A key component for controlling the flow of dry material in bulk powder throughout processing plants involves rotary airlock valves. Also known as rotary valves, rotary airlocks, and rotary feeders – rotary airlock valves also serve a crucial safety function, acting as flame barriers or eliminating the propagation of an explosion in connected processes. They do this by preventing air loss while transferring the bulk material, which they do at a controlled rate. Though not airtight, their precision design allows the metering of dried material with minimal air loss. This will enable manufacturers to control the feed rate while keeping air leakage from their pneumatic system to a minimum.
Because rotary airlock valves are instrumental to the bulk material processing industry, it’s important to size them correctly. This involves precise analysis of the product being handled and the system in which it will install. As Prater Industries understands the importance of rotary airlocks for both the safety and efficiency of dry bulk solids handling operations, we have introduced a new tool to help with rotary airlock valve capacity calculations. Now manufacturers can determine the best size and type for their equipment, material, and application.
Determining Type of Airlock
Before sizing a rotary airlock valve, it’s first important to determine the type of rotary airlock needed for the application. This depends on a variety of factors.
- Pressure differential of the application.
- The required displacement.
- Temperature range to which it will be exposed (including product, process, and ambient temperatures).
- Type of product the device must handle including its flow properties.
Industries that use rotary airlocks include:
- Chemical processing
- Food processing
Many industrial and agricultural bulk solid handling systems for bulk solids require rotary airlock valves, which are used to process materials in various forms, including pellets, crystals, granules, and powders.
Material & Production Conditions
Material characteristics will impact rotary airlock type and size. Customizations, options, and accessories needed during processing, such as cleaning solutions and product transportation methods will affect performance.
Additionally, the convey line pressure of pneumatic systems helps establish how durable the rotary airlock should be. A typical pneumatic conveying system, for example, operates at 4-8 PSI (pounds per square inch), while those operating on dense phase pneumatic conveyors need to withstand 15 PSI or more. As a general rule, the more pressure to which an airlock is exposed, the more robust its construction.
Extreme temperatures can also limit the material, rotor designs, and other capabilities of the airlock. If operations occur in below-freezing or extremely hot temperatures, equipment may require adjustments to rotor construction, drive motors, seals, bearings, and other components. The production environment will help determine the type and material from which an airlock is made, particularly if it needs to withstand both hot and cold extremes.
Determining Proper Size for Application
Once all these aspects are considered, the size of the rotary airlock can then be determined by matching its volumetric capacity to the application. Volumetric capacity relates to how much product a rotary airlock can displace, measuring in cubic feet or liters per revolution of the rotor.
Rotary Airlock Valve Capacity Calculations
To establish the required volumetric capacity (C) it is necessary to understand the rate of conveying (R) in units per minute, product density (D), valve efficiency (E) as a decimal percentage, and rotor speed (S).
Rotary airlock valve capacity calculations use the following formula: C = R ÷ D ÷ E ÷ S
Fill factor is normally stated at 80% for average conditions but can be adjusted to between 65% and 95% depending on product flowability and pressure differential across the airlock. In general, good flowing products will fill more and sluggish ones less. For airlocks on a positive pressure convey system, fill factor is often de-rated to between 65-70% to allow for disruption to fill created by the small amount of process air that leaks across the valve.
In summary, choosing a valve to meet or exceed the volumetric rate requirement needs to be determined by the application. Airlock calculations will need to factor in material flow properties and pneumatic pressure differential such that a correction to pocket fill factor can lead to the correct size.
In some cases, we have to consider a larger valve with a bigger feed throat when handling more sluggish moving materials that have a tendency to bridge the opening of a smaller model. This includes a trade-off as the larger model will require more complex construction (e.g., reduced pocket rotor) to meter (not exceed) the rate of a feeding application and will also leak more for a pressure differential application.
Other Factors to Consider
Though these are the main considerations for determining rotary airlock valve capacity, there are many other factors to consider when sizing a rotating valve airlock.
- Frequency of use or duty cycle
- Bulk density of the material being handled
- Flowability of the material
- Rate of flow required
- Whether the rotary airlock valve also functions as metering device, feeds a pressure convey system, or sits under a head of material
- The average size of the material