An on-line corrosion monitoring system, comprising linear polarization resistance, electrochemical noise, and high-resolution electrical resistance probes, has been installed in 3 stainless steel process vessels. Continuous corrosion rate data process information is also being obtained from the plant control system to determine the effect variations in process conditions have on the corrosion in the vessels. This paper describes the background to the system selection, the probes instrumentation and software used and the results from the first few months of operation.
INTRODUCTION AND SYSTEM DESCRIPTION
As part of a planned modification to a process system, an on-line corrosion monitoring system has been installed in 3 vessels at a chemical process plant in the UK. The vessels are part of the acetate ester production facility and are constructed of grade 316L stainless steel. At present the vessels are subject to a regular five-year inspection regime, accepted by the regulatory body on the basis of experience over many years. A planned modification to the vessels operation (planned for 2002) would require an increase in inspection for the vessels to an annual inspection (over the first 24 months at least) to confirm the impact of the changes on corrosion condition.
In order to forestall a major increase in inspection requirements, it was decided to incorporate an on-line corrosion monitoring system into the vessels. The intention being for the monitoring system to provide a footprint of the corrosion conditions under the existing operating protocol. Corrosion under the revised operating conditions would then be compared against the footprint to determine whether the new conditions were better, worst or the same with regards to corrosion of the vessels. The regulatory body has accepted that an on-line monitoring system will be able to demonstrate whether there have been any deleterious effects from the change in process. Subject to positive results and adequate analyses, they will then allow the existing 5-year inspection period to remain.
Because the known corrosion problems of the vessels relates to both general corrosion and pitting corrosion of the stainless steel, in both the liquor and vapor area, a number of different techniques and monitoring probes assemblies have been utilized. General corrosion is being monitoring using a Cormon Ceion? (1) highresolution electrical resistance (HR-ER) monitoring system. Localized corrosion is being monitored using a PRP Concerto? Electrochemical Noise multi-channel system (2), which also allows linear polarization resistance measurements to be taken on the same probe. This EN/LPR facility allows a greater flexibility in monitoring requirement.
