What You Need to Know About Rotary Feeders In Cement Plants

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PMCA cement plant

Cement manufacturing takes a toll on all of the equipment in a plant.  Rotary valves and rotary feeders are not exempt from the punishment.

This post begins with a look at the characteristics that make cement manufacturing so tough on material handling equipment.  Following that is an examination of nine typical applications in a cement plant where rotary valves and feeders are proven problem-solvers.

Finally, it presents practical guidance on how to size, configure, and select rotary valves and feeders for use in this challenging industry.

Let’s get started.

Cement Manufacturing:
The Tasmanian Devil of Process Industries

Of all the industries that use Precision equipment, none can match cement manufacturing for the demands placed on process equipment.  Factors contributing to the challenge include:

Long-Cycle Operation – Cement manufacturing is nearly a year-round, 24 hours per day business. Planned downtime is minimized. Unplanned downtime is not just inconvenient – it is costly.  The expectation is that equipment will run reliably from one outage to the next.

Materials – Many of the precursors and inputs into the cement manufacturing process are inherently abrasive, including limestone, silica, slag, ash, and others.  The critical intermediate cement form, clinker, is one of the most abrasive products in any process industry.

Volume – The production scale required for economic, financially-viable cement manufacturing is enormous, and plants have gotten continuously larger over the years.  Plants with a cement production capacity of 3 to 4 million tons per year are relatively common and far from the largest.

Particle/Piece Size – At the front end of the process, quarried raw materials can arrive at the cement plant in a size equivalent to a softball. To continue the comparison, raw coal and clinker are usually in the tennis ball size category. Achieving the fine, powdered condition that is associated with cement is a continuous size reduction journey.

Temperatures – Cement kilns operate at 2500° to 3000° F; pre-calciners can reach 1800°F. The various stages in the preheater tower and the clinker cooler operate at temperatures up to 700° F.

Rotary Feeder & Rotary Valve Applications in Cement

In most applications, no matter the industry, rotary feeders and valves have one or two functions.  The two primary purposes:

  • As an airlock to separate differential pressures, minimize transit of steam or vapors, or to isolate differential temperatures
  • As a metering device to discharge a known volume of material at a controlled rate

Airlock and metering applications are both found in cement manufacturing plants. Later in this post, a highly-specialized third function will be reviewed. 

Mill Feeding: Raw Mill

In North America, the majority of cement manufacturing plants use a vertical roller mill and an integrated separator (VRM) for grinding quarried limestone and other materials into raw meal.  Throughput is usually in the range of 150-750 tons per hour.  In raw mill feeding, a system of weigh-belts that discharge into the mill feeding device controls the material rate into the mill.

VRM’s are fed either with a rotary feeder or a flap-gate system.  In either, the primary role is to provide an airlock function to minimize the introduction of “false air” into the mill and separator. 

PMCA rotary feeder and technician

Excessive false air has negative consequences:

  • The ID fan must be larger; therefore, more costly to acquire and operate
  • Larger fans or faster-running fans draw more electric power
  • Unwanted turbulence inside the mill resulting in poor grinding: off-spec product or significant re-grinding

Key Advantages of Rotary Feeders

A crucial consideration for cement plants is raw mill uptime.  Rotary feeders, particularly new-generation designs such as the Precision PMCA Rotary Feeder, have proven to be much more reliable than flap-gates or older rotary feeders. 

A Precision customer recently shared some illuminating statistics.  Over a two-year operating cycle,  accomplished without rebuilding the feeder, unplanned downtime was limited to less than 34 hours during planned production of 13,350 hours.  99.75% uptime!

A second significant advantage of rotary feeders over flap-gates is a much lower frequency and intensity of maintenance.  The new generation of raw mill rotary feeders is made from heavier, thicker steel with a “we’re building a tank” attitude.  Periodic maintenance is limited to an occasional visual inspection, bi-weekly or monthly lubrication, and little else. 

Flap-gates require regular adjustment of the opening/closing mechanism and frequent replacement of the flap seals to maintain airlock performance.

