By: Heinz Feibert, Product Manager
Charles Ross & Son Company

We normally think of wear as a critical threat in high-speed, high-energy applications like dispersion. With shafts turning at 3,600 rpm and rotor tip speeds exceeding 7,000 fpm, abrasive materials can erase a steel edge in no time. But abrasive wear can also be a serious concern in slow-speed blending. Over the course of only a year or two of steady operation, abrasive powders can leave you with paper-thin ribbons and a shaft ready for the scrap pile – if the blender has not been designed specifically to withstand abrasion.

Left unchecked, abrasive solids will do much more damage than simply wear down ribbons and a blending screw. They will tear apart seals and stuffing boxes. Once particles have invaded the stuffing box, they grind away the shaft with an attack that becomes more ferocious with every revolution, as eroded area on the shaft spreads and deepens, producing more and more friction.

Maintenance becomes more frequent. Downtime increases. Overall blending efficiency declines. (In many applications, agitator cross-section and wall-to-agitator tolerances may degrade to a point that efficiency during the blending cycle is eventually degraded as well. In other cases, these tolerances are more forgiving and the cost of premature wear is mainly increased labor, stuffing box and shaft replacements, and a long-term loss in the value of the blender.)

The abrasive action we see in a blender is essentially the same as the abrasion that occurs in a high-speed mixer, except that in a blender the action takes place in slow motion. Most of the engineering strategies, technologies, and fabrication techniques that work in high-speed applications are also effective in slow-speed blending. By borrowing against our experience in high-speed applications, we can significantly extend the working life and long-term value of slow-speed blenders.

Strong engineering - based on smart business decisions.

There are many strategies in the design engineer’s play book for preventing excessive wear. But the best strategy for each application is not just an engineering problem. The correct solution strikes a perfect balance between long-term blender performance and cost. In other words, between the nuts and bolts of engineering and the dollars and cents of business.

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The questions you should ask in the beginning of the specification process reveal the relationship between engineering and business in blender design:

How abrasive are the materials you are blending?
Are they mildly abrasive, irregular crystalline solids, like sugar or salt? Or are they extremely abrasive, like limestone, tungsten carbide, or ceramic powders? How quickly will they grind down an ordinary blender?

How valuable is the product you will be blending?
High-value blends warrant a high-value blender, because the product will pay for the blender in a reasonable period of time. For lower-value blends, the decision is more difficult.

How long will you be blending this product?
A well-established product can justify a well-equipped blender, since you can confidently expect to operate the blender long enough to recoup those up-front costs ? and wind up ahead. But be prepared to adjust your focus as conditions change. For a pilot project involving an unproven product, you might spec out the blender conservatively, knowing that wear may be severe over a short period; but when the product has taken off and the time has come for scale-up, build the blender for the long haul.

These are questions that an experienced engineer asks. And this is the first place where an equipment buyer and his design engineer must think alike. Buyers are often tight-lipped about the nature of their application, because they are concerned about secrecy. This is often a great mistake, because it prevents the design engineer from recognizing the business context in which the blender will operate. Without a view of the whole landscape, he cannot contribute the full value of his experience.

Abrasion: Accept it or prevent it.
There are two ways to approach wear caused by abrasive solids: to recognize that it is inevitable, or to try to prevent it.

Many engineers believe that abrasive wear is as inevitable as taxes, or that prevention - using sophisticated coatings, exotic steels and composites - is generally prohibitively expensive.

Others believe that wear is just another engineering challenge to overcome. To them, the engineer’s task is to select the right technology to meet that challenge head on.

The best approach combines these two perspectives and reserves plenty of flexibility in the design process.

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Plan #1: Go with the flow.
The simplest strategy, for an application that will not allow much additional cost devoted to wear-prevention, is to expect it. In a ribbon blender, for example, we would specify carbon steel for the ribbon and trough in a gauge that is heavier (thicker) than usual. This simply provides more steel, allows for loss due to erosion, and extends the working life of the agitators and trough.

