Sizing and Selecting Air Piston Vibrators for Conical Silos

Mike Stratis

When turning to a pneumatic piston or air piston vibrator for bin clean out or bulk material flow out of your material storage vessel with a conical transition hopper, there are a few things we review to help determine the best unit for the job. Here are the various sets of criteria we review before making a recommendation:

  1. Find number of vibrators needed
  2. Find strength of vibrator needed
  3. Find best air piston vibrator for application

Step One: Find the Number of Vibrators Needed

To find the number of vibrators recommended for the conical hopper, we look at the geometry of the vessel being vibrated. The geometry, size and shape, of the vessel tells us the number of vibrators needed for the most effective vibration transfer into the material. There is about 5 feet of effective radius for the vibration energy from where the vibrator is mounted. Here is a visual aid which allows you to see how many vibrators are needed based on the size of your hopper.

Vibrator Sizing for Conical Hoppers

Step Two: Find the Strength of Vibrators Needed

To find the strength of the vibration needed, we look at the application and the overall goal of the vibration itself. Most applications we help with fall within three categories:

  1. Break free sticky material from one problematic area inside the vessel
  2. Promote bulk material flow
  3. Bin clean out

If the transition portion of the hopper is full when being vibrated, certainly for bulk material flow applications and occasionally when we need to help break free some sticky material from the inside of the vessel wall, we need to take both wall thickness and total material weight in the transition hopper of your vessel into consideration. In order to determine the total material weight, we need to know the material characteristics which include bulk density, preferably in pounds per cubic foot, and moisture content.

If the goal of the vibration is to only help with bin clean out after most material is out of the vessel, then the wall thickness is the major factor to consider. Without the material remaining in the hopper, we do not need to calculate the total material weight.

The formula we use for calculating total material weight in the transition hopper of the conical vessel is as follows:

Vibrator Sizing for Conical Hoppers - 2

Let’s walk through an example before moving to the next step:

  1. D = 12 ft
  2. H = 5 ft
  3. Material in bin = dried soybeans, approximately 50 pounds per cubic foot
  4. Application = promote bulk material flow
  5. Wall thickness = 3/16″
  6. WT = weight of material in transition hopper

WT = .261 x 12^2 (D) x 5 (H) x 50 (Material Density in lbs./ft.^3)
WT = total weight in hopper approximately 9396 lbs

There is a 10 lbs of material to 1 lbs of force rule of thumb, also known as the Vibrator Force Factor, that we use for getting material out of a vessel. With that in mind:
Total amount of force required from vibrators = 9396/10 = 939.6 lbf. 

Using the information shown in Step One above to determine number of vibrators based on vessel diameter, we recommend (2) vibrators. In order to find the proper force requirement needed per vibrator, take the amount of total force required for your application calculated above (939.6) and divide it by the number of vibrators needed (2).
Pounds of force needed per vibrator = 939.6/2 = 469.8 LBF

The vibrators should be mounted on opposite sloped walls (180 degrees apart) and at staggering heights. Vibrator #1 mounted 1/3 of the length of the sloped wall section up from the discharge while vibrator #2 should be 1/4 of the length of the sloped wall section up from the discharge.

Step Three: Find the Best Vibrator Needed For Your Application 

Basically, we need about 470 LBF from each vibrator. Once we have the pounds of force required from each pneumatic piston vibrator, we look at the application details to make a recommendation for style of piston vibrator. There are a few options available, single impact air knockers, impacting piston vibrators or non-impacting piston vibrators. Here are the common applications for each of these air piston options:

  1. Single impact air knockers are best for the lone problematic areas that need sticky material “unstuck” off the inside of the vessel and bin clean outSI
  2. Impacting piston vibrators are best for bulk flow of sticky materials (e.g. meals, DDGs, moist aggregates, etc.)01
  3. Non-impacting piston vibrators are best for bulk flow of drier materials (e.g. corn, wheat, dried aggregates, etc.)VMRAC

For this application, promoting bulk material flow of dried soybeans, we would suggest the non-impacting piston vibrator. Turning to our chart within our selection guide, we suggest (2) 1200 VMSAC non-impacting piston vibrators for this application.

If you need help with proper vibrator sizing and selection, or would like to review our selection guide and input your specifications, please let us know. We would be glad to review the details of your application!

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