Engineering Integrity Blog

How do I evaluate the design of an industrial agitator?

June 27, 2016

Customer Question: How are industrial agitators and blender designs evaluated and how can I be sure that my process goals will be met?

Answer: There are three inter-related, but distinct subjects that need to be addressed in order to answer your excellent question.

Specifically, these three interrelated areas of design are, mechanical integrity, mixing effectiveness and integration of the mixer to the tank mounting structure. Mixing effectiveness is most precisely defined as the “degree of mixedness or the removal of gradients in the fluid properties” (Uhl and Gray, Mixing, Academic Press 1967.)

Another important consideration would be the technical review of a particular vendor’s features. Are they truly unique and determinatively important toward reaching the end user’s overall intent? Or are the stated features non-essential to the overall performance, durability and value of the system?  Are these unique features truly beneficial or are they to limit competitive pressure?  For example, arcane details within a specific gear reducer design or who makes the gear reducer would tend to lower competitive pressure on pricing and technical qualification.

However, if a manufacturer has specific experience or specific tools to analyze a given mixing problem, those would tend to objectively lower the risk of a mis-applied mixer design.

Three Aspects of Evaluating a Mixer Design

I] Mechanical Integrity:

How does one determine the mechanical integrity of a low speed, turbine agitator?  For the sake of brevity, I have posed the following questions.

Impellers and Shafts

  • What is the critical speed of the impeller shaft assembly?
  • Is operating at less than 75% of first critical speed?
  • What is the assumed bearing support stiffness of the mixer mounting structure?
  • What is the calculated shear stress of the impeller shaft and what factor of safety is there from the yield value?
  • What is the thrust rating, torque and bending moment capacity of the impeller hub(s)? What are its factors of safety?
  • What is the calculated tensile stress of the impeller shaft and what factor of safety is there from the yield value?
  • What is the shear stress on the impeller blades and what factor of safety is there from the yield value?
  • What is the hardware used for the connection of the impeller blades, hubs and couplings?
  • What is the shaft coupling design? What are its factors of safety for tensile and shear stress? What is the welding procedure for said welds? What is the shaft coupling registration and how is shaft run out controlled?
  • What is the run out (TIR) of the impeller shafting?
  • What is the grade of impeller shafting used, what is its finish (RA) and what is the total calculated run out at the impeller? What are its dimensional tolerances?

For more information on the mechanical design of agitators, please refer to the text, Handbook for Industrial Mixing published by the North American Mixing Forum.

Gear Reducer

This is normally the most expensive component in a low speed agitator system. Because it must multiply motor torque and support the cantilevered impeller shaft, more attention needs to be spent on its design than on other components such as motors for example.

What is the mechanical input rating of the gear reducer? What is the safety factor?

  • What is the service rating of the gear reducer selection, AGMA I, II or III?
  • What is the gear reducer efficiency?
  • What is the thermal rating of the gear reducer?
  • What is the torque rating of the gear reducer? What is the safety factor of the torque load to the torque capacity?
  • Is the gear reducer equipped with a drywell?
  • What is the bending moment capacity of the gear reducer? What is the safety factor of the rated capacity compared to the calculated bending moment from the gear reducer.
  • What is the negative thrust rating of the gear reducer compared to the weight of the impeller shaft assembly?
  • How is the motor attached to the input shaft of the gear reducer and how much run out is the high speed coupling rated for?
  • What are the service intervals of the gear reducer and how is the service performed? What is involved in servicing the gear reducer? Does the impeller shaft need to be decoupled from the gear reducer?

Regarding the motor, our suggestion is to use a Severe Duty, Premium Efficient or IEEE841 motor design for any mixer service that is in the waste water treatment sector.

II] Mixer Performance:

The mixer manufacturer needs to answer these questions:

  • What was the design approach for the mixer as it relates to meeting the stated process goals of the end user. (A short narrative should be supplied.)
  • What are the following values of the proposed mixer design:  (This is correlational data. )
    1. Maximum Reynolds Number
    2. Total Invested HP
    3. Invested Power Per Volume (HP per 1000 gallons)
    4. Invested Power Per Mass (HP per 1000 pounds. )
    5. Turbulent Energy Dissipation Rate (m2/s3)
    6. Total Invested Torque (Inch pounds)
    7. Invested Torque Per Volume(in-lbs./1000 gallons)
    8. Total Mixing Intensity or Calculated Bulk Fluid Velocity (FPM)
    9. Total pumping capacity (GPM)
    10. Turnover Time (minutes)
    11. Total Thrust (lbs. f)
    12. Thrust/Area (lbf/ft2)
    13. Maximum Impeller Tip Speed (ft/min)
  • The mixer manufacturer should provide a computational fluid dynamics study that simulates the flow pattern and velocity generated from the impellers. The study should include fluid velocity contour plots in the X,Y and Z coordinates.  Fluid velocity vectors with their flow rates should also be shown.  The simulation should also show the shear stress contours. Finally, the simulation should show an animation of the fluid coursing throughout the vessel, displaying the flow pattern.
  • To audit the mixer sizing/selection of a given manufacturer there are three or more valid methods. Those would be:
    1. Full scale witness test and inspection of the mixer before shipment. This potentially could have the highest cost associated with it.
    2. Scaled test of at least 500 to 1000 gallons with a documented and verified scale up process. This would have a very low impact on overall cost of the proposed mixer.
    3. 3rd Party independent mixing consultant verifies the mixer design. There is a cadre of mixing consultants available throughout the country.

III] Integration of the mixer system into the mounting structure of the tank:

  • What is the assumed bearing stiffness of the mounting structure?
  • What does the tank manufacture calculate for the mounting stiffness of the mounting bridge? In this example of an older tank, what are the effects of fatigue and corrosion on strength?
  • What is the beam/nozzle design?
  • What is the load carrying capacity of the mounting structure and what is its factor of safety?
    1. What is the shear stress rating of the mounting structure?
    2. What is its bending moment capacity?
    3. What is the negative and positive thrust rating?
    4. What is the calculated deflection of the mounting structure and at what rate?
    5. Where and how is the mounting structure connected to the tank?
    6. What specific design(s) are used to mitigate beam displacement?

In summary, the above list of questions while lengthy, can still be added to.  If I were the purchaser of the equipment, my primary interests would be mechanical integrity, mixing performance and integration of the tank structure.  If any one of these three critical areas are not thoroughly analyzed, field based remedies could have costs well exceeding the original value of mixer.

 

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