Beware Thermal Cycling in Bioprocessing Part 2
Containment of seal material and control of compressive loads on gaskets can help reduce the impact of cleaning, draining and sterilizing.
By Dave Simko, Swagelok Corp.
Click here to read Part 1 of this series.
In bioprocessing, both the process (hygienic) and clean utility systems are connected by welding or sanitary connections. When the connections are to be repeatedly made and broken, gasket-sealed, clamp-type sanitary fittings are usually employed. The most often selected type is the ISO 2852. The fitting 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 (Figure 1a).
Figure 1: Sanitary Connections
Containment of the seal member and control of the loads on that member, both during make-up and in operation, are very important considerations. A seal which does not extend into the inside diameter of the tubing or pipe at the seal point is referred to as a bore-line seal and represents the ideal condition after make-up of the fitting. The standard, ASME BPE, requires that the gasket in a made-up sanitary fitting should be flush with the bore of the tubing or pipe.
However, the gasket seal in an ISO 2852 fitting is not fully contained and supported. As the clamp is tightened during make-up, the gasket is free to extrude radially outward and inward. The amount of extrusion will vary with how tight an installer tightens the clamp, and over-tightening is possible. Outward extrusion doesn’t create much of a problem, but the 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, CIP and steam sterilization systems, and in the processing system. Tests were conducted to determine the amount of extrusion possible at installation and to determine the impact of thermal cycling on the completed fitting assembly. Manifolds consisting of five 1-1/2 in. sanitary fittings, each separated with a short length of 1-1/2 in. x 0.065-in. tubing, were built. EPDM (ethylene-propylene diene monomer), silicone, fluorocarbon FKM and PTFE (polytetrafluoroethylene) gaskets were used, each in a separate manifold. Clamps were tightened to the maximum possible by hand. The thermal test, intended to simulate a sterilization process, consisted of heating the assemblies to 121º C in 30 minutes, stabilizing, holding at temperature for 30 minutes, and water quenching back to room temperature. The test was conducted for 250 cycles.
The assemblies were removed from the test rig 17 times during the test, dimensionally checked and helium leak tested. Based upon 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 (Figure 2) and hold-up volumes were calculated. 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, where excessive extrusion has occurred.
Figure 2: Retention Model
The dam 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) techniques. 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. Lastly, the dam can create 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.
In the sterilization system and the equipment and systems being sterilized, thermal cycling presents a further problem. When the test manifold was disassembled after thermal cycle testing, wear on the face of the gasket seal was observed. Wear and fretting of the gasket material was probably due to radial expansion and contraction of the gasket during heat-up and cool-down. The gasket material was characterized and the shape of the unconstrained portion of the gasket was modeled using finite element analysis (FEA) techniques.
The shape is bulbous. During expansion, the gasket extrudes radially, and the unconstrained portion expands axially. During contraction, as the gasket moves back into the constrained space between the ferrules, the surface of the unconstrained portion of the gasket is dragged over the relatively sharp metal corner formed by the bore of the ferrules and their flat faces. This explains the wear. In service, when gaskets are subjected to high temperature and high compressive loads, some gasket materials can take a compression set and become loose.
Compression set is the tendency of an elastomer to lose its memory under stress and not return to its original shape when the stress is removed. During thermal testing, regular leak tests indicated that the ISO 2852 fittings needed to be retightened after every fifth thermal cycle through the first 15 cycles, because the gasket had taken a set. These results can explain why containment is lost immediately following a sterilization cycle.
Another fitting design that addresses solutions to the issues discussed was also tested. In this fitting (Figure 1b), the configuration and cross-section of the gasket and the face of the ferrule are different from the ISO 2852 fitting. The gasket consists of two parts—the rib and the crown—each having a specific function. When clamped between two ferrules, the seal is made at the rib. The function of the large mass of material in the gasket crown is to control the amount of extrusion toward the bore of the fitting. The faces of the ferrules are machined to accept the crown and align the two ferrules for assembly of the connection.
A metal-to-metal stop is provided at the maximum outside diameter of the ferrules to limit the amount of load that can be applied to the gasket. The gasket was configured to maintain proper “squeeze” over its complete cross-section. At initial make-up of the connection, compressive force is applied to the gasket with the same type of clamps used with an ISO 2852 fitting. Controlled extrusion permits a small amount of extrusion into the bore of the fitting, creating a stable bore-line seal, and avoids undesirable concavity at the seal point. The majority of the extrusion is taken up in the crown contained in the chamber formed between the faces of two ferrules. The chamber is not completely filled in order to accommodate expansion of the gasket material during thermal cycling.
These fittings were assembled into a manifold identical to the ISO 2852 manifold discussed earlier and tested in exactly the same way. The results were quite different. None of these fittings required retightening during the thermal testing. Flow in this fitting was also modeled with the CFD technique, based upon a velocity of 5.5 ft/sec. The small amount of controlled intrusion at the gasket seal looked no different than the inside of a full penetration buttweld. There was no constriction of the flow through the connection and no increase of velocity through the connection. The CFD model showed no entrapment zone downstream of the gasket seal.
About the Author
Dave Simko is manager of marketing resources for Swagelok Co.