Selecting specialty elbows
Components. Service. Solutions.
How to Select Specialty Pneumatic Conveying Elbows
Wear is a common problem in pneumatic systems; it can cost an extraordinary amount of resources in labor and replacement costs. While there are simple ways to select good elbows for a given system, sometimes there is a need to select a specialty solution.
Throughout the years of pneumatic conveying, many people have developed various ways of handling the complex problem of wear. Just as there is an abundance of different conveying applications, there are also many different solutions. Each solution is geared toward solving a particular problem, while remaining within the boundaries of a set of important constraints. Obviously, the most common constraint is replacement cost, but there are also other critical factors, such as elbow material composition, corrosion resistance, food contact concerns, ease of replacement, and system space. Therefore, given all of these considerations – in addition to abrasion resistance – a review of specialty products and techniques is in order.
The first consideration when choosing a replacement elbow is typically cost. However, due to the labor required in replacing elbows, ease of replacement often goes hand in hand with cost consideration. Therefore, the most popular solutions are ones that fit into an existing system, just like the original sweep elbow. Below are some of these solutions.
Abrasion Resistance Elbows to Replace and Improve Wear
Taper Bored Elbows
The first solution people often consider in order to reduce wear is the use of thicker walled elbows. If you are currently using a thin-walled tube and moving to a wall that is only marginally thicker, you typically won’t experience a problem. However, if you are using a wall like SCH10 pipe before moving to a thicker wall, such as SCH80 pipe, then the difference between the materials can create a ledge where the two materials are joined. While this isn’t a problem in some applications, it may be undesirable in others. A ledge can cause problems such as product buildup or air turbulence. The solution to this issue is to use a thick-walled elbow, with the ends taper bored down, to connect with the other pipe material exactly, thereby eliminating the ledge. A taper bore slowly feathers the thickness of the elbow’s tangent material in the first few inches of the elbow. It allows a thick-walled elbow to smoothly transition down to a thickness level where a ledge will not occur.
Induction Hardened Elbows
Induction hardened elbows are sweep elbows bent from a special hardened pipe. About half of the inside wall is hardened for abrasion resistance, while the outside of the wall is left mild for bending. The hardened inside wall is about 600 Brinell to protect from wear. Mild steel elbows rate at 125 Brinell, so the induction hardened steel is several times harder than that of the standard elbow.
Square Elbow or Flat-Back Elbow
This is a square elbow made from sheet metal sides which use fabricated transitions that move from a round to square, and then back to round again. These elbows can be made on any centerline radius and made to replace a standard sweep elbow. Frequently, they are made with a replaceable metal back. Square elbows can work very effectively for mildly abrasive applications. There are also variations of these elbows which contain ceramic tiles inside – sometimes replaceable – that can be used for more intensely abrasive problems. However, one problem exists with this design – the transitions, which are the weak point. Because of the turbulence created, they can experience a lot of wear. Therefore, transitions on square elbows are often replaceable or reinforced against wear.
This is a regular sweep elbow with sides welded on and a replaceable plate bolted onto the back. It uses a similar “wear and replace” philosophy as the square elbow. Again, these can work great for mild abrasion problems. These elbows can be enhanced by using thicker plates on the back of the elbow or using AR steel material. This elbow doesn’t use transitions, so wearing them out is not an issue. However, they can wear out the sides before the top, compromising the structural integrity of the elbow. Furthermore, when the top of the elbow wears away, there may be some open-pocket areas between the round elbow and the square back, which may create an opportunity for materials to pack into that area. For this reason, food products and lines that convey multiple colors that contaminate should be avoided.
A hollow-back elbow is exactly as it sounds, with either an empty square box or an open-rounded channel welded onto the back of a sweep elbow. Once material wears through the elbow, the cavity can fill up with the product and protect against abrasion by wearing against itself, which can work really well with extremely abrasive materials that tend to pack. How effectively the material packs into the cavity depends upon the characteristics of the material, the velocity of the system, and the position of the elbow. Once again, because of the pocket area of the elbow that holds material, food and materials that can spoil or contaminate should be avoided.
A cement-back elbow is constructed like a Hollow-back elbow, except the cavity is filled with cement. For obvious reasons, these elbows are most popular within the cement industry. While cement is not the hardest of substances, it can still be quite effective in heavy industrial applications.
