The oil, gas and chemical industries commonly use reactors, process columns, high-pressure vessels, boilers, furnaces, storage tanks, exchangers, filters, piping, valves, dryers, ejectors, air coolers and other static equipment.
Static equipment is a term that is used to describe non-moving machinery. These mechanical components work in tandem with rotating equipment like pumps, compressors, blowers, turbines, agitators and centrifuges. Additionally, often included in the mix are equipment systems like chillers, cooling towers, raw water treatment units, sand filtration systems, chemical injection units, nitrogen generation systems, condensate polishing units and wastewater treatment units.
Process and thermal design are brought to life by operations that involve a combination of these elements at chemical and industrial processing plants.
According to the Business Research Company, the global static and rotating equipment market size is anticipated to increase from $24 billion in 2022 to $32.42 billion in 2027. Rapid industrialisation and technological developments are two major driving forces for these equipment markets, whose reliable functioning is vital for industrial machinery operation.
The key to good thermal design that optimises mechanical equipment is that it has a low overall cost, including initial and operating costs. Facility owners’ demand for more value-driven mechanical and thermal design services requires the delivery of tightly woven mechanical/process and thermal engineering that allows owners to better plan and manage overall operations.
Having thermal designers/engineers who are also knowledgeable about mechanical processing/engineering are most effective at helping facility owners meet their operations goals. Modeling, fuel system design, pipe stress analysis, pump selection, and building/site safety are among the areas of knowledge these dual engineers possess. They provide designs that are high quality, creative and code compliant. Ideally, each job begins with gathering input data. Important data required for optimal thermal design includes:
- Flow rate: the complete breakup of vapor, liquid, steam, water and non-condensable flow rates at both inlet and outlet.
- Inlet and outlet temperatures.
- Heat release profiles: plot of heat duty and weight fraction vapor versus temperature.
- Operating pressure: most importantly for gases and vapors because density changes significantly with change in pressure.
- Allowable pressure drop.
- Fouling resistance.
- Physical properties: Viscosity, thermal conductivity, density and specific heat are the fundamental properties. Whereas engineers need the values of these properties at both inlet and outlet temperatures for single-phase fluids, in the case of dual-phase exchangers we will need these properties at intermittent stages between inlet and outlet. This is because the physical property curves are linear for single phase but nonlinear for dual-phase exchangers.
- Type of service: whether condensing or vaporising.
- Surface tension.
- Critical pressure and critical temperature.
Determining the type of exchanger that will help with optimising design in terms of performance and cost is the next big step. Figuring out tube dimensions and layout, tube sheets, floating heads, girth flanges, bonnets, nozzle placement, nozzle-to-nozzle interference, support locations and baffle spacing must be completed with elaborate thermal-mechanical calculations using software like the Xchanger Suite, developed by Heat Transfer Research Inc. (HTRI).
The Art of Mechanical Design
For a given set of process conditions, an engineer can create many designs. Determining the optimum design is where science gives way to art.
Developing designs to meet heat transfer requirements is not confined to the configuration and mechanical details of heat exchangers alone. It involves optimising the entire system, which would consist of process columns, pumps and piping.
Heat duty, pressure drop, fluid velocity and other factors that are vital parameters have a bearing not only on the exchanger design but also connected equipment and piping. So, one has to visualise the system in its totality and see how the overall system can be designed to maximise performance in a cost-effective manner. With 3D modeling becoming a standard practice in plant design, designers of heat transfer equipment are better equipped to study equipment systems as a whole and effectively can intervene to help with equipment layout.
The Burns & McDonnell mechanical group works on projects by completing the full design of heat exchangers earlier than many would consider normal. With full thermal and mechanical design, there are important considerations — dimensional envelopes, weight and others — that can impact 3D modeling at various stops along the way.
Consider a thermosiphon reboiler, for example. Its operation is heavily reliant on the hydraulics of the piping circuits linked to the distillation column. Based on the difference in density between the bottom single-phase fluid and the two-phase fluid in the reboiler and outlet piping, the static head of the liquid in the column should provide adequate circulation through the intake piping, the reboiler and the outlet piping back into the column.
As a result, the intake piping from the column to the exchanger and the outlet-return piping from the exchanger to the column must be detailed in the thermal design of the reboiler. To match the static head available in the column, a thorough analysis of pressure drops throughout the circuit (inlet piping, reboiler and outlet piping) must be made. If the pressure drop in the circuit does not meet the static head, the flow rate to the reboiler will need to be adjusted.
Such thermal and hydraulic analysis helps in finalising piping sizes, reboiler design and column-skirt height. The feasibility of a design should be established well in advance in terms of equipment dimensions and nozzle locations. Teams like ours, who have proficiency in both thermal and mechanical engineering can offer clients a significant scheduling and pricing advantage by “early detailing” of static and rotating equipment design.
Producing a reboiler design compatible with column design without causing huge delays is possible if the mechanical engineering team begins full-fledged design early enough. Once a design is completed and the thermal rating is finalised, a datasheet is developed per HTRI standards. A datasheet and outline drawing helps suppliers quote firm pricing that by the end of the bid cycle should not change much. Our mechanical group, for example, produces outline drawings using RCS win software that can import the HTRI model directly. The outline drawings generated during the early phase of the project will help not only the supplier but also piping and civil engineering teams.
This engineering approach of getting thermal and mechanical engineers involved earlier in the conceptualisation process can result in final design moving into detailed design with fewer changes, resulting in 80% less rework. Teams like ours, who have proficiency in both thermal and mechanical engineering, can offer clients a significant scheduling and pricing advantage by “early detailing” of static and rotating equipment design.
Simply put, thermal design can indeed be a game changer in the engineering of a process plant in terms of cost and schedule when started as early as feasibly possible.
Thorough process integration is critical to the optimum functioning of the oil, gas and chemical industries.