Pipe bends are vital components in pellet pneumatic conveying systems that transport cereals, nuts, dry beans, wood chips, plastics, pet food and propellants. They appear to be a simple variation of the pipe. The curvature they provide allows piping to be routed around obstacles or height changes. The significance of bends, however, is not always properly considered and the difficulty of choosing the ideal bends for the system can be underestimated.
It may come as a surprise that pipe bends have a diverse structure and include pipe elbows (read up on the difference between pipe bends and elbows). How much the pipe bends curve, how they are designed and what materials they are made from can have a major impact on overall system performance and maintainability.
Did you know pipe bends in pneumatic conveying pipework for pellets are of utmost importance?
Pipe bends allow flexibility in the routing of the conveying pipework. Compared to straight pipe, however, the life of a pipe bend is variable. Some bends may need to be looked at each week or every few weeks; others might only need occasional care, and others require almost none. Why is this so? The main reasons are the pressure drop in the bend, transfer speed of the pellets and level of erosion.
As pellets traverse a bend, a 'pressure drop' occurs from a resistance to the change in the flow direction. Pellets striking each other and the pipe wall cause friction and generate energy losses. Such a pressure drop, however, will increase the pellet speed. The larger the drop the greater the increase, and pellet speed is a major factor.
Pellet speed has a massive effect on bend longevity. If speed is reduced by too much, the pellets will cease to move and the piping will block. Conversely, if the speed is too great, the internal surface of the bends will be eroded from increased friction. Ideally, therefore, there needs to be an equilibrium sort to achieve optimal bend life.
Pipe bend interiors erode from the abrasive grinding effect of pellets as they move along the curvature and change direction. Precisely where in the bend, and what level of erosion will take place, is contingent upon a number for factors, such as pellet composition, pipe material and pipe design. The level of erosion can be forecast from these factors and several other lesser factors specific to the system configuration.
With many kinds of bends, each offering has benefits and drawbacks. There is no ‘one size fits all’ solution for given pellet pneumatic conveying systems.
The curvature of a pipe bend is defined by the diameter of the circle that fits the bend divided by the radius of the pipe, ergo, a 3D bend is one where the curvature radius equates to three times pipe diameter. A bend under 1.5D equates most closely to a standard 90-degree elbow, whereas a larger bend, e.g. a 5D, equates to a point-to-point smooth bow connecting two sections.
The amount of curvature affects the level of friction, change in transfer speed, impedance in the flow and required length of pipework when conveying pellets. The lengthiest radius bend has little effect on impedance as the curvature is minimised; the length of required pipework, however, is increased due to the smaller rate of curvature per length of piping.
Testing of bends utilised in pellet transporting indicated that to achieve an equilibrium between lower pressure, wear and tear, and the power required for moving the pellets, a 3D bend is the best choice. While bends larger than 3D do not lose as much pressure, the pellets must be conveyed over a longer distance and more power is needed to achieve that.
There are a number of materials that can be utilised in the manufacture of pipe bends. There are also various kinds of inner surface treatments available, all of which have significant bearing on its wear and longevity. Greater longevity requires the use of more expensive materials, so the more economical substances such as common grey iron or carbon steel are commonly chosen. These types of bends are more readily available, but are likely to last only half as long as the costlier materials. The selection of material used to make the pipe bend certainly affects the long-term operating cost. The more costly bends may prove cheaper over the lifetime of the system given that the cheaper bends are likely to cause twice as much downtime.
The particular way a bend is made can make a big difference to durability, keeping the bend from eroding. An instance of this is the way Rhino Bends are made. These are designed to reduce speed in the outermost pellets by using an extra large radius, but without adding length to the radius. This creates a layer of pellets that forms a buffer over the inside of the bend and minimises its wearing.
Rhino Bends permit curving differentials in pipework to allow for turns and sharp corners, and fit into compact areas. Long radius bends greatly add to the aggregated distance of the entire piping system. This allows Rhino Bends to achieve huge reductions in the overall cost of the installation, and provide greater accessibility for service crews. In comparison to long radius bends, they offer great benefits such as:
Read on more about Rhino Bends.
Design options bend for pellet pneumatic conveying
Pipe bends need to be correctly selected as their final choice heavily impacts on performance and operating costs. Spending too much on bends for the sake of over-engineering may add unnecessary cost. Choosing the inappropriate pipe bends design can lead to problems like fines, angel hair and the development of streamers. Pipe bends are vital components of the design of pellet conveying systems.
An industry leader in the design and manufacture of bespoke pneumatic conveying systems, Pneuvay is ideally placed to help you with your pellet transfer requirements. Our clients include many large, major manufacturers that have turned to us for the design and successful provision of best fit and turnkey solutions for:
If you need help with pellet pneumatic conveying systems or assistance is solving pellet pneumatic conveying system problems, feel free to call on 1300721458 or contact us. You can even send us a message via our Facebook page if you like.
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