Structural Integrity Evaluation of the Heat Exchangers of a Hydrocracking

Abstract

The objective of this work is to evaluate the structural integrity of the feed-effluent heat exchangers of the Hydrocracking of the Luján de Cuyo Refinery, belonging to Repsol-YPF S.A. This equipment operates under a hydrogen load at a pressure of 2133 psi and at a temperature that varies between 300°C (572°F) and 400°C (752°F). An inspection performed during the plant maintenance shutdowns revealed several cracks on the channel of one of the equipment. These cracks had a depth of 2.4 in and a length covering almost all the circumference of the channel. It was concluded after several studies that during the 27 years service period and being exposed to temperatures of above 300°C (572°F), the material suffered an embrittlement process due to a phenomenon known as Temper Embrittlement, and that at room temperature, the material was within the range of brittle fracture. The commencement of the crack can be associated to the presence of hydrogen, to the effects of local stress concentration and to the plant maintenance shutdowns. It was estimated that probably the growth of the cracks was produced by the thermal stresses originated during the plant starting and the plant maintenance shutdowns, and that it is facilitated by the embrittlement of the material and by the presence of hydrogen. A risk evaluation was performed to analyze the feasibility of the continuous operation of the equipment under the described metallurgic conditions. By accepting a criterion of risk decision, it was concluded that the operation under such conditions was "not desirable" and that situation must be accepted by a high rank executive officer. A heat exchanger was shut down from service and used to perform several tests to determine its condition and therefore to provide the necessary technical background to the management decision making. This study was highly significant because the remaining four heat exchangers were in similar metallurgical conditions than the equipment removed from operation and they should continue with their service operation. Several studies were performed such as Charpy V tests, fracture tests and stress analysis calculations to determine the value of the stress intensity factor (J). These studies were then used as technical background and support material for decision making in regards to the continuity of the service operation of the equipment for a certain period of time and under certain operational conditions. Introduction The objective of this study is to evaluate the structural integrity of the feed-effluent heat exchangers of the Hydrocracking of the Luján de Cuyo Refinery belonging to Repsol-YPF S.A. and the associated risk of keeping this equipment in operation. This study was conducted with the purpose of determine if the mentioned equipment were suitable to continue providing service until the next scheduled shutdown of the unit. The Hydrocracking has two exchange ensembles with three heat exchangers each, named LX E4 A/A, A/B, A/C and B/A, B/B, B/C (See figure 1). These exchangers are subjected to a pressure value of 2133 psi, the circulating fluid inside the equipment is gas Oil with hydrogen. The temperature of operation of each exchanger is shown in table 1. The metallurgic of each one of the components is shown in table 2. The LX E4 A/A and B/A (see figure 1) were replaced in 1997 with new equipment, built with a similar engineering design than the original. After the non-destructive inspection tests of the other four pieces, several cracks were detected. In particular, the worst crack was found at the channel of the LX E4 B/C, contained on a plane perpendicular to the axis of the equipment and having a depth of 2.4 in and a length that covered almost the complete head circumference. (See figure 2). It is probable that the crack originated under the cladding on an area where stress concentrations might occur associated to the changes in the plate thickness of the head.