Acoustic emission testing is a proven advanced non-destructive testing method for to assess the condition of steel pressure vessels, pipelines and above ground storage tanks. Acoustic emission technology was first developed and commercialized for steel load bearing structures but now is a viable testing method for fiber reinforced composite pressure vessels (FRPs), pipelines, and storage tanks. This article reviews some of the key principles associated with the acoustic emission testing in American Petroleum Institute API-510 In-service Inspection of Pressure Vessels, API-570 In-service Inspection of Pipelines, and API-653 In-service Inspection of Aboveground Storage tanks.
Acoustic Emission Testing of Steel and Fiberglass Pressure Vessels
Acoustic emission testing is used to evaluate newly fabricated steel and fiberglass pressure vessels to American Society for Mechanical Engineering (ASME) Boiler and Pressure Vessel Code (BPVC) and during the fabrication of fiberglass pressure vessels to ASME RTP-1. In-service acoustic emission testing may be used to support American Petroleum Institute In-service Pressure Vessel Inspection code (API-570) Failure mechanisms in steel pressure vessels detected by acoustic emission include fatigue cracks, corrosion fatigue cracks, stress corrosion cracking and other deterioration mechanisms. A typical acoustic emission test setup in shown in the video below. The steel pressure vessel monitored with 14 acoustic emission sensors is 10 meters in length with a 2-meter diameter. Typically, 1-3 sensors are dedicated to the hemi-sphere heads depending on diameter. Equally spaces rows of acoustic emission sensors are placed on the cylinder body with the exact quantity dependent on the acoustic attenuation characteristics of the pressure vessel. The acoustic emission data, post filtering, is superimposed upon the pressure vessel. 3 acoustic emission events were detected during the various pressurization and hold stages during this test sequence. Two acoustic emission events on the cylinder body and one on the top head. Acoustic emission events are generated by acoustic emission hits received at three or more sensors and with differences in time time-of-flight that allow for source location.
Figure 1: Pressure vessel acoustic emission test
Acoustic Emission Testing of Steel and Fiberglass Pressure Piping
Acoustic emission testing of pipelines is typically a more challenging application compared to AE testing of pressure vessels due to difficulties in pressure control. In many cases, the approach may differ from traditional threshold detection acoustic emission testing where leaks are sought. For leak detection applications, the AE instrumentation may be setup to monitor continuous acoustic emissions from leak related sources. In this scenario acoustic emission average sound level (ASL) or root-mean-square (RMS) data is monitored. An example setup is shown in the figure below. A 20-meter long section of pipe was instrumented with eight different acoustic emission sensors from left to right along the pipe section. A cluster of acoustic emission activity was detected between sensors 1,2 and 3,4. It was recommended that this area be excavated for further investigation.
Figure 2: Pipeline vessel acoustic emission test
Acoustic Emission Testing of Above Ground Storage Tanks
An above ground storage tank (AST) bottom inspection is an expensive proposition due to the cost associated with emptying and cleaning the tank for an internal inspection. Tank floors are less accessible, and evidence of release remains largely hidden from view until the extent of the release is large. In-service degradation of tank bottoms is the result of internal, under-floor corrosion, and mechanical deformation. Acoustic emission generated from these tank floor degradation mechanisms provide the tank owner with a qualitative measure of a tank’s condition. This engineering data may be used to prioritize the future removal from service of internal inspection. Tank floor acoustic emission inspection is unique because the AE generated from the corrosion product, or leak, travels through the product to the acoustic emission sensor located at ground level around the outside of the tank. The correct product longitudinal wave velocity and acoustic emission sensor locations must be input to accurately locate AE emission sources from the tank bottom. An example acoustic emission test setup with results is shown in the video below for a 20-meter diameter diesel fuel storage tank. 12 low frequency acoustic emission sensors were installed along the lower course of the shell. The AE sensors were space equally around the tank shell. The correct product acoustic emission wave velocity for diesel fuel was loaded into the 2-D source location algorithms. During this test multiple hits were detected by all the acoustic emission sensors. However, only a four total acoustic emission events were detected and located during the test. 2 acoustic emission events were detected in the vicinities of sensors 4 and 10 in the critical area. The data shown is a 3-D view of the tank floor with acoustic emission event tracked in the vertical direction or z-axis. Follow-up short range guided wave inspection from the chime area was recommended.
Figure 3: Aboveground storage tank acoustic emission test