Abstract: A conduit is preheated with steam having steam drain valve(s) therein open to allow draining and control valve(s) therein closed to prevent steam downstream flow. A gas may backfill the conduit from a downstream gas source to further preheat, and gas drain valve(s) may be provided to drain the gas. A predicted exit temperature of the steam or gas for a respective drain valve is calculated using a linear regression model based on initial temperature and pressure of the steam or gas and a Joule-Thompson cooling effect for the steam or gas at the respective drain valve. The preheating is stopped for the respective section of conduit when one or a combination of the predicted exit temperatures is greater than or equal to a respective required exit temperature of the steam or gas at a respective drain valve. The system sequentially ensures adequate preheating of sections of the conduit.