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
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
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
, 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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.