Ultrasonic Tank Testing

Coincidental to the interest to assess piping condition is often a concern over the condition and useful remaining service life of various tanks and pressure vessels served by that piping.  For most building properties, the failure of a large domestic water storage tank, although rare, can produce significant service interruption.  Far greater threat exists for pressurized vessels, where catastrophic damage and injury is more likely.

For all tanks, there is a lesser ability to generalize their condition or assess their remaining service based upon visual indicators.  For many clients, an initial concern about old riveted storage tanks may reveal a far lesser threat than at its more recently installed piping.  An internally coated tank may show no leaks, although where that coating has failed, deep internal pitting will exist.

Like everything else, older tanks were constructed using substantially heavier steel plate.  The typical 0.500 in. wall thickness of older storage tanks from the 1920s, over designed in order to provide a greater safety margin, have today been reduced to their lowest wall thickness still acceptable by the prevailing code.

• • • Multiple Applications

The most common types of tanks under concern are:

• • • Domestic cold water storage
    • Domestic hot water storage
    • Pressurized fire sprinkler service
    • Steam condensate tanks
    • Expansion tanks
    • Make-up water tanks
    • Compressed air

Other vessels less frequently of concern are:

• • • Refrigeration machine shells
    • Tube and shell type heat exchangers
    • Make-up water deaerators
    • Steam surface condensers
    • Chilled water storage

Most steel domestic water storage tanks are typically square or rectangular in shape and are open to the atmosphere, whereas others such as for fire sprinkler service may be pressurized to 175 PSI or more.  While the same ultrasonic equipment is used following the same basic principles, procedures for tank testing differ from those addressing pipe.

• • • Same Technology – Different Procedures

In terms of measuring wall thickness, the same principles apply relating to calibration, probe selection, surface preparation, and data management.  Significant differences exist in the application of wall thickness measurement based primarily upon tank shape and function, as well as upon known corrosion characteristics.  A steam condensate vessel or domestic water storage tank typically exhibits highest corrosion activity at and below its water line.  A compressed air tank, however, shows most corrosion activity directly along the bottom where moisture condenses.  Lined tanks show condition characteristics entirely dependent upon the integrity of the coating itself, with age always their most significant limitation.

Depending upon the size of the tank, we typically perform thousands of wall thickness measurements in a standardized X by Y grid.  Every 6 in., for example, at a 5,000 gallon water storage tank.  For cylindrical tanks, the shell is addressed individually from its heads given the different thickness specifications for each.  Even differences in head shape, such as torispherical vs. hemispherical, are taken into account in our calculations.  For square and rectangular tanks, the walls are assessed differently than the bottom.  All surfaces are measured in a defined X by Y grid.

We follow formulas and calculations provided by both API 653 and the ASTM Pressure Vessels handbook in our calculation of minimum acceptable wall thickness, and then apply such values to the wall thickness data measured ultrasonically.  Each report includes all known characteristics of the tank, its level of corrosion activity, and estimated remaining service life.  A tank assessment is  somewhat similar to our piping evaluations with the exception that a substantially greater number of wall thickness measurements are taken due to the need to address a much greater amount of surface area.

• • • Same Thorough Reporting

Our reports are very thorough, and include a fitness for service assessment based upon current wall thickness to minimum acceptable limits, detailed calculations for the shell and heads at a pressurized vessel, or sides and bottom for a domestic water house tank.  All wall thickness data is then visually expressed in a series of 3-D summary graphs allowing us to provide an immediate understanding in conditions.

Below left we show a heavily deteriorated pressurized fire sprinkler water storage tank with severe wall loss along the lower 3/4 of its shell.  Graphs of its heads produced the same gradient profile.  Below right, the deep pitting at bottom dead center of a compressed air storage tank while the top remains at near factory specification – a side view best illustrating this condition.

In the below left example, general moderate pitting at a large domestic water tank is best shown in this side profile view.  Below right, a steam condensate receiver having significant deterioration along its bottom surface.

All visual observations made during the inspection survey are provided such as external corrosion of cold water storage tanks, prior repairs, failing supports, and any abnormal conditions.  As we have well documented in previous ultrasonic investigations, a high corrosion condition measured at the outlet piping to a tank will often indicate a near equal corrosion condition of the tank itself, and vice versa.  For that reason, any weakness shown at one should be assumed a potential threat to the other.

• • • Limitations To Successful Tank Inspection

The greatest difficulties in the assessment of any form of tank or vessel is its location, physical access around the tank, area conditions, and most importantly – insulation.  Many older tanks raising the greatest concern seem to have been constructed with the building itself – often up against one or more walls limiting full access.  Given that ultrasonic testing requires direct contact to the steel itself, assumptions then become necessary, along with the realization that a full assessment is not possible.

While most paints and coatings can be negated by “echo-to-echo” testing procedures, some heavy coatings, epoxys, or mastic type tar coatings will not allow ultrasound transmission.   A large insulated steam condensate tank, typically having a custom fabricated heavy outer metal covering, will exceed the cost of the UT inspection many times over in just removing and replacing that insulation.

© Copyright 2023 – William P. Duncan, CorrView International, LLC