The cryogenic plugging process consists in locally freezing the liquid contained in the pipeline by using the effects of a cryogenic fluid.
The most commonly used freezing fluid is liquid nitrogen (-196°C at atmospheric pressure).
A box suited to the tube's external diameter forms the cooling envelope. This box is made from stainless steel or polystyrene for small diameters.
The box is installed after having locally stripped the pipeline of its covering or insulation.
The freezing box is filled with liquid nitrogen until the top generatrix of the tube is covered. This level must be maintained for the whole duration of the intervention. The ice plug then forms with annular growth, developing from the tube wall towards the pipeline's axis.
When the ice plug has formed, the supply of nitrogen to the envelope is limited to maintaining sufficient level.
Liquid nitrogen consumption
The consumption of liquid nitrogen is linked to various parameters: pipeline diameter, ambient temperature, fluid temperature, type of fluid, plug holding time, etc.
Risks and limits of the method
Cryogenic plugging is a simple technology, which nevertheless has risks and must be used with their full knowledge.
- Nitrogen related risks:
The risks are due to the liquid nitrogen (cryogenic burns) and to gaseous nitrogen (danger of under oxygenation of the ambient atmosphere).
- Metallurgical risks:
In most cases the metal of the tube offers no guarantee of resilience at the temperature of liquid nitrogen and the frozen area is embrittled. During the implementation of cryogenic plugging (or holding the plug) it is necessary to prevent impacts on to the piping.
Similarly the freezing of areas with welds or shape parts (especially elbows) is avoided.
- Limits of the method:
As far as possible it is recommended to implement cryogenic plugging on water. Indeed certain frozen products do not have sufficient consistency to retain even low pressure.
There are practically no references to cryogenic plugging carried out on pipelines of diameter greater than 20".