Airlock Selection and Maintenance: Tips on Keeping Airlocks Operating Safely, Efficiently

This article is based on a presentation by Jeff Kice, technical sales, Kice Industries, Inc., Wichita, KS, (316-744-7151)/, given online at the 2020 International Association of Operative Millers’ (IAOM) All-District Virtual Conference and Expo, Sept. 15 - 17, 2020.

As a fourth-generation family member with Kice Industries, he joined the company in 2006 and also serves as a key account and major project manager.

IAOM members who registered and paid for the virtual conference still can view the presentation using their provided link, username, and password.

What follows is a partial excerpt of some key highlights of comments related to this presentation, which have been edited for clarity and brevity.

While rotary airlocks, which operate at low speeds, aren’t overly complicated equipment, and normally require minimal maintenance, they nevertheless are very critical for the efficient operation of a pneumatic conveying system.

Rotary airlocks utilize very tight machined internal clearances, which are necessary to maintain the proper pressure or vacuum differential for providing consistent feeding rates of material in the conveying line.

Airlock Performance

For example, the valves in airlocks are typically machined from .003 inch to .005 inch of clearance, which is a little bit thicker than a dollar bill. So, a dollar bill would barely be able to slide in between the tip and the side of the valve.

So, while the airlock valves are machined to very tight clearances, some amount of pressurized air (or the air under a vacuum) always will be lost through those clearances around the rotors’ edges and tips, and the pressurized air will fill any empty voids before being released to the atmosphere each time the rotor reaches the top of the unit.

This airlock valve leakage can be categorized as either static or dynamic.

• Static leakage refers to the leakage or movement of air that comes through the sides and the tips of the valve, regardless of whether the equipment is in operation or not. This is just air that’s going to leak out based on the worn out or open clearances of that specific rotary airlock. The higher the pressure, the higher the air volume the valve will leak. However, it’s still important to be mindful that as any valve wears out over time, the unit will leak more air.

• Dynamic leakage is generally air loss from the conveying line taken away by the empty pocket of the valve, which is usually only apparent when the system is being operated.

Minimizing these air leakages are all taken into account by pneumatic conveying system designers. Most rotary airlock valves are rated at 15 psi (or 15 inches of mercury on a manometer), and they are designed to maintain the necessary pressure or vacuum differential that facilitates the introduction of material into that conveying line.

Airlock valves also don’t rotate at super high speeds, with 10 rpm to 30 rpm being a typical range. If airlocks run too slowly, it can lead to “slugging” of material into the line which can cause conveying issues.

Plus, any speed above 30 rpm doesn’t really gain much in terms of productivity, because there’s not enough time for the incoming material to fill up the top pocket completely.

Airlock Selection

Since rotary airlocks are volumetric devices, there are a few key points to consider in choosing the valve size.

Rotary airlock suppliers focus on two key things when sizing this equipment.

1. Determine the needed conveying rate, which is calculated in pounds per minute, for the job.

2. Determine the bulk density of the product being conveyed.

For example, in considering a rate of 100 pounds per minute of a material with a bulk density of 35 pounds per cubic foot, the cubic-feet-per-minute rate would be 2.86 (100 ÷ 35 = 2.86).

This rate of 2.86 cubic feet per minute represents what the rotary airlock is expected to handle.

Rotary airlocks are sized based on cubic feet per revolution and typically at 100% capacity. While that capacity may be difficult to achieve, it still is an assumption a supplier will make when sizing the rotary airlock.

The capacity level also will depend on how the material is fed into the rotary airlocks, the bulk density of the product, and an assumed value for how much material will fill the spaces between the rotor blades.

For 35 pounds per cubic foot of material, it’s probably safe to assume a 70% pocket fill between the blades.

So, in this case, the quantity should work out to about 0.33 cubic feet per revolution, which would require the airlock to run at slightly more than 12 rpm to achieve the desired volume.

It also would probably be safe to increase the load capacity and run the airlock at 20 rpm, knowing that it can handle more incoming material.

However, as a rule of thumb, and from a manufacturer’s perspective, rotary airlocks shouldn’t be relied on to control the feed or the flow rate of material into a conveying line.

For example, in a choke-feeding situation, an excessive material load on the rotary airlock may hinder the flow rate to the point where it results in undue stress on the tips of rotors and ultimately added wear and tear on other equipment components.

Instead, using something like a metering screw conveyor or a drag conveyor would be better suited to control the flow rate, so that the airlocks aren’t overtaxed beyond their rated capacity.

In addition, rotary airlocks are available with either outboard or inboard bearings.

Outboard bearings offer good support to valves and allow for higher pressure differentials when conveying products. These types of bearings also can accommodate various shaft seal arrangements and rotor shaft diameters.

Inboard bearings provide the airlock shaft maximum support and help minimize rotor deflection; however, preventing potential contamination becomes more critical.

