Airlocks could be compared to the water pump on your car: a comparatively inexpensive component of a larger system that is out of sight and out of mind until a problem develops. Though usually a smaller item in the overall picture, its failure could threaten anything from a loss of efficiency to shutting down a production line.

It is important, then, to make sure the airlocks are properly built for the application it is running in, and maintained to ensure maximum efficiency and run time. This article will review three common issues airlocks can experience, and offer practical solutions to implement. The issues discussed are such that are found on either positive or negative pressure pneumatic systems. The majority of airlocks are found on positive air systems in which the blower is pushing the product through the system. The airlock's function is then to allow product to be dispensed into the pneumatic line, while at the same time preventing the air from blowing past, which would prevent material from falling into the airlock.

Though not as common, some airlocks are used on negative pressure systems. Here the material is being pulled through the system. Dust collectors are a common example of this. The airlock, usually at the bottom of a silo, allows material to be expelled while preventing air from being sucked up through the airlock. In a vacuum air system, the airlock acts more as a feeder, metering in material into the vacuum line, preventing material from being pulled out of the source above the airlock all at once.

Common Airlock Problems

Of the most common issues with airlocks, seizing of the rotor is at the top of the list. This is not surprising considering only the thickness of a piece of paper is what separates the housing from the rotor vanes. Aside from the obvious seizing due to an object falling into the airlock, there are three primary reasons why an airlock is going to seize.

The first has to do with thermo-expansion. This is when an airlock is running in an application hotter than it was built for, causing the rotor to expand into the housing. The second reason is product build up. If the material going through the unit has a tendency to pack or has a high moisture content, it will start to build up on the housing and quickly lock up the rotor. The third reason for seizing involves the rotor shifting to either side, galling up the endcaps enough to engage the rotor. This can occur when a drive sprocket is pounded on too hard, moving the rotor over, or when new bearings are installed.

The second most common issue with airlocks is bearing and seal failure. This is especially true of applications that run very fine material such as flour. The rest of the airlock might be in good shape, but if the seals and bearings fail, it could eventually lead to the rotor dropping into the bore. The final issue to consider is blowby. This is simply the condition of an airlock where it is no longer able to lock out the air, which is now blowing by between the rotor vanes and housing bore. This usually prevents material from being able to efficiently move through the airlock.

The following, then, are some practical tips offered to help keep your airlocks running efficiently.

Addressing Airlock Problems

Considering first the issue of seizing due to thermo-expansion, it is important to note the clearances of a particular unit before installing it. Any temperature spikes that may last for more than a few minutes should be taken into consideration. For airlocks used outside, insulation can be used to aid in helping the housing when hot material hits it during cold seasons.

If seizing due to product build up is an issue, it is a common practice to machine bevels into the vanes so there is less flat on the tips to bind with. However, sometimes the build up may persist despite the beveled edges. In such a case two solutions are offered. First, a new rotor with specially milled vanes with a knife-like leading edge can be installed. Second, a highly lubristic chrome plating applied to the internals of the airlock may be used alone or in conjunction with the new rotor.

To prevent the rotor from shifting side to side, place shims on the opposite side of the rotor when installing the drive sprocket, and remove when finished. When installing bearings, replace both of them at the same time. Alternate tightening both sides to prevent pulling the rotor to one side of the other.

There are two ways to address the bearing and seals issue. The first to be aware of is bearing conversions. In the case of inboardbearing airlocks experiencing bearing failure, it is possible to convert the bearing to another one more suitable to the application. An example would be going from a sealed bearing to an ERstyle to enable the airlock to be greased.

The second way to note is that of seal conversions. This may involve adding additional seals to existing lip seals, or converting an airlock from rope packing to mechanical seals. When blowby is an issue to be dealt with, there are three areas to address that can aid in obtaining longer life from the airlock. The first to consider is the rotor set up. For example if much wear is found on the endcaps, a change from an open-end rotor to a closed-end may be warranted.

Secondly, wear coatings from chrome to ceramics may be considered for the housing and endcaps. Communication is critical between the end user and the vendor applying the coatings, as some coatings perform better in certain application than others.

The third area to address is high-wear tipping for the rotor, which goes hand in hand with previously mentioned wear coatings. Again, knowing the abrasive product going through the airlock will help determine what material to use for tipping. Various options include stellite, tungsten, chrome, and AR adjustable blades.

Though it is good to be able to have solutions to common issues that arise with airlock usage, it is also helpful to consider some maintenance items that may aid in the prevention of the issues in the first place.

Preventative Maintenance for Airlocks

Schedule clearance checks during shutdowns to determine if the airlock is performing efficiently. Check with the o.e.m. for the original clearances machined or ask the company who services your airlocks for a clearance chart to help determine if an airlock needs to be scheduled for replacement. Catching a failing airlock early can save cost when remanufacturing the unit. Implementing this will alleviate additional labor needed to restore the unit back to o.e.m. specs.

If airlocks are changed to greaseable bearings, be sure to add them to your preventative maintenance programs to be greased two to three times per year. Even though product will eventually get through the seals, the airlock will still be able to stay in line longer if the bearings are being purged occasionally.

If possible, keep the airlock RPMs as slow as possible as this will reduce how quickly the unit wears out. With this in mind, it is also helpful to reduce capacity in the vane pockets as this will also prolong the run time. If production would negatively be affected by this, consider upgrading to the next size airlock. Though it will cost more than your current unit, in the long run, it could prove to be the most cost effective route.

Finally, if abrasive wear is a major concern, and the airlocks wear out quickly, take a look at possibly increasing the size of the pneumatic blow lines. Sometimes going from a 4" line to a 5" can make a big difference in the life of the airlocks and other components of the system.

Though airlocks are not always given much thought in the overall pneumatic system, proper care and understanding of how to keep an airlock in line as long as possible can go a long way to preventing a small item from becoming a big problem.