Specifying heavier-gauge steel is also appropriate for a vertical cone screw blender, but only in cases of extreme abrasiveness. The blending action in a cone screw blender is more gentle than in a ribbon blender (see the discussion below, “Gentle blending”), so the tumbling solids generate less friction.

Specifying the ribbon blender’s stuffing box is also straightforward. Although the common stuffing box is widely dismissed as old-fashioned technology, most stuffing box problems are actually caused by poor alignment, not by a deficiency in the basic design. When the misaligned stuffing box begins to leak, the operator typically responds by over-tightening it, which makes the situation even worse for the shaft and the stuffing box! For most ribbon blender applications, a basic stuffing box works well – provided that it is installed properly.

Occasionally we are asked to consider a mechanical seal, but they are seldom needed on a ribbon blender. A mechanical seal in a blender adds substantial cost in exchange for a doubtful return on the investment. Maintenance on a stuffing box is simpler, less time-consuming, and less costly. Even when the blender is equipped with vacuum ? for fast vacuum drying, for example ? a properly specified stuffing box is usually adequate.

Plan #2: Slow down!
Another common-sense strategy to reduce wear is to slow the blending process. By slowing the blender – especially a ribbon blender – we can substantially reduce wear, because we can greatly reduce the friction applied to the agitators and the vessel.
As appealing as this easy solution may seem, it has serious limitations. By slowing the blending action, we are also lengthening the blending cycle and cutting throughput. In applications where throughput is not a critical concern, this poses no problem. But in most applications these days, a reduction in throughput is unacceptable.

Ribbon blenders should not be slowed by much more than 5-15% anyway, because the blender is limited by a critical threshold in efficiency. (The precise threshold varies considerably from application to application, because every product behaves uniquely. The threshold must be determined on a case-by-case basis in the manufacturer’s laboratory prior to purchasing the blender.) A 5 cu.ft. ribbon blender operating at 70 rpm might be slowed to approximately 65 rpm, and a 155 cu.ft. blender operating at 24 rpm might be slowed to 20-22 rpm without threatening blender performance. Below that level, the fluid characteristics of flow in the ribbon blender drop off abruptly and proper blending is impossible.

One way to overcome this speed threshold in a ribbon blender is to switch to an another type of agitator. A paddle blender will suffer less abrasive wear than the ribbon blender, because it normally operates at only 2/3 the speed of a ribbon blender. The geometry of the paddles may also invite less wear than the ribbon.

Whether you select a ribbon or paddle blender, optimal blending speeds should be verified, using your ingredients, before you purchase the blender. But you will still want to reserve some flexibility to accommodate changes in your process. So, a variable speed drive is an extremely valuable addition to the blender design.

Plan #3: Protect the shaft.
One enhancement we often add to the stuffing box is a Lantern Ring, which provides an air purge and helps to prevent particles from penetrating the seal. With positive 10-15 psi pressure supplied to the gland, solids are effectively excluded and shaft problems are limited.

The next step is to further protect the shaft by adding a hardened sleeve to the shaft where is passes through the stuffing box. (In border-line cases, this can also be applied as an alternative to a step up to an air-purged stuffing box.) The material selected for the sleeve may include such engineered composites as Tungsten Carbide and Stellite

Plan #4: Think vertical.
Keeping abrasive solids out of the ribbon blender’s submerged seal is critical, and even with a Lantern Ring it is sometimes extremely difficult. In addition, some applications will not tolerate the possibility of solids collecting around the stuffing box – posing problems in cleaning and contamination. In cases of extreme abrasiveness, the prospect of accelerated wear on ribbons and trough, even with a heavier gauge of steel specified, may also be unacceptable. For situations such as these, the best answer is to switch from a ribbon blender to a vertical cone screw blender.

One of the essential differences between a ribbon blender and vertical cone screw blender is that the ribbon blender is a horizontal blender that always include a shaft seals submerged in the product zone. Since the drive in the vertical cone screw blender is located above the vessel, there are no submerged seals that contact the product.
For handling abrasives, the cone screw blender should also be equipped with an “unsupported screw.” That is, the drive assembly must be designed to support the screw completely ? while it revolves on its own axis and orbits the cone ? without any support bearing at the lower end of the screw. By eliminating this bearing, we eliminate a notorious source of maintenance problems. We also make complete discharge and cleaning faster and easier, since the lower end of the cone is clear of obstructions.

