Magnetic Flux Inspection: The Bottom of a tank floor tell you a lot with an MFL scanner.

by John Taylor

When it comes to insuring the safety of any storage tank, no news is not necessarily good news.  The problem with storage tanks is that even when everything looks good on the outside, inside the tank can be a ticking time bomb, where an unseen defect or spot of corrosion can lead to a catastrophic and costly failure.  That's why magnetic flux testing leakage (MFL) testing is vital to the continued longevity of storage tanks across the nation.

Most out of service inspections use magnetic flux leakage (MFL) as a great detector for locating underside floor corrosion. This is often referred to as soil side corrosion caused by the contact of the steel floor with the moisture and chemical make up of the soils underneath the tank. MFL however cannot give an accurate assessment of the remaining wall thickness due to some of the limitations imposed by the inspection environment. The only reliable and accurate quantitative assessment must be obtained by using ultrasonic thickness techniques. These techniques include appropriate operator training and not just the ability to take digital thickness measurements.

This discussion concerns the added requirements for the training and testing of NDT personnel who inspect storage tank floors during an API 653 out of service inspection. The America Petroleum Institute added Appendix G to the 653 code in 2003 to describe the precise requirements for this training and testing. It describes the number of training hours, the types of test questions and even details the construction of a tank floor mock-up with manufactured defects.

Magnetic Flux Inspection Picture 1: MFL (Magnetic Flux Leakage) Scanner.  Model: MFE 2412 MK2

It is essential that a quality A-scan be used to assess any corrosion detected. This equipment must be capable of determining location, size and the accurate measurement of remaining floor thickness. A 5 Megahertz dual crystal focused contact transducer of either .375" or .500" in diameter should be used. Either water or a gel type couplant should be used. It is essential that the amplitude of the signal from the corroded back wall must be raised to at least the same screen height as the calibration reflector. While it is true that a reflector from a straight back wall will give a clean signal the same is not true for a corroded surface. Since there is an irregular profile the energy being reflected back will be wider at the base and have much lower amplitude. In this case the operator's experience in adjusting the ultrasonic equipment is essential. This is the reason that code book now requires the advanced training for ultrasonic prove-up operators.

Here is a summary of the requirements now in place for API 653 tank bottom examiners.

1. A written procedure that address the essential and non-essential variables for a tank bottom inspection.

2. A record of the qualification test for a specific scanning operator.

3. A record of the qualification test for a tank bottom procedure.

4. Each scanning operator shall receive 40 hours of training. The training will include NDT principles and scanning equipment operation and calibration.

5. Specific dimensions of the tank bottom sample (70 square feet) and a minimum number and size of pits to be located during the operator testing.

Magnetic Flux Inspection Picture 2: Tank floor that been scanned and mark. Ready to be A-Scan

Recently we did a tank inspection at na air force base using MFL equipment.  The task, while routine involved the inspection of a tank contining JP-8 (Jet fuel). Everything looked good on the outside and on the inside.  Visually the tank would pass muster, sporting good plate weld lines, no obvious signs of corrosion, even a fresh coat of paint.  After our visual inspection, we took some thickness readings around the corner of the plates and in the middle in order to get an average.   Now the funs begins with the MFE.  As soon as I started to scan the surface of the plate I noticed that the MFE scanner’s screen was lighting up like a Christmas tree.  Usually we do a cross scan of the area so we can narrow down the area so it’s more accurate.  Once any defective area is found and marked, a technician will do an A-scan so we can get the lowest thickness reading of the area and note it in our report. Surprisingly all the areas that I inspected had a reading lower than the average. This meant that the tank was far from safe to refill with JP-8.  All I could say to myself as I climbed back out of the tank was, "Never judge a tank by its cover."

John Taylor is president of Nova Data Testing, a leading Magnetic Flux Inspection contractor.  For all your magnetic flux inspection needs go to http://magneticfluxinspection.com/

Ultrasonic Testing NDT - Risk Based Inspections

Ultrasonic Testing NDT - Risk Based Inspections 

  

By John Taylor 

Recently I had the opportunity to attend a three day class given by Equity Engineering Group on the American Petroleum Institute's Recommended Practice for Risk Based Inspections (API 580 & 581).

It was a great class held in a little Louisiana town known for great music and great food. While the petro-chemical industry is a major user of this inspection approach, it can be utilized in the pulp and paper, power generation and terminal storage industries as well as many others.

