Dead water pockets

Nkedi-Kizza et al. ( ). Even when shallow dead-end pores or thin immobile-water pockets exist, the diffusion rates may be so slow that a concentration profile is more realistic than an effective, average concentration in the immobile-water phase. 

Temporary circulation pumps are often necessary, as permanently installed pumps may be unsuitable or unable to provide the velocity necessary for flushing and cleaning work. Typically the water velocity necessary is 6 to 9 ft/sec (2 to 3 m/sec). Once in motion, debris velocity is virtually independent of particle size, but is clearly a function of water velocity. Relatively high velocities are needed to lift particles up from horizontal pipe surfaces and dirt pockets (dirt collection points at the base of riser pipes), and especially to overcome problems of particle retardation when moving from the horizontal through a vertically upward bend. Special provisions may need to be made to ensure the cleaning of dead-leg areas,... 

In addition, freezing and corrosion due to water accumulation in pipes, dead pockets, and instruments have caused numerous pipe failures. Some of these led to gas releases and fatal accidents. A detailed discussion on preventing water accumulation in column piping is presented elsewhere (210). Extensive discussions of winterization and freeze-prevention techniques are also available (120, 266). 

Include high pressure water purge for both forward and reverse flow at velocity > 1 m/s. Avoid dead ends and pockets. Arrange for flushing of the line with water place nozzles on the top of the primary loop. Avoid flow restrictions, including orifices. 

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