It is a common practice to monitor thickness of pressure boundary wall to estimate thickness loss due to general corrosion. The designer generally provides a corrosion allowance against which repair/replacement decision are normally taken. This is a very conservative approach as there is generally extra thickness available over and above the corrosion allowance due to use of standard plate thickness and additional thickness by virtue of manufacturing tolerances. Integrity engineers can actually consider this extra thickness available to extend the equipment life make use of the ‘hidden plant’ before deciding to repair/replace a particular piece of equipment.
In this post, I would like to offer an overview regarding setting various minimum thickness levels for remaining life calculations on equipment that undergoes general metal loss. This will also aid in determining the total useful life from the equipment while safely tracking the health of the equipment through optimized thickness monitoring schedules – from installation to retirement.
The figure below represents various minimum thickness levels that pressurized equipment will typically experience. These levels can serve as warning/alert levels for the integrity manager to monitor and manage equipment health.
Provided Thickness is the thickness originally provided by the designer; a result of design calculations that include the Corrosion Allowance (CA) and the additional thickness available due to a higher nominal thickness that was selected from standard plate thicknesses available that will include mill tolerances.
CA Tmin is the minimum thickness that would initially be used to calculate remaining life before one would consider the overdesign extra thickness available. This is the starting point for any kind of remaining life calculations during the early stages of an asset’s life cycle. It is derived by subtracting the design corrosion allowance from the initial measured thickness. This can be used as a first warning level and a trigger to explore the next Tmin stage to continued safe operation.
Overdesign Tmin is the minimum thickness that can be used by the integrity engineer to calculate remaining life once the design corrosion allowance is exhausted. This is typically available due to the higher standard plate thickness available in the market or rounding off the calculated design thickness value to a higher number. In some cases this Tmin level may not be available if a plate thickness is available that exactly matches the design calculation value. In typical cases where this is available, the integrity engineer can use this as the second warning level for the purposes of health monitoring.
NOTE: Thicker is not always better. In many cases rounding up and selecting thicker material may introduce other damage mechanisms, such as fatigue or brittle fracture when the additional thickness reduce the elasticity to a point of being detrimental to the design and operating conditions.
Pressure Tmin is the minimum thickness that is calculated by the equipment’s code design calculations. This Tmin can be considered for remaining life calculations after the corrosion allowance and overdesign thickness are exhausted as a result of corrosion. One can consider this as the third Tmin stage or Alert level for the purposes of asset health monitoring. However, it is to be noted that this Tmin may not be the governing factor for this stage in cases where the Structural Tmin is greater than the Pressure Tmin.
Structural Tmin is the minimum thickness that is required for the basic structural stability of the equipment considering dead load, fluid weight, insulation weight, support design, wind load, seismic consideration, etc. As shown in the diagram, depending on whether it is Case-1 or Case-2, this will serve (or not) as the third Tmin stage respectively for the asset’s remaining life calculation.
FFS Tmin is the minimum thickness as a result of fitness for service calculations. These calculations are carried out by plant inspection and engineering specialists as per Level 1, Level 2 or Level 3 methodologies as described in API 579, and also includes a Future Corrosion Allowance (FCA). This is the fourth or last Tmin stage, and the final resort to continue plant operations until the time of acceptable repair or replacement. This can be considered as the high critical alert zone from the purview of health monitoring & reporting, and the thickness is usually monitored with special attention at close intervals for the purposes of safe operations after reaching the third Tmin stage.
For those who already use Asset Performance Management (APM), it is time to leverage these concepts using the Tmin Calculator in the thickness monitoring capability of APM Integrity. For those who are in search of a new software strategy to improve/implement their mechanical integrity work process, this is yet another reason to consider an APM solution as one the prospective high value propositions.