3 Tips for Sanitary Agitation Design and Selected Mitigation Strategies
By Cole Weinland, Project Engineer, Bioprocessing
1. Start with the Vessel Design
Souring microbials can grow in even the smallest defects from fabrication. Fabrication methods offer mitigation strategy and should be considered when designing a sanitary vessel. Ensure the design is free of surface irregularities or pockets and is provisioned to be fully drained and cleaned. For extremely sensitive operations, consider specifying a surface roughness average, or Ra. Ra is the average profile roughness along the material and specifying a lower Ra will increase the smoothness by additional polishing. Specifying a lower Ra will increase fabrication costs but will decrease chances of infection.
2. Sealing for Success
The importance of choosing the best sealing approach can’t be understated. Using a double mechanical seal is a common sanitary way to isolate the process zone from equipment by use of a liquid barrier. The liquid barrier maintains a differential pressure such that any leakage occurs only outside the sanitary control volume. The liquid barrier also serves as a lubricant and a flush to keep debris cleared from the rotating mating surfaces. Use caution when choosing the fluid. Typical lubricants such as oils or utility water can increase the chances of contamination, even with a double mechanical seal. Two possible ways to avoid contamination by seal fluid are to use either sterile steam condensate, if available, or to maintain a sterile elevated temperature of the seal by indirect heating. Agitator shaft construction, hollow or solid, also plays an important role in agitator vibration and mechanical seal life. Work with your equipment supplier to select the correct combination.
3. Don’t Overlook the Importance of the Shaft Coupling
If your agitator size is large enough, the mixer element may require a coupling on the shaft. Choosing an appropriate shaft coupling can prevent pocketed areas where bacteria can grow and infect the process. Below are some advantages and disadvantages of common couplings that minimize pockets:
- Welded joint: Eliminates pockets entirely but creates one solid shaft assembly that may not be feasibly sized for maintenance or removal. Weld surface roughness, like in vessels, is important as well.
- Bolted joint: Eliminates pockets by use of weld neck flanges, O-rings, spacers, and fillet welds. However, the benefit of the bolted joint for maintenance can actually increase the need for maintenance by means of sterilization and O-ring replacement. Choose the O-ring elastomer that is most compatible with the process.
- Threaded joint: Internally threaded joint with O-ring seal eliminates pockets but will require sterilization at O-ring lifecycle. Choose the O-ring elastomer that is most compatible with the process.