This chapter concerns the practical design of a critical component for an existing plant which had to operate on a different fuel from that originally specified. It shows the need to study the interrelationship between the components of the system and how the analysis of the effects resulting from the change of fuel, together with an analysis of the system operation, enabled an outline specification for the component to be defined. From this specification, a design was developed on the basis of existing knowledge, employing rig testing where it was not possible to predict performance. The selection of materials and the detail design of the component were based on ensuring operational reliability under all phases of the system operation.