This is the last blog in a 5-part series about tanks. If you want to read the first entry about why tanks are important click here, or for the second part about tank design considerations click here, the third part about tank construction and the fourth part about inspection can be found here, and here
Now, we’re at the stage where we have designed and built our tank, its been operating for some time and after one of its inspections have found a defect, something that needs repair work, or possibly a change in the design of the tank, otherwise whatever is causing the defect will just continue to happen.
Storage Tanks – Defect Investigation
When a defect is found, the first thing that must be done is a complete investigation into why there is a defect in the first place. This isn’t something that is always carried out and some pIn some cases it’s something simple such as unavoidable corrosion however in others there is a flaw in the design that will over time likely result in a flaw or failure. Flaws in design aren’t necessarily only caused by the tank design, the design of the process or the plant in which the tank is used could be the cause of these problems. For example, if a tank is stored below something which causes corrosive material to leak on it from above, this is not something the original tank designer could have foreseen, however, it is a failure of design all the same.
It may help to ask the following questions during your defect investigation:
- How is the tank being used, and is this in line with the recommendations of the designer?
- What are the environmental conditions, does line up with those specified by the designer?
- What substance is being stored within the tank, is this within the tank’s specification?
- Is there a foreign substance of object that has come into contact, or is in contact with the tank?
- How is the tank currently maintained, what maintenance tasks are performed and how does this compare to the maintenance regime specified by the designer?
- How has the defect formed over time, is this the first time it has been noted or has it grown over a period of inspections?
- How do defects of this type typically form?
You’ll notice that many of the questions relate back to the designer’s recommendations or specifications, this is because when used and maintained as per the designer’s recommendations storage tanks should last at least as long as their design life, which is usually multiple decades. In most cases when a defect has been found it is most often caused by some form of neglect that has resulted in the tank being used, stored or maintained outside its specifications.
Storage Tanks – Repair Design
The repair process for tanks, is actually quite similar to the design and construction process we have discussed in previous articles, this is because the quality of the repair needs to be as good, if not better than the original design. Thus, when we design our repair, we must consider everything that was previously discussed in our Tank Design article (found here), additionally, we must consider all the findings of the defect investigation. For example, if it was found that a lack of maintenance caused the failure we must ask whether maintenance will improve in the future, if it will then we can repair the tank to the original design, however, if for some reason it will not be practical to maintain the tank as the original designer intended there may be a requirement to alter the design such that the tank won’t fail under the reduced level of maintenance.
There are additional design requirements related to how the repair can be performed. In some cases when a minor fault is found (such as a small leak) the tank will continue to be operated and if an opportunity for a shutdown isn’t able to be created soon the tank may need to be repaired during operation. In-service repairs are generally risky and should be avoided if possible, however in some cases it can be more dangerous to not perform the repair (for example, if a tank is holding a significant amount of hazardous material and there isn’t a safer location to store it). It needs not to be said that safety is a key concern during these sorts of repairs and as such it’s important the repair is designed in such a way that it can be performed safely on an operational asset.
Storage Tanks – Completing the Repair
As stated in the previous paragraph, the requirements of a tank repair are usually the same or greater than those present during construction. We discussed these requirements in a previous blog HERE.
The key thing that ties all the repair steps together is the inspection test plan (ITP), this will highlight all repair activities and specifically their acceptance criteria and testing requirements. By creating and following ITP compliant repairs are likely to occur. Part of the ITP will be selecting the acceptance criteria for many of the tasks, in many cases, this will be a standard or code, the most commonly cited repair code for tanks is one we’ve mentioned before, API 653. Although this standard might not always be the best choice, it often is and it does have a wealth of information on the different types of tank repairs that might be carried out and the best way to carry them out, it is highly recommended that a this standard is consulted prior to carrying out any tank repair.
Storage Tank – Document Review
Finally, just as within the tank construction process a document review is a crucial step and should be completed not only throughout the process (as specified in the ITP) but also upon conclusion of the job. By working through the relevant documentation it can be confirmed that the repair is of proper design and sound construction, thus the failure of a improper or poorly performed repair can be prevented by checking these documents prior to putting the tank back into service.
Conclusion
This final article concludes our series on tanks. I hope you’ve learned something, if you need any more guidance than was offered here it might be time to consider employing a professional. At Rapallo we have been dealing with tanks since our first year of operation in 1987, you can rest easy knowing that we know tanks. If you need any help at all please call our office and we will let you know what we can do for you.
