Tubes - Why some like it hot!

In an industrial environment, the reliability and performance of steel tubes used in Heating Ventilation and Air Conditioning (HVAC), mechanical services and general conveyance applications is paramount to ensure safety and reduced maintenance costs.

In this article, Stewart Jones, Senior Customer Technical Services Field Engineer, Conveyance Tubes at Tata Steel discusses the benefits of using hot-finished tubes over cold-formed alternatives.

HVAC systems expose pipework to a host of stresses, temperatures and pressures. Therefore, when specifying steel tubes to be used within such systems, a number of specific requirements unique to the HVAC setting need to be considered.

As well as reviewing the operating pressure, the composition of the water or fluid being conveyed in the pipework must also be taken into consideration as this can contribute to the deterioration of the tubes. For example, extremes of acidity or alkalinity in water can leave pipes exposed to corrosion. Additionally, an excess of oxygen within the water can degrade and rust metal, thereby exposing any imperfections or weaknesses in the pipework, while high levels of calcium in water (common in 'hard' water areas) may result in build up or blockages. High water temperatures and fluid velocity may also accelerate any corrosion of the pipework. .

Therefore, as well as using suitable water treatment or inhibition, the benefits of specifying and installing the most suitable pipework for a HVAC system cannot be overestimated. Choosing the right products for the right application not only helps deliver significant health and safety benefits, lower maintenance costs and lead to a more efficient and effective HVAC system, but also ensures full compliance with the required construction legislation and regulations.

With this in mind, when specifying steel tubes for use in HVAC systems, it is of paramount importance to consider the manufacturing process of the pipework to ensure the durability of any HVAC system against day-to-day wear and tear.

There are various ways of producing carbon steel tubes, and it is important to note that this can result in different levels of performances during service. Generally, products made to BS EN10255 (the replacement standard to BS 1387 that was withdrawn in 2004) are specified for HVAC applications. These are normally manufactured using a High Frequency Welded (HFW) process, but not all HFW products are the same as there are two possible product ion methods that can be employed - either hot-finished or cold-formed. These processes both create products of a similar specification, however there are several underlying factors and properties that distinguish and differentiate the two methods of production.

Cold-formed steel tubes are produced from steel strip, which is shaped when cold and then welded to produce the cylindrical tube shape. Whilst undergoing this process the material hardens, due to the cold working and the welding process, resulting in internal stresses being induced in the tube.

The main stress occurs in the region surrounding the weld seam, often referred to as the Heat Affected Zone (HAZ). Within this region, the steel has a distorted grain structure, resulting in a harder, less malleable material to work with. Any subsequent fabrication, or manipulation during installation, may heighten the level of internal stress, thereby increasing the risk of the product cracking and failing, or even further increasing the risk of corrosion whilst in service.

When used in HVAC applications, a defective tube will result in serious health and safety concerns as well as increased exposure to financial consequentials. Therefore reducing the risk of service failures should be of significant concern for designers, installers and facilities managers.

While the manufacturing process for hot-finished tubes is very similar to the cold-formed process, the addition of one vital step relieves any internal stress.

To provide a full-bodied hot-finish process route, the steel tubes are heated to very high temperatures using a furnace or induction process. As the temperature of the steel rises, its internal stresses are relaxed, the distorted grain structure is reorganised and the HAZ is completely removed. Hot-finished tubes present ten discernable and tangible benefits over their cold-formed alternatives.

Compared to cold-formed steel tubes, hot-formed tubes have:

A more ordered consistent microstructure
No internal stress that can promote cracking
More consistent and reliable mechanical properties
Improved structural integrity and ductility
Improved and more consistent toughness
A higher pressure integrity
A greater factor of safety
No loss of strength due to additional welding or heating
An improved performance against corrosion
An improved ability to be threaded, grooved and bent to tight radii without splitting, creasing or collapsing

Selecting the correct tube specification therefore plays a vital role in ensuring that you get the most suitable product for your project.

The Tata Steel multi-certified Install Plus® 235 hot-finished carbon steel tube range ensures each product is aligned with both the CE marking requirements of the Construction Products Regulations (CPR) and the Pressure Equipment Directive (PED). This is achieved by manufacturing to both BS EN10255 and BS EN10217 Part 2.

Only by requesting an EN10217 Part 2 compliant product, can you guarantee that a ‘GH’ (Get Hot) hot-finished steel tube will be supplied.

Furthermore, the Install Plus® 235 range is now available with the newly improved Tata Steel red paint finish as standard. Having undergone accelerated salt spray and aggressive humidity testing, designed to replicate the service life of the product, this external coating provides extra resistance against the forces of corrosion that are so prevalent in a HVAC setting.

Tubes created through a hot-finished process, produce an unparalleled level of reliability and durability within HVAC systems, helping to deliver safety and performance benefits throughout the lifetime of the pipework.

For more information on BISPA (Building and Industrial Services Pipework Academy), an initiative between Tata Steel and its Tubes business, and the School of Civil and Building Engineering, Loughborough University that supports the industry, provides training and improves the awareness of pipework innovations, please visit: www.bispa.org