Mill Feeding: Finish Mill

VRM’s are somewhat less common in finish mill applications.  There are still many ball mills in use for cement grinding in North American.  Functionally, the job of a finish mill feeder is the same as a raw mill feeder – minimize false air passage into the mill: the airlock function.  Finish mill feeders must handle large throughputs, 100-500 tons per hour rates are common.

Precision PMCA Rotary Feeder in plant

Finish mill feeders operate under even more challenging conditions than raw mill feeders:

  • The volume of air moving through a finish mill is typically higher
  • Materials added in with the clinker: gypsum, limestone and other additives such as fly ash, silica fume, natural pozzolans, or blast furnace slag, are sometimes even more abrasive than the clinker itself
  • The abrasiveness and fineness of clinker dust combined with high-volume airflow create a “sand-blasting” effect on the internal components of a rotary feeder or flap-gate

For both rotary feeders and flap-gates, minimizing and controlling airflow into the mill despite the prolonged exposure to the sand-blasting effect is critical to performance.  Improved rotary feeder designs, like the PMCA Rotary Feeder, and the use of abrasion-resistant steel can minimize gaps that allow false air infiltration.

Mill Feeding: Coal Mill

The #1 issue for coal mill operators is stickiness or coal build-up on all the material handling equipment surfaces, including the mill feeder.

Coal remains a common fuel choice for firing cement kilns in North America.  Most plants receive their raw coal in a form that requires on-site grinding, using VRM’s along with other mill types. Raymond bowl mills are particularly common. 

Like the raw mill and finish mill applications, the infeed device’s role here is as an airlock.  A weigh belt system controls the feed rate.  In most cases, coal mill feeders are smaller than raw mill or finish mill feeders with capacities of 10-100 tons per hour.

The #1 issue for coal mill operators is stickiness or coal build-up on all the material handling equipment, including the mill feeder.  Raw coal, which is usually 3-4” inches or smaller, can have inherent moisture in it.  If the plant receives its coal in open-top trucks or rail cars, surface moisture can accumulate.  Surface moisture becomes even more problematic if the raw coal is stored outdoors.

Both conventional rotary feeders and especially flap-gates, struggle with handling raw coal.  Maintenance and operations team members often spend hours trying to remove build-up to keep the system feeding and to maintain rate. 

Precision has developed the PMDS mechanical self-cleaning rotary valve specifically for the coal mill feeding application. 

Alternative fuels

Cement plants have been substituting alternative fuels in place of coal for years.  Liquid fuels and natural gas are outside the scope of this post.  Solid fuels – wood, tire shreds/chunks, carpet, plastics, nutshells, etc. – are important fuel sources for cement plants, either year-round or in-season.

Two types of alternative fuel systems are most common:

  • Pneumatic conveying of fuel for direct injection into the kiln main burner
  • Gravity-fed fuel dropping into the calciner

In both cases, a rotary feeder is an integral part of the system.  In most installations, the feeder functions as an airlock.  Feed rates are usually less than 25 tons/hour, often much less.

Dilute-phase pneumatic conveying systems operating at 9-10 psi or less require a rotary feeder to introduce the alternative fuel into the conveying line.  The material will be at ambient temperature.

Rotary feeders in gravity-fed fuel systems segregate the negative pressure in the calciner from the atmospheric pressure outside to maintain balance in the calciner.  The negative pressure is usually relatively modest.  However, the potential for the system to “go positive” and push high-temperature gas back up into the rotary feeder means that the rotor-to-barrel clearance inside the rotary feeder must allow for a thermal shock.  This increased clearance acts in opposition to what would otherwise be a tight clearance, low-leakage feeder.

A third rotary feeder functionality, mentioned in the introduction of this post, comes into play in alternative fuel applications.

Alternative fuels have a diverse range of caloric contents, bulk densities, and sizes.  Plant control systems can compensate for variations in the first two.  Variations in material size usually require a knife-style rotary feeder, like the Precision PMR Rotary Feeder, to minimize feeding disruptions.

Knife-style rotary feeders shear off oversized solid fuel pieces that would otherwise jam-up a rotary feeder and interrupt fuel flow.  This ability means that the plant can utilize minimally-prepared fuels that are less costly than other heavily pre-processed fuels.

