In industrial facilities where powders, granules, and similar bulk materials are processed, the airlock is one of the most important pieces of equipment for product transfer and maintaining air balance within the system. Also known as rotary valves, rotary gates, star valves, or star feeders, these devices operate continuously in silos, bunkers, filters, cyclones, and pneumatic conveying lines.
However, various malfunctions can occur if the rotary valve is used for an extended period without maintenance or if it is not properly selected for the process. Air leakage, rotor jamming, loss of capacity, abnormal noise, vibration, bearing overheating, and excessive wear are among the most common problems.
Some of these issues can affect not only the performance of the rotary valve itself but also the entire pneumatic conveying system to which it is connected. For example, increased air leakage due to wear between the rotor and the housing can cause a drop in pressure in the conveying line and an increase in blower energy consumption.
This article will discuss in detail the most common airlock failures, their causes, how to detect them, and which maintenance practices can be used to reduce risks.
What Are the Most Common Causes of Air Lock Failures?
An air lock failure is generally not caused by a single factor. Product design, equipment selection, process conditions, maintenance frequency, and usage patterns must all be evaluated together.
The primary causes of failure are as follows:
- Rotor and housing wear
- Incorrect rotor-housing tolerance
- Foreign object ingress
- Product buildup
- Excessively high rotor speed
- Incorrect motor or gearbox selection
- Bearing failure
- Wear of seals and gaskets
- Thermal expansion due to high temperature
- Use of abrasive products
- Rotor geometry unsuitable for the process
- Pressure or vacuum values exceeding design limits
For this reason, simply replacing the failed part is often not a permanent solution. It must also be investigated whether the failure is process-related.
What Causes Air Leaks in an Air Lock?
Air lock air leaks are one of the most critical performance issues, particularly in pneumatic conveying systems.
A rotary valve is not a piece of equipment that operates with zero leakage. A certain gap must exist between the rotor and the housing to allow for mechanical movement. However, when this gap increases over time or was incorrectly designed from the start, air leakage can exceed acceptable levels.
Main Causes of Air Leakage
- Wear on the rotor tips
- Wear on the inner surface of the housing
- Excessive tolerances
- High operating pressure
- Inappropriate rotor speed
- Damaged rotor vanes
- Shaft or bearing clearance
- Improper installation
- Product-induced wear
Especially with abrasive materials such as cement, minerals, ash, and similar substances, the clearance between the rotor and the housing can increase over the course of operation.
How Does Air Leakage Affect the System?
Excessive air leakage in a rotary valve can create a chain reaction affecting the process to which the air lock is connected.
The following problems may occur in a pneumatic conveying system:
- Drop in line pressure
- Decrease in vacuum level
- Decrease in conveying capacity
- Increase in blower load
- Increase in energy consumption
- Product being blown back at the feed point
- Irregular product flow
- Increased filter load
For this reason, air leakage should not be considered merely a maintenance issue specific to the rotary valve itself.
Pneumatic Conveying Systems—it is recommended to include an internal link to this section.
How Can Rotor Wear Be Detected?
Rotor wear is commonly observed in air locks, particularly in facilities where abrasive products are used.
The first signs of wear may include:
- Increased air leakage
- Changes in capacity
- Increased product leakage
- Thinning of the rotor blades
- Increased clearance between the rotor and the housing
- Irregular noise
- Unexpected pressure changes in the system
As wear progresses, the airlock performance of the rotary valve may decrease significantly.
Products That Increase Wear
In particular:
- Cement
- Ash
- Sand
- Mineral dusts
- Silica-containing products
- Hard granules
can cause high levels of wear.
For these products, wear-resistant materials or coatings should be considered instead of standard rotors and housings.
Why Does a Rotary Valve Jam?
Rotary valve jamming is one of the serious problems that can cause production to come to a complete halt.
The most common causes of rotor jamming are as follows:
Foreign object ingress
A bolt, stone, metal fragment, or other hard foreign object can get caught between the rotor and the housing, stopping movement.
Large Particles
If the product’s maximum particle size is not taken into account in the rotor design, material can become trapped between the vane and the body.
Product Buildup
Sticky or moist products can accumulate in the rotor cells and on the body surface, hindering rotor movement.
Thermal expansion
In systems operating at high temperatures, the rotor and housing may expand. If tolerances are not calculated correctly, the rotor may come into contact with the housing.
Bearing issues
Worn or damaged bearings can cause the shaft axis to shift, resulting in the rotor coming into contact with the housing.
Rotor deformation
Foreign objects or excessive mechanical loads can bend the rotor blades.
How Can Rotor Seizure Be Prevented?
Certain measures can be taken in process design and maintenance programs to reduce the risk of seizure.
