A biopharmaceutical plant in the U. S. was operating a cold/chilled glycol (CG) system to cool approximately 50 process vessels, mainly jacketed tanks (250 – 3500 L), and several heat exchangers to produce its products. The plant was approximately 18 years old and circulated propylene glycol (40%) and water (60%) as the heat transfer medium. Chillers in the basement generated 28°F (-2°C)cold glycol which was then circulated by three constant-speed centrifugal pumps to the distribution network. System differential pressure control was achieved with a bypass valve that short-circuited the CG supply to the return side during periods of low demand.
Globe-style control valves, located in the mechanical rooms, provided temperature control of the process vessels. The valves were automated (PID control) through the plant’s distributed control system (DCS).
However, being 18 years old, the original valves were providing inadequate positive shut-off as required for their application. Specifically, the plant experienced a temperature excursion in one of the tanks, because cooling fluid passed through a closed control valve, resulting in over-cooling of the tank and product. In addition, many of the valves were leaking externally through their (re-built) packing, causing accelerated corrosion of the carbon steel valves. Obsolete and non-maintainable, the original valves had become a real liability.
The biopharmaceutical company in question hired RPA Engineering to evaluate the existing process conditions and provide a solution for the passing control valves. The criteria was to maintain the existing temperature control scheme, provide tight shutoff (Class VI or better), and match the existing flow capability (flow coefficient, Cv), all with a standard platform and manufacturer that would be fully supported for the foreseeable future (10-20 years).
Globe valves require resilient seats mated to stainless steel plugs to achieve a class VI shutoff. However, the softer, resilient seats are susceptible to wear and tear and, consequently, more maintenance over time.
When new, the existing valves (equipped with stainless steel seats and plugs) provided Class IV shut-off. As an alternative, RPA engineers considered replacing the obsolete Class IV globe valves with similar Class IV globe valves and installing additional block valves into the piping. The block valves would handle the tight shut-off duty while the globe valves would continue with their true control function. This flow control strategy is well suited to short-term batch type operations with discrete start/stop of the temperature control function.
However, the plant required tank temperature control for extended and unidentified periods of time (up to 30 days), during which the valves would continuously cycle though closed and partially open positions. Inefficient and cumbersome, this solution required additional programming and associated change control documentation; task that generate considerable time and expense. RPA system designers opted for a less complex and therefore, less costly solution.
Because the original globe valves were outdated and were in a supporting utility system — as opposed to being directly involved a product processing train — it was feasible to replace them with functionally “like-for-like” valves in lieu of an exact match. For this application and under the circumstances RPA engineers evaluated the functionality of ball valves with a “V” cut, otherwise known as V-ball valves. Capable of both modulating control and positive shut-off, V-ball style control valves met the functionality requirements that would be provided by any “like-for-like” replacements of the failing, end-of-service-life control valves.
For this biopharmaceutical processor V-ball valves offered additional operational benefits including high turndown capability and longevity of service: The rotary motion of the V-ball valves swipes the seats clean with every stroke, which serves to help maintain a tighter seal. Due to its rising and falling action, globe valves are more susceptible to accumulating material on plug/seat surfaces and wire-drawing, thereby reducing sealing capability. Functionally, V-ball valves offer an appealing alternative especially since they are established in the market and have excellent performance records.
Globe valve packing is more susceptible to leaks due to the linear up/down action of the valve shaft versus the rotary action of the V-ball valves. As the globe valve shaft rise up, the cold, exposed shaft can sweat, and consequently this condensation drips down on to the carbon steel valve body and insulation. In addition, as the packing deteriorates, some of the glycol/water remains on the shaft. As the glycol/water mixture evaporates, crusty deposits form on the shaft. On the next down stroke, these deposits often enter the packing further impairing the valve’s ability to seal. As the cycle continues to expose the carbon steel valve body to water, corrosion accelerates. Because of its design, the rotating action of V-ball valve shaft mitigates these problems.