PIPE BENDING

Pipe bends are essential in acclimating the alignment and topography of the pipeline route. The pipeline

can have side bend, over bend, sag bend requiring bend degrees of various angles. For smaller angles

and longer radii, cold bending is utilized. But for shorter radii and angles less than 90 degree, hot

bending is utilized. For a pipe of particular thickness, the method that can be used for achieving the

required bending angle is determined by the wall thickness, diameter, material, angle etc.

Hot /Induction bend

Induction bends- commonly referred as “Hot bends” are factory made bends from straight mother pipes

using induction process. This allows production of piggable bends from bare pipe. In India, hot bends are

made for radii greater than or equal to 5D.

Process

Induction bending is simultaneously a shaping and heat treatment process. For a typical pipe of a

specified grade, the process involves careful evaluation of factors like pipe size and grade, pipe type

(seamless or welded), manufacturing parameters (cooling rate, induction temperature, speed), required

metallurgy and dimensional properties, post bending treatment etc . The process should be such that

acceptable material properties and required thickness should be achieved. Depending on the material,

diameter and thickness of the pipe, the process variables will change. So, the process variables have to

be selected carefully for the anticipated results.

Induction bending is done using an induction bending machine. This machine consists of an induction

coil, bend arm to bend the heated pipe, rollers and pushers. Induction coil generates a narrow

circumferential heat band of full wall thickness. Induction coil has the ability to heat the pipe in the

range of 800 to 2200 degree Fahrenheit. This helps to lower the yield strength of the narrow

circumferential heated region thereby reducing the force to bend the pipe.

Simplistically, induction process can be described in the following steps

1. Loading the straight mother pipe into the bending machine.

2. Clamping the pipe onto the bend arm at required bend radius.

3. Applying induction power to the portion of the pipe to be bent.

4. Bending the pipe at the required radius by the moment provided by the bending arm.

5. Moving/pushing the trailing end of mother pipe in pre-determined rate as the induction coil

heats the pipe and the pipe is bent by the bend arm.

6. Quenching the bend portion right after it advances out of the induction coil band by water spray

or cool air.

7. Post heat treatment of the bend pipe.

All the steps explained can be understood from the figure given below.

The objective of induction bending is to produce a bend of acceptable material properties and required

dimensions. To achieve this, an advanced control over the process parameters like temperature, speed,

cooling rate and a well-planned start/stop procedure are essential.

P= Push force

Fb= Bend roll force

R= Bend radius

1. Pusher

2. Bend roll

3. Heat transformer

4. Induction coil

5. Bend clamp

6. Bend arm

7. Pipe

SCHEMATIC OF INDUCTION BENDING MACHINE

Induction heating

Induction heating is a controllable non-contact heating technique. It utilizes a single induction coil to

heat a narrow band of the pipe to the full wall thickness. The induction coil generates a localized

magnetic flux and induces an electric current inside the pipe wall just beneath the coil. This induced

current and the material resistivity generates the heat necessary for hot bending. Optical pyrometers

are provided to measure the temperature in the heating zone. A cooling water spray or forced air is

applied onto the pipe right after out advances out of the induction coil. This can be understood by the

figures given below.

SCHEMATIC OF PIPE BEND AND INDUCTION COIL

Effect of induction bending on mother pipe

1. Ovality and wall thinning

Distortion of pipe in the form of ovality and wall thinning occurs in the extrados of the pipe and

a corresponding increase in wall thickness at the bend intrados. Proper analysis and calculations

should be done before start up for achieving required wall thickness.

2. Metallurgical effects

The process parameters like heating temperature, cooling rate and speed determines the

metallurgical properties of the bend pipe. Proper post bend treatments like annealing,

normalizing, tempering and quenching should be done for acceptable quality bends. Trends

related to these parameters are

Yield and tensile strength increases with induction bending temperature.

Hardness increase with induction bending temperature.

Increase in strength and hardness with an increase in cooling rates.

Lower toughness or elongation with increasing temperature or cooling rates.

Advantage

1. Ability to heat a specific narrow band of pipe will ensure minimal amount of distortion of

the bend.

2. Energy efficient process.

3. Induction bending takes less time compared to other bending process.

4. Capable to make 3D bends.