Ultrasonic Tank Testing
Capability and Benefits in Determining Corrosion Rate and Remaining Service Life
Ultrasonic testing, or UT as it is commonly called, is the procedure of introducing a high frequency sound wave into one exterior side of a material, and reflecting the sound wave from its interior surface to produce a precise measurement of wall thickness. The round trip duration of travel, divided by the known sound velocity through that particular material, provides a wall thickness measurement equally accurate to a micrometer or caliper reading.
Ultrasound is a well proven and respected diagnostic tool routinely employed for weld and flaw detection in critical applications such as aviation, aerospace, military, and nuclear power.
Yet, while improvements in instrumentation have moved this technology into other areas such as manufacturing and quality control, its benefits to plant engineers and property owners as a diagnostic and predictive tool are still widely underutilized.
The Many Advantages of Ultrasound
As a nondestructive method, UT offers obvious advantages over cutting out tank sections for thickness measurement or metallurgical inspection. It is non-intrusive, accurate, reliable, safe to both building and inspection personnel, provides immediate results, requires no system shutdown, and is extremely cost effective.
Depending upon the measurement technique, degree of testing, data analysis method used, and especially the competence of those performing the inspection, ultrasound can produce a general assessment of tank condition, provide direction for capital projects, or focus in on a specific area of concern.
Such advance information is becoming more valuable to plant engineers as the former “run to failure” mode of operation moves toward one where all known vulnerabilities of building or plant operation are known and monitored, and where long term planning has hopefully replaced unexpected failures and emergency repairs.
Establishing the condition of an aged water, condensate or air storage tank becomes especially important due to its critical function in any HVAC or building operations environment, and due to the wide variety of problems which can potentially develop. In most examples, ultrasound will reveal a hidden tank wall corrosion condition and enable further investigation and repair procedures to be carried out long before replacement is the only alternative.
Corrosion Protection Impossible
Unlike most HVAC piping system, chemical inhibitors cannot be used at storage vessels containing domestic water. Fire water storage tanks are rarely, if ever, chemically treated to reduce corrosion activity. Tanks and vessels serving as components to an HVAC piping system would be expected to benefit by the same chemical inhibitors, if present. Cathodic protection is provided to larger municipal water storage tanks and other more critical service, but not to building property tanks and vessels.
This means that corrosion losses to most commercial building properties are often entirely dependent upon the aggressiveness of the local water supply, the quality and durability of the internal coating protection if it exists, and the corrosion resistance of the steel itself.
Corrosion Monitoring Lacking
Corrosion monitoring is rarely if ever provided to water storage tanks, or those serving steam condensate, compressed air, and many other HVAC or building services. For more critical applications such as petroleum, nuclear, military, and aviation, tank condition monitoring is regularly performed. In many examples where the tank is a component to a chemically treated piping system, such as an expansion tank to a chill water system, corrosion monitoring provided at the piping can be expected to indicate similar wall loss at the tank itself. Otherwise, the first indication that a corrosion problem exists is only if internal inspection is performed, or when a leak or failure results. Often, the failure at an inlet or outlet section of piping, typically thinner than the tank wall, will offer the first indication to a problem.
High Threat Area
Most water storage tanks have a water line which varies. This is especially true for domestic cold water storage tanks, condensate tanks, and expansion tanks. Dedicated fire storage tanks are mostly static, but can still vary their water level due to expansion and contraction of the tank with temperature and due to draining and renovations. With few exceptions, it is at or near this water line where the highest wall loss takes place due to constant washing of the metal and the available oxygen to drive the corrosion reaction.
In the below graphic we illustrate this condition at a domestic water storage tank. Recording more than 1,200 equally spaced wall thickness measurements along the shell allowed plotting the data for a virtual three dimensional look at the tank interior. Here we show the result of taking measurements from the 12 o’clock to 6 o’clock positions at each side of the shell – the results separately illustrated for each shell side. This graph shows high and more uniform wall thickness above the water line, a deep channel of corrosion loss directly along the water line, and mild pitting below the water line.
Domestic Water Storage Tank
Separate ultrasonic testing at the front and rear tank heads produced identical results also showing highest deterioration directly at its water line.
Thorough Testing Required
In order to provide the greatest degree of reliability, any evaluation method must address as much of the tank as physically available. This is often difficult depending upon its construction and location. Taking substantial thickness measurements may be necessary for larger tanks or where a high degree of coverage is necessary. For most applications, taking a few wall thickness measurements every few feel is unacceptable, and unlikely to provide any worthwhile result.
Determine Corrosion Rate
With the original tank wall thickness and time in service known, calculations can be made regarding the approximate speed, as indicated in mils per year, that the metal has reached its current thickness level. Even though the tank wall is not likely to have corroded evenly over time, such corrosion rate estimates are generally accurate, and will fall within a certain range of values depending upon piping service.
A theoretical minimum acceptable wall thickness calculation, or an estimate of the lowest point the tank should be allowed to safely operate, can also be made based upon material strength, tank diameter or dimension, and operating pressure. This allows a further prediction of the remaining service life at the tank according to the time it will take to deteriorate from its current wall thickness, at the current corrosion rate, to its minimum acceptable value. From this point, a retirement date or remaining service estimate can be offered.
For those tanks having an internal coating, results will often vary widely. Where the protective coating holds, wall thickness is typically at new sheet metal specifications. In the generally random areas where it has failed, deep and often severe pitting can be expected. Our Tank Photo
Identify Overall System Status
Overall, ultrasonic tank testing offers tremendous benefits. For many building operators, an ultrasonic report will very often provide the very first suggestion of a corrosion problem or concern – and provide the advance notice required to address it effectively.
Ultrasonic testing can provide irrefutable evidence of a suspected corrosion problem, or document that a tank has fulfilled its useful service life and is in need of replacement or rehabilitation. At the high costs associated with any capitol piping replacement, an ultrasound report will provide the hard documentation necessary to move the project forward. For lower pressure domestic water and fire protection storage tanks especially, most will suffer only random corrosion loss making them ideal candidates for rehabilitation through application of a new internal coating at substantial cost savings.
For many water tanks now the same age of a 50 or 60 year old building, an ultrasonic survey can save money by confirming still high wall thickness and decades of reliable service life ahead. Where no problems exist, ultrasound will provide greater security, and most importantly, establish a solid baseline from which future and even more accurate and reliable estimates of corrosion rate and remaining pipe life can be made.
We offer on our site a large photo gallery of common corrosion conditions at domestic water and fire storage tanks. Although generally unseen, any older water storage tank has the potential to suffer similar corrosion loss to some degree.
For many domestic cold water tanks where insulation is not provided, surface moisture condensation is mild but sufficient enough to produce deep and severe pitting to the exterior over time. As we show through past examples in this Tank Photo Gallery, such outer tank corrosion, although easiest to observe and control through simple maintenance, will often cause the greatest damage.