Definition - What does Pipe Freezing mean?
Pipe freezing is a technique used to modify and repair pipelines. Pipe freezing cools the exterior of the pipe for a period of time long enough for the fluid within the pipe to solidify. The frozen fluid isolates a section of the pipe so that work can be performed in a more expeditious manner.
Corrosionpedia explains Pipe Freezing
Pipe freezing is a useful tool when shutoff valves are not available in the area of the pipeline that needs repair. Pipe freezing permits repairs in isolated areas. Because individual sections of a pipeline can be separated using the pipe freezing method, the entire pipeline system may not need to be shut down. Pipeline freezing is very practical for small diameter pipes, however large diameter pipes (150 mm or greater) may require special equipment and considerations.
To perform the procedure, the temperature of the pipeline must be reduced below the freezing point of the fluid it is carrying. This is accomplished with some type of refrigerant. A popular method is to apply liquid nitrogen to the pipeline, although different freezing methods may be required for different types of pipe materials and different types of fluids. After the cooling has begun, the fluid within the pipe will begin to form what is known as an ice plug. The ice plug prevents the rest of the fluid in the pipe from flowing, allowing repairs or other pipeline modifications to occur.