API Subcommittee on Inspection and Mechanical Integrity to Address the Auto-Ignition of NDT Ultrasound Couplants

By Gene Larson, President at ECHO Ultrasonics. April 24, 2019

In 2016 the API Subcommittee on Inspection and Mechanical Integrity (SCIMI) reviewed the increasing incidence of auto-ignition in NDT applications involving high temperature corrosion mapping and thickness gauging and requested an educational article be written and published in Inspectioneering Journal. This article, titled Auto-ignition of Ultrasonic Couplants During Thickness Gauging and Corrosion Mapping: Mitigating HSE and MHE Risk was published in the July/August 2016 issue.

In an effort to further reduce auto-ignition occurrences, The API Committee on Refinery Equipment (CRE) made a recommendation that SCIMI address the auto-ignition of NDT ultrasound couplants by including new paragraphs in the upcoming rewrites of API RP 572 (Inspection of Pressure Vessels) and API RP 574 (Inspection Practices for Piping System Components). These paragraphs will incorporate actions such as the following:

  1. Check the product label for the manufacturer’s operating range. If the operating range is not on the container, do not proceed, nor provide the product to inspectors.
  2. Know the surface test temperature where the couplant will be applied. This is of highest importance in ports and restricted areas.
  3. Provide access to a current product Safety Data Sheet listing the operating range and auto-ignition temperature to the inspector.
  4. Train inspectors in the mitigation of risk of couplant auto-ignition in high temperature inspections.

The auto-ignition of an ultrasonic couplant during inspection is a surprise event that puts the safety of the inspector at risk. Additionally, there is the potential of equipment damage and ultimately facility shut down. Auto-ignition is a serious and avoidable event.

Auto-ignition occurs when the temperature of the test surface exceeds the temperature at which the couplant will spontaneously ignite in normal atmosphere. Auto-ignition does not require a spark, flame or ignition source to occur.

The risk and occurrence of auto-ignition is widespread. Some NDT service companies have experienced over 20 incidents of auto-ignition at one site. Auto-ignition of couplant most often occurs above 650°F (350°C), under insulation, in ports and in restricted spaces such as flanges with adjacent structures. Under insulation and port temperatures are often 150°F (70°C) higher than exposed surfaces of pipes and, combined with their restricted air flow, often result in the auto-ignition of a couplant which performs adequately in an environment with open air flow.

Many ultrasound couplants will function without incident when used above their operating ranges providing there is adequate air flow to remove vapor as it forms. This can result in a misplaced confidence that the couplant is safe for use above the manufacturer’s operating range; which will lead to a surprise auto-ignition when the product is used in a space with restricted air flow. This problem has been compounded by manufacturers of high temperature ultrasound couplants changing formulations, changing operating ranges and not changing the name of the products, nor marking the current operating ranges on the product labels.

Some high temperature ultrasound couplants contain plastic powders which ignite as a result of the auto-ignition of the couplant base fluid and vapors. This results in a plastics fire which is difficult to extinguish quickly and depending on the location of the inspection site, can result in molten and flaming plastic dripping onto the inspector.

The expected updates to API RP 752 and API RP 574 will address this critical issue and provide clear recommendations to reduce or eliminate the risk of auto-ignition from ultrasonic couplants.

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