Tank Testing Overview

     Unlike the different reasons behind the need for ultrasonic pipe test testing, most inquiries about the quality and remaining service life of a tank or pressure vessel are related to concerns about its age. Other concerns also exist - See Technical Bulletin # C-11 about the hidden threats to many cold water house tanks. Tanks or vessels used for steam or steam condensate service often raise a greater level of concern due to internal corrosion, although age is still the primary factor precipitating such investigations.




The Advantages of Ultrasound

     Identifying the current status of a tank, pressure vessel, expansion tank, heat exchanger, cooling tower pan or other storage vessel, and the corrosion rate acting upon its metal surface, is often difficult or impossible through other destructive and nondestructive means such as x-ray, selective sample removal, or metallurgical testing, etc.

     Ultrasound technology, however, allows the precise measurement of the metal's thickness from the outside surface; thereby providing the means to produce a thorough corrosion evaluation at reasonable cost. Unlike for open or closed circulating systems, corrosion coupons are not a valid option for tank corrosion monitoring. In many cases, a leak or failure at the associated piping to or from a tank or pressure vessel is the first indication that a corrosion problem might exist.

     The advantages of ultrasound testing are many, and include:


Common Applications

     HVAC equipment for which ultrasonic testing is most frequently employed are:

     Ultrasonic testing is suitable for determining the thickness of any metal, and for tanks or pressure vessels used in HVAC applications, is most commonly applied to:


Recommended Levels of Testing

     Identifying the current status of any tank or pressure vessel generally requires a large number of sampling points upon which to base a final determination. This is primarily due to their size, and the different surfaces often having varying thickness values and minimum thickness limits. In addition, most tanks have varying physical parameters - with round tanks usually having a different shell and head thickness, and square or rectangular tanks having different wall and bottom dimensions.

     Since most tanks or vessels maintain a water line, it is extremely important to provide sufficient measurements to determine if any deterioration in that area exists, and to what degree. Under deposit corrosion at the bottom of the tank is also a serious concern due potential sediment and deposits, although this area is often difficult to physically access. In addition, cold water tanks have the added threat of developing outer surface corrosion due to moisture condensation - in some cases becoming the limiting factor in future service life.

     This high number of required test points generally eliminates cutting out samples regularly spaced along the wall for metallurgical analysis, which often leads to the dangerous practice of basing the evaluation of an entire system upon the results of just one or two individual samples.

     It also eliminates taking a few dozen random ultrasonic wall thickness measurements, printing them out on a standard spreadsheet, and calling it an evaluation. This is why each report produced by ECI is based upon a detailed investigation of 750 or more individual measurements per tank.

     As a general guideline, we recommend the following levels of testing:


Testing Accuracy

      The basis of each evaluation is, of course, the measurement of the remaining tank wall at the locations tested. The accuracy alone of the Panametrics equipment we employ is approximately 1 mil or 0.001 in. for most applications.

     Our use of on-site verification of each ultrasound measurement ensures the accuracy of each thickness reading, and eliminates the questionable practice often used by other testing services of discounting the highest and lowest readings within any set of measurements - in some cases discarding a significant percentage of thickness readings taken. Existing thickness readings can then be compared to original wall thickness specifications and minimum acceptable wall thickness values to produce average and minimum based estimates of corrosion rate and remaining service life.


Reporting Criteria

     Data is offered in summary form as well as in fine detail, and provides valuable and reliable information on corrosion rate, actual wall loss, percentage of allowable loss, remaining tank life, and estimated retirement date.

     Rather than produce retirement date calculations based upon some arbitrary percentage of the original wall thickness dimensions, a commonly employed practice having no basis in fact or theory, we refer to established engineering formulas for minimum acceptable wall thickness which take into account overall shape, size, diameter, head shape, base supports, ASME or API code, operating pressure, construction, material strength, thickness schedule, service provided, and other relevant physical parameters.

     The result - a report conclusion that does not recommend premature or unnecessary tank replacement, nor one that fails to recognize specific attributes of the tank or pressure vessel, or fails to include those operational factors which may lead to unexpected failure.


Field Testing

     Testing can usually be accomplished during normal working hours. Steam and steam condensate tanks and vessels, while they can be tested "live", is best performed during any available or scheduled downtime. Typically, it requires one day of on-site testing per large house tank, cooling tower pan, or steam condensate vessel and one half day to test smaller equipment. We require approximately 4-7 days to analyze the data and prepare the final report.

     Insulated tanks and vessels require the removal of such material, preferably throughout the entire unit. In some cases where management does not wish to remove and replace all the insulation, we can suggest cutting strips of insulation along the side of the tank to access specific locations only. Severely corroded outer surfaces may require mild grinding to produce an acceptable base measurement platform. Paint can generally be accounted for by a special feature of our test instrumentation which negates the paint or coating from the overall wall thickness - thereby eliminating the need to wire brush and repaint any test areas. This is always a savings to building maintenance. However, many "tar or mastic" like coatings often found at cold water house tanks must first be removed. Similarly, multiple layers of paint or coatings which have not held their adhesion, and offer microscopic separations and air gaps, must also be removed.


Pricing Guidelines

     Fees are based upon the total number of days of field work required, and include two copies of our full color report. Additional report copies are available, and all reports are archived by ECI for a period of 10 years.

     In general, fees depend upon the size of the tank or vessel, along with the number of anticipated measurements . This translates to between $1,500 and $2,000 per day of field testing - although the off-site data analysis and report preparation constitutes the bulk of time and effort spent for any tank or pressure vessel evaluation. The below listing offers a general guideline for different equipment types based upon past work experience, and may vary depending upon specific applications.

     Pipe testing that is located outside the immediate New York City area is priced at the same rate, but with the addition of actual expenses for travel and hotel accommodations.


Tank Testing Report Samples

     A selection of reprinted pages from a typical client report can be accessed from the navigation bar at the left, and is provided in order to illustrate the level of detail provided for each tank location tested. Although such detail is requested by many clients, summary information in the form of written text and a graphical format is also provided in each report. Related piping problems and visually observed defects are documented with a photograph and commented upon.

     Each report offers both an executive summary as well as a detailed discussion of our findings. Written recommendations and corrective measures are also made addressing any deficiencies identified in the report. A photograph of each tank location showing the actual test area is incorporated into each detail page of the report in order to better identify the test site to the reader, and to aid follow-up testing or future investigators.

     Since a high level of detail is provided in each ECI ultrasound report, and because we do not always have the opportunity to present our findings personally, we provide the written explanation and background information necessary to understand its content as best possible. All reports are written and arranged in an easy to read and understand format.

     Each report is preceded with a thorough explanation of the testing procedure, equipment used, basic theory of ultrasonic measurement, and necessary assumptions related to our testing and reporting procedure. Due to the fact that our tank testing reports are often submitted past a wide range of individuals including building owners, operating engineers, consulting engineers, and lawyers, among others, we have always strived to provide information ranging from the most basic conclusions to the most detailed technical information.

     For further, more detailed information regarding the actual testing procedures used, please view our Tank Testing Specifications and Procedures at the navigation bar at the left. A variety of Technical Bulletins related to tank and pressure vessel specialties are also offered.


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