Heat pipe repairs help keep ground cool beneath TAPS

About half of the 800-mile-long Trans Alaska Pipeline System (TAPS) is built above permafrost that needs to stay frozen to help keep the pipeline stable.

That's why engineers placed 124,300 heat pipes into TAPS' vertical support members that are on permafrost terrain, two per support. These tightly sealed pipes form a giant ground-cooling system containing anhydrous ammonia refrigerant, which vaporizes below ground level, rises and condenses above ground, transferring ground heat to the atmosphere whenever the ground temperature exceeds air temperature. The heat is conducted from the ground into the pipes, rises with the ammonia vapor and is again conducted through the heat pipe walls and out into the cooler air through aluminum radiators at the top of each pipe.

Over time, hydrogen gas has accumulated inside many of these heat pipes, especially at the southern end of the pipeline. Hydrogen is less dense than ammonia and condenses less easily, so it tends to accumulate in large bubbles at the tops of heat pipes. This reduces cooling performance of the pipe by blocking the ammonia from reaching the upper most levels of the heat radiators.

Alyeska engineers recognized this problem and used infrared photography to measure blockage caused by the non-condensing hydrogen. They discovered that some 84 percent of TAPS 124,300 heat pipes had lost some of their ability to transfer heat.

"These blockages are inevitable," said Steve Sorensen, an Alyeska engineer. "They don't completely stop heat transfer, but they make it less efficient."

Engineers determined that about a third of the heat pipes had blockages exceeding their original performance criteria. Tests were then conducted to determine how much hydrogen accumulation is unacceptable. It was discovered that most permafrost temperatures remain frozen and well below 31.5 degrees F at the end of summer even when hydrogen blockages far exceed the original specifications. Consequently, they developed new performance-based standards to determine which heat pipes do not keep permafrost cold enough and require repairs.

The repairs consist of replacing the existing anhydrous ammonia and accumulated hydrogen with carbon dioxide at 500 pounds per square inch pressure, ten times the ammonia pressure.

"The carbon dioxide removes ground heat like ammonia," said Sorensen, "but hydrogen accumulates in a much smaller volume in the higher-pressure carbon dioxide, so we believe this repair is permanent."

Crews started installing carbon dioxide in 2001 and have already finished 4,602 heat pipes. They will continue to recharge heat pipes as needed, but many may never need to be recharged, especially at the northern end of the pipeline where permafrost is colder and hydrogen forms more slowly.