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.
The Leading Cause Of Pipe Failure At Condenser Water Systems
Despite various corrective measure, advanced failures at condenser water systems are on the rise. Many problems are engineered into the system from the start due to the failure to recognize the impact rust deposits and particulates have on producing higher secondary corrosion levels. In addition, most corrective measure, if they are attempted, fail to provide a solution. Here is why.
Undersized Steel Pipe
A Simple Dial Caliper Measurement Of New Steel Pipe May Reveal Surprising Results
In addition to the many corrosion influences negatively impacting piping systems, many new building properties are constructed using carbon steel pipe which is below factory specification. To the surprise of many, an FM or UL approval, like its ASTM stamp, does not define that the pipe actually meets ASTM thickness standards.
The Decline In The Quality Of Today’s Piping Products Means Greater Corrosion Problems
A large number of negative influences have comined to produce a higher frequency of corrosion problems – often in spite of all precautionary measures taken. Of those, lower quality pipe, undersized pipe, more complex piping layouts, and generally less effective chemical treatment options have produced a “Perfect Storm” contributing to more piping failures.
When Pipe Corrosion At A Fire Protection System Can Cost Lives
The time to learn of a fire pipe corrosion problem is not during an actual fire emergency. Internal rust deposits can, and have, totally blocked water flow through the sprinkler heads – resulting in the loss of human life. More common at dry systems, internal deposits are a serious threat to all fire protection systems.
Fire System Failures
Major Misconceptions Within The Fire Protection Industry
Ignoring the obvious does have serious consequences when it comes to fire protection systems. From the use of thin wall schedule 10 & 7 pipe, to lower quality pipe products, to frequent flow testing which brings in new fresh water, clear and well documented reasons exist to explain the higher corrosion activity found at today’s fire protection systems.
Fire System Corrosion
The Threat Of A High Corrosion Condition To A Fire Sprinkler Line
Often viewed only in terms of water damage in the case of a corrosion induced pipe failure, far more serious concerns exist, although rarely considered. Unlike HVAC piping systems, corrosion activity at fire related piping can impede and in some cases totally block water flow – a potentially life threatening condition during any fire emergency
The Benefits of Ultrasonic Testing in Determining Corrosion Rate and Service Life
Ultrasonic testing provides the most comprehensive, accurate, and cost-effective tool to assess the condition and remaining service life of any piping system. Planned and performed properly, ultrasound offers the first step toward identifying a potential corrosion problem, or for certifying a piping system as fit for service.
Why Not All Pipe Failures Are The Fault Of Your Chemical Water Treatment Provider
Various design elements to any piping system can have dramatic impact upon its corrosion activity. Pipe origin, schedule used, physical layout, and many other unknown factors can produce a pipe failure. And yet they are completely beyond the realm of protection offered by chemical water treatment.
Corrosion By Design
Pipe Corrosion Problems No Water Treatment Program Can Protect Against
Various changes have occurred to mechanical piping designs over the past few decades, with virtually all HVAC, plumbing, and fire protection systems having been affected in some way. Many changes relate to the materials themselves. Major changes in piping design, however, have introduced new corrosion problems no chemical treatment program can stop.
The Benefits and Limitations of Corrosion Coupons
Relied upon for decades as an indicator of corrosion activity within piping systems, corrosion coupons are highly unreliable in most examples, and totally worthless in others. Many of the most damaging corrosion failures have occurred while at the same time corrosion coupons produced excellent results. Here is why.
If Corrosion Activity Is Only 0.4 MPY, What Is Wrong With The Above Picture?
Corrosion coupons reported a 0.4 MPY corrosion rate for 6 years where the actual rate exceeded 25 MPY. Believed implicitly in contrast to multiple leaks and failures, the slow but total destruction of the entire condenser water piping system was the net result. A case history illustrating the threat from relying exclusively upon this highly flawed testing method.
Dry Fire Sprinkler
Fire Protection Contractor – Antifreeze: The Fine Line Between Hero and Defendant
Antifreeze used in dry fire sprinkler systems may solve one problem, but has also proven deadly. Rated a Class 1 flammable liquid, antifreeze can accelerate a fire, create a fireball, and even cause an explosion. Although now restricted to lower concentrations, antifreeze still adds heat value to any fire and introduces new and unknown liability to any such system.
