Fusibond Piping
Systems, Inc.

Liner- Loose vs. Locked-In PIPE Plastic lined pipe can be fabricated with either a loose or locked in liner. A locked in liner reduces the effect of the difference in expansion coefficient between the metal and the liner. A loose liner, when extensively thermal cycled, can result in cracking at the flange surface since that is the only place where the liner is restrained. Loose liners tend to roll back and forth at the flanges when heated to an elevated temperature due to the differences in expansion between the metal and liner. Loose liners tend to collapse when operated under high vacuum at elevated temperatures. In addition, loose liners provide a space for gas build-up between the liner and the pipe and result in corrosion and liner collapse. Weep holes which can be used to initially prevent this problem are readily blocked by paint, rust, and insulating materials. Weep holes can also create problems of structural corrosion of the metal pipe. Weep holes are commonly used with PTFE and FEP due to the poor permeation characteristics. MPFA/PFA does not always need to be vented. FITTINGS For plastic lined pipe to be practical and viable, a total system is required including tees, elbows, crosses, standard reducing and special configurations. The method of fabrication of the plastic lined fittings is key to the total system reliability. The ASTM specifies that no welding is permitted on the fitting after the liner is molded into place. Some of the lined fittings on the market do not comply with this part of the ASTM with a resulting loss in reliability. PRESSURE AND VACUUM LIMITATIONS Plastic lined pipe is available in 150 and 300 pound pressure rated pipe. Higher pressure pipe is available on special order. Bonded lined pipe can operate at full vacuum. Vacuum collapse capability of loose lined pipe is dependent on liner thickness. Click on image for larger view