However, it’s important to remember that the National Fire Protection Association (NFPA) standards require bearings to be external to the body, in order to be compliant in reducing the risk of dust explosions.

Maintenance Issues

While airlocks are relatively maintenance free, there are a few maintenance issues to consider.

When working on any piece of power equipment, safety always comes first. So, reading and reviewing the operator’s manual periodically is always recommended to make sure that the information contained on the warning stickers is understood.

In addition, before any attempt is made to work on a rotary airlock, make sure that the facility’s lockout/tagout safety procedures are followed.

Examine Rotary Airlock Valve

Usually when a facility experiences pneumatic conveying or dust control issues, the tendency is to view the more complicated equipment as the culprit.

While that may be the case in many situations, it’s still important to not overlook the rotary airlock and its condition and performance.

So, take the time to examine the rotary airlock valve itself by checking the clearances on the valve, its age, and to ensure that it’s operating properly.

If not kept in tip-top shape, a worn out rotary airlock can cause big problems in the baghouse, as an example. Hearing a loud thud when the rotary airlock below the dust collector hopper is shut off is a common sign that the rotary valve has become worn out.

The material is not able to get down into the airlock valve. It’s been suspended inside of that hopper for an extended period of time.

In this situation, the airlock is typically the cause due to it being worn out and allowing too much air to come up through the bottom of the valve.


Obstructions caused by product buildup or from foreign materials are another issue that can potentially lock up the rotors inside an airlock, as well as stall the motor.

In addition, even the smallest of obstructions can cause problems due to the extremely tight clearances inside the airlock.

If this is something that occurs once, using the same valve probably should be fine, but if this is something that continues to occur, it’s recommended to check with the manufacturer about available options to remedy the problem.

For example, there are a variety of options to consider for specific applications, such as abrasion-resistant coatings or slick coatings similar to Teflon™ that can help prevent the product from sticking to the surface of the rotor.

Kice Industries also offers bevel tips and edges, which reduce the amount of surface area of the rotor that’s against the body and the end plates.

Removing Obstructions

Again, before any inspection is done to remove an obstruction, it’s important to follow proper lockout/tagout safety procedures.

Removing from service the airlock in question also is recommended to make it safer and easier to inspect and work on the airlock and to also check for any obstructions inside the chain guard.

If the blockage or obstruction can be identified, it’s recommended to use a sturdy but soft probe or claw retriever tool to dislodge it carefully, instead of trying to do it by hand.

It’s important to place a block in the airlock to prevent the rotor from turning unexpectedly. Even with these valves disconnected and out of service, injury to the hands can still occur if the rotor isn’t blocked properly.

Safe procedures for removing obstructions are well covered and illustrated in the operator’s manual.

If the blockage or obstruction can be identified, it’s recommended to use a sturdy but soft probe or claw retriever tool to dislodge it carefully, instead of trying to do it by hand.

Once the obstruction is removed and the rotor can be moved freely, it’s also recommended to check for any rotor damage, especially around the body housing.

If any rough edges or burrs are found, they can be removed carefully with a file.

Other Maintenance Issues

Other maintenance issues to consider include the following:

• Bearings and seals. Bearings and seals have a certain lifespan, so failure can occur, which often is preceded when the airlocks operate more loudly or start to squeak in a piercing manner. Sometimes this can occur if the rotor has shifted.

• Gear reducers. Although not very common, it’s still important to not overlook any issues caused by gear reducers and to confirm with the manufacturer whether or not the airlock setup is correct for the product application.

• Material temperature. Temperature is always something to consider, especially in situations involving very tight clearances, since prolonged heat leads to metal expansion of the valve components.

If such a condition exists, also let the manufacturer know so that it can account for that when sizing the proper clearance for the valves.

These high-heat conditions also can impact the performance of the rotor blades and expand them to the point where they may come in contact with the housing or head plates and ultimately seize up.

• Avoid choke-feeding. If choke-feeding occurs in a rotary airlock, the chain may start jumping more, or the motor mount may vibrate excessively. These conditions are very hard on a rotary airlock valve.

• Motor heater settings. Even if the rotor turns freely, it’s recommended to check the motor heater settings and confirm with the manufacturer that the correct motor is being used for the airlock.

Motor trips and overheats can certainly become an issue if the heat settings are not set correctly on the motor. If not set properly, the airlock may tend to kick out under varying loads.

So, it’s recommended to check the heat settings so that they’re set properly based on the size of motor that powers the rotary airlock.

Karl Ohm, contributing editor

Editor’s note: For further reading, see the article, “Central Vacuum Systems,” by former Milling Journal editor, Karl Ohm, in the Fourth Quarter 2017 Milling Journal, pages 54-58.

• “Measuring Air Velocity,” First Quarter 2017 Milling Journal, pages 50-52.

All the above articles can be accessed online by going to: or scanning the QR code.

From Second Quarter 2021 MILLING JOURNAL