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 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 :
- The body and rotor can withstand the pressure of an explosion - typically 10 bar g
- The clearance tip of the blades / housing must be less than 0.2 mm
- At least 2 blades in each side of the valve must be in contact with the housing (which means that the total number of blades must be > or equal to 8
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.
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.
Figure 3 : Typical capacity graph of airlock rotary valve
Different problems can affect a star valve during its operation. Common problems are among the following :
- Performance below design (lower throughput than expected)
- Damage by metal / metal contact
For each of these issues, possible root causes and remedy are given below
Table 3 : Troubleshooting - Main operational problems with airlock rotary valves
|Observation||Possible root cause||Possible action|
|Performance below design||The pockets are not completely filled
The product has a bad flowability and the hopper above the star valve is not designed properly. The product is blocked before reaching the valve.
|This could be adressed by using discharging aids in the hopper.|
|Performance below design||The pockets are not completely filled
In case of a valve used to feed a pressure conveying line, the air leakage is fluidizing the product at the star valve inlet, preventing it to fill its pockets : it could be due to an improperly specified valve with too large clearance, not enough vanes or a worn valve whose clearance are above specification. Another possible root cause is an improper degassing of the empty pockets before they reach the hopper to pick up again powder. Lastly, another possibility to be looked at is the air seal flushing : if it is set with a too high pressure, the air, leaking via the seal can oppose the flow.
|Check the specification of the valve regarding the pressure drop it has to overcome
Review the degassing of the valve and the hopper above (if existing)
Check the pressure of the compressed air sealing
Note : Airlock rotary valves can be designed with 6-8-10 vanes. The more the number of blades, the tighter will be the valve. However, a high number of vanes will also reduce the volumetric capacity of the valve : an optimum is to be found in between sealing and pocket capacity.
|Performance below design||The pockets cannot be emptied properly
A bad discharge of the pocket is linked to the flowability of the powder. If the powder is very cohesive, it can actually stay in the pockets of the airlock rotary valve, which reduces the volume available for new product at each rotation.
|Some rotor design are existing with the bottom of pockets plain. Powder cannot accumulate there and falls easier from the pocket.|
|Damage by metal / metal contact||"Scratching" a star valve corresponds to a punctual metal / metal contact that will lead to a damage of the rotor and the stator.
After such incident, the valve may be blocked, which prevents its use. During the incident, metal shavings can be released to the product stream which can be a problem for certain product applications.
|To prevent such damage, it is necessary to :
- Make sure no foreign body can reach the valve (use sieves and magnets prior to the valve)
- Make sure the valve is specified properly, especially the operating temperature since higher temperature can cause metal expansion and lead the valve to touching
- Train people to maintain the valve since most of the equipment damages are linked to rotors improperly put back in the valve after maintenance
- Use rotor / stator contact detection system
In case of damage, it is necessary to remachine the valve, or change it entirely if clearances are too high after machining.
|Wear [IAC]||Wear corresponds to medium / long term damage of the valve. 2 main phenomena can lead to valve damage :
- Material is trapped in between a vane tip and the housing during rotation
- Air leakage carrying at high speed particles of product which are eroding the valve
It can be possible to estimate which phenomena is damaging a particular valve : if the housing is damaged on the loaded side (pockets full of product), the 1st phenomena may be at cause ; if the housing is damaged on the other side (empty pockets), the 2nd phenomena may be at cause.
|The following actions can be taken to prevent airlock rotary valves wear :
- Select the right clearance to reduce chances of trapping product in between rotor and stator. It also reduces the air leakage responsible of erosion.
- Use vane tip chamfrened to reduce the potential contact surface with product trapped during rotation
- Use a closed rotor (the sides of the rotor are closed which is limiting the leakage and avoiding friction - but not all application can accept it)
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