Airlock Rotary Valve (Star Valve)
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.
Airlock 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, airlock rotary valves have the following advantage :
Table 1 : Star valve vs other dosing equipment
Simple (but rough) throughphut adjustment if motor on VFD
Low space requirements
Some design with easy access for cleaning
Chopping of large particles / pellets
Can be damaged by abrasive products
Limited operative pressure
Delicate maintenance - needs well trained staff
Can lead to Foreign Bodies incidents if not assembled properly
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.
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 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
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
|Main causes of scratches for rotary valves||How to avoid|
|Incorrect dismantling / reassembly||Training of operator / mechanics
Use of designs with extraction bars
|Foreign body trapped in between rotor and stator||Install sieve and magnet in upstream process|
|Thermal expansion reducing clearance||Proper specification of the valve and design of process (after cooler, temperature sensor|
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.
A low clearance will allow a good sealing and reduce the leakage through the star valve. 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 degassing 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
The sizing 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.
- A higher rotation speed generally means more throughput but the throughput will cease to increase past a certain speed
- The more fluid is the powder, the higher will be the throughput
Throughput can be estimated from supplier's abascus, but the knowledge of the product will be a key input.GRAPHS
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