Feb 14, 2020 Mining blog

Top 5 critical maintenance issues for rotary kilns

Andrew Wisner
Andrew Wisner
Product Specialist, Pyro
Your maintenance teams should include these top five (5) items in routine rotary kiln inspections. If any of these wear-and-tear issues exist, it is critical to catch and correct them as quickly as possible before these deficiencies become major problems.
metso-kiln-pyro-services

Improper Lubrication of Kiln Components

Proper lubrication of components is critical to the health of the kiln. It is imperative to check lubrication on equipment daily and to ensure the correct application is used for each kiln part.

When it comes to rotary kiln lubrication, both conditions—too much lubrication or too little lubrication—can equally contribute to problems:

  • Too much lubrication on certain parts can cause dangerous situations. Excess lubrication on kiln shells, foundations, guards, and housings can pose a fire hazard and can damage grout and concrete.
  • Lack of lubrication on critical parts (such as gear boxes, carrying rollers, bull gear, and pinion sets) can cause severe damage to a rotary kiln within minutes.

Kiln Misalignment

Left untreated, horizontal and vertical misalignment issues on rotary kilns can lead to unpredictable operation and decreased reliability. Contributing factors that can cause alignment issues include:

  • Component wear
  • Component replacement
  • Adjustment of carrying rollers
  • Foundation settlement

For kilns with three (3) or more piers, proper alignment evenly distributes loads to all the piers.

We recommend kiln realignment and leveling along both horizontal and vertical planes every one (1) to three (3) years depending on condition. Check alignment and kiln slope again before and after any major component replacement.

The most widely accepted method for alignment is performed under normal operating conditions and is aptly referred to as a hot-kiln alignment. Most services include some form of kiln mechanical survey where other measurements besides kiln misalignment are calculated.

Weak Thrust Balance at Each Pier

Thrust rollers are meant to stop the kiln from shifting too far in any direction. They absorb some of the thrusting force exerted by the kiln during operation. Additionally, most thrust force should be absorbed by skewing the carrying rollers to counteract the downward force exerted by the kiln.

Over time thanks to gravity, the kiln may become more susceptible to wear and imbalance. Therefore, it is key to inspect the thrust balance of carrying rollers at each pier for signs of overloading. In such cases, the kiln will need to be corrected through roller adjustments.

Adjustments on these rollers go by many names including thrust balancing, kiln float, or training the kiln. In general, the position of the carrying roller shafts must be evaluated to determine which direction the kiln is thrusting. Small adjustments are made to the rollers to equalize forces and distribute them as evenly as possible on each pier.

Kiln Shell Crank

Also known as dogleg or banana, shell crank is a condition where the kiln shell center is eccentric to the axis of rotation. Sometimes the kiln shell centerline exhibits an axial displacement somewhat similar to the crankshaft of an automobile.

Over time, kiln shell crank can have an adverse effect on carrying stations because it places higher loading on the carrying rollers during each rotation of the kiln.

The higher loading from kiln crank can then lead to many problems including:

  • Heavy gear vibration
  • Roller shaft flexing and eventual failure
  • Ineffective sealing
  • High cyclical drive amps
  • Increased ovality

Irregular Kiln Shell Ovality

Shell flex or ovality occurs at each pier. This condition is marked by a radial irregularity or deviation from the circular shape at the horizontal axis and an assumed equal and opposite deviation at the vertical axis.

Kiln alignment, kiln loading, kiln crank, and diametrical clearance (creep) and other factors affect the ovality of the kiln:

  • Tire ovality is determined by the design of the tire (thickness and width) at the pier.
  • Shell ovality refers to tire ovality that transfers to the shell plate. Thus, shell ovality is never lower than tire ovality. Shell ovality is also affected by the rigidity of the shell plate and the diametrical clearance between the tire bore and outside diameter of the kiln shell or filler bars.
  • Evaluation of the diametrical clearance during operation is completed by measuring the creep of the tire relative to the kiln shell on each revolution.

Monitoring both tire and shell ovality is important to help determine when refractory issues and shell cracking are likely to begin.

Metso Recommended Kiln Maintenance

In order to avoid these top five maintenance issues (plus many others), regular inspections and servicing for your rotary equipment cannot be overlooked.  As a starting point, Metso recommends performing a hot-kiln alignment and mechanical survey (HKAMS) every one (1) to three (3) years, This type of maintenance activity can be performed during regular operations, and should include the following at a minimum:

  • Assess kiln shell runout (eccentricity) and shell deformation (profile)
  • Measure roller shaft deflection
  • Record axial and radial gear runout
  • Calculate axial tire runout
  • Monitor ovality
  • Measure roller and tire diameters
  • Creep should be measured weekly along with uphill and downhill shell plate temperatures and tire side face temperature

Assessment and alignment services help kiln operators to budget for future service and equipment purchases and to plan their next maintenance shutdown. Proper inspections, assessments, and maintenance provide vital data for the scheduling of new or replacement parts, upgrades, and services.