Parameters Governing Steam Sterilization of Deadlegs

Abstract

Use of saturated steam for sterilization-in-place (SIP) is limited by factors effecting displacement of air from deadlegs. Effects of tube diameter, length, orientation and position within a deadleg were quantitatively studied by examining temperature profiles and rates of kill of Bacillus stearothermophilus spores. Tube diameter had the greatest effect on sterilization. For small diameter tubes, 0.4 cm inside diameter (ID), air displacement was minimal and due mainly to diffusion. 8.8 cm long tubes with 0.4 cm IDs could not be sterilized at 121°C. As tube diameter was increased and buoyant driven convective flow became dominant over viscous forces, sterilization was achieved and tube orientation became critical. Sterilization time, as defined by a twelve log reduction in spore population, was 75 minutes in a 19.0 cm long vertical tube with 1.7 cm ID, whereas 167 minutes were required for an 8.8 cm long tube with 1.0 cm ID. For 8.8 cm long tubes, only the 1.7 cm ID tube could be sterilized when orientated 5 degrees above horizontal.

Data show that length to diameter ratios, L/Ds, do not provide a general guideline which can be used to predict sterilization. In the absence of steam bleeders, equipment should be designed to assure strong buoyancy driven convective flow to assure adequate air removal. This requires elimination of small diameter deadlegs (0.4 cm ID and less) and vertical positioning of deadlegs.