Based on this information, the typical pitch of the lines in a system, and the size of the lines, a model of the puddle that
could be formed behind the extruded dam was constructed on a 3D CAD system and hold-up volumes were calculated (Figure 3).
Flow tests conducted with water confirmed that the model was valid.
The dam can create several problems in actual systems. In processing systems, after CIP and a final rinse, some of the rinse
water can be trapped behind the gasket in each fitting in a horizontal pitched line, if excessive extrusion has occurred.
The dam and the resulting puddles will not allow the system or equipment to be completely drained. During the sterilization
process, steam should be in contact with all surfaces, and a puddle of water behind the extruded gasket will not allow this
After sterilization, the puddles of rinse water—plus any steam that has condensed during cool-down and added to the puddles—are
locations where contamination could occur. The dams also are locations where expensive product can be trapped, resulting in
waste and making subsequent cleaning more difficult.
FLOW MODELS USING CFD
The dams can create problems during the operation of the systems as well. Using the information generated in the thermal tests,
flow was modeled using computational fluid dynamics (CFD). The model shows that, after the fluid passes over the dam, there
is no flow at the surface of the tube for a certain distance downstream of the extruded gasket. An eddy is created downstream,
immediately after the extruded gasket, where contaminants can become trapped and build up. The extrusion creates a dam, which
acts as an orifice placed in the line.
In the CFD model, flow was introduced at 5.5 ft/sec to simulate a CIP cycle. As the fluid passes through the constriction
of the extruded gasket or orifice, the fluid velocity is increased substantially (in one scenario modeled) to more than 15
ft/sec (Figure 4). Such an increase in velocity in applications where fluid shear is an important consideration, such as in
harvesting mammalian cell cultures, could present an additional potential problem.
The dam also can create potential problems in ambient pure water systems. Contamination and bioburden can become trapped and
build up in the dead spot that is created downstream of the dam. Under steady-state flow conditions, the probability that
the trapped material will be released into the fluid stream is minimized. However, when the flow is disturbed, as would occur
when a number of use points are actuated simultaneously, the resulting surges and disruption increase the chance that trapped
material will be released.
After release, the material travels through the pure water system as a "plug" of contaminants. It will be discovered only
if it passes a sample point at the precise time a sample is being taken. As it continues its travel through the system, it
will disperse, mixing with the pure water, until it contaminates the system, at which point expensive corrective action will
likely be required.