The process industry involves storage and processing of many hazardous materials. Failure of this equipment is unacceptable because of the potentially catastrophic consequences. With increasing regulatory requirements and ageing equipment, there is more emphasis on the reliability of this equipment. Acoustic Emission (AE) monitoring is playing an increasingly important role in the monitoring of this equipment.
To avoid failure, defects must be detected before they reach the critical size with regard to leak or unstable crack growth. Once found, fracture mechanics can determine if the defects are significant or if they can be tolerated in a vessel. AE is major tool in detecting active defects because of the significant time and expense savings of not having to enter the vessel for inspection. Ultrasonic examination (UT) is typically used to follow up on the AE sources to determine the size of and map the defects.
AE testing is a powerful, yet complex technique, making it essential in determining that the application to be inspected is suitable for AE.
The AE test method is designed to detect defects and damage in metal and composite equipment. Materials produce sound waves, acoustic emission, when deformed. The deformation is usually local, at a crack tip or corrosion, and occurs during loading or over stressing. These sound waves travel along the surface and in the equipment. The general procedure consist of subjecting the equipment to increasing stress while monitoring with sensors that are sensitive to acoustic emission caused by stressed defects. In-service equipment is typically tested to 10% over the maximum operating load for the previous six months. Pressure may be increased on-stream, with nitrogen or other gas pressure, or by hydrotest. The pressurization must be controlled and produce minimal noise from liquid or gas flow.
The preparation and the information given for the preparation of the test mainly determine the success of an AE test. It is essential that the previous operating history be provided, including maximum pressures and temperatures. The equipment must be slightly over stressed to produce AE, therefore the previous stresses must be known. Some general information needed:
For the time on site to go smoothly, it is necessary to make some arrangements in advance of the arrival of the crew.
Before the testing is performed, the following will be completed:
The general procedure for in-service vessels requires that the pressurization be monitored during a stepwise increase from 90% to 110% of the maximum operating pressure. Pressure hold periods will be required at 90%, 100%, 105% and 110%. The maximum operating pressure is the maximum pressure the vessel has been operated at in the last six months. This is perhaps the most critical information for the entire testing procedure.
For new vessels, the pressurization is monitored during a stepwise increase from 0 to 100% of the maximum test pressure. Maximum test pressure being 1.5 times design. Pressure hold periods will be required at 0, 50%, 65%, 85% and 100% on the first ramp and 50%, 65%, 85% and 98% on the second ramp.
For storage tanks, the procedure requires the level to be raised in a stepwise fashion during the AE monitoring. The monitoring would begin at 90% of operating level and then proceed to 100%, and to 105%. The maximum operating level is the maximum level the tank has been operated at in the last six months.
Fiberglass reinforced plastic (FRP) tanks are tested in similar fashion but do require some added preparation before testing. The owner shall verify that the vessel conditioning requirements have been met prior to testing. The conditioning requirements are shown below:
| Percent of Operating Maximum Load
(in past 12 months) |
Time Spent as Percent of Maximum Load |
| 10 or less | 12 hours |
| 20 | 18 hours |
| 30 | 30 hours |
| 40 | 2 days |
| 50 | 4 days |
| 60 | 7 days |
Example:
The maximum operating level of the tank in the past 12 months is 75%.
You can lower the tank to 10% or less of the 75% operating level.
Tank must be at 7.5% or less for 12 hours prior to the AE test.
The maximum operating level of the tank in the past 12 months is 75%.
You can only lower the tank to 40% of the 75% operating level.
Tank must be at 30% for 2 days prior to the AE test.
The procedure requires the level to be raised in a stepwise fashion during the AE monitoring. The monitoring would begin at 50% of operating level and then proceed to 75%, 87.5% and to 100%.
A preliminary site report is generally provided prior to demobilization. A final report of findings will be provided within three weeks upon completion of on-site work. The AE test report will include:
Applied Inspection Systems personnel are trained in, and follow, safe work practices. Hard hats, safety harnesses, fire-retardant clothing, safety eyewear, and sturdy footwear are standard elements of personal protection equipment when work is carried out at project sites.
The client is requested to provide reasonable access to the desired inspection sites and to provide for appropriate work permitting. Any special safety requirements should be communicated to AIS personnel prior to commencement of work.