Successful Volumetric Examination of a Modified Storage Tank Corner Weld

Introduction

In this article, a successful experience is presented using a volumetric examination on a modified storage tank shell-to-bottom weld, known in the industry as corner-weld. The experience resulted from developing a 20-year nonintrusive inspection strategy for a new ammonia storage tank designed and being constructed to API Standard 620, with the goals of improving reliability and extending the internal inspection interval [1].

In-service anhydrous ammonia storage tanks must periodically be inspected internally to assess their integrity. For instance, API Standard 653 requires a formal complete inspection that is supervised by an authorized inspector and is conducted at an interval from the initial service date not to exceed 10 years [2]. However, intrusive inspections of ammonia tanks bring multiple technical and integrity considerations, as well as operational and economic impacts. An intrusive inspection involves tank decommissioning, long downtime, high costs, hazards from tank entry, as well as the potential for off-service material damage. In ammonia tanks, internal surfaces exposed to atmospheric oxygen may increase the threat of stress corrosion cracking (SCC). Consequently, internal inspection of ammonia tanks is a complex task which requires an appropriate level of competence and expertise in tank design, operations, maintenance, and mechanical integrity. Some owners may only have one single ammonia tank without a spare tank, which implies additional considerations of project management to reduce downtime to compensate for the financial impact of production loss.

This case presents the design, construction, and inspection considerations for an ammonia storage tank, targeting a 20-year nonintrusive inspection strategy to minimize tank decommissioning. Several design, construction, quality verification, and operating considerations were implemented in an effort to improve reliability and achieve the desired inspection interval. The lower corner weld was considered a critical weld and was therefore modified to increase tank reliability, exceeding design code requirements. A conservative novel design used a double-sided full penetration butt weld instead of the code required dual fillet weld. Nondestructive testing (NDT) was used prior to commissioning to inspect the modified corner weld. One method consisted of a thorough volumetric examination conducted during tank construction to confirm full weld penetration, while the entire weld volume was examined for the nonintrusive inspection strategy. While both weld design and weld examination schedule exceed tank design minimum requirements for weld sizing and examination, the design code does not specify acceptance criteria for volumetric examination of double-sided full penetration butt-type tank corner-welds. Finite element analysis (FEA) and fracture mechanics were used to determine appropriate acceptance criteria and confirm the successful application of the modified corner-weld volumetric examination.

Tank Description

The tank is a double-wall carbon steel structure comprised of two concentric containers to make up a full containment cryogenic storage tank, with a nominal capacity of 8 million US gallons (30,000 cubic meters) of liquid anhydrous ammonia. The inner container is an open cup design, which is not considered gas-tight, with a diameter of 125 ft (38 m) and a height of 92 ft (28 m). The outer container has a diameter of 131 ft (40 m) and a shell height of 97 ft (29.5 m). The outer container is specified as a full containment, and as such is designed to contain product vapors in normal operation and full volume in case of inner container leakage. The material of construction is ASTM A516 Gr. 70, normalized carbon steel plate impact tested. The design stress was limited to 18 ksi (124 MPa) to improve resistance to ammonia SCC. The tank design lifetime is 25 years.

Corner-weld Sizing Requirements

API Standard 620 requires the attachment between the lowest course plate and the bottom to be a continuous fillet weld laid on each side. For half-inch or less bottom plates, the size of each fillet weld shall not be greater than half-inch, not less than the nominal thickness of the thinner plate, and not less than the values shown in Table 1. For bottom plates greater than half-inch, the fillet welds shall be sized so that either leg or the groove depth plus the leg for a combined weld are of a size equivalent to the bottom thickness (Figure 1).