| Liquid nitrogen piping |
Pipework - to convey liquid nitrogen from bulk store to use point
Thames Cryogenics Ltd manufactures Vacuum Superinsulated Piping often refered to as SIVL or VJ piping, suitable for most cryogenic applications.
The piping basically comprises an inner stainless steel pipe which conveys the liquid nitrogen or other gas, and an outer stainless steel pipe which forms the vacuum insulation jacket.
Special spacers, designed to allow minimal heat conduction, are used to ensure that the inner pipe is held centrally in the outer.
Multiple layers of high reflectivity "superinsulation" surround the inner pipe to minimise the heat inleak due to radiation, and a high vacuum in the interspace prevents convection heat transfer.
The high degree of insulation thus obtained ensures that liquid nitrogen losses due to evaporation in the line are very low (typically a factor of around 20 lower than conventional lagging, which normally occupies far more space). The low level of nitrogen gas generated in the line allows lower pressure drops, leading to further savings by reducing "flash" losses.
The outer pipe remains at close to ambient temperature, affording excellent personnel protection from the low temperature of the liquid nitrogen.
Different installation systems are available to suit different applications.
1. Site-Welded System
The line is made up of pre-made sections, each supplied fully evacuated, terminating in a long thermal path "re-entrant" end, to minimise conductive heat inleak. When the sections are installed, they are connected by means of site-welded, "muff" joints. These joints are leak tested and evacuated on site, to give a complete all-welded line, which is fully vacuum superinsulated for minimum evaporation loss.
The design of joint allows considerable site-adjustment of dimensions, so that building tolerances etc. can be accommodated. Where necessary, (for example, where a use-point or tank position cannot be accurately defined during a site-survey,) even greater dimensional variation can be provided by completing and evacuating closing sections on site
2. Mechanically-Coupled System
As with system 1, the line is constructed in pre-made, pre-evacuated sections, but in this case the sections terminate in the male and female parts of a low-loss bayonet coupling.
Sections are installed simply by inserting the male component on one section into the female component of the next. A very rapid installation is therefore possible.
3. Flexible Lines
Flexible vacuum superinsulated pipe sections allow installations in difficult areas where rigid sections would be impracticable. They are also very useful for temporary installations or where vibrations must be isolated.
In some instances, complete flexible lines may be appropriate, in others flexible sections may be incorporated in rigid lines.
Terminations may be compatible with either system 1 or 2, or may be screwed to suit standard pipe fittings.
4. Microbore Lines
Normally used for low-flow, short run applications, microbore vacuum superinsulated piping is a very compact means of transferring liquid nitrogen with minimum thermal losses.
In general, microbore lines are manufactured in complete runs, but mechanical or welded couplings are available for longer installations,
Standard Sizes
[internal (external) diameter in mm.]
Site-Welded : 18 (76); 30 (89); 45 (102); 57 (114); 85 (168).
Mechanically-Coupled : 18 (76); 30 (89); 45 (102).
Flexible : 10 (45); 12 (51); 25 (85).
Microbore : 5 (25); 8 (34).
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