High gas temperatures and potentially corrosive steam common in sulfuric acid production pose mechanical design challenges throughout a plant.
To help plant operators tasked with improving reliability, CG Thermal’s engineering team can design in reliability. Utilizing techniques including optimal ducting and plant layout, these common operational concerns can be minimized.
First, a thorough audit is conducted. This includes a review of the general arrangement, equipment drawings, data logs and operating procedures. Conversations with maintenance, project and reliability teams are also critical for gathering accounts of various historic plant failures.
Next, data from operations logs, Computational Fluid Dynamics (CFD) and Finite Element Analysis are used to create a digital twin of the process equipment. Using the digital twin, engineers can go beyond the calculations and assumptions to model temperatures and stresses unique to the area of concern, ultimately revealing the mechanisms that lead to the failure.
Once the failure mechanism is understood, the CG Thermal team draws upon years of experience to incorporate innovative and time-tested solutions that it’s developed. Solutions can include proprietary baffle and flow arrangements, optimized material selection and the selection of round transitions as opposed to rectangular.
The innovative design of CG Thermal’s AirBTU VPRR variable pitch radial recuperator/high-temperature gas recuperator, addresses and eliminates the conditions seen by cold-end and interpass exchangers, namely:
- Weld failures due to thermal expansion
- Cold-end corrosion
- Cold-end fouling
- Flow restriction due to unexpected pressure drop
A typical cause of weld failure is thermal stress caused by materials expanding at different rates, most commonly at the tube-to-tubesheet weld as some tubes expand more than others. Although a shell-side expansion joint is commonly recommended whenever the logarithmic mean temperature difference is greater than 150°F, it does not compensate for tubes expanding at different rates. Instead, the company’s unique variable pitch tube design is employed with the goal of uniformity of temperatures across the tubesheet.
Innovating for reliability in high-temperature gas streams
This large AirBTU VPRR unit was designed as a cold-end heat exchanger and is being transported for use in a sulfur catalytic operation.
Cold-end corrosion and fouling result from material temperatures lower than the dew point of the process stream constituents. During the design process, it is important to identify if and where those low material temperatures exist, and to incorporate features to eliminate their occurrence.
If the pressure drop through the heat exchanger is higher than expected, the design flow rate might not be possible. This reduction in flow rate will not only affect the performance of the heat exchanger but the entire process, resulting in a reduced rate of production.
Using methods developed through years of experience, material temperatures are evaluated in all the critical areas during CFD modeling. Tube pitch and baffle arrangements are modified until the desired temperature uniformity and pressure drop are achieved. Wall temperature mapping is used to identify any location in which the temperatures fall below the dew point. The tube pitch, baffle design and pass arrangement are evaluated. An array of design features developed through both design and operational experience are considered to determine the best solution. The final design will utilize the most effective solution with minimal cost impact.
Following this methodology will significantly improve the overall reliability of the heat exchanger, resulting in two-to-three times the operating life compared to commonly designed exchangers.
For more information, visit cgthermal.com.
