Pharmaceutical pipe dead-legs: what's that all about?
Purified water (drinking water treated using million pound purification plants) is frequently used in the pharmaceutical industry during the manufacture of tablets and medicines. This water is distributed throughout the manufacturing facility to points-of-use using high quality process pipework. The installation of a pipe tee in this pipework often creates a stagnant dead-leg zone. This dead-leg can contaminate the entire distribution network resulting in lost production, contaminated product and down time for cleaning.
Considerable basic research is required to address the lack of understanding of this problem and to assist during design, manufacture, installation and operation of these critical systems. Research work within the School of Mechanical and Manufacturing Engineering at Dublin City University involves the application of CFD software (Computational Fluid Dynamics) to the study of pipe dead-legs. The outcome of this research will benefit designers of next generation sterile and hygienic piping systems.
The formal definition of a pipe dead-leg as given by the Food and Drug Administration (the FDA) is:
Pipelines for the transmission of purified water for manufacturing or final rinse should not have an unused portion greater in length than 6 diameters (the 6D rule) of the unused portion of pipe measured from the axis of the pipe in use.
Left: Velocity contours – Right: Computational grid.
The FDA suggest the above 6D rule will help prevent contamination: however industrial experts are designing systems with dead legs limited to 3D or less. Some systems and fittings claim to have zero dead legs. The result of this confusion is an escalation in design and manufacture cost within an already highly expensive industry. Current research includes the development of a CFD model of a pipe tee and the application of turbulent models to analyse flow profiles within the tee branch (see illustration). Recent results indicate that the 6D rule is indeed insufficient to prevent stagnation and that areas exist within the branch of the tee which are undisturbed by the turbulent flow in the main loop pipe. Future work will involve the investigation of turbulence intensity and wall shear stress in the near wall region, and close investigation of the viscous sub-layer.
Contact: Mr Brian Corcoran;