Pressure Vessels: Analyzing Change
Patrick M. Nightengale
National Board Consultant
John T. Parson Sr., P.E.
Loss Control Specialist, Kemper National Insurance Companies
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (3 printed pages)
Vessel owners routinely expand and revise their plant layouts or change vessel content and production levels in order to adjust to changing market conditions. While controls exist in the National Board Inspection Code (NBIC) for the repair and alteration of pressure vessels, there is less industry recognition regarding changes in vessel location, service conditions, or contents where there is no physical change to the vessel itself. The level of engineering study associated with changing vessel service or contents, or moving and reinstalling existing pressure vessels including associated piping and supports, often doesn't get the same attention as projects where new vessels and systems are planned. The management philosophy regarding change must be appropriate to the circumstances, otherwise the resultant loss to the vessel, surrounding property, and to human life could be tragic.
Movement may include relocation within an existing facility or to a new facility by the current owner. It may also include purchase of used vessels for installation in another facility by a new owner. When the relocation crosses jurisdictional boundaries or where the ownership changes, the jurisdiction may regulate reinstallation based on ASME stamping and wall thickness measurement. In some cases, there may not be jurisdictional rules to govern the relocation of vessels. Provided the vessel is ASME-stamped and the wall thickness is still acceptable, what else needs to be reviewed?
When the vessel was originally designed, loadings beyond the actual vessel boundaries which had an impact on the vessel integrity were also considered. While the ASME vessel boundary is deemed to be the first flange face or circumferential weld for nozzles projecting from the vessel wall, the original design encompassed more than the pressure-retaining capability of the vessel itself.
In addition to the pressure load, nozzle thickness and the need for reinforcement may have been determined considering additional loadings as described in UG-22 of Section ASME VIII, Division 1. The weight of attachments, both internal and external, weight of attached motors, agitators, and drives, load concentrations due to the weight of attached piping at the nozzle, and wall junction are but a few of the original considerations that might be overlooked when planning a new location.
Temporary attachments, rigging, and transportation planning are also important considerations. Inadequate measures could cause hidden damage to the vessel. The effect of the damage may not appear until the vessel is reinstalled and under pressure, thus presenting a safety hazard. The new location may subject the vessel to environmental conditions such as wind loads which intensify at points of attachment. Corrosive atmospheres and/or temperature variations may also be present and could be harmful to the vessel materials.
CHANGE OF SERVICE
Changes such as contents, pressure, and temperature can be successfully adopted provided there is an understanding of the effect on the vessel. Can the vessel accept increases in flow rate, or will this change create impingement problems on internal surfaces, or loading problems at nozzle and wall junctions? Is the wall thickness still acceptable when the new contents are of a higher specific gravity?
When the new contents are of a higher specific gravity, there is an increase in the design pressure due to the additional static head pressure, without an increase in the stamped MAWP. Are the supports able to safely carry the additional weight of the contents? Are vessel materials compatible with new contents which may increase corrosion rates, perhaps accelerated as a function of changes in service temperatures? Do the new service conditions present cyclic pressure or thermal variations which could shorten vessel life? Will the pressure relief devices and their discharge piping arrangements function properly and reliably?
Owners have to ask themselves some searching questions. Is there a complete understanding of the causes and effects associated with changing service conditions or relocating vessels? Does the owner or potential buyer have the knowledge to analyze these changes, or must outside expertise be used? Should movement and reinstallation of vessels be performed by plant personnel, or should sources specializing in rigging, handling, and shipping be used?
Safety must always take center stage when deciding what to do, but the final decision will also consider jurisdictional requirements such as the NBIC, federal guidelines such as OSHA publication 8-1.5 Guidelines for Pressure Vessel Assessment , and the overall cost in terms of both time and money, should vessel modifications be required.
Ownership of pressure vessels carries with it the responsibility for safety throughout the operating life of the vessel. This includes maintaining a safe operating environment whenever changes occur. Don't allow the loading experienced "in service" to exceed the "design" loadings. Determine the design basis for the original conditions. Identify the new or revised conditions. Are they within a safe range based on the original design? Unsure? Contact the original vessel manufacturer if possible. They may provide original design information not otherwise included on the Manufacturer's Data Report. They may also be in a position to re-engineer the vessel for the new conditions. Alternatively, have a reputable engineering evaluation performed for the existing vessel, and compare the results with the proposed changes.
Be sure of jurisdictional or federal requirements before proceeding. A call to the chief inspector of the jurisdiction where the vessel is, or will be installed, will answer many questions. The insurance company providing pressure vessel insurance may provide a path to solving problems. When repairs or modifications to the vessel are required, select an "R" Certificate of Authorization holder, evaluated and authorized by The National Board of Boiler and Pressure Vessel Inspectors, to perform the necessary work.
Finally, perform the analysis, ask the questions, get the answers, then make an informed decision before committing a safe vessel to a new application.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.