We all love to have new equipment but economics dictates that we preserve our assets for long periods. Let’s take a look at the lifetime of some storage tanks. We will look briefly at the early stages of equipment and move on to extending the life of mature assets through Fitness For Service (FFS).
Large storage tanks for hydrocarbons are built to three Codes as follows:
- • API650 for low pressure service (<= 2.5 pig internal pressure)
- • API620 for internal pressure from 2.5 to 15
- • Anything 15 psig and over falls under the ASME pressure vessel code which is not typically used for large tanks. It is not unusual to have a bank of smaller vessels configured for large storage capacities and higher pressures.
API653 (Tank Inspection, Repair, Alteration, and Reconstruction) invokes required inspection intervals and techniques for large tanks. Many large tanks have been in service for decades. Some of us have worked with riveted tanks but they went out of favor after the 1940s when welding technology made great leaps forward. New tanks are almost exclusively welded around the world but there were still a few holdouts for riveting in some discrete geographies. Properly designed tanks built according to Code have some provisions for future corrosion, either higher grade alloy, a corrosion allowance or a combination of both alloy and corrosion allowance. Many of those first welded tanks form the 1950s are still in service but eventually, the tanks exceed their anticipated lifespan or experience some other degradation. Poor maintenance, contamination of the product, or a long list of other failure mechanisms can come in to play.
API653 sets inspection intervals based on risk and tends to be conservative (but not in all cases) for finding corrosion before it creates a leak. As the tank wall corrodes the corrosion allowance is consumed and the tank wall thickness approaches the design minimum thickness (also known as t-min or tmin). Once the thickness drops below tmin, the equipment is basically no longer Code compliant. Before API 579 Fitness For Service (FFS), which was first published in the year 2000, the tank could be taken out of service and repaired or the owner could continue to operate at their own risk.
API 579 Fitness-for-Service provides guidance and standards for accepting some degradation, even below the minimum required thickness for very localized damage. If the damage covers too great of an area or if the damage runs too deep below the component surface, then repairs may become necessary. API 579 is also valuable in that it can be used to make the determination on what to accept and what to repair.
In the author’s experience, the first-time consulting with owner or inspection organization, the FFS evaluation was a paper exercise and no field work was immediately required. For example, general metal loss or pitting corrosion at the minimum design thickness is based on weld joint efficiency (denoted by the parameter E in API 650). During original tank design, the API 650 Code dictates E= .7 for uninspected weld joints, .85 for partial inspected joints, and 1.0 for 100% inspected joints. In many cases, the pitting is on solid plate and remote from any weld joints. In these cases, the minimum thickness for solid plate can be adjusted and declared to be ~15% to ~30% thinner than the original calculations.
To further clarify this example, let us imagine a partially inspected weld joints with an E value of .85 and a t min calculated during design to be .57” with a 1/8” corrosion allowance (.57 + .125 = .695”) so the plate would most likely be ordered at ¾”. As the thickness corrodes and gets close to .57” remote from any weld seams, FFS can be implemented and the t min can be recalculated to be .5”. That .07” might not seem like much but when compared to corrosion rates, it might provide several more years of service before a repair is required. One word of caution here, corrosion rates tend to accelerate as equipment ages. This is especially true for pits that can collect contaminants that will not get flushed out or drain from deep pits. In such situations, the tip of the pit will start to grow at a faster pace.
For the tank in Figure 1, evaluating damage on the tank floor, shell, columns, rafters, and all other components can be time consuming. A level 1 fitness for service can be used when the damage is not to severe and limited to areas with boundaries that can be well defined. In the event the FFS Level 1 criteria is not satisfied; additional inspection data can be collected and FFS Level 2 can be implemented. Once again, the time required to gather all pertinent data increases with the complexity of details required for Level 2 evaluation.
The good news is that engineering time to evaluate the data has been drastically reduced with the use of software package. The picture above is an example from the evaluation license package of AMETank (one such software). Before computers were on every engineer’s desk, it took several weeks to manually calculate a new tank design and have it go through the checking and signoffs. Now, the tank can be calculated in a few hours and checked just as quickly. Likewise, the inspection data can be fed into the software packages for quickly determining where the critical dimensions and thicknesses are required. Many times, the FFS Level 2 engineering evaluation is sufficient and most tank components can be declared fit to be put back into service. Sometimes, renovations are required but a good engineering evaluation can limit the scope and therefore the down time to complete repairs is reduced. In today’s world of high reliability demands, every hour of repair converted to production time is valuable.
In cases where the Level 2 criteria cannot be satisfied, there is a choice to go to Level 3 which typically involves and Finite Element Analysis OR to just make the repairs. The time for FEA can be lengthy and even then, there is no guaranty that the tank can pass Level 3 FFS criteria. It can be disheartening to take the time and expense of FEA, only to find out that Level3 cannot be satisfied and the long down time and cost of repairs was inevitable. The cost and time of repair can be estimated and compared to the engineering evaluation in many cases, and if close, it is sometimes best to just make the repairs.
More good news is that the software packages are becoming more complex but also more useful as Finite Element Analysis is making its way into standard tank and vessel software packages. The Level 3 analysis can many times be completed in a very short time span.
With modern software tools and inspection techniques, it is many times possible to include a future corrosion allowance and add many years or even decades to the life of existing equipment.