This is the fourth 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 talks about tank construction considerations which can be found here
Once we have constructed our tank and have it operating, we need to take care of it, like we discussed in the first article in this series, it’s pretty important to keep tanks in good shape as failures can be incredibly expensive and dangerous. The primary we make sure out tank does failure is to perform regular inspections so we can identify issues and fix them before they cause a failure.
A quick note about the designer
Under current Western Australian legislation it’s a requirement that the designer provide sufficient information on the inspection and maintenance of assets to ensure the assets are able to be used safely, its also a requirement of owners to follow the inspection and maintenance regime set out by the designer. If you have the information provided by the designer or manufacturer of your own tank, you should follow that information, which would make most of what’s written here redundant.
In our experience, many assets are designed without any consideration given to the care, inspection and maintenance of that asset, and as such no instructions are provided by the designer or supplier as to how to maintain that asset. If you find yourself in this situation, read on.
Storage Tanks – When To Inspect?
The most commonly cited tank inspection standard in Western Australia is probably API 653, and thus we will write this article with reference that that standard, however this is not the only way to inspect, and assuming you can back up your inspection choices with sound engineering principals then many approaches are suitable.
As per API 653, it’s recommended that tanks are inspected externally at a minimum at least every 5 years, these inspections can be conducted with the tank in operation and are primarily to search for obvious signs of deterioration. Depending on the condition of the tank a more frequent inspection interval might be prudent, when choosing an inspection frequency, some (but not all) of the factors should be considered:
- The contents of the tank
- Corrosion allowance, corrosion rates, and corrosion prevention systems.
- Condition at previous inspections
- The environmental condition of tanks,
- Any leak detection systems
- Any changes in operation modes,
- Materials of construction
Much of this could be considered common sense, it’s obvious that a tank stored near the ocean that is filled with corrosive materials would probably require more frequent inspections than one in a cool dry climate storing an inert material. However we are reluctant to call this process easy, not all examples are as straightforward as that one, and with numerous factors to consider it can often be a tricky task to determine the most suitable inspection interval for a specific tank.
In many cases ultrasonic thickness testing (UTT) will be conducted at the same interval as an external inspection, however as per API 653, if the corrosion rate of the material tank has been determined it is possible to extend this out although its not recommended the interval is any greater than 15 years.
Finally we must consider internal inspection, the API standard as a few factors that go into determining an internal inspection interval. These intervals are usually greater than 10 years, and depend on what sort of protections the tank has (if any) to prevent premature failure. In many cases an internal inspection will be recommended following an external inspection or of UTT to confirm the condition of a tank.
Storage Tanks – Who Can Inspect?
We would recommend using a qualified person to perform tank inspections, this person is usually a structural engineer however not always, as anyone with appropriate qualifications and competence should be able to perform the inspection.
Th easiest way you can rest assured you’re using qualified people is to used API 653 certified inspectors. API 653 certification is verification that a person has a proper understanding of tank inspections, and understands the requirements of the API standards relevant to the inspection of above ground storage tanks. Rapallo does employ API 653 certified inspectors and can offer this service.
Storage Tanks – The Purpose of Inspection
The purpose of inspection is quite simple, to identify defects before they can cause a failure. External inspections focus on external defects as would be expected, these are primarily physical damage such as dents, divots, cracks and any evident forms of corrosion, as well as checking for failure of the coating.
Ultrasonic thickness testing is primarily focused on loss of material due to corrosion. By measuring the thickness of the material and comparing it to previous inspections inspectors are able to determine the approximate rate of material loss which is useful in determining expected tank life, and future inspection dates.
Internal inspection has a couple of purposes that can’t be evaluated via either of the other two methods, these are:
- To check the condition of the tank bottom for leaks, corrosion, or other forms of damage,
- To check the tank floor for settlement
- Gather data for minimum floor thickness assessments as per the API 653 standard.
Finally, when defects are found its important that they are dealt with, some specific considerations that must be taken into account when repairing tanks will be discussed in a future article however, one thing we can tell you here is that tank inspection is pointless if nothing is done to resolve defects. The key purpose for all tank inspections is to find defects such that they can be rectified and prevent a tank failure before it happens.
