Wednesday, August 13, 2014
What is a rotary airlock valve?
Integrating rotary airlock valves to a pneumatic conveying system is a method used when transferring set amounts of material from a material hopper. These valves provide the airlock between a hopper and the pressurized conveying pipe below.
How it works?
As the internal rotor of the valve rotates, it slowly discharges (metering) product into the conveying pipe at the same time. This provides a seal between the atmospheric conditions in the bin and the pressure conveying product in the pipe.
Rotary valves are made in many sizes and the speed of rotation is used to provide a governed material throughput. Pneuvay recommend to engineers that the volumetric filling capacity is 70% of actual volume and maximum speed should be limited to less than 35 rpm.
Types of rotary valves
Blow through valve
Using this configuration, you'll notice that the conveying pipes are connected to and from the end plate. This is purely used as a pneumatic conveying valve and is ideal when conveying non-abrasive powders such as Flour.
Drop through valve
This second type of rotary valve is often used to meter material from bins and hoppers. However, it is most often used in systems with dust collector hopper discharges which requires a seal between the atmosphere and the vacuum.
Both valves can be used in pneumatic conveying. However an ancillary pipe manifold underneath to connect to the conveying pipe has to be included to the system.
Do you need a rotary valve with a larger size? Talk to us at 1800 786 642 or
Factors affecting product feed rate
We have a selection spreadsheet that we can do for you to help you accurately and easily select the correct valve for the application. The variable inputs on the spreadsheet are in basic form are:
- Discharge rate kg/min of material
- Bulk Density kg/m3
- Efficiency of the feeder volume benchmark for materials over 500kg/m3 is 70%
- Efficiency for materials less than 500kg/m3 is 60%
- Efficiency for materials over 1000kg/m3 is more than 70%
- Keep the speed under 35rpm unless expert advice is sought
Bulk materials properties and behavior
Other factors that have to be considered to ensure efficient
product feed rate includes:
- Shape and size
- Degree of stress
- Surface and energy
- Particle uniformity (porosity and particle density)
- Angles of friction
- Moisture and velocity
- Solid concentration
Rotary airlock valve leakage solutions
Direct connection to storage vessel
This design shows the rotary airlock directly attached to the vessel. This allows the valve to act both as airlock and metering device. It is considered to be the simplest method in resolving air leakage which also requires the least amount of equipment and headroom.
Vented surge hopper
This design incorporates the use of surge hopper and dedicated vent filter to the rotary airlock valve. It is believed to one of the surest way to direct leakage air to the atmosphere.
Although effective, the use of static sock filter without including a cleaning facility may result to accumulated dirt. This in turn will increase the airflow resistance. Further, use of flood fed option in handling powder may also cause the material level to fluidise and the surge hopper to equalize.
Vent Pipe to Storage Vessel Top
This system design uses a vented surge hopper without a dedicated vented filter. As a replacement to the filter, a vent pipe, hopper vent stub and fan is added to it.
It is noticeable that air is not allowed to pass together with the materials, rather it is directed to pass through the vent pipe. This prevents the formation of gas bubble. However, this approach has been very sensitive to the conveyed materials.
Central dust collection
This system design requires the use of a vented hopper. It prevents venting leakage air by collecting the dust through the central dust collection system. It doesn't rely on the leakage air volume rather on the air volume produced by the fans of the dust collection system.
Although effective with low value materials, the amount of entrained particles that are easily collected together with the dust particles and is discarded as waste doesn't make it ideal for conveying high value materials.
Eductor driven capture
This method makes use of an eductor to capture rather than vent air leakage. This approach provides the necessary air volume to handle heavier material loadings.
Airlock body vent (boss vent)
This system requires the use of an airlock valve and body vent which produces two airlock sources. These airlock sources are then used to pressurize the empty rotor pocket and air leaking through the rotor tip-to-housing clearance. As a result, industries requiring a larger volume of pressurized air can now more efficient airlock systems with increased fill efficiency.
Pneuvay's design solution:
Simplicity and efficiency
Seven years has passed since this paper has been published. Much has changed in the field of pneumatic conveying, but airlock valve leakage remains a common problem to some. Here at Pneuvay we carefully study your industry needs and design the most fitting structure that delivers efficient and reliable flow of materials.
We keep our designs simple and energy efficient which favors the direct connection to storage vessel diagram as shown in the figure below:
We understand that no two systems are ever the same. So we spend time understanding your specific needs for us to draft the most effective solution to your rotary airlock valve leakage problems.
We can design a suitable system to your product conveying requirements. Contact us online
and schedule an appointment with us.
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