Updated: Jun 25, 2020
Spherical pressure vessels are manufactured in accordance with the American Society for Mechanical Engineering Boiler and Pressure Vessel Code (ASME BPVC) and are typically managed under the American Petroleum Institute API-510 Code: In-service Inspection of Pressure Vessels and and ASTM E 1930-02, Standard Test Method for Examination of Liquid-Filled Atmospheric and Low-Pressure Metal Storage Tanks Using Acoustic Emission. In-service testing of spherical pressure vessels using acoustic emission is an economical and technically sound approach to operating the vessel using risk based inspection (RBI) strategies. An example spherical pressure vessel that was tested with acoustic emission is shown below. The sphere was made in accordance with American Society for Mechanical Engineering Boiler and Pressure Vessel Code Section 8 Division 1. The spherical pressure vessel shell is 23 mm thick and all welds were tested with magnetic particle and radiographic inspection during fabrication and before hydrotesting. Once placed into service, acoustic emission testing of the sphere may be used to screen the weld for active fatigue, corrosion, and erosion based flaws.
Spheres that are tested with acoustic emission are commonly made from SA516-GR70 steel and are typically purchased with a corrosion allowance of at least 1.6 mm. In addition to the shell plates which can be tested with acoustic emission, there are numerous attachments that are welded to the sphere, shell that are also tested with acoustic emission. This includes welded reinforcement pads for the support legs, flanges inlets and outlets at the top of the sphere, a drain valve at the bottom of the sphere, and cooling water piping.
Acoustic emission testing may be used to detect fatigue cracks that form in and around nozzle and penetration welds due to numerous pressurization cycles. Acoustic emission testing may be used to also detect corrosion product, corrosion activity, and corrosion related cracks which may include stress corrosion cracking (SCC), corrosion fatigue cracking (CFC), and intergranular corrosion cracking (IGC) and transgranual corrosion cracking (TGCC) depending on the material of construction used. Generally speaking acoustic emission is not used to detect erosion related defect in spherical pressure vessels.
Prior to acoustic emission testing of spheres, a review of previous spherical pressure vessel documentation is required. This includes;
NDT reports – including acoustic emission inspection reports
Maintenance reports Operating historical records
Risk Based Inspection (RBI) analysis
Repairs, alterations, service or rating change
Fitness For Service (FFS) previous analysis
Construction drawings and calculations: MAWP, minimum thickness, etc.
Data sheet, data report, data book
Setting up a spherical pressure vessel acoustic emission test is a multi-team effort between the owner, inspection company and safety personnel. The most time consuming aspect of the acoustic emission testing is installing the acoustic emission sensors on the tank shell. The acoustic emission sensors used are typically in the 150-300 kHz range. The AE sensors must be distrubited over the sphere evenly to ensure that acoustic emission sound wave from fatigue cracks and corrosion related activity and cracks can travel to the sensor and be received with sufficient signal-to-noise ratio. And example acoustic emission sensor layout used during an acoustic emission test of a sphere is shown in the figure below. In this scenario, 24 sensors were used to monitor the sphere for acoustic emission 6 rows of sensors. Acoustic emission sensors may be installed at significant cost using scaffolding. Typically it takes up to 1 week to install the scaffolding and this approach may only be worthwhile if other inspection methods are leveraged into the process. It is more cost effective to install the acoustic emission sensors using a rope access technician. Acoustic emission sensors be installed and removed in a single day.