The Effect of Thermal Cycling on Clamp-Type Fittings - How to control compressive loads on seal materials. - BioPharm International

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The Effect of Thermal Cycling on Clamp-Type Fittings
How to control compressive loads on seal materials.


BioPharm International
Volume 23, Issue 6


Figure 3
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 to happen.

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.


Figure 4
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.


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