Passes USP Permanganate Test
<10 cfu/100 ml
Figure 5 summarizes a typical WFI storage and distribution system. Pretreated water is fed to the WFI still preheater by level control and on to an evaporator heated with the plant process steam. The evaporated water should go through filters/separators to remove entrained droplets. The steam is condensed with cooling water, and then partially reboiled to remove dissolved gases. The distillate is fed to a WFI storage tank. A conductivity monitor diverts under specification distillate to drain. WFI production is controlled by an on/off level control in the WFI storage tank. Boiler controls are incorporated in the WFI still. The still is vented automatically when on standby waiting for level control to request water.
The WFI recirculating loop velocity is designed to ensure turbulent flow and is generally 5-10 ft/sec. The WFI recirculation loop is designed to run continuously. At peak use rate, the water velocity in the pipes should be 2 ft/sec or more.
Standard control methods are used for the WFI tank. A water high level switch (LSH) turns the WFI still off or on depending on whether the water level is at or below the LSH point. A water low level switch (LSL) is interlocked to the recirculation pump to shut it off should the level reach the LSL point.
The WFI storage tank water is usually maintained at about 80°C. This water may be temperature controlled (heated) at the return end of the WFI loop via a WFI heat exchanger (shell and tube double tube sheet) or a hotjacketmaybeused. A heat exchanger is the preferred method. Where cool WFI is required, point of use coolers (double tube sheet) or a cool WFI loop is provided. Considerable control effort is needed for the point of use cooler design to meet the continuous flow non-stagnancy standards.
All pipelines used in a WFI generation, storage or distribution must be sloped to provide for complete drainage. No pipe segment not in regular use can be greater in length than six diameters of the unused pipe measured from the axis of the pipe in use.
A WFI system must be sampled and tested at least once a day. All sampling ports or points of use in the distribution system shall be sampled at least weekly.
It must be kept in mind that WFI is an extremely aggressive solvent, especially at 80°C. Therefore, the still, storage tank(s), and the distribution system are generally 300 series stainless steel with welded joints wherever possible. All surfaces that come in contact with the water are, at a minimum, smooth and manually polished to a #4 finish (150 grit) and passivated to prevent corrosion. Welds are made with automatic arc welders under inert atmosphere to prevent chromium migration, carbide and oxide formation, inclusions, or incomplete penetration of the joint. All connections that are not welded should be sanitary in design to eliminate crevices where corrosion can occur and bacteria can grow.
GMP's do allow storage at ambient temperatures, but if this option is chosen, the water must be tested on a batch basis, and can only be held for 24 hours before it must be discarded. Therefore, a hot loop may turn out to be less expensive than a system without the heated loop in the long term.
All maintenance on the WFI system must be performed by trained personnel and carefully documented. Maintenance personnel must be fully aware of any impact that their activities may have on the system and on the facility. All maintenance will require careful planning and coordination with manufacturing and quality control personnel.
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