MFL Limiting Factors
Factors affecting the reliability of Magnetic Flux Leakage Surveys Of Above Ground Storage Tank Floors
Over the past 15 years the preferred method for the inspection of Above Ground Storage Tank Floors has proven to be by the Magnetic Flux Leakage method. Unfortunately, service companies have been less than forthcoming with some of the limitations of this particular application of MFL. This has led to some confusion within the industry as to what this inspection method is capable of achieving. There are too many stories in circulation about significant defects being underestimated or missed completely. The following information addresses some of the limiting factors of Magnetic Flux Leakage surveys and what to expect from this inspection technique.
Adjustment of Equipment
It is vital that the equipment is adjusted in accordance with the manufacturers recommendations. These adjustments will vary in relation to both thickness of the plate to be inspected and the thickness of any coating. In order to create leakage fields from corrosion it is necessary to achieve near saturation of the magnetic flux in the material. If near saturation does not exist then no leakage field will be generated and therefore there will be nothing for the sensors to detect.
The top surface condition of the plate to be inspected absolutely dictates the achievable level of sensitivity of the inspection. The rougher and dirtier the surface, the more noise that will be generated from the inspection process and the lower the achievable sensitivity. Real signals may be masked by the level of noise generated.
Tank floors are seldom flat. The degree of curvature of the plates may have a significant impact on the reliability of the inspection. When the curvature of the plate causes an increase in lift off by either the magnetic bridge or sensor the sensitivity may suffer dramatically. This can often be overcome by scanning in more than one direction. Curvature of plates close to lap joints and/or the shell can make it virtually impossible to scan these areas reliably. In some cases smaller scanning heads can be used to overcome this problem.
The material itself can also have a significant impact on the inspection. Most floor plates are manufactured using low carbon steel, which lends itself very well to this technique. Higher grade materials require a much higher magnetizing force to achieve the same levels of saturation. This also applies to materials whose properties have been degraded by the service. i.e. Sour Crude Storage. In this case the material suffers from hydrogen attack and it may not be possible to achieve the necessary flux saturation of the material to achieve the required sensitivity. This can also be a major contributor to noise levels. The inspection direction can also have an impact on the amplitude of signals from the same sized corrosion pit. It has also been noted that noise levels can vary even on clean smooth surfaces due to either variations in the material properties or method of manufacture.
It has often been claimed that MFL can achieve 100% coverage of the floor area. In reality this is virtually impossible to achieve due to the overlapping nature of the plates and the number of obstructions in most floors. It is also difficult to inspect adjacent to the shell because of the curvature due to edge settlement. There are often many areas of the floor that are inaccessible to the regular scanners. Smaller scanning heads can be used to alleviate some of this problem but it is rarely possible to cover the entire floor using MFL alone.
Product Side & Underside Discrimination
Magnetic Flux Leakage is incapable of reliably discriminating between product side and underside corrosion. In most cases this is not a problem as it is possible to discriminate signals visually. In the case where there is a known topside problem that has been documented and passivated (by coating) after an earlier inspection and removal of the coating is not necessary then it is helpful to the inspection agency to have access to the earlier inspection information. This can speed the inspection process significantly. The very nature of MFL means that in some cases product side corrosion cannot be detected by the floor scanner. As a general rule the response from a product side pit with the same volume loss as an equivalent underside pit will be significantly less. This is due in part to the higher flux leakage levels being retained within the depth of the pit itself whereas on the other side (underside) of the ligament the leakage field will be above the surface and therefore in the direct path of the sensors.
In addition to the above limitation and in the case of thicker coatings, there is another factor that affects achievable sensitivity. While it is often possible to overcome the additional lift off of the magnetic bridge by lowering it within the carriage, the same cannot be done with the sensor array to place it as close to the surface as possible because the coating is in the way. This obviously means that it will no longer be possible to achieve the same levels of sensitivity that could be demonstrated on the uncoated plate of the same thickness. In all such cases, it is necessary to run a simulation prior to the inspection to see what can be achieved.
Some manufacturers employ a blind thresholding approach to this application in order to semi- automate the inspection. It is deemed a blind threshold if the MFE/MFL equipment does not provide a real time display that displays a viewable signal as the scanner passes across the tank floor. With blind thresholding tools there is no display and signals generated that may be significant will not come to the attention of the operator as the operator will not know the signal took place. In other words, the floor could have a defect that caused a signal that for whatever reason fell just short of the required threshold level and therefore the unit would not stop. With no display to see the signal, the unit would continue to roll across the floor and the operator would never know the signal occurred. The manufacturer’s claim is that the unit can be set up so that only defects depths of concern to the tank owner can be found and all signals for defects that are not deep enough to warrant further investigation can be ignored. This approach claims to save time and money.
