ASNT

Leak Testing Principles . Leak testing is concerned with the f low of liquids or gases from pressurized components or into evacuated components. The principles of leak testing involve the physics of liquids or gases f lowing through a barrier where a pressure differential or capillary action exists. Leak testing encompasses procedures that fall into these basic functions: leak location, leakage measurement, and leakage monitoring. There are several subsidiary methods of leak testing, entailing tracer gas detection (Figure 6), pressure change measurement, observation of bubble formation, acoustic emission leak testing, and other principles. Applications . Like other forms of nondestruc- tive testing, leak testing affects the safety and performance of a product. Reliable leak testing decreases costs by reducing the number of reworked products, warranty repairs, and liability claims. The most common reasons for performing a leak test are to prevent the loss of costly materials or energy, to prevent contamination of the environ- ment, to ensure component or system reliability, and to prevent an explosion or fire.

Infrared and Thermal Testing Principles . Conduction, convection, and radiation are the primary mechanisms of heat transfer in an object or system. Electromagnetic radiation is emitted from all bodies to a degree that depends on their energy state. Thermal testing involves the measurement or mapping of surface temperatures when heat f lows from, to, or through a test object. Temperature differentials on a surface, or changes in surface temperature with time, are related to heat-f low patterns and can be imaged using thermography to detect subsurface discontinuities or to determine the heat transfer characteristics of an object. For example, during the operation of an electri- cal breaker, a hot spot detected at an electrical termination may be caused by a loose or corroded connection (Figure 7). The resistance to electrical f low through the connection produces an increase in surface temperature of the connection. Applications . Thermography inspection applies to many material manufacturing operations for product anomaly detection and to in-service inspection for service damage due to wear, impact, moisture, heat or other sources. Infrared and thermal test applications include many industrial electrical and mechanical equipment facilities. Radiographic Testing Principles . Radiographic testing (Figure 8) is based on the test object’s attenuation of penetrat- ing radiation — either electromagnetic radiation of very short wavelength or particulate radiation (X-rays, gamma rays, and neutrons). Different portions of an object absorb different amounts of penetrating radiation because of differences in density and variations in thickness of the test object or differences in absorption characteristics caused by variation in composition. These variations in the attenuation of the penetrating radiation can be monitored by detecting the unattenuated radiation that passes through the object. This monitoring may be in different forms. The traditional form is through radiation-sensitive film. Radioscopic sensors provide digital images. Computed tomography is a three-dimensional, volumetric radiographic technique.

Envelope

Leak detector

System under test

Source of tracer gas

(a)

Envelope

System under test

Leak detector

Source of tracer gas

(b)

Figure 6 Leakage measurement dynamic leak testing using vacuum pumping: (a) pressurized systemmode for leak testing of smaller components; (b) pressurized envelope mode for leak testing of larger volume systems.

CHAPTER 1

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Part 1