API STD,API RP,API SPEC,API MPMS,API 570,API STD 619
API RECOMMENDED PRACTICE 575
In cases where API Std 653 shows preference for specic procedures in specic applications, these cases are noted. It is beyond the scope of this document to assess the specic performance characteristics of one method compared to another, or to cover the impact of multiple testing with mul-tiple technologies. More information on these topics has been published previously in such documents as API Publ 334. As with any NDE method, it is the responsibility of the owner/operator to make that assessment. It is anticipated that leak test personnel (examiners) have qualications con-sistent with API Std 653. Additional factors to consider include vendors and technologies api std 619 that have been qualied by third-party testing agencies or owner/operators. These methods may be required by various regulatory agencies or companies and provide other effective ways to evaluate the needed qualications.
When using information provided in this section, consider-ations for schedule, operational, economic and environmental characterizations should be reviewed. An owner/operator should be familiar with conditions under which the proce-dures can be used and in the case of developing technology, API Publ 334 should be consulted. There are eleven proce-dures associated with determination of the hydraulic integrity of a tank bottom. Of this total number of procedures there are six that are conducted with the tank typically out of service during the API Std 653 internal inspection and another procedures that are applied with the tank either partially or completely lled to its safe height with the service uid.
8.2 LEAK INTEGRITY METHODS AVAILABLE DURING OUT-OF-SERVICE PERIODS
8.2.1 Evaluation by Visual Examination
A Visual Test may be direct type when the surface is readily accessible to place the eye within 24 in. (61 cm) of the surface at an angle of not less than 30 degrees. The minimum illumination is 15-foot candles (25 lumens) for general viewing and 50-foot candles (100 lumens) for viewing of small anomalies. Visual test may be remote by using mirrors, cam-eras or other suitable instruments. The test would detect sur-face defects such as cracking, weld undercut, corrosion, dents, gouges, weld scars, incomplete welds etc. This method is applicable to all visually accessible portions of the tank bottom. Additional details on test implementation are described in API Std 650, 6.5. Paragraph 8.2.2 provides additional description of leak location by visually detecting areas of soil-side wicking from an otherwise clean bottom.
8.2.2 Evaluation by Wicking Examination of Shell-to-Bottom Weld
This test is a practical test because it provides information regarding the actual hydraulic integrity of the weld with a product less viscous than the product being stored. A leak could be easily located and repaired. Wicking test of the shell-to-bottom weld (corner weld) is the process of applying a highly penetrating oil or dye penetrant to one side of a weld (initial pass or completed weld as required by the applicable standard of construction or repair), then letting it stand for at least four hours (12 hours is preferred) and observing if it penetrates to the other side of the weld (see API Std 650,
5.3.5; API Std 653, 12.1.6). Personnel performing this test should api std 619 have the same visual acuity required for performing other visual tests (see API Std 653 and ASME Section V).
8.2.3 Evaluation by Bubble Test Examination?Pressure
For this method, the inside surface of the bottom is coated with an indicator solution. Air at not more than 3 in.
(0.75 kPa) of water pressure is injected by a hose under the bottom of the tank through the clay seal or through a drilled and tapped hole (or holes) in the bottom. The bottom is then inspected for bubbles, which will indicate any leaks. An alternative approach consists of pumping approxi-mately 6 in. (150 mm) of water into the tank and then placing air at not more than 9 in. (2.24 kPa) of water pressure under the tank. Leaks will be evident if air bubbles through the water in the tank. The effectiveness of these methods can be improved by tapping the entire bottom with an air-operated hammer. The sharp jarring of the bottom plates will frequently cause enough scale to pop out of pits to allow them to leak. When using 9 in. (2.24 kPa) of water pressure, the water must be pumped into the tank before air pressure is applied under the tank.
A variation on the bubble test method consists of pumping water under the tank to a depth of approximately 6 in. (150 mm) above the level of the highest point of the tank bottom and holding the water with the clay dam. Vents in the tank bottom are required to allow trapped air to escape. Leaks will then be evident as the water seeps through to the inside of the tank. This method can cause the tank pad to api 570 wash out or shift depending on its construction. It may also cause the tank to trapped water may later lead to accelerated corrosion. When using air under the tank, a con-siderable amount of plastering of the clay seal may be needed to build up the air pressure to the desired value.
8.2.4 Evaluation by Bubble Test Examination?Vacuum
Another method for nding leaks is the vacuum box method, which is particularly useful on the at bottom of a tank but can also be adapted to the shell and the shell-to-bot-tom joint. An example of a typical vacuum box is shown in Figures 62 and 63. In this method, the suspect area is coated with an indicator solution. In cold weather, it is impor-tant that the leak-testing liquid be formulated for use at the temperature involved. The open side of the vacuum box with soft rubber gaskets attached is then pressed tightly over the area. A vacuum is developed inside the box by means.