Porcelain Lined Elbow (Ceramic Lined)
While these are sometimes marketed as ceramic lined elbows, they are not. Porcelain Lined elbows have an industrial porcelain lining fired into the interior of the elbow. This lining is only several thousandths thick and protects by lowering the coefficient of friction inside the elbows, allowing the product to slide inside them. Additionally, industrial porcelain does possess significant abrasion resistance due to its hardness – a 6 on the Mohs scale – which is harder than mild steel. Porcelain can offer some measure of corrosion protection, however all provided protection is very thin and susceptible to cracking with hard impact. There can also be pinholes and imperfections in the kilning process and because of this, they are not considered to be a food grade solution.
The Ceram-Back® elbow is a sweep elbow with ½-in. of hard ceramic compound on the outside of the elbow. The idea behind this design is that the metal of the elbow is left to wear out, which will then transfer the abrasion to the much harder ceramic material. The advantage of these elbows is they offer ceramic protection to any size or centerline radius of elbow without mitering – something difficult for ceramic tile lining. Additionally, the Ceram-Back® elbow only offers protection where it’s needed – the back and sides of the elbow. They offer excellent protection for sliding abrasion. Ceram-Back® elbows can also be used in conjunction with other techniques, such as taper boring, induction hardening, porcelain lining, and tungsten carbide lining. However, the downside is that they are not as effective as ceramic tiles for protecting against large particle or heavy impact problems. Plus, they also have some heat limitations.
Ceramic and Basalt-Lined Elbows
Ceramic and basalt-lined elbows offer superior protection for almost any application. They’re inherently impact resistant and are offered in high-heat and corrosion resistant variations. The tiles used to line the elbows range from thinner protection – such as ½ in. – to bricks that are several inches thick. However, they can be difficult to get in smaller sizes and longer centerline radii. Additionally, it is not unusual for these elbows to require special oversized flanges in order to fit into a system, due to the unusual size of the outer diameter of the elbow. Ceramic-lined products also tend to have longer lead times and are usually the most expensive of all the solutions.
Cast Ceramic Flat-Back Elbow
This is an elbow with a box back on a sweep elbow – like a cement-back elbow. Instead of being filled with cement, it is filled with solid cast ceramic tiles, which provides heavy resistance to wear and impact. It can be easily patched on the outside, if needed, and is much cheaper than a ceramic tile lined elbow. Furthermore, unlike a ceramic lined box elbow, it doesn’t have transitions that wear out or cause turbulence. It is also available in most sizes, in any degree, or in any centerline radius.
The Cast-Back Elbow offers the protection of a Cast Ceramic Flat-Back Elbow, but with a standard thicker tile and built-in side protection. This elbow is perfect for course, abrasive products with high impact.
Solutions to Small Spaces: Short Radius Elbows
Sometimes space is a huge factor. For those who have systems that are short on space – or for those who don’t mind refitting their system – there are specialty elbows that are offered only in a short radius. Here are a few of these solutions:
Tungsten Carbide Clad Elbows
Tungsten carbide is an ultra-hard, abrasion-resistant metal coating applied to the inside of elbows. It contains excellent wear properties, particularly against sliding abrasion. However, due to the way it is applied, the coating is usually fairly thin and limited to shorter radii elbows. It should also be noted that tungsten carbide usually offers some corrosion protection.
Dead-End Tee (Blind Tee)
These elbows mechanically protect against abrasion by filling one end with material that protects from wear by having the product wear against itself. These tees can work very well in many different situations, however they should not be used if the product is moist, cohesive, or sticky. Dead-end tees are available in mild steel, abrasion resistant materials, and can also be ceramic coated.
These are short radius elbows that are cast out of Ni-Hard alloy (around 600 Brinell). They are usually thick-walled, very tough, and good for high heat applications. Some of the elbows even have replaceable back designs. That said, they are fittings, so they are only available in short radii and are limited in size and degree. Additionally, while Ni-Hard is tough, it is not nearly as tough as harder materials, such as ceramic tile.
Vortex elbows, such as the Progressive Products Aero-Back™, protect against abrasion by mechanical means. They have a chamber in the back of the elbow which causes the material to spin into a vortex. This vortex deflects material and causes the material to move through the elbow in a way that reduces wear. This style of elbow is available in different materials and alloys, and some of these cast materials offer extra abrasion or corrosion resistance. There are, however, some definite limitations. There is a minimum amount of line pressure and product flow required in order for the vortex action to work properly. And, because they are fittings, they are limited to certain sizes, configurations, and degrees.
As this brief overview shows, many different solutions are available to solve wear in pneumatic conveying systems. Some are direct replacements, or fit in small spaces, and some are not. Some work well with high heat or corrosion, and some do not. There is not one solution that solves all problems. In pneumatic conveying, the best solution is always the one that both fits the import criteria of a particular system and offers the best overall value for the customer.