By John Delahunt
As discussed in the October 1996 Newsletter, an important evaluation of bottom linings after application to aboveground storage tanks (AST) is holiday (i.e., discontinuity) detection. Linings are principally applied to ASTs to prevent internal corrosion that may be severe. Therefore, holidays must be detected and repaired prior to the newly lined tank being returned to service.
To review, two types of holiday detection are employed. For thin film bottom linings, under 20 mils dry thickness film (DFT), low voltage - wet sponge detectors are specified. For those linings equal to or greater than 20 mil DFT, high voltage spark detectors are employed. Previously the advantages and disadvantages of low voltage - wet sponge detectors were described; the following describes the advantages and disadvantages of high voltage detectors.
Briefly, a high voltage (greater than 800 volts) spark detector is an electronic device used to detect holidays in thick film linings applied to tank bottoms. It consists of an electrical direct current energy source, an exploring electrode, generally for tank bottom linings the electrode is brush shaped employing thin copper “bristles,” and a ground wire connection to the substrate. The detector can either be a pulsating direct current or continuous direct current type. The pulse type discharges a cycling type high voltage while the other direct current type discharges a continuous voltage. The operation of the two differ so it is important to know the type to be used. The most commonly used is the pulsating type. A holiday is detected by electrical discharge from the exploring electrode through the holiday to the grounded substrate.
Primarily there are several advantages to the use of high voltage spark detectors:
High voltage spark testing has several disadvantages, including the following:
There is controversy surrounding the use of high-voltage evaluation of linings; however, most users agree that personnel performing spark tests should be trained in the correct methods and voltage requirements. Guidelines for voltage selection are given in standards such as ASTM D5182 and NACE RPO 188. Others specify 100 volts per mil of lining thickness and third choice is to purposely damage a lining by causing a pinhole and then adjusting the voltage of the high voltage detector until a spark is detected. Lining manufacturers are also a source of information concerning voltage selection.
In one recent study of spark testing practices, a reviewer concluded that there is not enough known about spark testing and its use may create defects hence the need for qualified inspectors. One well known authority in fact states that “spark testing should only be used to confirm defects found during close visual inspection.”
NACE Recommended Practice RPO 188-90 is presently being reviewed and any major changes to this document will be reported in further issues of this newsletter.