- Use of a screen at the product inlet
- Installation of a metal trap or magnet
- Determination of the maximum particle size
- Selection of appropriate rotor geometry
- Proper rotor-housing tolerance
- Motor overload protection
- Torque control system
- Speed sensor
- Regular cleaning
- Periodic bearing inspection
Forcing the rotor by increasing motor power after a jam may not be the correct approach. This can lead to greater damage to the rotor, shaft, or gearbox.
What Causes Capacity Loss in an Air Lock?
A decrease in an air lock’s capacity over time compared to its initial installation can result from various causes.
Product buildup in rotor cells
If the product does not completely empty from the cells, the actual volume that can be transported per rotation decreases.
Decrease in rotor speed
A motor or gearbox problem can reduce the rotor speed.
Changes in Product Properties
If the moisture content, density, or particle size of the product used in the same facility changes, the rotary valve’s actual capacity may also change.
Insufficient Product Flow at the Inlet
If bridging occurs at the silo or bunker outlet, the rotor chambers will not fill completely.
Rotor Damage
Bent or worn rotor blades can affect chamber volume and product flow.
Air backflow
In pneumatic conveying systems, high pressure can send air in the opposite direction into the rotor cells, preventing the product from filling them.
Why Do Theoretical and Actual Capacities Differ?
Rotary valve capacity can generally be calculated theoretically based on rotor volume and rotational speed. However, under actual process conditions, the rotor cells do not always fill to 100 percent.
Key factors affecting actual capacity:
- Product density
- Flowability
- Rotor speed
- Cell geometry
- Product level
- Pressure difference
- Product moisture content
- Particle size
- Air backflow
Therefore, if a drop in capacity is observed, one should not only check the motor speed but also examine the product’s filling behavior within the rotor cells.
What Causes Bearing Failures?
Bearings ensure that the rotary valve rotor shaft rotates smoothly and evenly. A bearing failure can cause rotor misalignment and serious mechanical damage.
Main causes:
- Insufficient lubrication
- Excessive load
- Product dust entering the bearing
- Improper installation
- Shaft misalignment
- High temperature
- Vibration
- Lack of maintenance over a long period
Signs of bearing failure
- Abnormal noise
- Increase in bearing temperature
- Increased vibration
- Difficulty in rotor rotation
- Increase in motor current
- Product leakage around the shaft
The system must be inspected when these symptoms are detected.
Seal and Leakage Problems
The seals and sealing elements used at the points where the rotary valve shaft exits the body can wear out over time.
As a result of wear:
- Product may leak out
- Dust may reach the bearing
- Bearing life may be reduced
- Dust may accumulate in the work area
- Sealing performance may decrease
When selecting seals, product temperature, particle structure, chemical properties, and operating pressure must be taken into account.
Motor and Gearbox Failures
The air lock motor and gearbox operate under constant mechanical load.
Main causes of failure:
- Rotor jamming
- Excessive product load
- Incorrect gear ratio
- Insufficient motor power
- Mechanical strain due to an oversized motor
- Lubrication issues
- Electrical problems
- Insufficient motor cooling
Why is monitoring motor current important?
When the rotor is under strain, the current drawn by the motor may increase.
An above-normal increase in motor current:
- rotor seizure,
- bearing problem,
- product buildup,
- foreign object
may be early signs of such problems.
For this reason, monitoring motor current or torque in automation systems can be beneficial.
Abnormal Noise and Vibration in the Rotary Valve
A normally operating rotary valve typically produces a regular and stable mechanical sound.
Sudden onset of:
- knocking,
- scraping noise,
- metal-on-metal contact,
- irregular vibration
may indicate a mechanical problem.
Possible causes:
- Rotor contacting the housing
- Foreign object
- Damaged bearing
- Loose connection
- Rotor balance issue
- Shaft deflection
- Gearbox failure
Forcing the equipment to continue operating when abnormal noise is detected may lead to more serious failures.
Product Buildup and Clogging
Product buildup inside the rotary valve can occur, particularly with sticky, moist, or cohesive products.
Buildup can occur in the following areas:
- Rotor cells
- Housing corners
- Inlet area
- Outlet area
- Shaft area
As buildup increases, the rotor’s usable volume decreases and capacity drops.
Solution Options
Depending on product and process suitability:
- different rotor geometry,
- surface coating,
- low surface roughness,
- maintenance access door,
- regular cleaning,
- vibration or flow-assistance system
may be considered.
What Failures Can Excessive Temperature Cause?
In high-temperature applications, the rotary valve design may differ from standard operating conditions.
Due to temperature:
- the rotor may expand,
- the housing may expand,
- the rotor-to-housing clearance may decrease,
- the seal material may degrade,
- bearing temperature may rise,
- and the lubricant may lose its properties.
