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Issues such as sterility, isolation and contamination must be considered when choosing seals for use in biotech and pharmaceutical processes. This overview describes common types of seals and offers guidance on selecting the appropriate seal for various applications.
Many process steps in the pharmaceutical and bioprocessing industries require mechanical agitation. Some, such as fermentation, involve growing a culture of living organisms. Others, such as blending of injectable drugs before packaging, must have no living organisms present. Most process vessels must avoid both chemical and biological contamination.
Except where a magnetic-drive mixer is employed, the normal configuration of an agitator consists of a motor, gear drive, shaft and impeller (Figure 1). Where the shaft penetrates the agitator mounting flange, some type of seal is normally employed. Various seal types, differing in functionality, cost, durability, reliability and maintainability, are available.
The pharmaceutical industry emphasizes zero batch contamination, process hygiene, and clean-in-place (CIP) and sterilize-in-place (SIP) requirements. High equipment reliability is also needed to minimize production costs. Correct selection of agitator seals is vital to achieving these objectives. This article explains the many differences and guides the user in selecting the most appropriate style for specific bio-pharma applications.
The functions of seals
Agitator seals may perform several functions, including:
* containment of tank contents to protect plant personnel or the surrounding community from biohazards, toxins, etc.
* isolation of tank contents to protect against exposure to the outside environment
* avoidance of contamination, which is similar to isolation, but includes particle shedding from the seal and material left over from previous processing
* static and dynamic sealing to maintain the vessel pressure, whether operating or stationary
* personnel safety from physical hazards, such as sudden pressure venting.
Agitator seal applications for pharmaceutical service are typically characterized by: strict leakage-prevention requirements; high process purity - i.e., zero contamination; hygiene, CIP and SIP requirements; exposure to hazardous, corrosive or volatile fluids (including chlorides); large shaft size; low speed (typically 30-350 rpm); high radial shaft run-out (up to 1.5 mm); and high vibration velocity (up to 20 mm/s on large fermenters).
Shaft entry position
The position of the agitator mounting has a major influence on selection and design. Agitator mounting is classified as top, side or bottom entry.
Top entry...