Airlock Rotary Valve
Do you have a question, a remark ? Please contact the author at email@example.com
# Types of pneumatic transport
# Conveying phases
# Dilute Phase transport
# Dense Phase transport
# Air mover
# Roots Blower
# Roots Blower performance curve
# After Cooler
# Airlock Rotary Valve
# Product inlet / Injector
# Choking velocity
# Conveying speed / velocity
# Air volumetric and mass flowrate
# Pipe Equivalent Length
# Solids particle velocity in pipe
# Solids Breakage
#Pipe Diameter or Bore
# Selecting dilute or dense phase
An airlock rotary valve, also called rotary feeder or star valve, is a mechanical conveying device that allows to control a bulk solids flow (powder, pellets, granules...). Such airlock feeders are use in pneumatic transport system to act as a transition in between pressure areas. They are often used prior to a pneumatic transport system in order to introduce solids to the conveying line (dilute phase) or at the receiver where air is separated from the solids conveyed.
Airlock rotary valves are used at solids handling processes interfaces, typically when it is necessary to separate 2 areas under different conditions (pressure most of the time) while letting the solid go from one condition to another.
Rotary valves, also commonly called star valves, are therefore used at the beginning and at the end of pneumatic transports. They allow to bring the solid from a zone of low pressure to a zone of low pressure at the beginning of the line while helping to disengage the solid from the air flow a the end of the line.
Such valves are able to perform a rough dosing, thus, they can also be installed as dosing equipment, although it is not a good practice.
Compared to other dosing equipment, rotary feeders have the following advantage :
Table 1 : Star valve vs other dosing equipment
2 types of airlock rotary valves are available : a drop through type and a blow through type. Both types are basically giving the same results, however, the way they do it and their characteristics are slightly different.
2. Drop Through rotary valve and Blow Through rotary valve
Drop Through airlock rotary valve
Drop through airlock rotary valves are "dropping" the product to the pipe or equipment below. There is an entry flange and an outlet flange.
Figure 1 : Front view of a drop through star valve
Blow through airlock rotary valve
Blow through star valves are directly connected to a conveying line. The air used in the conveying line is therefore directly going through the alveoles of the valves, sweeping the product away
Typically, blow through valves are used either when there is a very limited height or when the product has a tendency to stick inside the rotor. For other applications, the drop through model is quite preferred.
Having the rotor directly in the pipe flow can lead to larger breakage of the product being transported, it is especially the case if several drop through valves are in series in a same piping. For this particular case, drop-through valves may be considered in order to preserve the product.
Figure 2 : Front view and side view of a blow through star valve
3. Star Valve Clearance and Contact detection
Star valves have typically very small clearance in between the rotor blades and the stator, it is necessary in order to provide an air sealing in between upstream and downstream areas that are not at the same pressure.
Typical clearance for airlock rotary valves is 0.1 mm and usually ranges from 0.05mm to 0.25 mm depending on the service expected for the valve (high difference of pressure from each side of the valve or not). This is a very small clearance which explains that rotary valves often suffer of scratches due to contact rotor / stator. The following table is summarizing common causes of contacts.
Table 2 : Troubleshooting - Main causes of scratches for star valves
Scratches can have different consequences : blockage of the valve, reduction in air sealing, foreign bodies generation. It may be necessary to repolish the valve after a scratch, which has as a consequence to extend locally the clearance and reduce the sealing capacity of the valve.
To be noted that some designs have been developped where the blades have an adjustable tip bolted. If the tip is made of soft material like Nylon, it allows to touch the stator without damage. It is however subjected to wear and has a limited range of applications.
4. Explosion protection
A rotary airlock can be used as an isolation elements to prevent dust explosion to propagate in an installation. For this, the airlock rotary valve must be certified to be explosion shock resistant and flame proof.
In order to get those characteristics, the valve must be
designed so that :
It is very important to monitor the clearance regularly since wear of the valve can cause the clearance to exceed 0.2 mm which will affect the flame proof properties of the valve.
5. Rotary Valve Degassing
A low clearance will allow a good sealing and reduce the rotary airlock valve leakage. However even reduced a leakage will happen. As well, the air trapped in each pocket will also be released when the pocket is opened to the low pressure area. This leads to leakage of air.
The air leakage is increasing with the difference of pressure and increases with the rotation speed of the valve. It can be very detrimental to the performance of the valve, especially with light powder, since the air released will actually fluidize the powder and prevent it to fill the pocket.
This phenomena can be witnessed in the performance curves of airlock rotary blades : the capacity will reach an assymptot and even decrease at high speed since the pockets cannot be filled anymore by the product, too much fluidized to have time to fall in the pockets.
To control this phenomena and improve the performances of the valve, a proper venting of the rotary valve must be implemented. A degassing channel is mounted on the side the pockets are returning up in order to empty them from the air prior they pick up new product. The channel is sending the air to a filter to be released.
Figure 2 : Star valve equipped with degassing hopper feeding a pneumatic conveyor
6. Airlock rotary valve sizing
The capacity calculation of a star valve to achieve a given throughput is a function of the star valve diameter, its target rotation speed and the nature of the product,
- The bigger the star valve, the higher will be the
Throughput can be estimated from supplier's abascus, but the knowledge of the product will be a key input.
Figure 3 : Typical capacity graph of airlock rotary valve
Important notice : the airlock rotary valve throughput is not linerar. The throughput ceases to increase or can even decrease past a certain speed. It can be due to different causes, mainly it will be due to the reduced time for the pockets to be filled and empties. With light powder, the degassing of the pockets when returning to the low pressure side will prevent the powder to flow in the pocket, this phenomena will be increased by the pressure drop through the valve and can be mitigated thanks to a proper venting system of the pockets. For cohesive materials it will be difficult to flow in and difficult to flow out from the pocket in the dischage zone.
Equation 1 : Airlock Rotary Valve capacity calculation
Typical rotation speed for correctly sized airlock
rotary valve : 20 rpm
Different problems can affect a star valve during its
operation. Common problems are among the following :
For each of these issues, possible root causes and remedy are given below
Table 3 : Troubleshooting - Main operational problems with airlock rotary valves
|Do you want to
know more ?
www.powderprocess.net is covering all aspects of solids and powder processing and handling
The pages below will probably interest you !
Answers ALL your questions on solids and powder mixing
of MIX / MEASURING DEGREE OF MIXING
All you need to know about measuring the degree of mixing (Coefficient of Variation) of a dry mix
OF POWDERS / DEMIXING
All on the physical phenomena causing segregation of particles and how to avoid them
All on main size reduction processes, key equipement, design consideration..etc..
All on powder key physical characteristics, among others : flowability, density...etc..
All equipment found in bulk solids handling industries, explained, with design tips...
|You can find much more through the menus at the top of the page !|