Shell and tube heat exchangers are well favored by plant engineers for their long life spans and minimal maintenance needs in addition to their ability to efficiently transfer heat. While stainless steel heat exchangers are reliable and durable for the most part, there are times when stress and extended use take their tolls on the equipment’s components.
Mechanical failure can appear in several different forms, each with its own symptoms and consequences. When you know what to look for, you can work to prevent mechanical failure in your exchangers or identify an issue quickly so you can solve it promptly. Here are three types of mechanical failure that shell and tube heat exchangers can sustain in time:
Repeated thermal cycling can lead to fatigue in shell and tube heat exchangers. Fatigue can cause cracking in the tubes beginning with small, hard-to-see striations that can quickly grow larger. Eventually, fatigue cracks can span the diameter of the tube and completely sever it.
There are several factors that can increase the risk of fatigue or cause it to escalate more quickly. A report on failure analysis of shell and tube heat exchangers from the College of Engineering Pune in Maharashtra, India, explained that the stress ratio can play a big role in contributing to thermal fatigue. As may be expected, when stress increases, so does the risk of failure due to fatigue.
“Small weld defects can lead to fatigue and severe damage.”
Another factor that can lead to fatigue faster is poor welding practices, as well as other fabrication shortcomings. In one failure analysis published in Case Studies in Engineering Failure Analysis, engineers determined that a faulty weld joint where the tube met the tubesheet was the catalyst that resulted in failure. After careful review, the team identified a small welding defect, just 0.4 millimeters long, which was the first crack that would eventually lead to dozens of small fractures throughout the tube. With use, the cracks grew and propagated.
In addition to the welding defect, issues with thermal expansion also caused serious stress on the tube-tubesheet joint. The report noted that it’s best to have expansion positions 15 or more millimeters away from the tube end to lessen the stress expansion would have on the tubesheet. In the failed exchanger, the expansion position was very close to the tube end, which likely caused even greater stress on the already faulty weld joint.
Avoiding fatigue must begin with fabrication. Even a tiny 0.4-millimeter mistake can lead to severe damage to the exchanger. Additionally, understanding how to adjust your process to reduce stress within the equipment can help.
2. Metal erosion
Metal erosion in stainless steel shell and tube heat exchangers negatively impacts the quality of the equipment in two ways. First, as metal erodes away from the tubes, they become weaker and more susceptible to damage. Second, as the protective outer layer of the tubes wears down, the tubes face a higher risk of corrosion. Any corrosion already forming will only get worse as metal erosion takes place, according to Plant Engineering.
Metal erosion can be caused by excessive speed of flow within the exchanger and by abrasive solids suspended in slurry streams. When a high-velocity stream is divided into thin, sharp jets of fluid upon entering the heat exchanger, it can also lead to metal erosion. In cases like this, the erosion pattern is horseshoe shaped and very localized. Additionally, high temperatures, such as those that allow for flash steam to occur, can also increase the risk of metal erosion. Plant engineers are most likely to see metal erosion occur on the bends of U-tubes or at the tube entrances.
To prevent metal erosion, it’s important to understand the maximum velocity fluid within the exchanger can reach without causing harm to the components. This largely depends on the materials of construction, the fluid type and temperature, among other factors. Stainless steel fabrication, for example, can handle much higher velocities than a copper shell and tube heat exchanger. Other alloys that contain steel, stainless steel and copper-nickel combinations are also sturdy and can handle higher flow speeds.
3. Thermal expansion
As the fluids within the shell and tube heat exchanger transfer heat, the tubes and shell begin to heat up. Depending on the materials of construction, the temperature change, how fast it occurs and other factors, either the tubes, the shell or both may swell, a process called thermal expansion.
Thermal expansion is fine, as long as you know how to prepare for it. If your exchanger is not equipped or built to handle thermal pressures that cause the metal alloys to widen, it can sustain extensive damage. Plant engineers will most commonly see thermal expansion in exchangers where the cold fluid that’s being heated is valved off, most often in steam-heated exchangers.
In fixed tube heat exchangers, thermal expansion of the tubes can cause them to grow too large for the tubesheet, causing pull out, warped tubes or a damaged tubesheet. A U-tube shell and tube heat exchanger is a popular deterrent for the potential damages of thermal expansion because the side of the tubes with the U-bend is expected to absorb the heat that leads to swelling, thus saving the fixed side from the stress. However, that doesn’t mean this is an infallible strategy, Mechanical Design of Heat Exchangers explained. In process dealing with very high temperature changes, it’s important to consider all possibilities of thermal expansion and ensure the tubes can handle it.
Thermal expansion can also be a problem when the fabrication material for the tubes is subject to faster expansion than the fabrication material for the shell. When the tubes swell but the shell doesn’t, it can cause severe damage to both the tubes and the shell.
Preventing damage due to thermal expansion must begin with the heat exchanger design. First, it’s important to understand the properties of various alloys and how they react in certain conditions. As such, engineers building the exchanger must have a thorough idea of how the exchanger will be used, including information about planned temperatures and the types of fluids that will be introduced to the equipment. If it’s determined that thermal expansion is highly likely, the exchanger can be built to include an expansion joint.
When designing and fabricating shell and tube heat exchangers, it’s important to have a full idea of what types of mechanical failure the equipment will be at risk for, and to take measures to prevent it. The engineers at Enerquip are well-versed in the many different design options for heat exchangers and can construct a high-quality exchanger for your operation. If you’re looking for the right exchanger for your process, reach out to Enerquip.