This situation can lead to rotor seizure or loss of sealing.
In high-temperature applications, the equipment’s actual operating temperature must be specified during the design phase.
Air Lock Periodic Maintenance Checklist
Regular air lock maintenance is important for minimizing unplanned downtime.
Daily or operator inspection
- Are there any abnormal noises?
- Is there excessive vibration?
- Is there any product leakage?
- Is the motor operating normally?
- Is the product flow steady?
Periodic mechanical inspection
- Does the rotor rotate freely?
- Is there wear on the rotor blades?
- Is the inner surface of the housing worn?
- Is there play in the bearings?
- Are the seals intact?
- Are the bolts loose?
- Is the motor-gear connection secure?
Process Performance Check
- Has capacity decreased?
- Has air leakage increased?
- Has the pneumatic line pressure changed?
- Has energy consumption increased?
- Is the rotor speed correct?
Maintenance frequency should not be the same for all facilities. Maintenance intervals may differ between rotary valves handling abrasive products and equipment handling food products with low abrasiveness.
When Should an Air Lock Be Overhauled?
A detailed overhaul should be considered in the following situations:
- Significant increase in rotor-housing clearance
- Constant air leakage
- Wear on the rotor blades
- Damage to the housing’s inner surface
- Bearing and shaft clearance
- Recurring jamming
- Significant drop in capacity
- Constant motor overload error
- Increased product leakage
During overhaul, not only should worn parts be replaced, but the root cause of the wear should also be investigated.
When Should a New Rotary Valve Be Selected?
In some cases, it may be more economical to select a new rotary valve better suited to the process rather than continuously overhauling the existing equipment.
For example:
- If the product has changed completely
- If capacity has been significantly increased
- If the new system pressure exceeds that of the existing equipment
- If the system is operating at a higher temperature
- If hygiene requirements have changed
- If a new, abrasive product has been introduced
- If the existing housing is severely damaged
- If air leakage cannot be reduced to an acceptable level through refurbishment
the selection of new equipment should be considered.
At this point, it is recommended to provide a link to the Rotary Valve Selection Guide.
Quick Check Table Based on Failure Symptoms
Symptom Possible Cause
Air leakage has increased Rotor/housing wear, increased tolerances
Capacity has decreased Product buildup, low speed, insufficient filling
Rotor is not rotating Foreign object, bearing failure, motor problem
Abnormal noise Rotor contact, bearing, gearbox, or foreign object
Vibration has increased Balancing, shaft, or bearing problem
High motor current Rotor jam or overload
Product leakage Seal wear or connection issue
High bearing temperature Lubrication, alignment, or overload
Reduced pneumatic capacity Increased air leakage or supply issue
This table is for initial troubleshooting only. A definitive fault diagnosis requires a joint examination of the equipment and the process.
Frequently Asked Questions
Why does the air lock leak air?
Air leakage may increase due to clearance between the rotor and the housing, wear, high pressure differential, or a damaged rotor.
Why does the rotary valve jam?
Large particles, foreign objects, product buildup, tight tolerances, thermal expansion, or bearing failure are the most common causes.
Why does the air lock capacity decrease?
Product buildup in the rotor cells, low rotor speed, changes in product properties, or air backflow can cause a loss of capacity.
How often should rotary valve maintenance be performed?
It is not appropriate to specify a fixed interval. Maintenance frequency should be determined based on the product’s abrasiveness, operating time, temperature, and process conditions.
Can rotor wear be repaired?
Depending on the extent of wear and the equipment design, the rotor may be overhauled or replaced. The inner surface of the housing should also be inspected.
Why does the airlock’s motor draw excessive current?
Rotor jamming, product load, bearing failure, foreign objects, or mechanical friction can increase motor current.
Conclusion
Air lock failures can affect not only the rotary valve’s own performance but also the entire system of connected silos, filters, cyclones, and pneumatic conveying systems. In particular, air leaks, rotor wear, jamming, and capacity loss can lead to increased energy consumption and production downtime if not detected in a timely manner.
Regular maintenance, proper product selection, and monitoring of process conditions help prevent a significant portion of these problems.
The airlock’s rotor-housing clearance, bearings, seals, motor-gearbox system, and air leaks must be checked regularly. Additionally, whenever the product being used or process conditions change, the existing rotary valve should be re-evaluated to ensure it is suitable for the new operating conditions.
Let’s Conduct a Technical Evaluation of Your Air Lock and Rotary Valve System
If you are experiencing air leakage, rotor jamming, capacity loss, or wear issues, please share information about the product being conveyed, capacity, operating pressure, temperature, and equipment details with the MASTRON engineering team. We can jointly evaluate the operating conditions of your current system and explore suitable solution options for your application.