Automatic Operation of Transfer Line and Decoke Valves, Flexibility & Process Safety

Decoking is an important function in the operation of a furnace.  This paper describes the decoking system, focusing on the transfer line and decoke valves and their operation. Switching the plant’s operation between decoking and process mode is a critical task, as at least two valves have to be moved simultaneously in a defined and synchronized way. This is usually performed by means of a mechanical linkage, to prohibit backflow of hydrocarbons into the decoke system and prevent the furnace from being totally blocked in and over-pressured.

Automatic control systems are designed to regulate the valves' positions that mimic the actions of a mechanical linkage from process to decoke mode and vice versa. The same control philosophy is applied via pre-calculated stroke, as already known from the mechanical linkage.

The system permits the valves being controlled to be placed in more convenient locations, which is not possible with mechanical linkage design. The control system compensates for the varying furnace delivery flow rates and alters the valve movement profiles accordingly. It also adds flexibility to the operation of the valves while at the same time ensuring process safety.