When Selecting a Rotary Valve, Leave Nothing to Chance

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July/Aug 2022

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Rotary Valve Application Data

What We Ask and Why It Is Important

When selecting a rotary valve, a little time upfront can save you a lot of time later. Other than normal wear and tear, the most common cause of operating problems is an incorrectly configured rotary valve. Valves have many components and they can be configured in hundreds of different ways. Here are some of the considerations that go into customizing a trouble free rotary valve for your application.

Material Characteristics: Bulk Density, Particle Type (powder, pellet, dust, etc.), Abrasiveness, Flowability, Tendency to compact, absorb moisture, aerate, corrode or ignite.
Operating Conditions: Flow Rate, Temperature, Equipment above and below the valve, Pressure above and below the valve.

MEYER HDX ROTARY AIRLOCK VALVE

In addition to improperly selected valves, changes in your operating conditions, your material properties or poor maintenance may also lead to problems.

Here are the four most common.

Material Jams

Material buildup can overload a motor, break a chain or bend the rotor to the point that it breaks. This is more common with certain products - powdery or sticky materials like sugar. Moist or humid conditions can cause those materials to stick inside the rotor.

Material caught between the rotor blades and the housing or headplates jams the valve and wears the components. Stringy or fibrous materials, such as wood fibers, hard materials, like some plastic pellets and large, chunky materials are more susceptible to jamming.

What to Check?

Check the valve's rotation. Is it turning in the direction the manufacturer recommends? (View the rotation from the driven end). Correct valve rotation is especially important when utilizing beveled tips. With beveled tips, the wrong rotation can cause excessive drag and force between the rotor and housing. This can result in a failure of weld between the vane and rotor hub even to the point of the vanes separating from the rotor hub.
Does the rotor turn smoothly without contacting the housing or headplates?
Does the valve shudder? Shuddering can indicate material is jammed between a blade tip and the housing.
Is your valve undersized? Material must fit comfortably in the rotor pocket.
Has a foreign object entered the valve?
Are changes to the material's properties affecting the flow?
Is the material temperature higher than expected?

What to Do?

Bevel blade tips to ease flow between the blades and housing or headplates.
Install flexible seal strips.
If water vapor is reaching your your material and causing it to lump, vent moist air before it reaches the valve.
Higher than expected material temperature may cause valve components to expand reducing valve clearances. Ideally, reduce material temperature. If not possible, valve clearances may need to be increased.

Note: Beveled blade tips and flexible seal strips are much more easily added by the manufacturer when building the valve, rather than by maintenance staff on site.

Excessive Air Leakage

Rotary valves are designed with clearances between the rotor, housing and headplates. Typical blade-to-housing and blade-to-headplate clearances are between 0.003 and 0.012 inch, varying by manufacturer. As a result, no rotary airlock valve is 100% leak-proof. However, excessive air leakage may result in a drop in material throughput or in an increased air requirement in your conveying system.

There are two types of air leakage in rotary airlock valves:

Clearance leakage: leakage through the blade-to-housing and blade-to-headplate clearances which usually decreases after the material enters the valve.
Displacement leakage: the amount of air entering each rotor pocket as the pocket passes the material outlet. This leakage increases with the valve's running speed and actual displacement.

How do you know if you have excessive air leakage? Sum the clearance leakage and displacement leakage estimated by the valve manufacturer and go from there.

Ways to minimize air leakage

Install flexible seal strips to reduce clearances
Select a valve with more rotor blades
Install a housing or headplate vent
Install a vent adaptor at the material inlet

Avoid air leakage problems by discussing your clearance needs with a TEC Engineer before selecting the valve. He or she will consider the material's characteristics, the operating temperature and application conditions (such as pressure differentials).

Noisy Operation

Some operating noise is common at startup. If your valve makes more noise than usual when running, or makes sounds you've never heard before, it's time for an inspection.

Ensure the rotor is running in the right direction and doesn't contact the housing or headplates.
Was the valve properly stored? If not, check for rust. Rust can reduce the clearances inside the valve.
Is there build up inside the valve. Material, especially hygroscopic material, can build up on the housing and headplates, causing noise. Coating the valve interior with nickel or chrome often prevents material buildup. Or, try venting moist air before it reaches the valve.
Bevel rotor blade tips to reduce drag.
Check for a mechanical problem with the drive chain or drive shaft.

Material Throughput is Insufficient

Check for excessive air leakage. If that is not the problem and all of the valve components check out, your valve may be the wrong size for your application. If your conveying system’s operating speed dropped or your material’s properties changed since you installed the valve, you may need to transition to a smaller or larger valve.

We can work with you to choose the right size and configuration for a rotary airlock valve unique to your application. By considering your material's characteristics - bulk weight in lbs/cu ft, maximum particle size and the tendency to compact, aerate, absorb moisture, corrode or ignite - along with the device and conditions above and below the valve, you can avoid these common operating problems.