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Pipefreezing for repairs, modifications, maintenance, leak detection, pressure testing, etc.

What is Pipe Freezing ?

Pipefreezing is a means of isolating part of a pipework system by freezing the pipe contents over a short section, so that the contents form a solid plug. This avoids having to drain the system down, and in many cases freezing can be carried out without shutting the down the system.

Once a specific part of a system has been isolated with pipefreezing, it is possible to carry out repairs, modifications, maintenance, leak detection, pressure testing, etc.

How is pipefreezing carried out ?

A container (usually referred to as a jacket) is placed around the pipe, and then filled with liquid nitrogen. Liquid nitrogen boils at -196oC, and so boils very readily on the surface of the pipe within the jacket. This removes heat from the pipe and its contents, which will in time freeze solid. As the nitrogen boils off, it releases nitrogen vapour which is vented away to the atmosphere. In certain situations, solid or gaseous carbon dioxide is used. This is not as cold as liquid nitrogen, and so freezing takes longer, but has the advantage of not exposing the pipe material to such low temperatures.

What pipe materials are suitable for pipefreezing ?

Metallic pipes.
Metallic pipes are generally all suitable for pipefreezing using liquid nitrogen.
Care must be taken when freezing carbon steel and cast iron (ferritic BCC structures) pipes because they are embrittled at liquid nitrogen temperatures. Pipes in this condition should not be subjected to shock loading. Allowances must also be made for thermal contraction in cases where pipes are axially constrained. Nickel alloys, stainless steels (austenitic FCC structures), copper alloys, aluminium alloys, are all suitable for freezing.
Non-metallic pipes
GRP and polymer pipes. Extreme caution must be excercised when freezing these pipe materials: low conductivities and high thermal expansion coefficients can lead to very high thermal stresses on cooling. Our tests indicate that GRP pipes can be frozen using liquid nitrogen, but that some crazing of the outer surface is likely. Freezing un-reinforced polymer pipes using liquid nitrogen is best avoided: controlled temperature techniques using refrigerants or Carbon Dioxide are more appropriate in these cases.
Lined/coated pipes.
In our experience, concrete, bitumin, ceramic, glass, and rubber lined pipes can be frozen without any apparent damage to the lining. Allowances have to be made for the lining thickness, which may considerably slow the freezing process. There are of course risks associated with subjecting pipes made with components of differing expansion coefficients to very low temperatures, and the consequences of damage must be bourne in mind.

What pipe contents are suitable for pipefreezing ?

Water and water-based liquids are by far the most commonly encountered pipe contents. Water is almost unique in that it expands on freezing - the cause of burst pipes in cold weather - and allowances have to be made to accommodate this expansion. Freezing times are dependent on liquid temperature, pipe size, and local pipework geometry, and in the case of water, are usually predictable. Additives such as glycol which suppress the freezing point make pipefreezing more difficult, but allowing additional time and liquid nitrogen usually means that freeze isolation is possible, even in very large pipelines (see our examples pictures below)
Water based liquids such as slurries and sludges behave in a similar manner to water.

Heavy oils (e.g. fuel oil, diesel) and crude oils can usually be frozen in favourable conditions, given sufficient time and liquid nitrogen, provided that pipe sizes are not too large.

Light hydrocarbons present a much greater problem due to their low viscosities and low freezing points. In small pipe sizes, freezing these materials may be attempted, although it is usually prudent to carry out experiments, or computer-based numerical simulations before attempting operations with this class of substance. Recent experimental work has used styrene (with which we have also used our heat-flux system), tetradecane, exxsol-d140. Contact our London Office if you have a pipefreezing requirement which involves freezing something unusual.

Heat flux monitoring for pipefreezing safety
Heat-flux monitoring gives ultimate safety and assurance in critical applications:
Non-invasive ice-plug detection
Flow detection
Ice-plug integrity monitoring

"Is it ready yet ?"
When attempting to freeze small pipes, this question can usually be answered by our technicians, based on their experience, and various observable physical signs. With larger pipes, it becomes a little more difficult to answer. Quite often, the physical signs may be indicative of a complete ice plug, but another means of testing is required. In many cases, a physical test point is either present, or can be introduced into the system.
Heat-flux measurement gives us a non-invasive means of monitoring the development of an ice-plug during a pipefeezing operation, tells us when a complete plug has been formed, and allows us to monitor the condition of the complete plug during any subsequent pipework intervention. On large scale pipefreezing jobs, it is also possible to detect any flow in the system, and determine its direction.

Heat-flux monitoring is carried out by attaching heat-flux gauges to the outer pipe surface within the cooling region. These gauges measure the amount of heat being drawn out of the pipe by the liquid nitrogen in the jacket. The amount of heat that can leave the pipe is related to the pipe contents, and the amount of ice that has been formed within the pipe, and from these measurements we can deduce the state of the plug and detect the presence of flow.

We believe that Cyril W Bishop Engineering is the only company in the world that can offer the full benefits of heat-flux measurement technology.

This picture shows the pipefreezing jacket quite clearly: it is built in two halves and bolted around the pipe. Each half is a double-skinned welded aluminium construction, with foam insulation between the two skins. Liquid nitrogen is fed into a fitting in the lower part of the jacket through a vacuum insulated hose attached to a pressurised vessel. The nitrogen vapour produced in the jacket escapes through the collar at the top of the jacket

Monitoring pipe surface temperatures at the jacket extremities can give useful information about the extent to which the ice plug has grown beyond the jacket. Here, a hand-held digital thermometer is being used. In spite of the foam insulation between the skins of the jacket, the extreme low temperature of the liquid nitrogen within the jacket causes considerable frosting on the outer surface.

The new valve being lowered into position. The freeze site is in the background.