Optimus Engineering Solution
The waste heat recovery systems on sulfuric acid plants designed with technology by MECS, Inc. (MECS) typically feature a superheater downstream of the converter first pass operating in high temperature, high pressure service. These superheaters, usually designated as Superheater 1A or 1B, are required to handle gas-side conditions around 1,150°F and 7+ psig (621°C and 0.49+ barg), which can induce high mechanical stresses in the outer casing that are challenging for the superheater to accommodate.
Because superheater heat transfer coils themselves are a rectangular configuration, for most units the casing that houses them is also rectangular with stiffened flat sidewalls and header boxes with square corners. Particularly on large Superheater 1A/1B units, the welded corners where two flat casing walls intersect are subjected to high stresses due to internal pressure and thermal expansion. Even stainless steel casing material has relatively low strength at design temperatures of around 1200°F (649°C).
As an alternative to a “rectangular”-style superheater, in 2007 Optimus developed its first “cylindrical” casing design for the Superheater 1A used in the PCS Phosphates Aurora Plant #7, which is the largest sulfuric acid plant in North America. Because of the history and critical nature of the superheater casing, the specifications from MECS required a cylindrical casing design and that a Finite Element Analysis (FEA) be performed on the final design. Even though the main body and ends of the superheater are cylindrical, the FEA is important because the design of the gas inlet and outlet connections are very tricky and the most susceptible to failure. At that time, the most popular computer aided engineering (CAE) tools for structural design were useful for rectangular casing design but were not suited for sophisticated FEA analysis of the cylindrical-style casing. So, Optimus shifted to a more versatile set of 3D modeling/CAE tools and designed the Aurora cylindrical superheater solely with FEA methodology, and has made it the company standard for superheater/economizer casing design ever since. The Optimus cylindrical superheater design has evolved over the years with our 3D FEA tools helping to find and reduce stress concentrations more effectively with each new Superheater 1A/B produced.
Since the PCS Aurora Plant #7 project, Optimus has participated in several large capacity Superheater 1A/B replacement projects where an old rectangular unit is replaced with a new cylindrical superheater. Also, the majority of the RFQ’s we’ve seen for new-build MECS® sulfuric acid plants have specified cylindrical Superheater 1A/B’s. This is not to say a rectangular-style superheater cannot operate reliably for 10+ years… Optimus has used its 3D FEA methodology to improve those, also… but apparently some plant operators prefer a well-designed cylindrical unit for the high temperature/high pressure operating conditions of the Superheater 1A/B units.
Optimus is currently manufacturing a cylindrical Superheater 1B for the Mosaic Uncle Sam Plant to replace an existing rectangular unit on its “E Train” plant. The engineering project managers for Mosaic consulted with MECS for the concept and specification package of this replacement superheater. Nick Darling the MECS mechanical engineering supervisor has had an active role in recommending cylindrical superheaters in many of these hot, high pressure applications such as Mosaic Uncle Sam. “As sulfuric acid plants continue to get larger MECS is recommending and specifying more and more cylindrical stainless steel superheaters in place of the rectangular designs. We feel that when designed, built, and installed properly these cylindrical superheaters are more reliable than the rectangular type, especially when used in conjunction with MECS’ gas bypass and duct design.”
For plant operators with rectangular Superheater 1A/B units nearing their end-of-life, it is certainly worth considering whether a cylindrical style unit could enhance your plant’s long-term reliability.