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More reasons to consider a switch to a vertical cone screw blender?

1. Less aggressive blending, reduced wear.
The cone screw blender is always a more gentle blender, and this can sharply reduce wear. This derives partly from the geometry of the vessel: solids gently lifted by the screw tumble and slide down along the cone wall. The high-energy blending action in a ribbon blender is more violent.

The cone screw blender also presents a lower peripheral speed to the solid being blended. For example, a 14-inch diameter screw in a 500 cu.ft. vertical blender operates with a peripheral speed of 220 surface feet per minute. In a ribbon blender of the same working volume, the 72-inch diameter ribbon turns at a peripheral speed of about 300 surface feet per minute ? almost 50% faster. We don’t need a long equation to tell us that we can reduce the mechanical energy applied to the batch, along with friction and wear, by switching to the vertical cone screw blender.

2. Energy savings.
The vertical blender can also deliver a substantial savings in energy consumed. Consider a cone screw blender and a ribbon blender, each with a working capacity of 100 cu.ft.. In the cone screw blender, the blending screw is driven by a 20 hp motor and the orbital arm by a 2 hp motor. The ribbon blender is driven by a 50 hp motor ? drawing more than twice as much power!

3. Available space.
Finally, the choice between a ribbon or cone screw blender is often made based on available floor space and overhead space. A ribbon blender with a compact drive is still a horizontal blender, and a cone screw blender is a vertical machine, with a much smaller footprint. When space on the floor is tight, the vertical blender provides a welcome alternative when you are looking for an increase in production capacity.

The bottom line: test both!
Before settling on either a ribbon blender or a vertical cone screw blender, be sure to test both in a controlled laboratory setting, using your own ingredients. Surprises often come to light in the lab, and when they do, you are the beneficiary.

Plan #5: Upgrade to wear-resistant steel.
Be prepared to discuss steel grades with the design engineer. An upgrade in steel, to provide greater abrasion resistance on the wearing surfaces, can make an enormous difference in the durability of your blender. It will also increase your up-front cost, but perhaps by less than you might expect. In some cases, the added hardness and strength of the steel in a higher-quality plate will allow a reduction in gauge (thickness).
You don’t have to become an expert in steel plate. But you must at least be aware of the gauge and quality of steel specified in the blenders you are considering. Two blenders that appear comparable may actually be quite different in terms of the steel specified for each of them – and in terms of the lifespan you can reasonably expect. If you are thinking of buying a used or refurbished blender for an application involving abrasive solids, never buy a unit unless you know for sure what kind of steel you are buying. Otherwise, you may watch your investment wear away overnight.
From the manufacturer’s perspective, the choice of abrasion-resistant steel is not a perfect solution, because many varieties of abrasion-resistant steel are brittle and hard to handle during fabrication ? driving up costs. The manufacturer must balance the value of wear resistance against the need for a steel that will allow efficient, high-quality fabrication.

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Plan #6: Wear-resistant coatings.
Technology available today offers many options for applying hard coatings to steel surfaces. Tungsten carbide has been around for years, and it remains a strong shield against abrasive wear.

Stellite is another effective coating to consider, and it represents a family of cobalt/chromium/tungsten-based alloys with remarkable properties. With many grades to choose from, they offer excellent hardness, smoothness and uniformity. They are extremely resistant to wear, galling, corrosion and erosion, even at high temperatures.
They also offer a variety of options for deposition. A high velocity spray produces an especially smooth and uniform surface. The coating may also be welded on, using Stellite supplied in the form of welding rod, then ground to a smooth finish.

Except in applications involving extremely abrasive solids, it is generally unwise to coat the entire ribbon, screw or vessel wall. Coatings like tungsten carbide or Stellite are cost-effective when they are concentrated where they are needed most ? on the surfaces that are subjected to intense abrasive action. In a cone screw blender, for example, we would coat only the leading edge of the screw.

