ABSTRACT

Petrochemical process plant construction materials deteriorate when the plant is in service. In consequence, planned maintenance is undertaken to repair and replace equipment when it is predicted or detected that the facility has sustained damage. Typically, the operating environment does not normally vary very much. However, from time to time, process conditions may change rapidly and unexpectedly, due to contamination by water or other impurities, which can result in severe and widespread damage to the materials of construction. One such example concerned a process plant operated by our company. In this instance, damage to a process heat exchanger could result in moisture ingress that, in combination with an anhydrous hydrocarbon process stream that contained hydrogen chloride, produced a highly corrosive downstream environment, due to ionization of the HCl, even though the bulk liquor stream was relatively non-conductive. The low conductivity of the bulk environment meant that conventional process and corrosion monitoring instrumentation was unsuitable to detect the onset of the fault condition or provide an indication of the likely corrosion rate. In consequence, it was desirable for corrosion combination of monitoring techniques to be developed that could detect changes in the service environment in real time in order to reduce the severity and duration of such attack. This paper describes the development of the monitoring approach, and in particular on the application of electrochemical noise analysis, which, in combination with certain process chemistry parameters, could be used to detect the onset of the fault condition and prompt timely remedial action in such complex and/or low conductivity hydrocarbon processing systems.

INTRODUCTION

The maintenance management regime that is applied in petrochemical plants operated by our company is intended to ensure stable operation and to minimize downtime and repair costs. When implementing maintenance management, predictions of plant degradation are made, based on data from the literature, or from inspection results, or from plant tests. However, it is difficult to make accurate estimates of the degradation rate as the service environment in such plants is varied. Furthermore, the service environment may change, due to a failure in other equipment or as a result of operational error, etc. A timely and appropriate response to these sudden occurrence events is one of the key issues of concern for good operational control of petrochemical plants. In these processes, many organic solvents contain halides and while the processes normally operate without difficulty, severe corrosion can occur, for example, if a tube leak occurs in a shell and tube heat exchanger unit, which allows a small volume of moisture into the process stream where it is mixed with the process liquor, resulting in ionization of the halides. If the detection of the fault condition is delayed for any reason, the damage may extend to the whole facility and the consequences may be catastrophic. It is very helpful, therefore, if process surveillance systems can be developed that can quickly detect such abnormalities. An example in which combustibles leaked from the damaged section of a process plant is shown in Figure 1. The release resulted in a fire accident on an organic solvent process unit, which involved a solvent release that contained halides. In this case, moisture from upstream equipment was able to mix with the process stream containing chloride and hydrochloric acid was generated, which rapidly corroded the downstream process piping. In consequence, the combustibles leaked a short time after the moisture was introduced.

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