Ice in my dewar! (Liquid helium transport dewar)

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Photographic Record:
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<P align="center">[[Image:Whole_dewar.jpg]]</P>
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<P align="center">[[Image:Whole dewar.jpg]]</P>
<P align="center">[[Image:Neck_assy.jpg]]</P>
<P align="center">[[Image:Neck_assy.jpg]]</P>
<P align="center">[[Image:Loosening_clamp.jpg]]</P>
<P align="center">[[Image:Loosening_clamp.jpg]]</P>

Revision as of 07:00, 13 March 2009

Ice in My Dewar (Liquid helium transport Dewar)

This is of special interest to sites which are far from their liquid helium supplier, and who perform their own liquid helium fills.

Warning. Cryogenic Liquids and Associated Vapor are Dangerous. Wear Protective Clothing and Eye Protection


The Situation:

The Rhodes University in Grahamstown, South Africa, is 120 km from the nearest (Port Elizabeth) Cryogen vendor’s depot. This depot does not always have a cryogenics expert available. We are 1000 km from the supply point, Johannesburg. Another Support point is Cape Town, 800 km distant. Liquid helium (and liquid nitrogen) is delivered by road transport and the transfer of cryogens to the superconducting magnets is performed by university staff members.

The Problem:

A 100 litre liquid helium delivery Dewar was found to only pressurize with difficulty and it was not possible to vent via the vent-valve (See Figure 1 for terminology). In additional cause for concern and alarm was the discovery that the lower safety valve did not release any gas; neither did the travel valve port . Nevertheless the upper safety valve was able to vent gas, and this was the clue to the puzzle. It was possible probe the helium tank and measure, by sounding tube, that there were close to 100 Litres of liquid helium in the Dewar. The suggested course of action suggested blowing warm (room temperature or warmer) helium gas into the Dewar. He also suggested removing the delivery port / guide pipe assembly.

The Structural Details of the Dewar:

After making some measurements, and with some hind-sight after having dismantled part of the Dewar, the structure of the Dewar is believed to be similar to that shown in Figure 1. This is totally the author’s idea of what goes on inside, and if any reader would like to correct this sketch he will be delighted.

Image:Liquid_Helium_Transport_Dewar_Internals.jpg

The sketch shows the Dewar to have two regions, one insulated and containing the cryogenic liquid and the other un-insulated and containing much of the valving. Access to the liquid is via either of two paths.

Image:Paths_1_and_2.jpg

Path 1 passes through the delivery port and the guide pipe, while path 2, passes through any of the pressurization port, vent port, the lower safety port or the travel valve port, passing outside the guide pipe to access the helium tank. Under normal circumstances these two paths are freely connected.

To expel liquid Helium one applies ‘over pressure’ to the liquid surface by connecting pressurized helium gas to the pressurization port. In response to the over pressure liquid is forced up the delivery pipe. Under no-delivery (storage or transportation) conditions the delivery port and pressurization port valves are closed. Normal evaporation of the liquid raises the internal pressure and helium gas is vented via the travel valve, If the Dewar is in delivery mode (travel valve removed and port used for pressurization), but with the delivery port valve closed, excess pressure is released via the lower safety valve.

Should path 2 be blocked, internal pressure can still be released via the upper safety valve.

The image below shows what appeared to be the situation with the Dewar. Ice had blocked the path from the pressurization port to the liquid surface. Similarly any pressure build-up over the liquid could not be released by the vent valve.

Image:Liquid_Helium_Transport_Dewar_Internals_With_ICE.jpg

The Repair Procedure:

Warning: Open the delivery valve before attempting to remove the delivery port / guide pipe assembly. This should avoid any sudden release of pressure, cold gas and / or liquid when the neck seal is opened.

On the instructions of the vendor the neck-clamp was removed and an attempt was made to withdraw the guide pipe. It was completely stuck. Room-temperature helium gas was applied to the pressurization port at increasing pressure until the upper safety valve opened. Pressure was then rapidly reduced via the delivery port. In this way it was hoped to cause turbulence and allow some warmer helium to reach the blockage. After some minutes of this treatment there were signs of some pressurization of the helium tank but the guide pipe assembly remained stuck.

The system was allowed to rest for 2 hours and then a further attempt to move the guide pipe assembly was made. This time it slowly moved. After some minutes of struggle the assembly was removed. A cylindrical collar of Ice surrounded about 3 cm of the guide pipe, some 10 cm from its free end. Apparently the ice had sealed the access to the helium tank via path 2 as shown in Figure 2. The Ice fizzed and sublimed rapidly once in the open air. The guide-pipe/delivery port assembly was then replaced without difficulty. Thereafter the Dewar functioned normally.

Photographic Record:

Image:Whole dewar.jpg

Image:Neck_assy.jpg

Image:Loosening_clamp.jpg

Image:Clamp.jpg

Image:Withdraw.jpg

Image:Guide-pipe.jpg

Image:Throat.jpg

Image:Plug.jpg

Image:o-ring.jpg

Image:0-ring_seated.jpg

Image:Tank_position.jpg

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