Introduction: For the process fluid from the distillation column, a temperature reduction is needed, and originally an air-cooled heat exchanger has been used. In order to increase the energy efficiency, a plate heat exchanger is designed, manufactured, and put into operation as a substitute in this position. Several attempts to obtain the best applicable materials and welding technology for the plates have been made. The best solution has lasted eight years. In order to optimize performance and service life, two heat exchangers of the same construction and materials but with different joining methods of the plates are compared.

Method: Material for the construction of the air-cooled heat exchanger is X6CrNiTi18-10 (EN 10088-2). The initial plate heat exchanger is made of X2CrNiMo17-12-2 (EN 10088-2) and has a service life of five years. Its substitute is made from super austenitic stainless steel Avecta SMO 254 X1NiCrMoCuN20-18-7 (EN 10088-4), but it fails within a year due to defects in the large area of laser-welded seams. Detailed consideration of all operating conditions is performed, as well as corrosion evaluation. Comparison is made between the two applicable types of plate’s welding processes—Tungsten Inert Gas (TIG) and Laser welding. The resulting structure and manufacturing technology are investigated. Possible post-weld treatment is proposed in order to reduce the remaining residual stresses. Consideration is made for heat utilization, and subsequently, a new material is proposed: NiCr23Mo16Cu (Hastelloy^® C-2000^®). After detecting leaking, the defective plates' edges are examined with visual and penetrant testing. Positive material identification is carried out. Microhardness of the base metal and welding zone is performed, together with microstructure analysis.

Results: The heat exchanger is operating on gasoline/crude oil with process inlet T^о = 135 – 152 ^оС and process outlet T^о = 78 – 95 ^оС. For the crude oil, we have an ambient inlet temperature and an outlet of around 100 ^оС. pH is 6 +/-0.5, with an operating pressure of gasoline 5,4 bar and crude oil 12,5 bar. The overhead vapors of gasoline are directed to the plate heat exchanger. A comparison is done for two different welding methods/regimes for samples of the plates in order to simulate and detect the initial conditions that led to the defects. Both have used PA positions, autogenous weld without edge preparation for a thickness of 1.2 mm. TIG welding was performed at 50 A and 10 V, with a tungsten electrode diameter of 1.6 mm. Laser parameters were as follows: power 1800 W, speed of application 25 mm/s, distance to workpiece 120 mm, with Ar 100% gas at a flow rate of 4 l/min. For the TIG welding, the achieved values of microhardness in the welded zone are 200 - 220 HV1, and in the heat-affected zone, they were 191-208 HV1. Laser welding measurements yield, correspondingly, 230 - 246 HV1 and 174 - 193 HV1. For the laser method, the received hardness values are above the permissible limit and over 1.4 times higher than those shown by the base metal. It is vital to perform treatment after welding, which shows that in the welding zone the hardness value falls to 190-192 HV1.

Conclusion: The aim of the investigation is to define and propose the best mechanically reliable heat exchanger option for a critical refinery position. Resulting is that either additional filler material has to be specified for the welding or mandatory treatment for stress relief has to be applied in order to retain the corrosion resistance and operational life of the equipment.

Funding: The author acknowledges support from project BG16RFPR002-1.014-0005.