Mixer seals are critical for pharmaceutical, chemical, and food process plants. These are highly engineered seals designed to keep contaminants from entering the process fluid and keep the process fluid from reaching the environment. A leak in the process fluid or contamination can result in a batch of product being ruined, which can be extremely costly depending on the application. This makes mixer seal performance extremely vital and offers very little room for error. Mechanical seals are available for all types of mixers, such as top-entry, bottom-entry, and side-entry mixers.
Top-entry mixer seals need to be designed to handle excessive shaft movement as the impeller is at the bottom of the shaft. To safeguard against this movement, manufacturers often design top-mount mixer seals to have one face with a much larger cross-section than its mating face. Another consideration for a top-entry seal is that the process fluid will be at the bottom part of the vessel. This means the mechanical seal is actually sealing the gas phase of the product and may never actually come into contact with the process fluid. This seal is also designed to keep any contaminants from entering through the top of the vessel which is of special consideration for this type of mixer due to gravity. Top-entry mixer seals can be designed in single dry-running, dual dry-running, dual lift-off, or dual wet-running configurations. If a dual seal is used, a compatible, pressurized/unpressurized gas or liquid buffer/barrier fluid may be utilized if deemed necessary for the application.
Bottom-entry mixer seals are, as anticipated, mounted to the bottom of the vessel. Since the process fluid naturally settles at the bottom of the mixer, the seal will need to be designed to seal the process fluid as opposed to its gas phase. Some process fluids have solids that can collect near the seal faces. To counteract solids accumulation from damaging the seal, many bottom-mount seals are designed with a longer sleeve that extends up into the vessel. If the seal is not removed after the vessel is drained, bottom-mount seals are often designed with smooth, tapered edges and wider spaces to assist with equipment cleaning. Since many applications require FDA certification, everything that can be done to prevent any bacteria build up is always a plus. For example, many bottom-mount seals have an electropolish finish on the metal parts. This extremely smooth finish helps prevent bacteria from sticking to parts within the seal and guards against product contamination. Bottom-entry mixer seals can be designed in a single wet-running or dual wet-running formation. If a dual seal is used, a compatible, pressurized/unpressurized liquid buffer/barrier fluid may be used depending on the application.
Side-entry mixer seals have aspects of both of the top-mount and bottom-mount seals. The mixer shafts are typically shorter than top-entry mixers. However, the horizontal shaft raises concern about shaft runout. Depending on the height of the liquid, the seal may or may not be exposed to the process fluid. If the seal is exposed to the process fluid, the seal can be designed similarly to the bottom-mount seals to assist in cleaning procedures and improved bacteria growth resistance. If the impeller is not fully immersed in process fluid, it could “slap” the fluid and cause the shaft and seal to bounce. This often leads to premature seal failure. In this instance, a bearing can be installed in the seal to remedy some of the shaft movement. Side-entry seals can be designed in any single or dual configuration available for the top and bottom-mount seals. Again, if a dual seal is utilized, a pressurized/unpressurized gas or liquid buffer/barrier fluid may be used depending on the application.
There is much to consider when designing and installing mixer mechanical seals given their popular use in critical applications. Correct seal selection and design is extremely important in very expensive and highly regulated operations. Not all top-mount, bottom-mount, and side-mount seals are the same. There are many different types of each seal to assist with the needs of each application. Something to note, additional bearings can be integrated into the mechanical seal to help resolve potential shaft runout and protect the mechanical seal.
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