Source: The Pressure news (This newsletter is a publication of
ABSA) Volume 25, Issue 4, December 2020
One issue that is commonly seen on submitted pressure vessel drawings is the indication of a 'design pressure' for the vessel, in addition to the vessel's maximum allowable working pressure (MAWP). Although from a process engineering perspective, a 'design pressure' for the vessel may have been initially important in specifying the vessel's minimum purchase parameters, from a regulatory point of view and within the scope of the ASME code, the vessel's initially-specified 'design pressure' loses its relevance, and indicating it on a vessel drawing has the potential to cause problems.
From the process engineering perspective, when a pressure vessel is required to be designed for a certain MAWP, this minimum required working pressure is referred to as the vessel's 'design pressure'. The pressure vessel is then designed to suit the specific application: although the vessel is designed to accommodate this minimum specified MAWP, materials are typically chosen based on available plate thicknesses, and parts chosen based on supplier availability. Once material thicknesses, vessel geometry, and detailed component designs are finalized, the vessel can often be shown to be suited to a higher MAWP than the purchaser had originally specified. Quite often, the manufacturer seeks design registration for this higher MAWP, and marks it on the vessel's required nameplate and Manufacturer's Data Report. This does not present a problem, because in order to do this, the vessel must be fully suited to this designated MAWP, exceeding the purchaser's specified requirements.
Once a vessel has a designated MAWP, that designated MAWP for all intents and purposes is the vessel's MAWP, and the vessel must meet all applicable code and regulatory requirements with respect to this actual designated MAWP – the minimum MAWP that had previously been required by the purchaser is no longer relevant with respect to the vessel's required compliance with the code and associated regulations. Any revisions to the vessel's design, along with any future alterations made to the vessel, must meet all requirements with respect to the vessel's designated MAWP. Indicating the purchaser's originally-specified MAWP on the vessel drawing and identifying it as the vessel's 'design pressure' can then be a source of problems.
The main danger in indicating a 'design pressure' on a vessel drawing is that it is typically lower than the vessel's MAWP, and can be mistaken for the vessel's MAWP when that drawing is relied upon for subsequent repairs and alterations. There are known cases in the past where alterations have been made to suit a vessel's 'design pressure' based on a value shown on the vessel drawing, rather than its MAWP, leading to the vessel inadvertently being derated for the lower pressure. This can turn out to be more than a minor inconvenience to equipment owners, as overpressure protection may have been selected to suit the vessel's MAWP rather than its lower 'design pressure', or there may have been modifications made to the surrounding process to take advantage of the vessel's higher original MAWP. Fixing the problem in these instances could involve undertaking an additional alteration of the vessel to return it to its previous MAWP, or making new adjustments to the process in order to suit the vessel's lower, as-modified pressure capacity.
Confusion can also come from the fact that although ASME Section VIII-1 does not define 'design pressure' for a vessel as a whole, it does define the term with respect to a vessel component, or with respect to a location within the vessel: the 'design pressure' for a vessel component is required to be at least equal to the MAWP of the vessel, with additional allowance provided for any pressure caused by static liquid head at the location being considered. This 'design pressure' that is within the scope of the vessel code is thus required to be at least equal to the vessel's MAWP, whereas the 'design pressure' parameter from the process engineering perspective is a minimum specified MAWP, and thus it is typically less than the vessel's MAWP. This creates an apparent paradox between the terms that has led to some confusing discussions over the years.
A 'design pressure' specified by a purchaser as the vessel's minimum required MAWP ideally should not be included on a vessel drawing or Manufacturer's Data Report, or marked directly on the vessel, as it is no longer relevant to code compliance of the vessel once the vessel design is finalized. Experience has shown that indication of this purchase parameter on official vessel documentation or on the vessel itself can contribute to errors that can lead to unsafe situations and potential rework. If such a parameter is required to be placed on a vessel drawing, it should be clearly labeled as a "process design pressure" or "minimum MAWP required by purchaser", and kept away from the drawing's design data block in order to avoid potential confusion. Appropriate word choice and careful planning of drawing contents can serve a vessel owner well, by helping to avoid the potential for confusion and costly mistakes.
-- https://materials-welding.blogspot.com/
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