Material Movement & Control: Pneumatic Conveying

The alternative fuels section of this post discussed one pneumatic conveying application, and there are numerous others.  For most pneumatic conveying applications, the rotary valve or rotary feeder will function as an airlock, but some applications may demand both airlock and metering functionality.

Properly designed pneumatic conveying systems are balanced and require the consistent introduction of the material into the air stream.  A rotary valve or feeder with excessive internal clearance, either when new or after use, will reduce transport efficiency.  Excessive internal clearance leads to “blow-by,” where the conveying air passes through the valve or feeder and out of the conveying line. 

Loss of efficiency will cause the blower to consume more power and material transport rates to fall below target. The conveying air that passes through a wide-clearance valve or feeder also can disrupt the material being fed. This disruption can manifest itself as a fluctuating feed rate – material, particularly something light, may not drop into the valve or feeder at the designed or assumed rate. A further issue is the possibility of “dusting” when the leakage through the valve or feeder is so great that material is blown out of the top of the vessel.

Proper sizing and matching of the valve or feeder and the pneumatic line injector (a “T-injector” in Precision’s parlance) is critical to proper feeding in a pneumatic conveying application.  A very common concern is the operating pressure of the pneumatic system – higher pressure will wear out all the conveying system components, including a valve or feeder, more quickly than a low-pressure system.

Excessive or increasing blow-by is the issue in pneumatic system feeding devices.  A well-built rotary valve or feeder is preferred over a flap-gate in the cement industry.  Tight initial clearance and resistance to clearance-widening wear in a rotary valve assure a lower level of blow-by than a flap-gate. 

Material Movement & Control: Silo Discharge

Common in storage silos or silo “farms” that hold raw meal, additives, finished cement, or a pulverized fuel, rotary valves in silo discharge applications are usually a metering device.  The valve may also serve as an airlock function when a silo is discharging into a pneumatic line or air-slide. As the silo location may be in a less-traveled area, durability and reliability in the rotary valve are essential – it may take time for personnel to notice if a valve isn’t working or is leaking. 

As a metering device, the proper sizing of a silo discharge rotary valve is crucial. It is important to consider rotor pocket fill and the possibility for the material to bridge in the silo cone.

Silo discharge applications often require large capacity rotary valves.  Units sized for 5 to 8 CFR (cubic feet per revolution) are reasonably common, which would produce a transfer rate of more than 7000 cubic feet per hour.  Depending on the bulk density of the material in the silo, this could equate to 250 to 300 tons per hour.  Very high throughput silo discharge applications dictate a robust, long-lived rotary valve.

Rotary valves, such as the Precision PMV Modular Rotary Valve, outperform flap-gates in this application, can be quickly activated and, with an inverter-duty motor, ramped up or down to meet required transfer rates. 

Material Movement & Control: Pulverized Coal

The most common method of dosing pulverized coal or pet coke to the kiln main burner is to use a Pfister DRW rotor weighfeeder. A rotary feeder is often installed above the DRW to meter fuel, allowing the Pfister unit to be neither starved nor flooded.  For accurate weighfeeding, the DRW must have a full-head of fuel available to it at all times.

The Precision PMV and other rotary valves in this application have to work without fail and operate over long operating cycles.  Failure of the rotary valve would take down the DRW and the kiln.  Durability is the key in this application, making rotary valves with upgraded materials of construction highly recommended.  The successful operation also calls for an inverter-duty motor in the drive package – the same signal relayed to the DRW for more or less fuel controls the rotor speed of the valve.

Material Movement & Control: DSI/FGD Systems

DSI (Dry Sorbent Injection) systems, which are sometimes known as FGD (Flue Gas Desulphurization) systems, are a specialized subset of pneumatic conveying.  These systems treat flue gas to remove SOx and other contaminants.  The most common sorbent is hydrated lime, but other sorbents, including trona, activated carbon, or some types of ash, are used.

The rotary valve functions as an airlock only and the metering function is handled by a screwfeeder or weighbelt.  A few systems rely on the rotary valve to be both an airlock and a metering device.

