Fig. 2. Tank shell to nozzle fillet weld crack.

Tank Tales - 4 cases of failures, damage and inspections of storage tanks

Some tank failures are well known by the public: 1. The distilling tank with molasses, which killed 21 in Massachusetts in 1919; 2. The Bhopal tragedy in 1984, where people were exposed to methyl isocyanate gas with 3787 or more deaths; 3. The sulfuric acid storage tank failures in 2001 in Delaware City , where one person was killed, eight others were injured, and significant damage ensued to aquatic life.
 
^ Fig. 2. Tank shell to nozzle fillet weld crack.

Article By Ana Benz, Chief Engineer, IRISNDT, Canada
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1. Welding the drain nozzle in large steel tanks

During hydrostatic testing a NPS 3 (Nominal Pipe Size, NPS, is a North American set of standard sizes for pipes tank drain nozzle leaked. The tank was fabricated to carry diesel. The drain nozzle had one fillet weld joining it to the shell and another joining it to a ‘repad’ (a reinforcing pad which is a plate formed to the shape of the tank or vessel around a nozzle for extra strength). The welds had multiple cracks, porosity and non-fusion. See Figs. 1 and 2.

Lesson learnt about welding the drain nozzle in steel tanks


Tanks are welded from the floor up. This means that access to this location for welding is challenging. To prevent leaks, the welding passes can be deposited in multiple stages to prevent the formation of continuous leak paths. The hydrostatic test hold times should meet and exceed the standard requirements. Fluorescent liquid penetrant could make identifying a minute leak easier.

2. Concentrated sulfuric acid tanks require internal inspections

Sulfuric acid (H2SO4) is the largest volume and one of the most important industrial chemicals in use today. It is ubiquitous for water treatment and so important that the standard practice1 NACE SP0294- 2006 “Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures” has detailed guidelines for maintaining these tanks. The standard lists recommended intervals for performing internal inspections. These practices state “Most tank leaks are caused by localized effects on the acidcontacted surface, such as inadvertent dilution, excessive turbulence, or localized overheating… such areas are almost impossible to locate with certainty by external inspection.

Therefore, internal inspection shall be performed at the interval.” The necessity of internal inspections and profi le radiography of nozzles smaller than NPS 8 is illustrated in Figs. 3 to 5.

Lesson learnt about concentrated sulfuric acid tanks

These tanks require internal inspections.

3. Corrosion under insulation (CUI) of tanks

CUI is a common problem for refi neries, petrochemical, offshore and onshore facilities, power plants, chemical, and fertilizer plants. One’s tendency can be to think of CUI as a problem next to a marine environment, but this is not always the case.

CUI can happen in dry industrial sites as far away from the sea as the Canadian prairies. Once equipment is insulated, the location of cooling towers, leaking, heat tracing, rain, mist from melting snow that saturates insulation, and other leaks/steam impact CUI. CUI can stay undetected until a leak develops or until the insulation and cladding/jacketing is removed or damaged; it is challenging to find.

Today Pulsed Eddy Current (PEC) can be used to assess some of the CUI of Tanks as it can decrease the need for insulation removal:

• A PEC probe scans the insulated equipment and identifies general (averaged) losses, but isolated pits can be difficult to detect.

• The detection sensitivity of this electromagnetic inspection technology depends on the footprint of the probe, liftoff/insulation thickness, and on the steel thickness.2

• As for other electromagnetic techniques, edges (i.e. nozzles, flanges, or the end of structure) pose a challenge for inspection.

Fig. 6 illustrates an application of PEC on a tank. The tank at one time had been insulated, but after the insulation was removed, corrosion products were identified under the ladder rungs. Inspection personnel scraped the products to assess the remaining thickness, but the tank started to leak. Consequently, PEC was used to assess the overall losses. Though the inspection did not give remaining thickness values, the loss trends are illustrated in the figure and based on them, a repair was planned and made.

Lesson learnt about CUI of tanks

Tanks develop CUI (even in the dry Canadian prairies) and need detailed inspection plans when assessing CUI.

4. Non-intrusive non-destructive tests

Acoustic emission of FRP and of ammonia tanks

AE tests of FRP and ammonia tanks provide volumetric tests of shells of the tanks while in service. Entry is not required, avoiding the potentially damaging process of shutting and exposing the tanks to air (when ammonia tanks can develop stress cracks) and thermal stresses for ammonia service. The tests aim to detect and locate areas of concern; Fig. 7 shows tank bottom pits identified with acoustic emission. Follow-up inspection with a complementary NDT method being needed to identify and size any AE indications for ammonia steel tanks. FRP tanks require visual follow-up.

This technology was proven and implemented by Monsanto personnel. As stated by Martin Peacock,3 “the initial round of testing led to shutting down several tanks for inspection and repair of fabrication defects detected by the AE test. However, once the tanks were repaired, no further inspections were required. One tank in the USA has been in continuous service with regular AE testing since 1984. This tank is tested every five years with the last carried out in June 2011 with no indication of service related damage to the shell.”

Lesson learnt about non-intrusive acoustic emission of ammonia tanks

Used wisely, these tests can keep FRP and ammonia tanks operating reliably without causing inspection related tank damage.

Robotics

Today, we are performing robotic inspections to monitor the thickness of tank shells and roofs.4 Robotics is also used to inspect water tanks. The future holds the promise of robotics being used for tank thickness, visual and various other internal inspections for multiple other services (see Fig. 8). This would reduce the need for personnel to enter confined spaces and the time and budgets needed for emptying and storing tank contents elsewhere. However, cleaning and navigational challenges are today the obstacles that need to be overcome.

Lesson learnt on robotics non-intrusive tank inspections

This field promises revolutionary changes.

Overall summary

Tanks are part of our everyday life and they appear, deceivingly, simple. However, they require a great deal of know-how and specialized knowledge to operate reliably.

Acknowledgements

I am thankful for the great collaboration from many of my colleagues and customers who have allowed me to show their images. Special thanks to Chris Bishop, Martin Clements, Dustin Loveland, Dexin Lu, Martin Peacock and Marten Sales.

References

  1.  “NACE SP0294-2006, Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures,” electronic source.
  2. API RECOMMENDED PRACTICE 583 “Corrosion Under Insulation.”
  3. Martin Peacock, “Improving the reliability of an ammonia tank AE test,” Insight - Non-Destructive Testing and Condition Monitoring, Volume 54, Number 12, December 2012, pp. 681-687(7), The British Institute of Non-Destructive Testing.]
  4. Electronic source. Equipment being used for tank roof inspections, https://inspection-robotics.com/ bike-platform/
 

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