The right selection of the material is a fundamental parameter when designing the bellows and other parts of an Expansion Joint, and becomes even more important in high temperature applications which, in many cases are associated with aggressive fluids and large movement compensation requirements.
When a material is used at high temperatures, its strength, as reflected in tensile strength, stress rupture life or fatigue life, is of vital importance. Corrosion processes are also quite likely to affect strength; therefore, corrosion resistance at elevated temperatures is critical.
The selected material must have a good compromise between mechanical properties and good resistance to high temperature corrosion by the time it ensures good fatigue life in the creep range zone.
There are several appropriate metals for high temperature applications, but the best selection for each depends on several aspects. Below are some of the best available heat-resistant metals and alloys used for the production of Expansion Joints for elevated temperatures:
Austenitic stainless steels with high carbon content providing good performance at moderate costs:
Nickel Alloys for special applications such:
Other refractory heat resistance metals for high temperature applications:
The selection of heat resistant steel for a particular application is based on the level of the heat resistance required and the needed mechanical properties from the steel. The use of a higher alloyed and hence more heat resistant may be disadvantageous in some cases because of embrittlement besides having a higher cost. Heat resistant steel must not be exposed to flame and a direct contact with carbon must be avoided to prevent the lowering of heat resistance due to carburization.
High temperature Expansion Joints are used in a wide range of applications, including:
Bellows are usually the most critical part of an expansion joint, and whenever the application and design allow it, it is recommended to reduce their operation temperature as much as possible.
Controlling the level of temperature is essential to protect bellows and hardware and to guarantee the optimum performance and lifetime of the Expansion Joint.
In general, reducing the operating temperature of bellows increases their lifetime. This can be achieved in many ways, being the following the most used and effective.
Internal thermal insulation is used in Expansion Joints in a wide range of applications, especially when extreme thermal conditions are involved.
It is generally used to:
Systems like purge rings or air injectors have been widely used in the past but the complexity, maintenance and requirement of secondary installations made this system obsolete.
The evolution and appearance of new insulation materials led to the development of “packed-bellows” design. A simple, reliable method with practically zero maintenance that is currently the most widespread on the market.
Likewise, this design allows a precise thermal analysis of the heat transfer, being able to ensure that bellows temperature is always within the desired parameters.
Thermal control is what allows maintaining the bellows temperatures within desired range during its lifetime.
The internal insulation is usually composed of ceramic blanket, silica cloth and stainless steel wire mesh. The different densities available in the market of the ceramic blanket make it possible to adjust the heat transmission to each specific process, while the silica cloth and the wire mesh perform a protective function and allow the assembly to be given the desired shape.
Internal and external ceramic fiber insulation is sized so that the bellows temperature is controlled.
The purpose of the refractory lining is to protect the steel from high temperatures.
In certain applications, it is also necessary to reduce the operating temperature of the pipes where the different support systems are welded. The best and most widespread solution is the use of interior refractory linings or brick. The thickness of this insulation is a critical parameter and will determine the surface temperature on the metal parts of the Expansion Joint. In addition to the thermal properties, the resistance to abrasion may also be interesting for certain industrial processes.
The insulating or abrasion resistant refractory linings can be installed by casting, gunning (pneumatic application), hand-placing or pneumatic ramming. The abrasion resistant refractory installation involves several critical activities like anchor system (hexmesh or equivalent), QC of materials, prequalification, application and heat dry outs. It is very important that all refractory installation activities are, during all the process, carefully controlled to ensure a good quality control through a good method, materials selection, testing frequency, installation systems and dry out curve.
Refractory lined Expansion Joints are typically used in FCC units, steel mills, furnaces, hot gas turbines, styrene plants, fluidized bed boilers, etc. Refractory lining reduces the tube wall temperature and protect the bellows from abrasion caused by the flow of abrasive particles.
Thermal expansion due to high temperature can cause cracks in the welds of the external hardware and supports. This effect is especially important in the case of dissimilar materials (with different coefficients of thermal expansion) or with different operating temperatures (usually the external supports act as a heatsink).
In certain cases, to avoid this phenomenon, it is convenient to use floating supports, which are not welded to the pipe. A gap is created between pipe and the support, allowing their independent expansion, so that stresses would not be generated in the weld avoiding possible cracks.
There is a growing trend towards exhaustive control and monitoring of the different parameters present in industrial processes based on real-time digital data acquisition systems.
Expansion joints in critical applications (such as those with high service temperatures) can be equipped with this type of system and sensors and by means of thermocouples, pressure and movement sensors, so operation within service conditions can be verified and predicted to avoid possible failures.
Thermal analysis is an important tool of modern design. Many materials and structures have characteristics that are directly dependent on temperature and that affect their strength and performance. For these reasons, a thermal analysis tool is used in the early design phases of many critical high-temperature Expansion Joints.
Modern CFD and computer heat transfer analysis models represent a remarkable advance in predicting heat flow to the surface and correctly selecting and sizing internal insulation materials.
Likewise, it makes easier to detect possible “hot points” that would lead to premature failure of the expansion joint.
At MACOGA our engineers carry out heat transfer analysis to predict temperature distribution, rate of heat flow and heat flux in a design.
In specific and critical cases, we carry out thermal tests and trials with thermo-mechanical simulation. A heat transfer analysis can reveal, among other things, the upper and lower temperature fields, how a material conducts or insulates heat, how the solid reacts to thermal load or how increasing the pressure load causes temperature changes in the solid.
Performing these analyses helps to design a product that is safe, reliable, durable and high performing at the temperatures and conditions for which it is being created.
Expansion Joints design challenges engineers when it comes to high temperatures. The specific environment requires special (custom) designs and high-quality heat resistant materials. MACOGA is the specialist in this field and has developed a wide range of Expansion Joints that are being successfully used in the most demanding processes.
We offer a complete range of Expansion Joints for any high temperature application and all our Expansion Joints combine the latest technologies with decades of industry expertise.
MACOGA Expansion Joints are certified by the world’s leading classification authorities.
From design to final tests and shipping, all production phases are regulated in accordance with the requirements of the MACOGA’s Quality, Health, Safety and Environment management system.
MACOGA has type approved Expansion Joints available certified with:
We provide our customers safety and reliability through a certified quality management system.
Additionally, MACOGA holds numerous industry approvals, including: