Short radius elbows have many advantages. They are readily available, less expensive, lighter, easy to install, easy to replace, and take up a lot less space. For some applications, they can be the perfect elbow. However, they do have some major disadvantages, too. Short radius elbows – which have a severe angle of impact – take a lot of punishment in one spot just off the centerline of the elbow, making them more likely to blow out if the product is abrasive. Additionally, the greater impact in short radius elbows often contributes to greater product degradation.
Long radius elbows, by comparison, are more difficult to handle but offer some innate wear protection. By having a shallower angle of impact – and allowing product to drag all the way along the back of the elbow – impact is reduced and wear spread more evenly down the back of the elbow. Product degradation is also reduced when compared to a short radius elbows. (The exception to this rule are products that smear and/or create streamers, like polyethylene pellets. These applications have a worse time with degradation because of the amount of time they spend dragging the back of the elbow. When this happens, an elbow with a grooved/surfaced interior or a fitting/specialty elbow is used). As a final consideration, some products could bounce and cause multiple wear points in a long radius elbow. This problem is rare in dilute phase conveying. When it does happen, it can be avoided by using a short radius or impact elbow, but it then usually results in the bounce still occurring and wearing out pipe on the outlet side of the elbow instead of the elbow itself.
Impact elbows are the dead-end tee (blind tee) and the vortex style elbow, both of which work well to minimize abrasion and product degradation. Moreover, the dead-end tee also has the extra advantages of being readily available and inexpensive. However, neither elbow should be used if the products are moist, cohesive, or sticky. Also – and we should acknowledge that there is conflicting data and opinions on this point – using these elbows may have an adverse effect on system pressure and conveying efficiency in some situations. While this is not a reason to avoid impact elbows, use caution before changing out a bunch of radius elbows for impact elbows in an existing system.