Modernising Options for Steam Locomotives
As can be deduced from the Principles of Modern Steam pages, there is a wide range of “modernisation” modifications that can be applied to locomotives, both new and old. This section offers guidance to locomotive owners as to the types of modifications that might be relevant to their locomotive and to outline the sorts of improvements that each might be expected to deliver.
The following table offers a quick guide to modernising options that might be adopted and the benefits that they offer, in reference to which it is recommened that note be taken of the extract below taken from page 144 of Wardale’s book.
The following page offers guidance with respect to the potential relevance of each option.
|Improved exhaust system||Reduce backpressure in cylinders to increase cylinder efficiency & power output, & reduce specific steam consumption (SSC)||√||√||√||√|
|Fit Kordina below blast pipe||Reduce back-pressure in exhaust passages to increase cylinder efficiency & power output, & reduce specific steam consumption (SSC)||√||√||√||√|
|Convert firebox to GPCS operation||Reduction of: unburned fuel carry-over; smoke & spark emissions & clinkering;||√||√||√|
|Increase superheat temperature||Increase thermal efficiency, thus reducing SSC, fuel consuption and carbon emissions.||√||√||√|
|Install feedwater heater(s)||Reduction in SSC & fuel consumption and/or increase in steam production.||√||√||√|
|Cover all hot surfaces with high quality insulation||Minimise heat losses and reduce condensation on cylinder walls.||√||√||√|
|Improved valve and piston lubrication
||Reduce piston and valve ring wear and cracking of lubricant oils by applying them directly onto rubbing surfaces.||√||√|
|Multiple narrow valve and piston rings||Minimise steam leakage; reduced ring wear.||√||√||√||√|
|Multiple element piston and valve rod glands||Eliminate steam leakage from glands between overhauls||√||√||√||√|
|Modify valve ports and valve heads||Improve streamlining to reduce triangular losses & improve cooling of valve liners, valve heads and valve rings||√||√||√||√|
|Saturated steam cooling of valve liners||Reduce liner temperature – recommended for very high temperature superheat||√||√|
|Increase steamchest volume and steam pipe diameter||Reduce pressure drop of admission steam at high steam flow rates, improving efficiency and SSC.||√||√||√|
|Optimise valve events||Improve steam flow into and out of cylinders by adjusting valve motion, lap and/or lead.||√||√||√|
|Optimise clearance volume by modifying pistons and/or cylinder covers||Raise SSC, efficiency, etc by reducing incomplete expansion losses without impeding steam flow.||√||√||√|
|Fit roller bearings wherever practicable
||Minimise wear, knocking and loss of valve motion.||√||√|
|Fit Franklin wedges to hornguides||Postpone development of knocking as hornguides wear||√||√|
|Fit Franklin-type buffing mechanism between engine and tender||Reduce buffing shocks between engine and tender; reduce fore-aft rocking from unbalanced reciprocating masses||√||√|
|Fit air-sanding equipment||Reduce risk of slipping and consequent delays, damage etc||√||√|
|Adopt mid-gear drifting||Maintain steam pressure against admisssion edge valve rings, and prevent smokebox contaminants from entering cylinders.||√||√|
|Adopt Porta water treatment||Reduces or eliminates corrosion and scale formation; dramatically reduces boiler maintenance costs.||√||√|
The Holistic Nature of Steam 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.
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.
Improving Locomotive Performance and Efficiency
On the subject of improving locomotive performance and efficiency, Wardale offers some erudite observations on page 144 of his book, drawing attention to the fact that the four catagories listed in the above table all impinge on locomotive performance and efficiency in the broadest sense of the terms:
Before considering the modifications in detail the underlying philosophy upon which such an integrated rebuilding scheme was based must be explained. Improving the performance and efficiency of steam locomotives was essentially a question of minimizing avoidable losses, called ‘loss control’ in modern parlance. This applied both in the thermal and mechanical sense. Taking the latter, maintenance and less-than-perfect reliability were both due to losses – wear, the consequences of which consumed much of the maintenance effort, is by definition a loss of material, and any kind of failure is a loss of the ability of the component concerned to function correctly, which might be the loss of the clamping force of a bolt which stretches or fractures, or the loss of thermal conductivity in a heat transfer surface which becomes scaled, etc. In the broad sense mechanical deterioration meant a loss of the ability of a locomotive to perform as designed and a locomotive which was loss-free in this sense would have been perfectly reliable. Although such mechanical losses could not all be reduced to zero in the real world – for example traction required unlubricated creep between wheel tyres and rail heads which had to lead to wear – there was nothing in the laws of nature which said they could not be made much less than typically found in steam locomotives (for example the wear rates of steam locomotive cylinder liners, pistons, rings, etc.. could be some thirty times as much as for the corresponding diesel engine components).
Considering thermal losses, the performance limit of any heat engine is dictated by the laws of thermodynamics and the related heat loss is unavoidable, its magnitude at any given work output depending on the upper and lower boundaries of the thermodynamic cycle on which the machine operates. The discrepancy between this theoretically attainable efficiency and that actually achieved represents losses which are not imposed by the laws of thermodynamics and which are therefore potentially avoidable (or at least largely so) or, put another way, it is a measure of the extent to which the performance of the actual machine falls short of that which can ideally be achieved. In the case of steam locomotives avoidable thermal losses were much too high, and at worst could be simply appalling.
Taking the 25NC class, the Rankine cycle efficiency for the thermodynamic limits between which the engine operated, i.e. the potential efficiency allowed by the laws of thermodynamics, was 17%, yet it can be deduced from dynamometer car tests that at 90 km/h and maximum power the thermal efficiency at which useful work was produced at the drawbar was only 3.3%, or one fifth of this theoretical maximum. The carpet of coal which lined railway tracks wherever coal burning steam locomotives worked hard was a silent testimony to this unfortunate fact. Therefore a scheme to improve the performance of existing locomotives had to attack avoidable losses – especially where they were highest.
It was clear that there was great scope for reducing the thermal losses from the 25NC class: moreover cost data showed that in normal service between Kimberley and De Aar the fuel cost of these locomotives was about three times their maintenance cost, hence the priority given to attacking thermal losses. As input minus losses equals output such loss control would achieve both improved thermal efficiency and power. This philosophy was exactly what had guided Chapelon and Porta but not, unfortunately, many other locomotive engineers, and much of the rebuilding work could be classified as simply correcting the mistakes in the original design, such as poor internal streamlining. In the thermodynamic sense it was bound to succeed if properly applied, and was the reason why apparently astounding improvements in performance could be made without altering the overall size of locomotives nor many of their important parameters, such as boiler pressure, grate area, evaporative heating surface area, or cylinder dimensions.