The failure to employ a reliable valve with a suffcient seal can put an entire system at risk. Ultimately, the bottom line may be affected if materials are released, cleanliness and sterility are compromised, or the operation of process instrumentation, control, and sampling systems is hindered. There are two distinct types of valves used in applications where cleanliness and entrapment control are of paramount concern. The weir-style valve, developed about 70 years ago, is the industry standard-bearer, with a track record of solid performance in validated systems. In weir-style valves, the seal is made outside the bowl rim.
Another type, the radial diaphragm valve, uses a boreline seal, which is created when the lip of the diaphragm is clamped between the valve body and the actuator around the 360° circumference. This type is gaining increased use as many design engineers and biopharmaceutical companies are exploring alternative ways to safely regulate the flow of water, steam, and product through their clean processing systems.
Recent research has shed light on the question of which valve seal design is better suited for high-purity applications. Through a series of riboflavin tests, the entrapment potential of the weir-style valve and a valve with a boreline seal were examined. The tests showed that weir-style valves have a potential entrapment area at the valve bowl edge between the body and diaphragm; this entrapment area is not present in radial diaphragm valves that utilize a true boreline seal.
These findings raise significant questions as to the impact this residue has on overall system cleanliness and yields. The tests further indicate that a valve with the boreline seal design has the potential to better ensure cleanliness than the traditional weir-style valves.
Test Assesses Valve Cleanliness
To assess the cleanliness of weir-style valves and those that utilize a boreline seal, an independent test laboratory undertook riboflavin cleanability comparison tests on five weir-style, 1/2-inch diaphragm valves with manual actuators, five weir-style, 1-inch diaphragm valves with manual actuators, five Swagelok® DR series radial-style, 1/2-inch diaphragm valves with manual actuators, and five Swagelok DR series radial-style 1-inch diaphragm valves with manual actuators.
Samples of both valve styles were tested using electrophoresis-grade powdered riboflavin to assess and compare the cleanability of both the weir-style valve and the radial diaphragm valve. While there is no standard test for cleanability, riboflavin was chosen because this yellow crystalline B vitamin is known to leave a residue that can be readily detectable under the testing methods. Powdered riboflavin was used instead of liquid riboflavin because the powdered material was easy to apply and gave an even coating on all interior valve surfaces.
Procedure for Cleanability Comparison
As part of the testing procedures, all the valves—the weir-style and radial diaphragm models—were disassembled and the components were cleaned with isopropyl alcohol. They were air dried and then examined under ultraviolet light to verify that no riboflavin was present prior to testing. The valves then were reassembled according to the manufacturers’ instructions and specifications using PTFE diaphragms and manual actuators. The valves were actuated to the closed position.
Riboflavin powder was placed inside the inlet of each valve, with approximately 0.75 grams of powder for the 1/2-inch valves and 1.0 grams of powder for the 1-inch valves. To simulate throttled flow, the valves were actuated to the half-open position followed by the application of a 1/2- to 1-second burst of pressurized argon at 50 psig (3.4 bar).
The inlet and outlet of the valves were examined to verify even distribution of the riboflavin powder. The inlet was connected to a flow test bench, and the valve was mounted with inlet and outlet oriented horizontally and the bonnet oriented in a manner that located the valve in its most drainable position, according to the manufacturers’ instructions. The valves then were purged with deionized water for seven minutes and allowed to drain. The flow rate during purging complied with the recommended miniGum, according to ASME BPE-1997 Bioprocessing Equipment Specification.
Valves Minimize Entrapment
ýach of the valves was removed from the flow test bench and placed on a hot plate to enable the interior surfaces to dry. The valves were disassembled and the wetted surfaces of the valves (body and diaphragm) were visually inspected under standard illumination and ultraviolet light for the presence of riboflavin residue.
The test results clearly indicated that the weir-style valve bodies consistently showed riboflavin residue, while the radial diaphragm valves regularly exhibited no or very little residue. Specifically:
* Riboflavin residue was found on ten of the ten weir-style, 1/2-inch diameter valves. The residue was present on the ledge above the valve bowl and was visible under both standard illumination and ultraviolet light.
* Riboflavin residue was found on ten of the ten 1-inch, weir-style valves. The residue was located at the edge of the seat-sealing bead that contacts the valve weir and was visible under both standard illumination and ultraviolet light.
* Trace residue was found on only two of the ten 1/2-inch diameter radial diaphragm valve bodies. When present, the residue was located on the valve bowl. This residue was visible under standard illumination and ultraviolet light.
* Trace riboflavin residue was found on only three of the ten 1-inch diameter, radial diaphragm valves. When present, the residue was located at the diaphragm center, or near the outer sealing rim. This residue was visible only under ultraviolet light.
Design Differences Are The Key
The design of the DR series radial diaphragm valve represents an improvement on the existing technology and may provide a cleaner, more compact way to manage sterile flow streams. The valve reduces entrapment because the geometry at the point of the seal between the diaphragm and the valve body creates a boreline seal. There is no flexing at the point of seal, which maintains seal integrity. The outer diameter of the diaphragm is contained by a counterbore in the valve body, a design that controls diaphragm extrusion and further sustains seal integrity during thermal cycling.
In the weir-style valves, the seal is made outside of the bowl rim, creating the potential for entrapment as evidenced by the riboflavin tests. Flexing can occur at the point of the seal.
The importance of these test results cannot be understated. They clearly show that weir-style valves are susceptible to entrapment to a greater degree than the valves with the boreline seal. This not only raises questions as to the long-term effects this residue can have on a system, but also strongly indicates that cleaning time can be reduced significantly through the application of the boreline seal valves. By minimizing entrapment areas, processing facilities can reduce residue, thereby limiting the risk of contamination in clean processing systems while also reducing potential costs associated with cleaning valves.