Super-hard materials like Stellite and tungsten carbide are also available in pre-formed wear plates, and these can be welded onto critical surfaces. This is worthy of consideration, but they are generally hard to keep clean and for many applications this problem rules them out.

Great engineering starts with great communication.
Although, at first glance, the ribbon and cone screw blenders sold today look much like blenders sold a decade ago, the technology of blender manufacturing is actually changing fast. As demand for greater productivity and versatility rises, equipment manufacturers must respond with blenders that are more capable - and offer greater value - than ever before.

They must blend faster.
They must last longer, even when blending difficult ingredients such as abrasive or corrosive materials. And they must be more versatile. Today, we commonly build blenders that are really hybrids. They are often equipped for multiple functions ? blending slurries, for example, then vacuum drying the product to complete the process.

Reaching this level of productivity requires an engineering process that is equally productive.

1. Find an equipment maker who welcomes two-way communication - who wants to do more than just fill an order.
2. Don’t conceal too many details about the application and the business challenge in front of you. You’ll be surprised to discover how much more a seasoned design engineer can contribute when you don’t keep him (or her) in the dark.
3. Make sure that everyone in the design process thoroughly understands the materials being blended and all the details of the process.
4. When assessing competing engineering proposals, compare design elements meticulously, right down to such particulars as the gauge and grade of all critical materials and the long-term value they represent.
5. Before purchasing a blender, consider testing it in a well-equipped customer service laboratory, using your own ingredients.
6. Don’t presume that either a ribbon blender or a cone screw blender is necessarily the right answer for you until you test them both and weigh short-term costs against long-term value for your application.
7. Once you have narrowed your choice, use the manufacturer’s lab to optimize your blending technique, too. A well-controlled setting is the perfect place to fine-tune all your process parameters prior to installation.

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Steel that stands up to abrasion.
Choosing the right steel for your blender is a crucial step in the design process. To make sure that your blender is tough enough to withstand the continuous attack of abrasive materials, the design engineer will certainly recommend a grade of steel that is specially formulated for high wear resistance.

“Abrasion resistance is mainly a product of enhanced surface hardness,” says Tom Perry, manager, product applications, at Bethlehem Steel, the largest producer of plate steel in the USA. “Steel can be described by its surface hardness and its ‘through-thickness’ hardness ? hardness throughout the entire cross-section.

“Surface hardness is a function of both the steel’s chemical composition and the way it is manufactured,” says Perry. “Increasing concentrations of carbon and manganese generally produce an increase in surface hardness.”

But a blender manufacturer like Ross requires steel with other mechanical properties that are equally important.

“We need steel that can be rolled and welded into ribbons and troughs, screw agitators and conical vessels in our plant,” says Dave Hathaway, Vice President of Operations at Ross’s 75,000 square foot fabrication plant in Savannah, Georgia. “To meet our performance and fabrication standards, we need top quality plate that offers hardness and exceptional formability and weldability.”

The family of alloy steels produced by Bethlehem Steel includes product with a remarkable combination of hardness and formability. At Bethlehem Steel, Q&T (quenched and tempered) plate is heated to about 1,650oF, then roller-quenched with high-pressure water over the entire plate to cool it quickly and develop uniform surface hardness. In the tempering cycle, the plate is heated again, but to a lower temperature than before.

Surface hardness is measured in Brinell Hardness Numbers, or BHN units, and plate used to build blenders for normal applications is generally specified at 200-250 BHN. For abrasive applications, your blender manufacturer should recommend plate with a surface hardness between 300 and 500 BHN, depending on the degree of abrasiveness involved.

Assessing abrasives and wear potential
The threat posed by some “abrasive” materials is obvious. But for others, the threat must be considered carefully, based on: (a) the size and other physical properties of the material itself; (b) the action of the blender in which is will be processed; and (c) changes that will take place during the blending cycle. For instance, a powder blended with a liquid may pose a small threat while the mix remains a slurry. But when the liquid is forced off during the vacuum drying phase and the material becomes a fine, lightweight dust, its potential for abrasive damage may increase sharply. Be sure that all the details of the process are considered while your the blender is being designed.

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