As a service, nitrogen pipe freezing for isolations has been on the market regularly since the 1970’s, but the first patents have been documented as early as 1940’s. The main purposes of the ‘freeze plug’ have been to create isolations in the following applications:

General Pipe Freezing Principle

The basic theory behind a ‘freeze plug’ is the cooling of the outer wall of the pipe to such an extent that its contents (usually water) form a plug inside the pipe, adhering to the wall and creating a solid isolation. Tests have been carried out with plugs holding in excess of 8,000 psi. To accomplish the freeze, a ‘jacket’ (cryogenic heat exchanger) is placed around the pipe, and liquid, usually liquid nitrogen, is circulated within. Earlier jackets were essentially a contained void space where liquid nitrogen came into direct contact with the pipe; modern jackets work more on the principle of a heat exchange, with the liquid nitrogen circulating through veins in the jacket. Jacket length is typically twice the diameter of pipe and approximately one inch thick.

Metal Strength at Low Temperature

At low temperature, the elastic module of materials increases. In general, the tensile strength and yield strength increase as the temperature decreases. Therefore, there is no additional risk to the pipe being pulled or bent slowly at low temperature. Due to increased brittleness at low temperature, however, any impact on the pipe should be avoided.

Since most stainless steel pipes are designed for cryogenic conditions, exposure to low temperatures during the freeze plug will not create any risk to their metallurgy.

Gel Freeze Plugs

In many cases, isolations are required, in which there is no internal medium to freeze. The most common applications for this process are:

1. Modifications or additions to flare headers, where the intention is to keep the plant and therefore flare system fully operational
2. Any gas line where a temporary isolation is required to enable repair of an existing valve
3. New tie-ins on gas or steam lines
4. Temporary blocks for hydro-testing

Gel Freeze Plug Methodology

Cross-linked gel water systems are ideal as a medium to allow for the creation of a stable double block and bleed freeze plug. These systems employ a high-viscosity water-based gel, which, due to its cohesive nature can displace oil or fill a void space in a gas line. The gel itself is a biodegradable product adapted from the food industry and does not require any special handling for injection or disposal. Due to the extremely high viscosity of the gel, it does not behave like a conventional liquid and spread across the bottom of the pipe, but instead forms a mound. Following the first injection, the gelled fluid will remain as a raised mound, likely filling up to 45-65% of the cross-sectional area of the pipe below the injection point. The remaining void space will be filled with gel during subsequent injections.

The location of a freeze plug is carefully selected to ensure that the freeze jacket sites are free of welds or any kind of pipe damage or stress. The chosen site must also have a fitting or valve between intended jacket locations to allow for gel injection, pressure testing of the plugs, and continuous pressure monitoring during the operation. Once the pipe has been inspected and cleaned, the freeze jackets are installed on either side of the injection point and connected to the nitrogen supply by a cryogenic hose.