Study on the corrosivity of the recirculating cooling water during shutdown in LNG plant

In order to better control the water quality so as to inhibit the corrosion and scaling, the characteristics of the recirculating cooling system during shutdown state were studied. Related to chemical dosing cases and operating procedures, chemicals were screened and chemical dosing program were optimized. The designed chemical scheme were proved to be feasible by analyzing the water quality data and coupon corrosive test results. The study showed that since there were more dirts such as slime and corrosion products depositing in the pipeline under the shutdown state, under-deposit corrosion is prone to occur and the effects of corrosion inhibitors are degraded by reducing the contact between the metal surfaces and inhibitors. Cycles of concentration (CoC) is the key factor for the function of the corrosion inhibitors. Considering the high residence time of the chemicals in water at high CoCs, it would be better to choose the stable and persistant chemicals. Correspondingly, the chemicals need to be replaced when the CoC is reduced significantly.