Should you check your rotor clearances? Most mechanical engineers, machine operators, and others who know a thing or two about rotary airlock valves needn’t read any further, as you already know the answer. For those working around rotary valves but who are a bit fuzzy on why rotor clearance is so important, this post is for you. 

Let’s get something clear about rotor clearances: they must be checked regularly. Rotor clearances are the gap between the rotary valve’s housing and the rotor. This is one of the easiest ways to increase the lifespan and efficiency of a rotary airlock valve.

Importance of Checking Rotor Clearance 

Rotary airlock valves are comprised of vanes rotating within a housing. To maintain differential air pressure, it’s important that there’s minimal clearance between the rotor and the valve body. The valve cannot maintain this pressure differential when rotor clearances are too large. Wider rotor clearances also produce greater wear, shortening the valve’s lifespan, especially when dealing with abrasive substances. 

Numerous variables contribute to the best rotor clearance for the application:

  • Capacity and application of the valve.
  • Properties and type of the product being made.
  • Temperatures during operation and of the product during processing.

Current design and manufacturing techniques can achieve uniform rotor tolerances tighter than 0.004 inch (0.1 mm). Yet if the valve isn’t serving an airlock function to maintain a pressure differential, rotor clearances generally won’t matter as much. Additionally, broader rotor clearances are normally preferred for materials that are hydroscopic, stick easily to surfaces, or tend to glaze.

Consider Flow Rates

It’s also important to consider the rate of material flowing through a system when monitoring rotor clearances, as rotary valves become inefficient over time. When rotor tips wear out, they don’t convey the correct amount of product necessary. For example, if the airlock is feeding a milling system, the mill can be overfed with too large clearances. The material would leak through the airlock, causing throughput to change. 

Maintaining Rotor Clearance Compliance 

One of the most important guidelines that bulk processing manufacturers are required to follow is the National Fire Protection Agency (NFPA) standard on explosion and fire prevention systems. NFPA-69 states the requirements for systems that control and prevent explosions from dust, gas, mist, vapor, and admixtures within enclosures. This standard is often used by design engineers, machine operators, and other authorized individuals. 

When rotor clearances are too big, the rotary airlock won’t operate properly, leading to flame suppression deficiencies. When working as an airlock, the rotary airlock valve isolates oxygen sources to keep flames or explosions from spreading. Rotor clearances, according to NFPA-69, need to be 0.008” or less to function as an isolating device.

When There’s No Rotor Clearance

While tight rotor clearances provide better system efficiency, it’s not a good thing when a rotor contacts the housing. Should this happen, the power to the drive motor should be shut off, and the valve should be inspected. This is particularly true in the pharmaceutical and food processing industries.

This helps: 

  • Detect bearing failure promptly.
  • Prevent improper reassembly following cleaning or maintenance.
  • Protect against any internal damage within rotary valves.
  • Reduce the possibility of contamination from shedding metal.

Recommendations on Checking Rotor Clearance & Replacement

Generally, rotor clearances should be checked every three months, and maintaining a maintenance history is imperative. Should these checks reveal that the rotor remains intact after multiple safety tests, scaling back scheduled maintenance a month or so more usually won’t matter. However, without regular inspections planned, there’s always a risk of system failure, leading to unplanned downtime.

By regularly monitoring rotor clearance, manufacturers can schedule a rotor replacement to coincide with planned downtime. Additionally, depending on the materials being conveyed, different types of steel should be considered for any replacement rotors. This is especially true for applications dealing with abrasive substances like cement powder, where a stainless steel rotor usually works best.

We invite you to contact us today to learn more about rotor clearance and Prater’s rotary airlock valves.

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