Bends and Rubber Lining
The use of bends in a designed piping system is critical to managing wear in piping directional changes.
In short the longer you can make the directional transition, the less wear you will ultimately have in your elbow. Induction bent elbows provide engineers and designers an infinite degree of adjustment and allows the elbow to have any sweep dia, limited only by the sweeping arm of the induction bending machine.
Although there are very little limitation of what can be fabricated as an elbow, lining these elbows with rubber can be a challenge.
Here are some general rules of thumb.
Positive: Larger diameters elbows are easier to line as you can reach the inside to stitch and work out possible air pockets.
Negative: Large diameter tubes are very heavy and hard to manage, sometimes difficult not to create wrinkles
Negative: Small diameter long swept bends are challenging as you cannot manually adjust the tube in the swept section. Difficult to stitch in corners.
Negative Small long swept bends can hide bubbles and wrinkles as they can be hidden in the inside radius which is impossible to see with the naked eye.
Positive: Small diameter tubes are generally easy to handle.
Thick liners in small diameter bends are difficult, due to the tube not wanting to turn the corner and kinking when folded in half. The liner has tendencies to create large wrinkles.
New trends is to have thick liners in pipe and elbows. Engineers are using the additional rubber thickness in order to match flow calculations and standardize pipe size. An example of this would be using a 30″ dia. pipe and lining it with 2″rubber. The engineering design recommended 28″ pipe for the flow. But 28″ pipe is not common and 28″ flanges and valves are not commonly stocked and are very expensive. Using 2″ rubber would accomplish the reduction in interior diameter.
Other times the belief is that a thicker liner will make the elbow last longer, this is inaccurate. If using the correct compound for the fluid with particle size and flow taken into consideration than 1/2” rubber sub-straight is ample for best performance.
Lining with 2″ thick material is extremely in-practical. The weight of the tube would exceed the tear strength of the silk required to pull the tube through the elbow. The tube would weight in the area of 700 lbs. The other challenge with such a thick liner, the sag created in the material during the vulcanization process will create an uneven liner. The top of the part would thin and the lower section would increase dramatically. An uneven liner generally created uneven wear.
From a designing perspective, 3D bends should generally have a minimum 1D tangent. Anything less and the end of the pipe will result in an elliptical form. An elliptical end will create challenges with the termination, flanges and grooved ends. This generally doesn’t affect rubber lining but 1D tangents are good design practices.
-Although 6 D bends are great for wear, the most common swept bend ordered are 3D 45’s and 90’s. These are common and stock in most location have a 1D tangent. Even if the site has a 1 deg slope try to standardize all bends to normal standard angles for ease of replacement and interchangeability.
-3D Bends 8″ and below 1/4″ liners
-3D Bends above 8″ 1/2″ Liners.
-3D Bends above 16″ if required 1″ liners are possible
-On cyclone under-flows where the material if very coarse like 3’/4″ ,minus allow for the longest possible Radius such as a 5D or 6D elbow.