By Ray Chao
Hot tapping has been a generally accepted method and, in some cases, the only practical way of adding nozzles in an operating atmospheric storage tank (AST). API Standard 653, Tank Inspection, Repair, Alteration and Reconstruction, provides guidelines on making hot taps in ASTs. However, in our opinion, the requirements contained therein are unduly restrictive in some respects and prohibit hot tapping in certain situations where hot tapping has been done successfully in the past without incident. Hot tapping can be performed safely in situations prohibited by API 653 when appropriate consideration has been given to all relevant factors.
The requirements given in API 653 cover hot tapping connections on existing in-service ASTs constructed with shell materials that do not require postweld heat treatment (PWHT). Furthermore, API 653 requires new shell penetrations to be installed in accordance with all requirements of API 650. These requirements essentially limit hot tapping on ASTs to the following:
Note that the Material Groups are as defined in API 650. These restrictions are based on the API 650 requirements for PWHT of nozzle assemblies. Therefore, hot tapping is not an option when PWHT is required.
The API 650 PWHT requirements are more stringent than those of other codes or standards used for equipment in similar or even more severe services. Specifically, ASME Code, Section VIII, Division 1 or 2, for pressure vessels, and API 620, for low pressure storage tanks, do not require PWHT of nozzle connections in carbon steel shell plates up to 1-1/4 inches thick, or up to 1-1/2 inches thick if a preheat of 200°F is applied during welding. Therefore, making a hot tap on this equipment is not as restrictive as on ASTs.
The PWHT requirement for nozzle assemblies in ASTs was introduced into API 650 more than 30 years ago when the use of a higher design stress was permitted. The higher design stress raised some concerns regarding the stresses in the bottom shell course and, in particular, around the nozzles due to piping loads and shell differential settlement, in additional to the hydrostatic head pressure. Therefore, as a precautionary measure, PWHT of nozzle assemblies to minimize the welding residual stresses appeared to be justified, since the additional cost is insignificant for new tanks.
The PWHT requirements for nozzle assemblies in existing ASTs, however, cannot be justified on the same basis as for new ASTs because of the following:
Therefore, PWHT requirements based on the well established rules for pressure vessels in accordance with the ASME Code should be adequate. This will allow hot tapping on ASTs which are generally fabricated from carbon steel plates up to 1-1/4 inch thick, or 1-1/2 inch thick with a 200°F preheat.
Hot tapping, nevertheless, must be undertaken very carefully, especially on old ASTs. The safety aspects as outlined in API RP 2201 must be observed, and suitable welding and inspection procedures developed and implemented. In addition, as a general safe practice, the liquid level should be drawn down as much as practicable during the hot tap operation, but not less than a minimum of 3 feet above the highest point of welding. This will also facilitate fit-up of the hot tap connection in the lower portion of the bottom shell course, since a significant bulge will occur at this location in a large AST when it is full of liquid.
The most serious concern with any alteration of an existing AST is the risk of a failure due to brittle fracture. This concern is addressed in API 653 which limits hot tapping to only steels of known acceptable toughness or steels of unknown toughness that have a minimum shell metal temperature at or above the exemption curve given in API 653. This curve shows that the safe minimum shell metal temperature at the time of hot tap increases with increasing shell thickness. While this will prevent brittle fracture at the time of the hot tap, consideration must be given to the minimum shell metal temperature subsequent to the hot tap if it can be lower in order to ensure that the tank will remain safe from the risk of brittle fracture.