Beware of the couplant

A long, long time ago, when I first started using ultrasonic testing for corrosion evaluation in refineries, we were limited to accessing equipment at ambient temperatures only. This, in turn, meant that we could only evaluate critical piping operating at temperatures ranging from 300°F (149°C) to over 1000°F (538°C) during maintenance outages.

The need for thickness information on a current basis drove the process of ‘on-stream’ inspection, which required the ability to obtain measurements at operating temperatures. Further demands for knowledge of equipment condition led to the development of weld examination and corrosion scanning at operating temperatures.

Ultrasonic transducers depolarised at the Curie temperature and had to be separated from the hot surface by a stand-off, made either of temperature-resistant material or using a flowing liquid. This created the market for high-temperature couplants.

Couplants are critical to the success of ultrasonic testing. They facilitate transmission of the sound energy into the material for thickness measurement or damage detection. Ultrasonic instruments and transducers are currently capable of operating at temperatures up to 950°F (510°C) and up to 1100°F (590°C) under controlled conditions.

Autoignition of ultrasonic couplants during high-temperature inspection is a major concern. It is critical that users of these couplants are fully aware of the proper use and application of every couplant available to them.

Couplant manufacturers address the autoignition risk by specifying the following information:

The autoignition temperature of the couplant;
Any limitations in performance from vaporisation of the base fluid and the melting point of any plastic polymer in the couplant; and
Product liability considerations.

It is critical for users of couplants to be aware of the limitations of each couplant product and ensure that they are never applied outside of manufacturers’ temperature limitations. If couplant products not specifically designed for use as high-temperature couplants are used, the entity making the ultrasonic examination must obtain the relevant information and provide it to the technician conducting the examination. An example is the use of peanut oil, which, in addition to providing a fast food-like odour, was never intended for couplant use. We have had experiences where the peanut oil impregnated the lagging material and subsequently caught fire, fortunately without disastrous consequences.

Refineries in the USA require the submittal of safety data sheets (SDSs) or material safety data sheets (MSDSs) for all materials brought on-site by contractors. Users of the commercial couplant products must determine the operating range and autoignition temperature and be extremely vigilant not to exceed these temperatures.

In order to operate within the appropriate limits, the operator must have the ability to accurately measure the temperature of the material surface being examined. Temperature measuring devices include the Tempilstick®, a temperature indicating stick that provides a range of temperatures when the stick melts, and the Fluke 568 Ex Intrinsically Safe Infrared Thermometer, which can be used in Class 1 Division 1 and Division 2 or Zone 1 and Zone 2 hazardous environments around the world. The Fluke 568 has a measuring range of −40°F to 1472°F (−40°C to 800°C).

The accuracy of the temperature measuring device must be factored in and the device must be maintained within acceptable calibration.

The following information must be provided to the technician for each couplant product:

Autoignition temperature
Initial manufacturer’s specified operating range
Current manufacturer’s specified operating range
Manufacturer’s limitation for confined space (ie inspection ports)
Current SDS information.

This information must be included in the method procedure. The technician must not proceed if the operating range is not available or if the test object is above the operating range.

Autoignition of couplants must be regarded as a critical function whenever ultrasonic testing is being conducted in a potentially explosive environment. So, be forewarned and be aware of the couplant.

Definitions

Flash point: The temperature at which the vapours produced from a fluid will ignite (flash off) with the presence of an ignition source (the fluid will not burn at this point). While some applications and revised fire safety standards may require a high flash point fluid to be used, it is still quite common to operate systems at temperatures above the flash point of the fluid.
Fire point: The temperature at which the fluid will sustain a fire if ignited by an outside ignition source. Heat transfer systems are commonly run at temperatures above a fluid’s fire point as the fluid is contained within the system, far removed from ignition sources.
Autoignition: The minimum temperature at which a fluid will spontaneously ignite without an external ignition source, such as a flame or a spark. Never operate a system above a fluid’s autoignition temperature.

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