacturer or vendor. |
