To few engineers' surprise, shell and tube heat exchangers can get very hot at times. When that happens, the rising temperatures can cause the unit to swell, a process called thermal expansion.
If a shell and tube heat exchanger is not equipped to handle thermal expansion, the added pressure and changing dimensions of the tubes or shell can cause serious damage to the unit. Tubes and shells often do not expand at the same rate due to differences in material composition, varied temperatures and other factors.
If, for example, tubes begin to expand faster than the shell, they can cause damage to the interior of the shell. Conversely, if the shell begins to expand, the tubesheet can be negatively impacted.
Luckily, there are numerous ways to deal with thermal stress before it comes a costly issue. One such method is to install expansion joints. However, these add-ons aren't a one-size-fits-all solution. There are numerous categories of expansion joints, and each one has its own set of considerations to take into account.
Two of the most common types of expansion joints are metal expansion bellows (sometimes called packless expansion joints) and packed slip expansion joints.
Metal expansion bellows: pressure placement
Metal bellows can either be internally or externally pressurized. The type chosen depends on the application and environment of the equipment. These models are also sometimes called packless expansion joints, though that title has become somewhat outdated as this design has become more popular.
Internally pressurized expansion joints
Internally pressurized expansion bellows are excellent for operations that require frequent inspections of equipment. It's easier to review the condition of internally pressurized expansion joints than their externally pressurized counterparts, Chemical Processing explained. Additionally, they're less expensive, making them a cost-effective option.
However, there are a few downsides to investing in an internally pressurized expansion joint. These designs can't accommodate large amounts of thermal stress or expansion, particularly when it comes to axial movement. Internally pressurized versions tend to "squirm" under too much pressure, or when accommodating high pressure on long bellows.
For these reasons, internally pressurized bellows are best used as a cost-effective method to combat small amounts of thermal stress.
Externally pressurized expansion joints
Externally pressurized bellows have a few advantages over internally pressurized models, even though they are often more expensive. These are built with a strong outer pipe and an expanding inner lining. This construction also makes them safer because the pressure is contained within the outer pipe, preventing any material inside the unit from escaping, should the bellows fail. Additionally, the outer pipe absorbs shock if there's an anchor failure.
These models can handle much greater amounts of thermal stress and expansion, making them ideal in most operations that encounter these types of issues. In situations where internally pressurized versions might squirm or outright fail, externally pressurized bellows can hold strong.
They can be customized to minimize leakage in the event of an equipment malfunction or damage. Additionally, they can be designed to be self-draining, adding yet another layer of protection against cross-contamination.
"The size, shape and method of fabrication all determine key characteristics of the bellows."
Expansion bellows: wall thickness and construction
The size, shape and method of fabrication all determine key characteristics of the bellows. To determine the best wall thickness and other qualities for the bellows, fabricators need to know details of the operation, such as how much pressure the expansion joints will need to withstand.
Thick-walled expansion bellows
Thick-walled expansion bellows are typically between 4 and 13 millimeters thick, often matching the material and thickness of the shell itself. These designs are primarily used in fixed tube sheet shell and tube heat exchangers, according to Thermopedia. Though they are highly durable, they're also very stiff and have very limited flexibility. They usually only have two or three convolutions, but the convolutions are exceptionally tall – between 75 and 150 millimeters – compared to thinner walls. Thick-walled bellows are usually made by pressing and welding.
Thin-walled expansion bellows
Thin-walled expansion bellows are usually between 0.5 and 2 millimeters thick. While this means they are much more flexible than thick-walled versions, it also means they're more prone to damage. Because the thin-walled bellows are more pliable, manufacturers can include more convolutions than with thick-walled bellows, and the convolutions are typically between 25 and 75 millimeters tall. Thin-walled bellows are usually made through cold rolling or hydraulic forming.
For operations that involve high pressures but require the thin-walled bellows, fabricators might opt for a multi-ply construction, where multiple layers of the thin metal lie atop each other to bolster and strengthen the bellows. Another method of reinforcing thin-walled bellows is through installing restraining rings, which are usually located outside the bellows at the root, or the lowest point of the curve, according to a research paper from the International Association for Structural Mechanics in Reactor Technology.
Medium-walled expansion bellows
In certain situations, neither thin- nor thick-walled bellows are ideal for accommodating thermal stress, and a compromise needs to be made. Medium-walled models are typically between 2 and 4.5 millimeters thick, offering greater durability than thin-walled versions. They are also more flexible than thick-walled bellows, and have convolution heights of between 50 and 65 millimeters. Medium-walled bellows are usually made by hot rolling.
Packed slip expansion joints
Another common type of expansion joint is the packed slip expansion joint, sometimes simply referred to as slip expansion joints or packed expansion joints. This design includes a sleeve which extends outward from the body of the exchanger as thermal expansion increases, as well as packing installed between the sleeve and the body that helps prevent leaking.
"Packed slip expansion joints are ideal when the thermal expansion causes axial movement."
These are more often used with packed floating heads, and are ideal when the thermal expansion causes axial movement. Because these expansion joints are composed of two cylinders – the sleeve and the shell – they can't expand in any other direction other than left and right (or up and down, depending on the orientation of the unit). Any lateral expansion or rotation due to thermal stress would most likely break these joints, according to Piping Guide, a blog written by engineer Ankit Chugh.
One of the downsides of packed slip expansion joints is their susceptibility to leakage. Though the packing is supposed to prevent this, it can't provide a guarantee that no leaks will occur. Additionally, as the material wears away over time through many uses, a leak will become more likely. Because of this, operations where no cross-contamination can be tolerated would be better off using a different type of expansion joint.
However, where small leaks won't cause too much concern, a packed slip expansion joint can be an economical way to deal with thermal stress.
What is the best expansion joint for your equipment?
Clearly, there are many factors that impact your decision when weighing your options for expansion joints. To determine the best choice for your shell and tube heat exchanger, it may be most helpful to speak to an engineer well-versed in the design and fabrication of these units.
The knowledgeable engineers at Enerquip enjoy taking on new problems and coming up with the perfect solution for their customers. They'll work with you to determine your unique needs and find the right expansion joint for your operation. To connect with an Enerquip engineer, request a quote with our simple online form.