How to coil stainless steel tubing

With more than 35 years of expertise in the global metal markets, our UK-based company is a trusted leader in supplying top-quality nickel alloys and stainless steel. Keep reading to find out how to coil stainless steel tubing with advice from our expert team.

Get in touch for more information about Special Metal Alloys and our exceptional products. You can call us on +44 161 278 2386 or email sales@specialmetalalloys.co.uk

What is stainless steel tubing?

Stainless steel tubing can be made from different combinations and ratios of metal alloys, depending on the requirements for a particular application in a variety of industries.

Different stainless steel alloys are used for tubing to improve corrosion resistance, heat and cold resistance, ductility, malleability and flexibility, tensile or compression strength, and weldability.

It is manufactured in different shapes and thicknesses, from round to oval, rectangular or even almost flat. Depending on the application, it is produced without any seams, or with welded seams.

It can be formed into various coil configurations for diverse industries and specialist applications. Some require sophisticated machinery and expertise, while some coils can be created by hand with minimal equipment.

What is stainless steel coil tubing?

When stainless steel tubing is manufactured into exceptionally long continuous lengths for high-performance applications where corrosion resistance, strength, tolerance and resistance to contamination are important, it is coiled or spooled on a large reel to avoid joints, which can leak, change the dimensions, alter the flow resistance inside, build-up of particles, or lead to contamination of liquids and gas conveyed in the tubes.

Stainless steel tubes are also coiled in a variety of configurations for diverse applications in instrumentation, machinery, supply lines, control lines, heating or cooling systems, and more.

What are the advantages of stainless steel coil tubing?

Stainless steel coil tubing can handle extreme hot and cold temperatures, is highly rust and corrosive resistant, can withstand high pressures, and is stronger than most other types of tubing of the same wall thickness and diameter, and generally conducts heat better than copper or other materials.

In domestic and commercial applications where aesthetic appearance is important, stainless coiled tubing is more attractive than other metals.

What is stainless steel coil tubing used for?

Stainless steel coiled tubing is incredibly versatile and is used in a vast array of equipment in a large variety of industries, commercial enterprises and residences.

Coiled tubing has become a vital part of processes requiring deliquification, cooling or heating, production strings, cleaning wells, hydraulic fracturing, and modern drilling operations.

It can also be found in gas plants; power plants; refrigeration; domestic and commercial appliances; medical devices; OEM instrumentation; mining, automotive, and chemical injection lines; aviation and aerospace; communications, laboratories; and many more.

What you have to know about bending and coiling stainless steel tubes.

Can stainless steel tubing be bent and coiled?

Stainless steel tubing of virtually all sizes can be bent and coiled, but it necessitates a thorough understanding of how the tubing reacts, and requires special handling and the correct tools, to perform the job properly and to allow free flow of the liquid or gas inside the coil.

Any kind of stainless steel tubing can be coiled, but seamless tubing responds better because a seam might be weakened and leak, or do not line up when coiled.

Special Metal Alloys has been a reliable supplier of professionally coiled stainless steel tubes for every conceivable application. However, in both industrial and DIY projects, it will sometimes be necessary to repair herm coils, heat exchangers, boilers, moisture condensers, and other systems, and it will be valuable to understand how to coil stainless steel tubes.

In this article, we attempt to help you understand the process and methods of bending and coiling stainless steel tubing, both professionally and for small DIY jobs.

In essence, coiling is a continuous bending process, but with unique challenges. We will therefore describe both the bending and coiling process.

What makes stainless steel tubing bendable and coilable?

Metal composition

It is mostly the chromium content that makes it less brittle and more malleable.

Plasticity

Over-simplified, the metal becomes plasticised at the point where it bends.

During the bending process, the outside radius is stretched (elongated), causing wall thinning, while the inside radius is compressed.

Yield strength

The thicker the wall and the larger the tube, the more force is required to bend the pipe and the greater the external forces that can deform the metal. Springback also increases with the wall thickness and pipe size.

Bend Radius

The tighter the bend radius, the more complex it becomes to coil tubing, requiring greater skill and care, and often more sophisticated machinery.

Proper interior and exterior support

Because tubing is hollow, if not supported correctly, the outer radius stretches and becomes too thin and may distort or even crack, while the inner radius is compressed, pushing towards the centre, and may buckle, crack, dent, or kink.

The centre needs to be filled with a pliable mandrel (a core around which the tube can be bent and coiled) is used to support the inside of the tube, and a draw die or wiper die (curved support with a hollow of the correct shape and bending thickness) is required on the outside to apply the bending force.

Seamlessness

Stainless steel tubing with seams can be bent and coiled, but there is a risk that the welds or seams may be weakened, or do not line up properly.

General principles and guidelines when coiling or bending stainless steel tube

Bending and coiling tubing is very tricky and can easily go wrong, even right at the end as it gets more difficult as you progress. Remember, the entire process is significant and each step requires attention to the minutest detail.

Design the coil on paper and make model/template from a soft malleable material if at all possible.

Carefully calculate the entire length of tubing required to complete the coil as you cannot add to it. Also carefully consider the wall thickness and thickness as the methods will vary greatly.

If a tight radius is necessary, the coiling process is exponentially more difficult and requires greater care and accuracy to prevent damage.

Remember extra support as each outward spiral is completed and the form and draw dies have to be adjusted.

Choose the correct material to use as a mandrel and select the correct working pressure die, clamp die, and wiper die

The mandrel supports the inside walls of the tube

-For DIY projects, various materials have been used to fill the tube, sealing it at both ends. These range from water to frozen soapy water, gels, paste, hydraulic fluid, water-soluble oils, liquid metal such as gallium, and even very fine sand.