American Welding Society – Understanding Pipe Corrosion Problems
A piping system that satisfies service life demands, requires the recognition of piping design vulnerabilities, effective corrosion monitoring, and the adoption of corrective measurements. With corrosion related failures on the rise, and with generally lower quality pipe being installed, advanced planning and an awareness of potential threats becomes more important.
World Pipelines – Investigation vs. Procedure
Substantially different findings are likely where ultrasonic pipe testing is approached as a forensic investigation based upon known system problems and vulnerabilities, rather than simply a linear based measurement procedure. A critical importance is understanding the inherent corrosion related problems to various piping systems. An adapting investigation will also produce a more definitive answer to any piping problem.
Fire Pipe Corrosion
Fire Protection Contractor – When Pipe Corrosion In a Fire Protection System Can Cost Lives
The time to first learn of a pipe corrosion problem is not during a true fire emergency when lives are in jeopardy. Thinner pipe, more corrosive steel, lower quality galvanizing, foreign pipe, dry systems, MIC – all such negative factors are driving toward higher internal corrosion deposits to render your fire protection system worthless.
Fluid Handling Systems – Finding The Remaining Service Life
Ultrasonic testing is, by far, the most informative diagnostic method available for determining pipe status, as well as extremely cost effective. An effective piping analysis is much more than a spreadsheet of a few wall thickness measurements – requiring careful statistical analysis and practical interpretation of the data.
World Pipelines – Multiple Metering And Monitoring Needs
With no single form of corrosion monitoring capable of proving full coverage to the many different forms of pipe corrosion possible, multiple testing methods are always advised. These should include ultrasonic testing, spool pieces, LPR, regular internal inspection, and a close observance to the often obvious but missed signs of a problem.
A 30+ Year Knowledge Base
Unfortunately, the above is a very common progression of events for many of our clients. Often, greater attention to chemical corrosion control and corrosion monitoring could have saved the system and avoided the problem. In others, a decades prior design flaw or poor choice of pipe supplier may be traced back as the primary fault.
During the 20 years that we have been involved in the field of ultrasonic pipe testing / corrosion monitoring, we have authored various Technical Bulletins for the benefit of our clients. These Technical Bulletins address frequent problem issues to any building owner or operator, and offer both insight as well as reasonable and proven solutions.
We offer below the various categories available, and continue to add new bulletins as time permits.
Interior Rust Deposits, Common Threats, Corrosion Types, Winter Lay-Up, MIC, Corrosion Monitoring and Testing, CUI, Corrosion Coupon Failures, Rust Removal, Reducing Corrosion Threats, Roof Level Corrosion, Drained Pipe, Corrosion Trends, Fire Sprinkler Corrosion, Corrosion At “Free Cooling” Systems
The Impact Of Flow Rate To Higher Corrosion, Inadequate Water Filtration, Piping Layout Design, “Green” Piping Designs
Corrosion Threats, Design Misconceptions, Interior Rust Deposit Threat, Dry Fire System Corrosion, Schedule 10 Pipe, Premature Failures, Clogged Fire Systems, Chemical Control Options, Remediation Choices
Condition Assessment, Due Diligence, Preparation Prior To Renovation, System Evaluation, Expert Witness
Heat Exchangers, Benefits of UT Testing, High Pressure Water Jet Cleaning, Filtration Errors, Chemical Treatment, Condenser Tube Coating, Mold Concerns, Chromate Removal, Growing Threat of Corrosion, Heat Exchanger Tub Coating, Nondestructive Testing
Schedule 40 Limitations, Piping Trends, Hidden Corrosion Threats, Dielectric Insulators, Clamped Grooved Piping, Piping Schedules, Pipe Testing Specification, Roof Pipe Draining, Low Corrosion Guidelines, Dual Temperature Piping Failure
Cold Water Threats, External Corrosion Issues, Fire Reserve Tanks, Interior Pitting, Protective Coatings, Rehabilitation
Improving Heat Transfer Efficiency, Improving Filtering Efficiency, Filter Placement, Poor Performance Causes, Filter Selection Considerations
Chemical Treatment Challenges, Limitations to Water Treatment, Corrosion Coupon Reliance