By Doug Stelling
Frequently when debottlenecking process plant equipment, the possibility of increasing the equipment's design temperature, design pressure, or both comes up. This process of re-evaluating the equipment for more severe design conditions is usually referred to as uprating.
In other cases, after a piece of equipment has been in-service for a number of years and corrosion has taken its toll, it may be necessary to re-evaluate whether the equipment has sufficient corrosion allowance until the next inspection or whether the equipment should be retired from service. Again, rather than assuming that the piece of equipment must be replaced when its corrosion allowance is used up, it may be worthwhile to investigate downrating the equipment to allow its continued use although at somewhat less severe design conditions. In some cases the term rerating of the equipment may also be used to cover changing the throughput or service of the equipment.
Many different types of equipment can be rerated, such as pressure vessels, piping systems, heat exchangers, and tanks to name a few. Since the rerating of each equipment type has certain nuances, this series of articles explores some of the “tricks-of-the-trade” that relate to rerating each type of equipment, as well as some of the pitfalls that may be encountered. This article covers various aspects of rerating pressure vessels.
The idea for uprating a piece of equipment usually comes in the form of a request from either the process engineers during debottlenecking or the process operators based on improving the operation of equipment. Most often it is desired to uprate the pressure vessel from its current design pressure and/or temperature to some new higher pressure and/or temperature. Sometimes they want to know if the vessel can be filled above a given level or whether the flow rate through the vessel can be increased. At first glance, it may seem that if the vessel was properly designed from the start it should not be able to be uprated; however, in many cases there is some “fat” in the design due to the manufacturer rounding up from minimum thicknesses to the next available plate thickness, etc. The problem is to recognize the fat and not get caught in any pitfalls.
The first step is to gather all of the readily available information. This includes the original manufacturer's calculations, drawings, manufacturer's data reports, the Code papers, and equipment inspection reports.
The Code used for rerating must also be determined. For some vessels it may be permissible to rerate in accordance with API-510, the Pressure Vessel Inspection Code, or the NBIC, National Board Inspection Code. In other cases, local Codes or rules may apply. In most cases, the vessel can be rerated in accordance with the original Code of construction. This may be a problem if the vessel was built to a very old edition of the Code, and it may require a trip to the company's archives or the Engineering Societies library to obtain the relevant Codes and Standards. As an alternative, the vessel may be rerated to the current revision of the original construction Code if all essential details of the design are similar to current requirements.
Although it is probably true to say that we know more today about the design of process equipment just by the sheer weight of the Codes, the question that always comes up is:“Why should the original Code of construction be used?” Over the years most material allowable stresses have gone up, and only in a few cases have they gone down. Some details that were suggested based on good engineering practice in earlier Codes are now required and visa-versa. Note that if rerating to the current Code is considered, the allowable stress used in the original Code of construction may still be required. This is because the material's allowable stress in the original Code of construction was based on material specifications that were applicable at the time the Code was written. While the current Code may indicate a higher allowable stress, justifying the use of the higher allowable stress to the Authorized Inspector may be difficult unless it can be proved that the material would meet the current material specification with respect to all essential aspects. Also in some cases, a higher allowable stress can be problematic due to the way in which some components are designed using the Code formulas.
The next step is to consider basic information such as the design pressure and temperature, the MAWP, and the vessel's test pressure. In general, if the new design pressure is less than the MAWP of the vessel and the vessel was tested to a pressure based on the MAWP, then the rerate is usually not too complicated. In some cases when the equipment is made from carbon steel, and only the design temperature is being increased to some temperature below 650°F, rerating may appear to be simple. This is because most carbon steel materials built to Section 1 and Section VIII, Division 1 of the ASME Code have the same allowable stress up to 650°F.
From a mechanical standpoint, some of the pitfalls in what may seem to be a simple temperature uprate may be that the flange pressure-ratings may decrease too much as the temperature increases. The higher temperature may cause problems with thermal expansion of the vessel or differential expansion with the platforms and ladders that may be overlooked. Corrosion rates may also go up as the temperature is increased. In some cases temperature limits on various types of construction should also be considered, such as temperature limits for slip-on flanges, gasket type, bolting, painting, and insulation. If the rerate is possible, the pressure relief valve set pressure and capacity requirements should also be checked.
When rerating a pressure vessel, a review of the original vessel manufacturer's calculations should always be made. While some manufacturer's are very good, anybody can make a mistake. Some mistakes that we have found when rerating equipment are as follows:
In some cases, the original calculations may not be available or thorough enough for the rerate and new calculations must be made. In this case, the use of a good pressure vessel design program like COADE Inc.'s CodeCalc or proVESSEL can come in handy.
An important piece of information that is sometimes overlooked is the equipment's inspection history and recent UT thickness data. From reviewing this history, the pressure vessel engineer can usually tell if the equipment has enough remaining corrosion allowance for its intended service life (or at least enough to get to its next inspection). In most cases, spot UT thickness data is available from the last internal inspection and the rerate can be based on the minimum thickness found. If the indicated shell or head thickness are above the required thickness, less the nominal corrosion allowance, then a remaining life can be determined. If the remaining life is less than the time required until the next internal inspection, then the vessel may be downrated; however, it is sometimes possible to conduct a more extensive UT survey and base the rerate on this data.
Another consideration in rerating is whether a pressure test is required to accomplish the uprate. If the uprate is only on paper (i.e., no physical modification of the vessel), and the new minimum test pressure calculated for the uprated conditions is no more than the original or last pressure test, it may be permissible to uprate the vessel without a new pressure test. However, this should be checked with the Authorized Inspector and local authorities prior to the rerate.
If a pressure test is required, a hydrostatic test is preferred; however, hydrostatic testing can also be problematic. The vessel or its foundation may not have been designed for hydrotest in its erected condition with all of its internals installed or piping attached. For older vessels, the possibility that past repairs may have introduced critical flaws in the vessel or that corrosion or embrittlement of the materials has occurred may make brittle fracture a consideration.
Finally, it is important that the rerate be properly documented. This usually requires updating the drawings and calculations, preparing revised Code papers and other documents, contacting the Authorized Inspector, and preparing new name tags.
The next installment in this series will discuss rerating of heat exchangers.