In one of our CDUs, the wash water to desalter is being heated in a heat exchanger with LP steam on tube side and wash water on shell side. Normally we use service water as wash water. Provision is there to use stripped water also. But whenever we are using stripped water the exchanger is getting fouled with salt scales on tubes and pressure drop across exchanger is increasing within 2 days. We are not using complete stripped water also, while using stripped water, service water is also going as wash water through exchanger. The metallurgy of tubes is CS SA179. How to proceed for analysing the root cause of fouling. It is not observed in other CDU desalters where same type of facility is there.