INTRODUCTION
This paper describes the experience gained in “corrosion monitoring” in the overhead systems of three crude distillation units of a refinery in Haifa, Israel.
The data of electrical resistance (ER) probes, connected to an on-line data acquisition system were compared with the mass loss method and chemical analysis of accumulated sour water tier condensation in the overhead system.
SEM & EDS analysis of films and deposits formed on the coupons showed that iron sulfide with impurities of chlorides are responsible for the corrosion extent. The corrosion was less than 5 MPY when a uniform tenacious iron sulfide films of 10-50 microns thickness were formed. The severe corrosion occurred when deposits and non-uniform films of more than 80- 100 microns thickness were formed.
Special attention was given to ER probes which were connected on-line to the process units and enabled the operators to react immediately to any change in corrosion rates.
Crude oil contains different species which while being processed reacted or converted into substances corrosive to alloys.
Water is condensed during the production processes and gases like hydrogen chloride (HCI) and hydrogen sulfide (H2S) are dissolved in this water, various salts (chlorides, sulfides, hydrosulfides) and oxide-hydroxide deposits formed on the metal surfaces result in severe corrosion in the overhead system of the crude distillation unit. Air coolers, heat exchangers, condensers and pipes made of carbon steel suffer km general corrosion, pitting corrosion and under deposit corrosion.
Prevention of corrosion damage and possible detrimental effects on the environment depend on knowledge of the corrosion situation at the units, that is “corrosion behavior” of metallic equipment, especially their corrosion rates and corrosion forms.
Various types of corrosion monitoring methods were recommended fix follow-up in the overhead system of distillation units [1,2]. We did not find literature comparing the various types of monitoring results and how effective it is in evaluating real time corrosion.
SEM & EDS method is widely used for the identification of corrosion products [3], but we did not find in the literature, how this method is particularly used fir the chemical identification of corrosion products and morphology of coupons’ surfaces from the overhead system.
Given this apparent lack of well published basis for using various corrosion monitoring techniques, we attempted to use our practical experience to define ways how to utilize these tools to monitor Real Time Corrosion in the Equipment.
Corrosion follow-up in the overhead systems of three crude distillation units consisted of on-line monitoring with ER-probe measurements, long-term mass loss coupon measurements, visual examination of coupons, identification of deposits formed on Coupons and investigation of coupons, surfaces by means of SEM & EDS, and chemical analysis of accumulated condensed sour water.
The collected information is analyzed by experts and conclusions are drawn as of the required changes in the anti-corrosion treatment program The data their treatment, the analysis and the conclusions are presented in a periodic report by the corrosion engineer to the management, to the operation people, to the maintenance and to the technical services.