MFE Enterprises does not support this blind threshold approach and feel that this is a major reason why significant defects have been missed. The amplitude of signals generated from corrosion pitting is more a function of the overall volume loss than the remaining wall thickness. This indicates that three dimensions (length, width and depth) affect the signal response and not just pit depth. Defects exhibiting largely different amplitudes can have the same through wall dimension. Couple this with the need to control and monitor all the inspection parameters (virtually impossible in this application) and it means that this approach is seriously flawed both theoretically and in practical terms. MFE Enterprises provides a real time display that must be monitored by the operator. If the same above described situation occurred with an MFE stop on defect unit, the operator would see the signal even if it did not exceed the threshold level. The operator could investigate as many of these signals as deemed necessary to gain confidence in the current therehold or could make changes in the threshold as necessary to reflect the proper threshold based on the ultrasonic readings obtained when investigating signals.
Corrosion Mapping of Flux Leakage Signals
Systems that use a multiple threshold and supposedly accurately map the results in the form of a color coded output related to wall thickness are available on the market. It is fashionable to computerize everything these days. Unfortunately this is one application that could suffer by this approach. First of all, Magnetic Flux Leakage is incapable of accurate quantification of remaining wall in this application owing to the volumetric nature of the defects as well as other factors such as lift off of sensor and/or magnetic bridge.
Originally MFE Enterprises, Inc. did not offer mapping scanners due to the false perceived advantage of being able to determine pit depth with MFE/MFL as opposed to using ultrasonic tools to obtain quantitative information. Competitors made (in our opinion) false claims. Through input from customers, it was later revealed that they found out or already understood the problems as regards to false claims. However, customers liked the automated reports and color map presentations of the tank floor overall condition.
They learned they could not use the equipment with ultrasonic data as regards large numbers of small repair patches but they could make repair decisions as regards the overall condition of the tank floor as “in general” larger signals were “usually” indicative of deeper pits. These factors led to a determination by MFE to build a mapping scanner but of offer it without the false claims of MFE/MFL providing pit depth.
As with any equipment the operator is responsible for its correct use and the validity of any inspection is absolutely dependent upon the operators integrity. It is probably better to have a good operator with the worst equipment rather than a bad operator with the best equipment. Of course, the best situation is to have a certified operator, equipment, and procedure. All operators should, as a minimum, be able to show proof of training in the use of the equipment they are operating along with a reasonable understanding of the technique. MFE Enterprises provides 8 hours training free of charge other than for the reimbursement of expenses when the equipment is purchased from MFE. MFE also provides training according to API 653 Appendix G. The options exist for test plate training and/or 40 hours of training based on API 653. NO TWO PIECES OF EQUIPMENT ARE THE SAME. It is imperative that the end user goes for the best equipment and a good level of confidence in the person using it if they want to get the best inspection possible. This is not the type of equipment or technique that can be made to be independent of the operator. Some manufacturer’s claim equipment is “technology driven not operator dependent”. All MFE/MFL equipment and most NDT equipment operation are to some degree “operator dependent”.
No two tank floor inspections are the same and can range from smooth, clean, flat, defect free, rough, dirty, buckled and corroded throughout. It is obvious that a good inspection of the latter is going to take significantly longer and require more effort than the former. In spite of this situation many of these inspections are bid on a lump sum basis? It is impossible to foresee the amount of time and effort involved. A tight bid on a bad floor with extensive corrosion is bound to put additional pressure on the contractor. It may induce pressure to make a financial decision to limit the time spent on that particular project. What normally happens in a case like this is that some of the corrosion detected goes un-reported as the contractor tries to make the best of a bad job. As already discussed, if the decision is made to select spots to mark on the floor on the basis of amplitude of signal alone, it stands to reason that significant defects will be overlooked. This process may lead to missed defects and leaking floors. This should not happen if the inspection is based on time and material with the end user taking some responsibility for progression of the work. Invariably the best inspections are carried out by this method.
Note to end users and tank owners:
On a daily basis hundreds of tank floors are being successfully inspected by competent well trained operators using reliable equipment who have a full understanding of the above and are not afraid to mention the limitations of the technique. There is nothing else out there that can compete with the reliability and speed of Magnetic Flux Leakage provided that the technique is properly applied and that all the limitations are fully understood.
Make sure the operators are trained and certified on the particular unit they will be using to inspect your tanks!
Make sure the inspection company has a certified procedure in place for inspecting tank floors!
Make sure the operator can provide a “function test” to prove the equipment works on the thickness of floor and coating that will be inspected!
Are you ready to move forward?