This is another application where the rotary valve has to be available to run at all times. The treatment of flue gas is so vital that DSI systems are often set-up as a so-called “dual train” system with duplicate blowers, rotary valves, and metering devices. These pull from the same sorbent silo and discharge into the same pneumatic line that feeds the lances in the flue.

As with other pneumatic conveying systems, excessive blow-by can be the major challenge in DSI systems.  Flue gas treatment systems rely on a precise ratio of sorbent to gas.  If some of the sorbent is not reaching the lances or nozzles in the flue, the system won’t function properly.  Hydrated lime presents unique problems due to its tendency to react chemically if over-heated and its tendency to build-up on metal surfaces if not kept very dry.

Tight-clearance rotary valves, such as the Precision PMV Modular Rotary Valve, are very desirable for DSI applications and are often equipped with optional upgrades to combat the issues with sorbents.  These options include air purges, beveled rotor vanes, and zero speed sensors.  Rotary valves for this application are generally small with capacities under 0.35 CFR (cubic feet per revolution.)

Dust collection

In many industries, rotary valves in dust collection systems can run for years and years with little attention as the products are mildly-abrasive, the loading is generally light, and there is minimal pressure drop.  To some extent, these characteristics are similar in the cement industry except that products can be abrasive, particularly CKD (kiln dust), and at times at an elevated temperature.

Precision PMV Rotary Valve

While this application may be somewhat less demanding than others in a cement plant, it can be quite problematic if the rotary valves in a dust collector aren’t working.  Housekeeping and clean-up can consume maintenance or operations man-hours if the valves aren’t working correctly or appropriately sized.

Rotary valves in dust collection systems are airlocks as they are discharging to atmospheric pressure – usually to a screw conveyor or belt conveyor.  The valves typically are relatively small with capacities generally less than 0.65 CFR; roughly a 12” valve. 

Rotary Feeder & Valve Selection Guide

There are numerous applications for rotary feeders and valves in cement manufacturing.  The raw material and inputs, the existing process equipment, and the finished products at each plant vary considerably.  So, it is difficult to apply hard-and-fast rules to selecting the best rotary valve or feeder for your application.  However, here are some broad guidelines.

1. Large & Slow vs. Small & Fast

This is primarily a philosophical choice about costs.  Smaller equipment running at a higher rate of speed usually requires a lower initial investment.  However, the high speed will cause faster wear than a larger piece of equipment running more slowly.  As a result, rebuilding or replacement costs will be incurred on a shorter cycle and measured over a medium-term horizon, overall equipment investment will be higher.  Higher maintenance costs, for bearings, seals, and other components, also tend to correlate to operating speed – further accentuating the expenses associated with the Small & Fast choice.

At Precision, our experience leads us to believe that valves and feeders will last longer and provide an improved total-cost-of-ownership if operated at 20 rpm or less and sized accordingly.  We also have confidence that slower speeds and larger equipment will improve feeding performance.  At higher speeds and with small diameter rotors, consistently filling the rotor pockets is difficult in Small & Fast.

Other valve and feeder manufacturers and some system design firms don’t subscribe to the Large & Slow philosophy as the better choice.  Our company’s Mission Statement states, “We provide solutions that deliver an exceptional return-on-investment…” and we are convinced that Large & Slow is a key to stronger ROI.

2. Pocket fill assumption

The hardest-to-know parameter in rotary valve or feeder sizing is pocket fill.  For example, a dry, flowable material, coming out of a well-designed silo, with a consistently full-head of material in the silo, and into a valve with a sufficiently large inlet should fill the rotor pockets to at least 90%.  Lots of qualifiers in that description, no?

Understanding all the details of the application is vitally important.  Material moisture content can influence the fill assumption as can the tendency of the material to bridge.  Particle size, specifically maximum dimensions, will have a significant impact.

Relatively small differences in pocket fill assumption can have an outsized impact on valve and feeder sizing.  For example, in alternative fuels, changing the pocket fill assumption from 50% to 30% will push the feeder up at least one size larger or possibly two sizes.  This change will impact equipment cost, the horsepower requirement, and the installation envelope.

