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

SANITARY CONNECTIONS

Both process (hygienic) and clean utility systems are connected by welding or sanitary connections. If the connections are intended to be permanent and not meant to be made and broken during the intended service life of the system or equipment, automatic orbital welding may be used to make consistent, high-quality welds.


Figure 1.
The ISO 2852 gasket-sealed, clamp-type sanitary fitting—used most often for connections that must be made and broken—consists of four components: two flanged ferrules, which are to be welded to the required lengths of thin-walled tubing, thin-walled tubular shapes, or other components; an elastomeric or plastic gasket located between the flanged faces of the two ferrules; and a clamp to hold the connection together (Figure 1A).

The containment of the seal and the control of the loads on that seal, both during make-up and in operation, are very important considerations. The ideal condition after make-up of the fitting is a bore-line seal. This type of seal does not extend into the inside diameter of the tubing or pipe at the seal point.

The standard, ASME BPE, requires that the gasket in a made-up sanitary fitting must be flush with the bore of the tubing or pipe. However, for a gasket seal used in an ISO 2852 fitting, this often is not the case. As the clamp is tightened during make-up, the gasket is free to extrude radially outward and inward. Installation methods and techniques vary from installer to installer and company to company. The amount of extrusion will vary with how tight an installer tightens the clamp. Because the compression on the gasket is not controlled, overtightening is possible.

Outward extrusion doesn't create much of a problem, but inward extrusion can. As the gasket material extrudes into the bore of the tubing or pipe, it creates a dam in the flow path of the lines, which are pitched to facilitate draining. The dam can create problems in pure water, clean in place (CIP), and steam sterilization systems, making cleaning, draining, and sterilization more difficult. Also, it can result in product holdup in the processing system during harvest, recovery, and downstream purification and refining.

THERMAL TESTING

Tests were conducted to determine the amount of extrusion that is possible at installation and to determine the impact of thermal cycling on the completed fitting assembly. Manifolds consisting of five 1.5-inch sanitary fittings, each separated with a short length of 1.5-inch x 0.065-inch tubing were built. Ethylene-propylene diene monomer (EPDM), silicone, fluorocarbon FKM, and polytetrafluoroethylene (PTFE) gaskets were used, each in a separate manifold. Clamps were tightened to the maximum possible by hand.


Figure 2
Before thermal testing, all pertinent dimensions were recorded, including the gasket intrusion into the bore of the tubing. The assemblies were vacuum helium leak-tested to confirm that a proper seal was made. The thermal test, intended to simulate a sterilization process, consisted of heating the assemblies to 121 C in 30 min, stabilizing, holding at temperature for 30 min, and water quenching back to room temperature. The test was conducted for 250 cycles for an extensive view of the assemblies' performance; most companies run only 25 to 100 steam cycle tests. Ideally, gaskets should be replaced after every sterilization cycle, but many companies reuse gaskets between batches. The assemblies were removed from the test rig 17 times during the test, dimensionally checked, and helium leak tested. The results are shown in Figure 2.


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