Failure of an ammonia vessel is unacceptable because of the potentially catastrophic consequences of a highly toxic gas release. Cracks caused by stress corrosion cracking (SCC) have been found in ammonia storage vessels. However, the existence of cracks does not mean that failure is imminent. With the possibility of failure in aging equipment along with increasing regulatory requirements, there is more emphasis on the reliability of this equipment.
To avoid failure, cracks must be detected before they reach the critical size with regard to leak or unstable crack growth. Internal inspection is common practice, but can be less desirable, as opening the vessel can initiate or aggravate a SCC condition by admitting oxygen to the vessel. Another alternative is the combination of acoustic emission testing (AE) to detect the active cracks, and external automated ultrasonics (AUT) to map and size the defects. An engineering assessment can determine the critical crack size for various locations on the vessel and estimate the crack growth rate.
AE has become a major inspection tool because of the time and expense savings of testing an entire structure in a short period of time without having to enter the vessel and of the increasing regulatory requirements.
On storage tanks, AIS’s technique of installing the AE sensors requires the need of only two small diameter holes at each sensor location with basically no insulation removal. These small holes can be filled with spray foam after removal of the equipment.
For double wall ammonia storage tanks, AIS uses a custom-built waveguide device that has been used successfully on these types of tanks. This device allows for testing the inside tank when the wall space between the inner and outer tanks is up to three to four feet apart.