-Consider the pros and cons of each. Some, like salt, may cause a chemical reaction and become very difficult to remove when wet.

The pressure die holds the tangent (straight section just behind the bend.) A bend die forms the pivot around which the coil is formed and a clamp die rotates the tube around the bend die.

Some might work well during the process but may be difficult or even impossible to remove because they get stuck in the coil, or cause a chemical reaction that turns them solid.

Choose the correct tool – this may be something as simple as a bending coil or pipe bender, to highly sophisticated tools designed specially as a tubing bender.

Some tools will require lubrication to coil or bend tubing. Make sure you choose the correct lubricant with the right viscosity for both the mandrel on the inside and the bending machine on the outside. A lubricant that is too thick can cause drag, while it will not properly lubricate the surface if it is too thin. Chlorinated paraffins are often used because they lubricate and form a protective barrier film to protect the surface of the tubing.

Remember to correctly adjust the tooling for radial growth with each coil. Every time the clamp die is released, the clamp and drawing die should be adjusted, or substituted, to allow for radial growth, otherwise, the beginning of the coil bend will be different from the end radius.

Heat makes it easier to bend or coil the tubing, but it may weaken the material.

Properly prepare the tubing by making sure it is clean and free of any debris. Check for dents or distortions.

Thoroughly secure the tubing by clamping it into a vice or using clamps that will firmly hold it in place without damaging the outer surface.

Check for spring back and correct as necessary. Springback is the tendency of the stainless steel to straighten and not hold the bent shape.

The most common methods of coiling and bending stainless steel tubing 

Bending and coiling by hand.

Using some form of soft mandrel (a core around which the tube can be bent), a bend die (centre spindle, pivot bloc, or ram block),  and a draw or wiper die (on the outside perimeter to prevent the metal wrinkling) with the correct diameter, thin stainless steel coil tubing can be carefully shaped by hand.

  • Completely fill the tubes with the appropriate material to act as a mandrel and seal both ends
  • If a liquid is used, this process is also called hydroforming
  • Various materials have been used, from water to frozen soapy water, very fine sand, hydraulic fluids, water-soluble oils, liquid metal, and even very fine sand
  • Some use fine salt, but it can cause a chemical reaction and become very difficult to remove when wet
  • Make very sure you will be able to remove the mandrel when you have finished the coil.
  • Fit around a spindle or ram block with the correct diameter and carefully wind the pipe around it, either horizontally or vertically
  • In horizontal coils the coil diameter increases with each “wind” and you need to ensure you support the expanding coils to prevent them from slipping off and buckling or bending the entire coil out of shape
  • In vertical coils, the spring back affects both the tightness of the coil diameter and the distance between each winding and needs to be corrected
  • When finished, carefully remove the coil from its supports, cut off the sealed ends (or remove the seals) and drain out the mandrel material.

Bending methods and machines

Tube benders can use mechanical, hydraulic or electric energy to supply the force required to bend the tubing. Here are some of the most common methods for coiling and bending stainless steel tubing.

Ram bending stainless steel coil tubing

A ram tube bender is the simplest and least expensive tool, which can generally achieve a centreline radius of three to four times the outside thickness of the tube.

A disadvantage is that the inside is not supported, which will stretch the outside of the bend and crimp the inside of the bend. This method works better on square stainless steel tubing than on round or oval-shaped coiled tubing.

Roll bending stainless steel coil tubing

A roll bender is used typically in large workpieces with a large radius. They have two or three pinch-style rolls, with sometimes a fourth that acts as a guide.

With two rolls, the long metal tube is fed between the upper and lower rolls with adjustable guides to produce the desired bend angle and coil outer diameter.

With three rolls, there are two methods.

  • The tube is fed along two of the rolls, with the third applying the bending force. The bend diameter is achieved by adjusting the position and distance of the rolls
  • The centre roll acts as the pivot and the other two rolls apply the bending force
  • To produce coils, the tube is lifted after each revolution. Often, a fourth roll guides the coil outward as it is formed.

Compression bending stainless steel coil tubing

A compression bender clamps the tubing just behind the rear tangent point (where the bend of the circle begins) and uses a roller for pulling the tubing around a stationary bend die and the pipe is moved and reclamped for each coil winding.

The disadvantage of this method is that the outside surface may be flattened because the inside diameter is not supported.

Draw bending stainless steel tubing

A clamp die holds the outside tangent of the tube while a pressure die pulls the tube around the centre bend die. A wiper die reduces the wrinkling and deformations of the coiled tube.

Rotary draw bending stainless steel coil tubing

Rotary draw benders are used for more precise work and tighter radii. The tube is filled with a mandrel, and a precision clamp along the tangent (straight section of the tube) rotates the tube around the bend die, with a wiper die contracting the tube just before the tangent point. The tooling is adjusted for each coil winding.

The main advantage is that rotary draw bending gives maximum control over wall thinning and distortion. Sometimes articulating balls are used in conjunction with the mandrel to support the tube interior, and wiping dies to prevent wrinkles on the inside radius.

Common mistakes to avoid when coiling stainless steel tubing

·   Springback

·   Flattening or ovaling

·   Buckling and wrinkling

·   Incorrect clamping

·   Wrong mandrel (or no mandrel!)

·   Wrong bending tools and ill-fitting dies

·   Working too fast

·   Too little or too much pressure, deforming the tube or the coiled tubing.

Contact us

Get in touch for more information about Special Metal Alloys and our exceptional products. You can call us on +44 161 278 2386 or email sales@specialmetalalloys.co.uk

How to coil stainless steel tubing
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