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Unit 26
Pipe Bending
Objectives
• Learn when when pipe pipe bending bending is used. used. • State State the methods methods of bending. bending. • Do bending bending calculations calculations.. Every fitting is a possible source for a future leak. Vibration, pressure, corrosion, and chemical breakdown of the fitting materials are causes of fitting failure. In some circumstances pipefitters and plumbers wish to have as few fittings and welds as possible to avoid future sources of leakage. This can be important in heating pipes which are to be buried in concrete, boiler piping inside boiler casings, hydraulic piping, and others. The best solution to these situations is often to substitute pipe bends in place of fittings wherever possible. A pipe bend bend can be visualized visualized as an arc of a circle (see Figure 26-1). When a pipe is bent, the outside of the bend is stretched and the inside is compressed. The stretched outside has safety considerations. When the pipe wall is stretched it becomes thinner. The general practice, with standard weight pipe, is to Figure 26-1. The bending circle minimize this thinning by limA is the bend angle iting the radius of the bend to no less less than five times times the the pipe bending wheel is a steel steel size. A bending or cast iron circle that is pressed into the pipe to make the bend. A groove is machined into its perimeter to a depth of one-half the pipe diameter. If the bending wheel is measured across its greatest width and this is divided by 2, the radius of the bend that this particular bending wheel will make is obtained. Bending on too short a radius will flatten the pipe and reduce its carrying capacity. capacity.
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Pipe Bending
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Making the Bend
There are two kinds of bending methods. In the first, the bending wheel is pressed into the pipe, which is held in position by two “bending blocks” (seeFigure26-2). The beginning contact point between the machine and the pipe is the center of the arc of the finished bend. The other kind of bending machine grasps the pipe to the wheel at the beginning of the bend and then rotates the wheel, forcing the pipe to bend with the wheel as it rotates (see Figure 26-3). This makes all calcuFigure 26-2. A bending table lations start at the beginning of the arc of the bend. The mathematics is slightly different in each situation. This is the type process found in small hand benders used for hydraulic, gas, and air pressure lines, but it is also used in heavy pipe fabrication shops where pipe sizes 8" IPS and more are bent on a machine weighing many tons.
Visualizing the Math
If we make a 90° bend in the center of a piece of pipe 24 inches long, how long will the pipe tangents be? We can’t be exactly sure, except to say that they will not be 12 inches long. We can’t be certain because the size of the pipe is not known and, therefore, the radius of the bending wheel is not known. But the reason that the length of the pipe tangents added together is greater than 24 inches is that the pipe takes a short-cut across the angle of the bend. This shortcut results in what pipefitters call gain. Gain is illustrated in the following paragraph. To make this calculation, calculate the difference between the straight line length of the pipe from where the bend starts and stops and the length of the arc made in the Figure 26-3 pipe by the bending wheel. In the case illustrated by Figure 26-4, a piece of 2-inch pipe is bent at a 90-degree angle. The pipe follows the 20-inch diameter bending wheel (5 times 2 inches times 2). The pipe length con1 sumed by the bending arc is ⁄ 4 of the perimeter of the 20-inch circle. The perimeter 1 of a 20-inch circle is 20π or 62.832 inches, ⁄ 4 of which is 15.708 inches. So if an abrupt 90-degree angle were possible, as shown in Figure 26-4, it would consume 20 inches (10 plus 10) between the intersection points with the bending wheel. The pipe, however, follows the wheel and consumes only 15.708 inches, a difference of 20 – 15.708 or 4.292 inches. Note:
The radius of the bending wheel can be specified to be anything larger than 5 times the pipe size. If the radius is not specified it will be 5 times the pipe size.
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Pipe Length Calculations
A Formula for Calculating Pipe Involved in a Bend
P = pipe size. D = number of degrees in the bend. Pipe length = the length of the pipe actually incorporated in the bend. Pipe length = P
D 5 2 π 360
The number 5 represents the standard bending radius multiplier. This would be changed to suit for special cases. The number 2 is the multiplier necessary to change the radius into the diameter of the bending wheel. This is so that the circumference of the bending wheel can be calculated using the formula C = π D, i.e., the circumference of a circle is equal to the value of pi times the diameter of that circle. If a bending radius different from 5 times the pipe diameter is not going to be used, the formula can be shortened to
This calculation is helpful in determining how close together bends can be made for a particular angle.
D Pipe length = P 31.4159 or 360' Pipe length = P D 0.0873
A Common Pipe Bending Problem
Figure 26-4
Problem: Make a 10-inch offset in 2-inch pipe using 20° bends. Visualize: Figure 26-5 shows the centerline of the 2-inch pipe offset. In practical bending practice with the push-type bending wheel, two strips of metal are joined at one end with a clamp screw. This creates a tool that can be fastened at any degree setting. We use a protractor to set this tool at 20°. Then we press the bending wheel into the pipe until the desired degree of bend is reached by comparing the bend with our Figure 26-5 tool as the work progresses.
Figure 26-6 shows the piece of pipe that is going to be used to make the proposed offset. If the clamp wheel is used, the center of the wheel is fastened to the points marked “B” (for beginning). If the push-wheel type of bending machine is used, the center of the wheel is pressed into the Figure 26-6 points marked “M” (for middle). The wheel is pressed in at the first mark and then the pipe is turned over and the wheel is pressed in at the other mark from the opposite direction, forming the offset. The distances B-B and M-M are exactly the same. With either type of bending machine it should be obvious that the student must first calculate the diagonal (travel) of the offset.
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Unit 26
Pipe Bending
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The Process:
1.
Consult Unit 25 and calculate the diagonal for a 20°, 10-inch offset. a.
Look at the example offset chart to find out which function to use when the offset is known and the diagonal is needed. In this case, it will be the cosecant of the bend angle.
b. Look up the constant in the trigonometric function chart in the Appendix. The cosecant of 20 degrees is 2.9238. c.
The example offset chart also tells us to multiply the function value by the offset. 2.9238 10 = 29.238 inches. This answer is the length of the diagonal in our offset.
d. Convert the decimal fraction to an English measure fraction. 29.238" 1 4 inches. becomes 29 ⁄
Problems
2.
Put lines on the pipe indicating the point at which the bending wheel makes first contact with the pipe.
3.
Bend it, using some kind of gauge to check progress. You will have to overbend somewhat because there will be a certain amount of spring-back in the pipe.
Calculate the length of pipe that will be needed for the following pipe sizes and angles using the standard minimum bending radius.
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Pipe Length Calculations
How far apart should the bends be made for the following offsets?
