What type of rotary airlock are you using? Some are designed better than others. A round hopper bottom tends to direct the flow to the center of an airlock. If the flow is high and the material piles up in the center so that the rotary vanes have to shear off the pile every time the pocket closes, there will be high wear in the center both on the wall and the blades.
Some models of airlocks have "shoes" that are pulled against the rotor with spring action. The intent is well intentioned; to maintain a tight seal. However, I have never seen those work well; the springs always end up with unequal force and the shoe gets "cocked" causing unequal wear, IMHO.
I prefer a square hopper botton into a square rotary valve. That way the flow from the hopper is spread as evenly as possible throught out the rotary valve, not just toward the center.
Rotary valves are going to wear. That is a fact of life. However, you get to pick the wear point. I always specified and operated rotary airlocks with hard chrome plated bores and soft (sacrificial) steel vanes or vane tips. When I could, I used soft steal replacable wear strips. In theory the wear strips can be reversed, using the good edge when the other edge becomes worn, but in real life, it is just as cost effective to replace the wear strips when the valve is out. Wear strips are cheap, labor to change them is expensive where I came from. Set the gaps with feeler gages and the valves will get remarkable life.
I used rotary valves where the material being collected contained combustables so any back leakage into the airlocks could cause hopper fires, so maintaining tight airlocks was critical. I found Wm. W. Meyer in Skokie, IL a good company to work with. They knew their stuff and wouldn't mislead you. I hope something here helps.
Many thanks for your reply, it was of real help. What you think of using a secondary airlock? Will closing the damper a bit will help? How can we predict the faliure? Will this always be a guess from experience or we can install some instruments in adittion to diff. pressure? Ours is a circular type airlock provided by Rotolock.
Consult with your vendor or with a competitive vendor. Show him the failed rotors. Give him a mason jar full of the product (with an MSDS Sheet) so he can send it back to the factory wizards for testing. Chances are when the rotary airlocks were originally specified the service conditions were not well described. Pressure, temperature, flowrate, pressure-or-vacuum conveying, start-stop or continuously running..all these parameters matter.
Or the purchasing clerk just went cheap. Not a slam against Rotolock, but everybody makes "economy" units and higher-featured units.
I had some experience with Premier Pneumatics rotary airlocks a few years back and I was impressed. They make rotors with 6, 8, 0r 10 blades. They put different tip profiles on the blades depending on the material flowing through. And of course they offer the units in different materials depending upon the parameters of the application.
Also, don't tie the vendor down to a bolt-in replacement of the existing unit. Ask him what would provide the best service in the application. Your problem may be solved by a different size unit or different piping configuration. A couple-thousand dollars spent now is well spent if it avoids even a single future unscheduled outage.
I wasn't familiar with rotolock and googled it and took one look and instantly didn't like it. It is the round inlet type valve that I described above. It requires a round hopper outlet and it means that the material will try to pile in the center of the valve in my opinion. Sorry, but I just don't like round inlet rotary feeders for abrasive materials.
Amplifying on what JimCasey said above, if the rotary feeders came with the original equipment you always have to remember that in the absence of a real good specification an OEM will put the cheapest thing possible on their product in order to be competitive and maximize profits. If you want to gold plate your equipment write a bullet proof spec of plan to retrofit.
Now to your question, I have seen double airlocks and they worked well. Most of my airlock experience was with combustion control air pollution control equipment and the potential for hopper fires with air in-leakage was very real. Good airlocks were essential.
You haven't described your process so it is hard to answer your question regarding how to predict failure. In the combustion experience of my experience a tight valve would be hot while a leaking valve would be cooler and the hopper above it would be cool to the touch. A serious failure would usually be indicated by a glowing red hot hopper bottom because of the burning combustible material inside.