Sizing for airlock-only applications is somewhat easier than for a combined airlock and metering device situation.  In airlock-only apps, the sizing math is concerned with only the maximum anticipated throughput.  Metering applications must consider minimum, maximum, and typical throughput rates and the turn-down that is achievable with the drive package.

In the cement industry, it is relatively common to run sizing calculations using the assumption of 30% pocket fill. This assumption provides for lots of growth in throughput in the future and provides reasonable assurance that the valve or feeder won’t become a bottleneck.

One final subtlety regarding pocket fill assumptions in cement bears mentioning.  Capacities are often provided to Precision in terms like “250 tons per hour.”  This sort of hourly rate is critical to proper sizing, but it does not describe the mini-surges or pulses that can develop in operation.  Plant control systems frequently maintain feed rates within tight upper and lower bounds, but the potential for very short-term surges in rates are a reason not to assume a high-percentage pocket fill.

3. Tight internal clearances

For many cement applications, particularly those where the sole function of the valve or feeder is as an airlock, the temptation is to assume that the tighter the internal clearances, the better.  Up to a point, that is useful guidance. 

Tighter clearances do reduce blow-by.  For example, a Precision PMV-12 with a rotor-to-barrel clearance of 0.005” has 44% less blow-by than an identical valve with 0.010” rotor-to-barrel clearance, reducing blow-by from 108 CFM to 61 CFM.

There are two caveats to consider regarding internal clearances:

  • It is imperative to understand the material temperature.  With elevated material temperatures, the valve will have to be manufactured with greater internal clearance to account for the thermal expansion of the valve components in operation.
  • A well-made valve using wear-resistant steel will maintain its close-clearance longer and minimize blow-by better over time.  Consistent low-blowby airlock performance under heavy use is a very desirable trait.  Evaluating valves based on the tightest internal clearance when new misses an important factor.

Tight internal clearance (rotor-to-barrel) in rotary valves and feeders will reduce the pathways for blow-by or false air to migrate.  Minimizing airflow is, by definition, the valve or feeder’s job as an airlock and will result in improved material handling and transport.

4. Serviceability and Maintenance Frequency

Rotary valves and feeders, once purchased and installed, become the responsibility of the plant maintenance team.  Most cement industry insiders would agree that maintenance is the most overloaded department.  Troublesome, maintenance-intensive equipment is something that they could happily do without.

Some rotary feeders and valves, by nature of their design, require frequent maintenance “intervention” to operate correctly. 

Chief among these design features are such things as replaceable rotor tips, adjustable rotor tips, and feeder inlet flaps.  Each has a certain theoretical appeal and can have a beneficial impact on performance when new. However, the downsides of frequent maintenance attention and the difficulty of even accessing these components to adjust them or change them out seem to us to strongly outweigh any benefits.

5. Up-Armoring

Heavy steel body panel inserts, bulletproof glass, and other “Up-Armoring” features increase occupant protection in limousines used by presidents, prime ministers, and other dignitaries. The same concept protects rotary valves and rotary feeders against abrasive wear.

Up-Armoring has its place in material handling equipment.  However, too often, the Up-Armoring lacks a coordinated, thoughtful design.  For example, Stellite-tipped rotors sound like a robust, durable feature.  However, if that Stellite-tipped rotor is running inside an ordinary cast-iron valve housing, the gain in durability will be illusory or non-existent.

Up-Armoring should be one part of a product plan that includes excellent manufacturing workmanship, proper sizing, consideration of system design, and site-specific factors.  Otherwise, it is just slapping expensive materials onto a valve or feeder for marketing purposes.

Conclusion

The kilns, the mills, and the control systems receive much attention in cement manufacturing.  Rightfully so, as they are the keys to producing a saleable product. We’ve described some of the considerations and complications in several feeding applications that, if not done correctly, have the potential to bring cement manufacturing to a halt, significantly disrupt production, or needlessly consume energy, fuel, and manpower. 

Precision’s strongly-held position is that rotary valves and feeders cost-effectively address all of the applications discussed here.  If you’d like to learn more about our solutions, please contact us.  Precision is focused on solving the toughest cement industry’s material handling challenges. We’d be delighted to help you overcome the material handling challenges in your plant.

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