Tests of cryogenic pigs for use in liquefied gas pipelines

Pipeline pigs are a key element in the design of a proposed spill test facility whose purpose is to evaluate the hazards of large spills of liquefied gaseous fuels (LGFs). A long pipe will run from the LGF storage tanks to the spill point; to produce a rapid spill, the pipe will be filled with LGF and a pig will be pneumatically driven through the pipe to force out the LGF quickly and cleanly. Several pig designs were tested in a 6-inch-diameter, 420-foot-long pipe to evaluate their performance at liquid-nitrogen temperature and compare it with their performance at ambient temperature. For each test, the pig was placed in one end of the pipe and either water or liquid nitrogen was put into the pipe in front of the pig. Then pressurized drive gas, either nitrogen or helium, was admitted to the pipe behind the pig to push the pig and the fluid ahead of it out the exit nozzle. For some tests, the drive gas supply was shut off when the pig was part way through the pipe as a method of velocity control; in these cases, the pressurized gas trapped behind the pig continued to expand until it pushedmore » the pig the remaining distance out of the pipe. The tests provided information on how the effectiveness and velocity of the pig and the flow rate of the expelled fluid changed with pressure and shutoff time of the drive gas and with temperature. The pig designs that left the least liquid during the water tests were a polyurethane foam pig and a cylindrical metal pig with flexible metal wipers. In the liquid nitrogen tests, the metal pig with wipers performed best. It removed all the liquid nitrogen and survived most of the tests well.« less