Over the last 80–90 years, the
pipeline bending process has developed from rudimentary press bending
techniques to the advanced induction bending processes available today. This
article provides an overview of the current techniques available for bending
pipeline in the workshop, and what is in store for the future.
Many pipe bending methods are
available, with considerable overlaps in capacity. Deciding on a suitable
option ultimately depends on quantity and the quality required. All the
processes have been described below with a suggested range/suitability, etc.
While some companies have quite successfully developed processes that exceed
the ranges suggested below, having processes outside of these ranges is not
common.
Press bending (hot or cold) Pipe size range: 0.5–60 inches Radius range: 10D+
Despite the fact that this is the
oldest method of bending, it is still used for some jobs. Two fixed dies/tools
hold the pipe and another presses against the pipe to form a bend and, with
multiple ‘hits’ on the pipe, it is possible to bend to various radii. There is
often a considerable amount of deformation with this process, which greatly
increases as the radii become tighter.
This process has also been linked
with sand packing and forming, a process that is rarely used now due to the
cost. Sand is packed into the pipe, which is subsequently heated to between
800°C and 1,100°C, and then pressed into shape. This was usually on a metal bed
with the pipe subsequently jacked into position.
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Rotary draw bending (cold) Pipe size range: 0.5–6 inches Radius range: 5D–10D
This process wraps the pipe around a
former die. This method is limited in radius to that of the die radius. It is a
very quick process; however, due to the tooling cost and its very limited
application, it is rarely used for applications above 6 inches nominal bore
(168 mm outer diameter). As there is no internal support, the quality of the
bend is poor for tighter radii, and thicker pipe is generally used to counter
the ovality effect.
Mandrel bending (cold) Pipe size range: 0.5–6 inches Radius range: 1D–10D
This process is similar to rotary
draw bending, however, the pipe is supported internally using a mandrel. The
mandrel is a hardened steel rod, often designed with linkages, to allow it to
curve with the bend. This prevents ovality and rippling of the internal wall.
As with rotary draw bending, the radii is limited to the former die, so it is
generally limited to 6 inches and under. When linked to a CNC unit it is a very
efficient process for multiple bends on the same pipe and large batches.
Three-roller bending (cold) Pipe size range: 0.5–26 inches Radius range: 5D+ Three
rollers drive the pipe through the formers. Depending upon the machine style,
either the front/centre roll or the two lower rollers will move to increase
press.
This method has an infinite range of
radii and is generally a quick and economic process. It is the best process for
radii 7D and above with a three-roller mandrel, and 10D+ without a three-roller
mandrel. It is possible to bend in multiple axes and multiple radii per bar
with this process. Water filling and sand packing is also still used by some
companies to support the section during bending to give a better end result.
Induction bending (hot) Pipe size range: 0.5–88 inches Radius range: 3D–20D
(large radii attachments are available but not common)
Induction bending is generally the
most common process for pipelines due to the quality of the bend. The pipe is
heated at one point using an induction coil; the bending temperature will
depend upon the material grade, but generally it is between 800°C and 1,100°C.
The high frequency-induced current creates a heat band within the electrically
resistant material, which then conducts though the section to create a uniform
temperature both inside and out.
The pipe is held in a press, clamped
to a pivot arm, and then gradually pushed through the induction coil. The
material is as low as 10 per cent of the strength of the metal at room
temperature at the induction coil area due to the heating, and therefore yields
at this point. The radius of the pipe is determined by the pivot arm position.
To retain the cross section, the
pipe is immediately quenched in water after bending. This creates cold sections
adjacent to the heat band, which supports the heat-softened section. Thicker
pipe is often air-cooled.
The bending temperature is monitored
throughout the bending process, and controlled using the bending speed and
power input.
Challenges associated with pipeline
bending
The main challenges with pipe
bending are naturally cost and lead time. As the world’s energy requirements
grow, and resources become more expensive and scarce, the need for a more
efficient pipeline bending method increases. In terms of cost, the reduction of
welds has always been considered; it has not always been a cheap option to
create multiple bends due to their set-up and transportability.
The future of bending technology Over the last 20 years, computer-controlled CNC machines
have been developed, and are now generally commonplace in the workshop. CNC
machines calculate the bending para-meters based on data inputted by the
operator on the required shape.
These days, intelligent cold bending
machines are able to calculate the ‘spring back’ or bending characteristics of
a pipe during bending, enabling the computer to alter the roll or tool setting
to give the same bend on differing pipe strengths. Combined mandrel and roller
machines that are able to bend in three dimensions to multiple radii and angles
very quickly are also available now.
In the near future, a number of new
developments will be seen, including:
- Multi-axis induction bending machines, which will be able to create three-dimensional induction quality bends in a single operation.
- Greater control of bending processes with instant material analysis, which then controls the bending parameters.
- Mandrel benders which are able to bend to multiple radii without the need for expensive tooling, as they will use adaptive tooling.
Pipes bends are often described as
bent to a certain ‘D’ (e.g. 3D or 5D). This relates to the centreline radius of
the bend in relation to the diameter of the pipe. For example, 24 inch diameter
pipe bent to 3D would have a 72 inch centreline radius (CLR), (24 inches x 3 =
72 inches), 8 inch pipe to 5D (8 inches x 5 = 40 inch CLR).
As the world’s energy requirements
grow, and resources become more expensive and scarce, the need for a more
efficient pipeline bending method increases.
