Ground freezing installation accommodating thermal contraction of metal feed pipes
Joseph A. Sopko
In some ground freezing applications, it is necessary to create frozen barriers that extend to considerable depths. For example, ground freezing to depths of 1000 feet or more has been achieved. However, the need to freeze the ground to substantial depths leads to complications. One example is that thermal contraction effects become increasingly pronounced as the pipes increase in length. The feed pipes are typically constructed of high density polyethylene (HDPE). HDPE has a relatively large coefficient of thermal expansion, so long pipes contract significantly when cooled by the refrigerant. As a result, the bottom discharge end of the pipe can rise so much that the lower end portion of the freeze pipe fails to receive refrigerant. Then, the lower part of the bore remains unfrozen.
Although metals such as steel contract much less than HDPE due to thermal effects, such metals are also much heavier and must be supported from the bottom. However, when long metal feed pipes are cooled, they contract enough to rise above conventional bottom support systems such that the pipe weight is then borne by a fitting at the top end of the feed pipe. This can exert undue stress on the top fitting and cause the feed pipe to detach from it, resulting in a structural failure.
This invention provides a solution to the problem of feed pipe thermal contraction in a ground freezing system. In accordance with the invention, the feed pipe is constructed of metal, preferably steel, so that its thermal contraction is much reduced compared to HDPE pipes. To accommodate the thermal contraction that does occur, a resilient mounting element such as a compression spring supports the feed pipe at the bottom of the bore. The spring or other resilient mounting structure is able to expand as the lower end of the feed pipe is drawn upwardly due to thermal contraction. The mounting system is thus able to maintain support of the feed pipe as thermal contraction occurs so that undue stresses are not exerted at the top fitting of the pipe system.