Holistic Nature of Locomotives
When considering any modifications to a locomotive, it should be born in mind that these are holistic machines in that alternations made to one part can have unexpected (and sometimes undesirable effects) on other parts. Both Porta and Wardale have taken some pains to point out that particular care be taken when planning modifications to locomotives to consider possible the effects of any change on the rest of the locomotive.
On page 145 of his book, Wardale explains this point when describing his decision-making when planning the modifications to 25NC No 3450 (later to emerge as The Red Devil:
Examples have already been given in Chapter 2.2 of the integrated nature of the functioning of the various parts of a steam locomotive, i.e. how the workings of many important parts could not be considered in isolation from each other. Therefore in a rebuilding scheme as extensive as the one being considered there had to be a clear view at the outset of how the overall scheme fitted together and a reasonably accurate prediction from calculations of the values of parameters which influenced the functioning of the modified components. Confidence was also needed that all such functionally interdependent parts could be designed and fitted, for failure to do this would have caused malfunctioning of some parts of the locomotive and given one or more weak links which would have limited the worth of the whole scheme. To give one example, the maximum possible superheat was desirable and a figure of 450°C was designed for, requiring a larger superheater. This temperature dictated that the valve and cylinder lubrication be improved and the valve liners be cooled, but cooling could not be incorporated without changing the design of the valve liners and steam chests. The valve liners had to be compatible with the new valves and their motion, the latter depending on the feasibility of making the necessary valve gear alterations and providing Herdner starting valves, the design of which was in turn limited by the vehicle gauge. Altering the superheater altered the boiler tube bundle gas flow resistance and exhaust steam temperature, both of which affected the draughting. The modified main steam pipes had to suit both the altered steam chests and superheater header and the latter had to fit into the altered smokebox. The size of the superheater for a given steam temperature depended on the feedwater temperature, i.e. whether or not a feedwater heater was to be fitted. Therefore the certainty of being able to incorporate all these alterations had to be established and where necessary calculations made to predict their performance when all were working together.