|(a) Bolted connections shall be designed to withstand thermal contraction and expansion. (b) Pipe joints larger than two-inch nominal diameter shall be welded or flanged. Joints of four-inch nominal diameter or less may be threaded where necessary for special connections to equipment provided that the special connection is not subject to fatigue-producing stresses. The number of threaded or flanged joints shall be kept to a minimum. Dissimilar metals shall only be joined by flanges or transition joint techniques which will not be adversely affected by LNG. (c) Gasket material shall withstand as much as practicable exposure to fire. (d) Piping and tubing shall be installed as directly as possible with provisions for expansion, contraction, jarring, vibration, and settling. Underground piping shall be buried at least 18 inches below the ground surface unless otherwise protected. Refrigerated piping shall not be buried unless the surrounding soil is heated.