Since API 580/581 is based on risk based inspections the question is: how do you define risk? Quite simply it is the probability of failure multiplied by the consequences of failure. This analysis of risk is not only a common definition which relates to all of us and at any age but one that we use when making daily decisions. Do we sky-dive or play pool? It all depends on how well prepared we are to jump out of an airplane or what the environment is like at the bar where we shoot pool. The more you think about those choices the more information is needed to decide on the proper course of action. So the key to any good risk based program is information.

That was the great thing about the Equity Engineering's course. It led you through the initial steps to develop a program that would be appropriate for the facility and the types of equipment that was being utilized there.

There are two references that RBI uses. The first is API 580 which gives the minimum requirements and the basic elements for implementing an RBI program. It also details what makes up a good RBI team and many of the problems that the team can encounter while assembling information. API 581 is the procedure that gives the details and formulas for assessing the condition and inspection frequencies. It assigns the probability of failure and consequence in a logical mathematical sequence.

Insert Ultrasonic Testing NDT Video here. http://www.youtube.com/watch?v=YPZ2ulbn6Lo

Top ten lessons learned about RBI:

1. The mechanism for corrosion and deterioration must be known before an adequate RBI program can be started. It is important that a capable corrosion specialist be a member of the team.

 2. Inspections must be designed to reduce uncertainty. Without certainty probability becomes too elusive to measure. Inspections alone do not reduce risk.

 3. Everyone involved must understand and appreciate the measuring systems that are designed to reflect both the area risk and the financial risk associated with equipment failure.

4. In any RBI study, consistency is more important than accuracy.

5. The RBI program must follow a defined working process that will assure both consistency and reproducibility.

6. Always match the inspection technique with the damage mechanisms that are associated with the equipment type and process. Otherwise you may not find any defects.

7. While the probability of failure changes over time, the consequences of failure are not controlled by time.

8. Since risk is dynamic it is vital that the RBI process chosen can be updated easily when new information is discovered.

9. The consequence and risk results for atmospheric storage tanks are in financial terms alone. The consequences considered are from component damage, product loss or environmental penalties.

10. RBI can only define an acceptable level of risk to the owner-user. A Risk Based Inspection program is a blend of solid communication, quality information and good judgment. And good judgment comes from experience and experience as we all know comes from bad judgment.

Just as in any of life’s adventures, mitigating risk is all about understanding how to assess and manage the probability of failure. 

John Taylor is President of Nova Data Testing a leading Ultrasonic Testing NDT specialist.  For all your testing needs, go to http://ultrasonictestingndt.com/

    

Nondestructive Testing – What I Did on my Summer Vacation

Most people are unfamiliar with the world of nondestructive testing.  In fact, more people have heard of Quantum Physics and String Theory then have heard of the science of NDT. Like these other fields of scientific inquiry, once introduced to NDT, most folks immediately want to know more. Unlike theoretical physics, which is something that few people come into contact with on an everyday basis, the practice of NDT is one of the little known processes that keep our technological world spinning day in and day out.

Having just returned from a service trip to the North Rim of the Grand Canyon while working with the US Park Service, I met a number of other volunteers from all over the country. There was a doctor, two lawyers, a librarian, some teachers and even a retired park ranger. But of the twenty or so people who took part in the project, only one had any experience at all with NDT.

As is customary with groups like these, it wasn’t long before we started to introduce ourselves and what we did for a living.  When I told the group that I was an NDT professional, they all looked at me as though I had just admitted to being an alien from another planet.  Once I showed them some photos of the equipment used in my line of work, they were convinced of it.

 http://youtu.be/YPZ2ulbn6Lo 

Whether you realize it or not, NDT professionals are an integral part of the US work force. They are involved with all types of industries and are a necessary requirement for industrial safety and reliability. The reasons that bridges don’t fall and pipelines and storage tanks fail to burst is due to legions of nameless testing professionals who are responsible for inspecting the nation’s infrastructure.  The other big beneficiary of this kind of work is the environment.

NDT helps to prevent accidental releases and spills of all sorts of nasty products and chemicals into our soil and water sources of this country.  Maybe that’s why when talking about NDT to a group of volunteers for the Park Service they seemed so interested in the techniques and procedures surrounding nondestructive testing.

While there were many occupations represented by the group, the person most familiar with NDT was a retired park ranger. A few years back, some underground gas lines had been discovered on a parcel of land the park service was looking to acquire. As part of the land deal, the gas company that owned the pipeline was required to inspect the line.   The park ranger I met on my trip had been assigned the task of insuring the inspection was completed. After hearing what I did for a living, she questioned me at length on the various types of NDT inspections that I performed.  

I took the time to give her some technical insights about the inspection requirements for different types of above and below ground piping.   But all I kept worrying about was that