WFI Distribution End of Loop Pressure Control
When you need a back pressure control valve (BPCV) at the end of a WFI loop.
I had an interesting application call from our great rep in the Carolinas that I want to share with you. His customer wanted a back pressure control valve (BPCV) at the end of his WFI loop. As indicated in the photo below, the majority (not all, as I found out) of return lines terminate inside the vessel at a spray ball. The BP control valve or regulating valve is usually mounted directly on the riser as shown.
When selecting a Cv (Kv) for these applications, valve manufacturers need the expected min and max flow rate, the valve’s inlet pressure set point (P1), and the outlet pressure (P2). That information is critical to selecting the correct Cv (Kv). Since WFI and US PW vessels are vented, you would think that the valve’s outlet pressure is atmosphere, but that is usually not the case because of the spay ball. Spray balls are designed to completely saturate the inside of the vessel with hot WFI. They require a certain differential pressure to do that That pressure *is* the BPCV/BPRV outlet pressure.
Pharmaceutical water distribution systems for medium to large scale manufacturing plants are complex with multiple loops and takeoffs. The figure above shows a simplified view of two separate, but connected, parallel loops; a hot storage vessel with a hot distribution loop with reheat. One or more storage tanks may be used.
Parallel loops are very common and are most advantageous where multiple temperatures are required, or where the area served is so large that a single loop becomes cost prohibitive or hydraulically impractical.
The major concern is to balance the various loops to maintain proper pressure and flow in each so as to ensure Point of Use (POU) pressure and flow, and keep turbulence, pressure and temperature within ideal limits to minimize microbial contamination.
- The pressure in each loop is maintained using back pressure control valves (MK95s or MK978s with positioner and pressure transmitters) at the end of each loop to maintain upstream pressure at a value that insures a positive pressure at even the most remote point of use on the loop.
- Flow is controlled by using either a single variable speed pump with a manifold and multiple flow control valves (MK978s with positioners and flow transmitters) or multiple variable speed pumps (one for each loop).
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