Preventing erosion in pneumatic conveying lines
Q: What are my options to mitigate erosion in my pneumatic conveying lines?
Pneumatic conveying lines come in two types, dilute- and dense-phase conveying. Most conveying lines are dilute-phase, which means the particle concentration is relatively low, but the particle velocities are high and range from 15 to 60 feet per second. Hence, some parts of the conveying line, such as bends and elbows, are prone to erosion. Any point where the conveyed bulk solids have to make an abrupt change in direction can lead to a point of erosion. The points where erosion can occur include long-radius bends, blind tees and bends, and mitered bends.
As solids move through a dilute-phase conveying line, bends and tees are often a source of particle-to-wall impacts. The higher the particle velocity, the higher the force of impact. If the impact force is high enough, erosion can occur. The rate of erosion in a bend depends on the particle velocity to the second or third power, which means a small increase in particle velocity can result in a significant increase in erosion.
What many might find confusing is that not all bends will erode because the particle velocity (speed and direction) isn't always the same for each bend or tee. For example, bends at a straight conveying line's end may be more prone to erosion than at the beginning because of the increasing gas velocity at the end of the line. As gas and solids move through a pneumatic conveying line of the same pipe diameter, the gas velocity is increasing because the pressure is decreasing. With each foot of conveying line, the pressure is lowered because there are frictional effects and particle acceleration. The 20-feet-per-second particle velocity at the beginning of a conveying line can easily increase to 30 feet per second as the material reaches the end of the conveying line. Sometimes the pipe diameter is increased (or stepped) in a long conveying line to better manage particle velocity increases.
Erosion problems can occur when there's a bend too close to another bend. As particles exit one bend, a rope pattern can form that takes time, possibly the entire line length, to dissipate. If a subsequent bend is too close, the concentrated particle stream can promote higher erosion rates in that subsequent bend despite the similar gas velocities at each bend.
Fortunately, there are several bend configurations in a dilute-phase conveying line that can be used to reduce erosion. These bends are radius bends, tees, and specialized bends.
Radius bends are commonly used in pneumatic conveying. However, the bend type used can significantly impact erosion rates. Long sweeping bends with a bend-radius-to-pipe diameter of 20 can have lower erosion rates than an elbow that has a bend-radius-to-pipe diameter of 1 to 2. The long sweeping bends have the most gradual change in direction for the oncoming particles. Hence, the risk of erosion (and particle attrition) can be minimized.
Along the same line of thinking, erosion from a 90-degree elbow can be significantly reduced by replacing it with two 45-degree elbows, which have a smaller angle of impact. Even though one point of impact has now been replaced with two points, the smaller angle of impact has more of an effect on erosion than the number of impacts. If possible, it would be best to space the two 45-degree bends 10 pipe diameters in length from each other to minimize any possibility or roping effects.
If substituting a longer-radius bend or replacing a single bend with multiple bends won't fit into your allotted space or work with your conveying system, you can also use a bend specifically designed to have the point of erosion be a replaceable part. These replaceable-elbow bends have removable backs so, instead of changing the whole bend, you change only part. The part removed can also be made with a more erosion-resistant material, which saves capital costs as the whole bend no longer needs to be made of this more durable material. The drawback is that if the conveyed material is a health or environmental hazard, the system will have to be shut down and completely evacuated to replace the removable component.
Blind tees can also reduce erosion in a bend, although blind tees have a much higher pressure drop. Nevertheless, a blind tee is the most common bend used when erosion is an issue in pneumatic conveying. Before installing a blind tee, make sure your blower can handle the increased pressure load. A blind tee is a tee with one end capped off. Particles build up in the capped off portion of the tee and provide a buffer for the oncoming particles. In other words, the incoming particles impact a loose collection of relatively stagnant particles and not the rigid wall of the tee. This will prevent the erosion the particles would have caused in a different bend type.
Finally, specialized bends have the blind tee's advantages and address its disadvantages, such as higher pressure drops. A specialized bend is designed to have multiple impact zones in one bend or bulbs that induce particle rotation, which can reduce the pressure drop as compared to a blind tee.
If you're experiencing erosion problems with your dilute-phase conveying line, there are several equipment changes you can make to increase the system's reliability. Which one is right depends on your operation type (such as material velocity and pressure drop within the system), potential hazards, environmental requirements, and cost.
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