Certain painted, coated or lined plant and equipment is subject to aggressive working duties or harsh environments, such as a chemical process or a buried pipeline and require a continuous paint, coating or lining film to protect the underlying substrate, maintaining its integrity.
A paint, coating or lining must be applied as a continuous film to perform its intended function. Early failure will occur if there is a discontinuity – a holiday, as it is commonly called, such as a pinhole, void, crack, area of low dry film thickness, inclusion or contaminant in the paint, coating or lining film. Many of these defects are not seen by the naked eye, but they can be located using holiday / spark test detection equipment. Holiday detection is typically performed on coating systems designed for critical service such as immersion or chemical processes.
When Should Holiday Detection Take Place?
Holiday detection should be carried out when the paint, coating or lining film has fully cured to ensure there is no misinterpretation of results due to the presence of moisture or solvents that may remain within the film.
Holiday Detection Methods
Once full cure of the paint, coating or lining has been established it can be tested for holidays. There are two main holiday detection methods; the first being the Wet Sponge Method, the second being the more widely used High Voltage DC Method.
It is vitally important that the correct test method is selected together with the appropriate test voltage. A general rule of thumb within the industry is acknowledged as 4 volts per micron of material being tested – for example, a coating or lining applied to a dry film thickness of 1mm (1000 microns) would be tested at 4kv [1000 microns x 4 volts]. The speed of the test is also critical and should be carried out at a rate of approximately 0.3m/second.
Wet Sponge Method (Low Voltage Detection)
A low voltage – typically 9v – is applied to a moist sponge, and when the sponge is moved over a coating flaw such as a pinhole, the liquid penetrates to the substrate, completing an electrical circuit, and triggering an audible alarm and the alarm LED to flash. KUE Group use the Wet Sponge Method where specified or the dry film thickness is typically below 200 microns.
Spark Testing (High Voltage DC Method)
A high DC voltage, minimum of 1kv (1000v) is passed through the selected probe, typically a drum brush type or circular brush and passed over the painted, coated or lined surface. Should a flaw such as a pinhole, void, crack, area of low dry film thickness, inclusion or contaminant be located then the electrical circuit would be complete, again triggering an audible alarm, the alarm LED to flash and, if at a high voltage, a visible spark.
A high-voltage holiday detector causes the air between the drum brush/circular brush and the substrate to conduct electricity or break the air gap in the holiday.
Holidays have a direct and significant effect upon coating integrity, and it is important that they are accurately detected. By seeking the advice of an impartial, independent professional company such as KUE Group you can be safe in the knowledge that your plant and equipment will be thoroughly inspected prior to being placed in service.
To discuss your options please call 01274 721188 or contact us through our website.