The topic is discussed in some detail in two of the papers published in L.D. Porta's "compounding" paper published in Camden's book "Advanced Steam Locomotive Development - Three Technical Papers" including his short paper titled "Some Steam Leakage Tests on Locomotive NORA of the Ferrocarril Austral Fuegino". Avoidance of such leakage is covered in much greater detail in his unpublished papers on Tribology and the Design of Piston Valves.
Porta describes the worst offenders being locomotives with American-type Duplex rings, and quotes Chapelon under whom measurements were carried out by SNCF that showed steam leakage losses of up to 12% of the maximum boiler evaporation in the case of a very well-maintained 141R. He even mentions a figure of 20% steam loss from these locomotives, presumably in less well-maintained examples, and compares it with figures of 1.4 to 1.7% loss on the Rio Turbio Santa Fes when fitted with his design of valves and pistons, measured when the rings were life-expired.
The paper about FCAF's Garrant "NORA" reveals even worse losses that by Porta's estimation amounted to some 50% of steam generated by the boiler, adding that "when one considers that wall effects (on this unsuperheated engine) increase the indicated steam consumption by AT LEAST 100%, the actual steam/fuel consumption is roughly FOUR TIMES GREATER THAN WHAT IT COULD BE. In turn this means that the boiler, and (water/fuel) tanks, etc, are four times larger than they should be - same for the annual fuel bill."
[Following these and other findings, FCAF comprehensively rebuilt NORA, and under the guidance of Shaun McMahon, they incorporporated many "Porta enhancements", as described by Shaun on a separate page of this website. At the same time, the locomotive was renamed "Ing L.D. Porta".]
In his "compounding" paper Porta comments about steam leakage as follows (with edits):
"To the best of the Author's knowledge, only in France, after World War II, has leakage [past piston and valve rings] been measured. As a matter of course it has been much studied in Internal Combustion engine technology through which it has been well established that leakage occurs through the area left between the cylinder, the piston and the ring joint, not along the circumference, and this is confirmed by Chapelon. In the case of the Author's design, the leakage is very small. However, because of it being constant, its importance increases in the case of slow moving machines, such as those used in shunting
As a first approximation, leakage can be considered to be a constant volume of steam by-passing directly from the steamchest to the exhaust. It is proportional to the steamchest pressure and inversely proportional to the absolute steam temperature. But if conditions are such that the parts determining it are below the saturation temperature, its importance increases considerably because what leaks is condensate, whose density is more than a thousand-fold that of the steam. The same is obtained during the warming up period, which lasts roughly 20 minutes at full power, or may last indefinitely at low power.
Poppet valves are heavy offenders where leakage is concerned. The various claims against this statement have never been sustained by measurements or serious reasoning. Chapelon measured heavy leaks as reported in his book. The reason is that except in the Caprotti gear, the valves seat on the cylinder block, itself subject to widely differing (and varying) temperatures, and hence distortions. This aspect is so important that Stumpf developed elastic seats and the corresponding theory for them.
Because of it being constant, leakage may be significant for low-speed engines showing low piston speeds and low volumetric power. This is the case with shunting engines and many ships. The author's technology since early times incorporated the concept of long strokes (American/GWR practice) and high rotation speeds - 504 r.p.m in AAR standards."
In a later section of the paper, Porta goes on to summarize how "leakage is reduced through the application of the [Porta's] advanced cylinder tribology, whose basic points are as follows:
- narrow, diesel type piston rings,
- as many rings as possible per valve head or piston (6 rings per valve head on Wardale's 'Red Devil'),
- the valve resting on the liner so as to have a theoretical zero leakage,
- best "diesel" quality materials,
- elimination of abrasive material entering via blast pipe and moisture (use of antifoam: diesteraliamide),
- some piston rings made out of bronze (to aid lubrication),
- piston rods,
- oil injection "between" the valve rings (NEVER mixed with the steam),
- paraffin based oils,
- light weight piston valves,
- packing rings for the rods,
- liner cooling,
- adoption of wheels of the smallest possible diameter as allowed by the AAR standard (504 r.p.m.),
Sincere thanks to Adam Harris of Camden Miniature Steam, publishers of "Advanced Steam Locomotive Development - Three Technical Papers" for allowing the sections of the book to be published on this website.
For further information on tribology and piston and valve ring design can be found on the following pages: