Beware Thermal Cycling in BioProcessing Part One
Its impact on ball valve seals and clamp-type fittings poses a
serious contamination risk, for utilities and processes
By Dave Simko, Swagelok Corp.
Those connections will be discussed in Part 2 of this article (to appear in September’s issue of Pharmaceutical Manufacturing) on sanitary fittings. The critical seals in ball valves are at the stem and seat and are part of the basic design of the valve. They are dynamic seals and must retain leak-tight performance during and after both mechanical and thermal cycling.
Stem seals are intended to prevent leaks into or out of containment. Leaks are driven by pressure and proceed from a high-pressure to a low-pressure region. In pressurized clean utility systems, leakage of pure water or sterile air into the surrounding environment normally will not be hazardous; however, it can be expensive, in terms of lost fluids and clean up.
Leakage of pure steam out of the system can present a safety issue as well as an expense. Leakage into the steam system creates an even worse situation. For example, clean steam is used for sterilization. After the necessary temperatures are reached, stabilized, and held for the prescribed length of time, the equipment or system is cooled.
During cool down, a vacuum is created inside, and if the stem seals in ball valves used for isolation have failed, contaminating microorganisms may be drawn into the system. Generally, ball valve stem seals consist of a ring of polytetrafluoroethylene (PTFE) having a square or rectangular cross-section and are contained on the outside diameter (OD) by the packing bore wall, on the inside diameter (ID) by the valve stem, and on the top and bottom by washer-shaped glands (Figure 1a).
Figure 1 (A and B): Figure 1A shows a ball valve stem seal that consists of a ring of PTFE with a rectangular cross-section. Figure 1B illustrates the
live-loaded, 2-piece chevron stem packing, which requires less operating torque, improves performance, and compensates for stem wear.
Most manufacturers choose PTFE for stem seals. It is relatively easy to deform the material to make an initial seal. However, it can cold flow, or continue to deform under load—a condition which worsens with increasing temperature. Because PTFE has no “memory,” once it is deformed under load, it does not return to its original shape when the load is removed.
The seal is made by deforming the seal ring by applying sufficient force through a packing nut to deform the material inward against the stem and outward against the packing bore. It is necessary to fully encapsulate the PTFE seal material on all surfaces with metal—the packing bore, valve stem, and glands—as described. The clearance between the gland ID and the valve stem OD should be an absolute minimum.
Otherwise, during thermal cycling, when the temperature of the valve is increased from ambient up to the sterilization point and back down, the material can migrate or cold flow out of the seal area, eventually loosening the seal. In addition, the stem seal must maintain its integrity during the rotation of the stem within the PTFE seal member. Surface finish on the valve stem is important in terms of reducing wear on the inside diameter of the seal.
ASME BPE addresses requirements for rotating valve stem seals but does not specify surface finish requirements for the stem. The metal surface of the stem will have a certain level of roughness, in the form of “peaks and valleys,” from the machining operations. Under load and with thermal cycling, the PTFE seal material cold flows into the valleys.
Then, when the valve is actuated, the small amounts of material in the valleys are sheared off and migrate out of the seal area. As the valve is cycled, more material is removed, the initial load that made up the seal is reduced, and the seal becomes loose. Both of these situations result in rapid wear of the seal, leading to potential leakage.
The degree of encapsulation of the PTFE seal in various ball valve designs and the manufactured surface finish on the stems in valves from different manufacturers impact how quickly stem seal failure might occur—in many cases after only a few thermal cycles. If suitable head pressure is not maintained in the system or equipment during cool down from sterilization temperatures, unwanted microorganisms can be drawn in and destroy the sterile condition.
Although pressurizing with sterile air can help avoid this situation, these microorganisms can still migrate across the loosened stem seal and contaminate the process. General improvements to ball valve stem seal reliability can be accomplished by improving the containment, or encapsulation, of the seal member and by improving the seal surface on the stem during manufacture. Further improvements in seal reliability have been made by improving the typical configuration of the seal member and by adding a live-loading mechanism.
Liveloading means that as the stem seal wears, sufficient load is consistently applied to maintain the seal. One approach to live-loading uses a seal ring with a two-piece split chevron configuration (Figure 1b), rather than the typical one-piece square or rectangular crosssection. The two angled, conical pieces of the chevron create a wedging action to achieve a seal against both the packing bore and the stem. The force required to initially make-up this seal is lower than the force needed to make a seal with a solid, one-piece packing.
A group of conical disc springs placed on top of the seal member provide the live-loading force and compensate for expansion and contraction during thermal cycling. As the seal wears during normal use, the springs continually “retighten” the seal, ensuring its integrity and reliability over a longer service life.