By Ray Chao and Doug Stelling
This is the second in our series of articles covering rerating of refinery and chemical plant processing equipment. Heat exchangers sometimes must be uprated due to an increase in design pressure or temperature when a plant is debottlenecked. At other times, corrosion in excess of the original corrosion allowance may have occurred and a decision may have to be made whether to repair, replace, or downrate the exchanger. In either case, a mechanical design engineer is usually responsible for making what is sometimes referred to as “rerating” calculations.
From a mechanical design standpoint, heat exchangers are really special purpose pressure vessels, and thus have similar considerations with respect to rerating. Formally, rerating of an exchanger is considered by the National Board Inspection Code (NBIC) or API Standard 510 as an “Alteration.” Therefore, new calculations must be made to verify that the exchanger is suitable for the new design conditions. The available corrosion allowance should also be sufficient to account for the amount of corrosion that may take place until the next inspection. In addition, a determination must be made as to whether the rerate would require a new pressure test.
In order to satisfy the NBIC or API 510 requirements, new rerating calculations must be made in accordance with the original Code of construction, typically the ASME Code Section VIII, Division 1. In addition, the exchanger will typically be designed in accordance with an industry standard such as the Tubular Exchanger Manufacturer's Association (TEMA) Standard and API 660, if it is a shell and tube heat exchanger, or API 661 if it is an air cooled heat exchanger. While many of the equations in the Codes have remained the same over the years, allowable stresses and joint efficiencies may have changed. Therefore, it may be necessary to check the allowable stresses and joint efficiency in the edition of the original Code of construction under which the exchanger was built.
In addition to calculating the thicknesses of simple pressure vessel type components like cylindrical shells and heads, heat exchanger rerating may involve complicated calculations for other components such as girth flanges, tubesheets, and floating heads. In many cases, the required calculations can be readily made using a computer program like Coade's CodeCalc. However, this assumes that the original fabrication drawings, Code papers, and inspection data are available. Unfortunately, in some cases, the exchangers to be rerated are old and the original calculations and/or a complete set of drawings and Code papers may not be available. Therefore, it would be necessary in these cases to obtain the component dimensions by taking field measurements.
In the case of shell and tube heat exchangers, the rerating may involve a change in the design conditions on the shellside, tubeside, or both. Although not obvious, some components on the tube side may be affected if the shell side design conditions are changed and visa versa. For instance, if the design pressure or temperature on the shell side of the exchanger is raised, it is obvious that the various components that make up the shell should be checked for the new conditions. In addition, all other components that are in contact with the shellside fluid (e.g., tubesheet, tubes and floating head) should also be checked although they are normally thought of as being tubeside components. Finally, some components, such as the channel flange at a fixed tubesheet, should also be checked if the shellside design pressure is increased. This is because the channel flange will also be subjected to the higher pressure loads on the shell side since these loads are transmitted by the common bolting across the tubesheet.
In some cases, the rerate may show that a component, such as the floating head, is not thick enough for the new design pressure or temperature on the shell side. In this case, it may be possible to use a more exact analysis, such as the Soehren's Method, to show that the floating head is sufficiently thick. The use of such a method is permitted by the ASME Code in Par 1-6(h). The Soehren's Method is more complicated than the ASME Code procedure since it accounts for interaction between the floating head and the flange ring; however, in many cases this analysis can be used to show that the head and flange are satisfactory for the new design conditions.
After the minimum required thicknesses are determined, these thicknesses should be compared to the actual thicknesses obtained from recent inspection data. The remaining corrosion allowance and corrosion rate for each component in the exchanger should then be determined. At this point, decisions can be made regarding whether each exchanger component is suitable for the rerate, if any modifications must be made, and when the exchanger should be next inspected. A physical modification or replacement of an exchanger component may be required, or the inspection timetable may require revision. This is especially true if the exchanger is nearing the end of its life. Note that some exchanger components are easily replaced, which can economically prolong the exchanger's useful design life.
Finally, as with pressure vessels, the rerate should be properly documented. This documentation should include:
Revised drawings and new drawings for the rerate nameplate should also be prepared.
The next article in this series will cover rerating tankage.