<The Risks Associated with High-Pressure Steam Locomotives>
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Dear Reader,
When examining steam locomotives, one might conclude that they were less efficient than later locomotive types. This article discusses manufacturers' efforts to enhance steam locomotive efficiency through high-pressure steam boilers.
The Bell Locomotive Works succeeded in producing models that operated at 325 psi, while other attempts yielded varied outcomes. Join me as we delve into the world of high-pressure steam locomotives.
A high-pressure steam locomotive operates with a boiler that functions at significantly elevated pressures compared to conventional locomotives. By the end of steam locomotive production, many were utilizing boiler pressures between 200 and 250 psi, while high-pressure variants began at 350 psi and some even reached up to 1,500 psi.
The Motivation for High Pressure
Achieving maximum efficiency in a heat engine involves increasing the temperature at which heat is absorbed (the steam in the boiler) and decreasing the temperature at which it is expelled (the steam exiting the cylinder).
To enhance efficiency, two strategies can be employed: raising the acceptance temperature or lowering the rejection temperature. For steam engines, increasing the acceptance temperature involves generating steam at higher pressures and temperatures—a relatively straightforward engineering task.
Conversely, lowering the rejection temperature may require larger cylinders to allow for more exhaust steam expansion. However, this option is constrained by the loading gauge and might involve condensing the exhaust steam, which can lead to frictional losses due to the greater volume of exhaust steam.
High-pressure systems can improve fuel efficiency, but experiments often faced setbacks due to the higher costs of procurement and maintenance. A simpler alternative to raising acceptance temperature is to use moderate steam pressure combined with a superheater.
A superheater converts saturated or wet steam into superheated or dry steam, which is utilized in steam turbines for generating electricity, steam engines, and processes like steam reforming. There are three types of superheaters: radiant, convection, and separately fired.
In locomotives, the fire tube type is the most common superheater. Saturated steam from the dry pipe enters a superheater header in the smokebox, passing through several superheater elements (long pipes inside larger fire tubes).
Hot gases from the locomotive's fire flow through these tubes, heating the water and steam within the superheater elements. Typically, steam travels through the system four times—twice at the fire end and once at the smokebox end—before entering a separate compartment of the superheater header and subsequently into the cylinders.
Drawbacks of High Pressure
Complexity
High-pressure locomotives are more intricate than standard designs. They usually employ water-tube boilers with smaller diameter steam drums and interconnecting tubes that have thick walls for enhanced strength.
Scale Deposition
High-pressure boiler tubes often face issues with scale deposition and corrosion. Scale buildup can lead to overheating and potential tube failure.
Safety Concerns
A sudden leak of high-pressure steam into the firebox poses significant safety risks.
Jacob Perkins
Jacob Perkins was an early innovator in high-pressure steam technology. He introduced the “hermetic tube” system to steam locomotive boilers, resulting in several locomotives built in 1836 for the London and South Western Railway.
To mitigate corrosion and scaling problems at high pressures, distilled water is often employed, a practice common in power stations. Dissolved gases, such as oxygen and carbon dioxide, can also cause corrosion under high temperature and pressure, necessitating their exclusion. Most locomotives lacked condensers, which meant pure feed water was unavailable. One solution was the Schmidt system.
Layout
The Schmidt system featured a sealed ultra-high-pressure circuit that transferred heat to a high-pressure circuit using heating coils inside a high-pressure boiler. While using ordinary water could cause scale formation on the heating coils, it would not lead to overheating. Ultra-high-pressure tubes could withstand their internal steam temperature but not the flame temperature from the firebox.
Pressures
The sealed ultra-high-pressure circuit operated between 1,200 and 1,600 psi, depending on firing rates, while the high-pressure boiler functioned at around 850 psi and the low-pressure boiler at 200 to 250 psi. The low-pressure boiler followed the conventional fire tube design typical for steam locomotives.
Examples
The French PL241P, German H17–206, and British LMS 6399 Fury all utilized the Schmidt system and shared similar designs. The New York Central HS-1a and Canadian 8000 also adopted the Schmidt system but were considerably larger, with the 8000 weighing more than twice that of the Fury.
The Schwarzkopff-Löffler System
Another method to transfer heat without scaling in the high-pressure boiler is by using steam. Saturated steam from a high-pressure generator can be pumped through superheater tubes lining the firebox. This steam can then be superheated to around 900 °F and its pressure raised to 1,700 psi. Only a quarter of this steam is sent to the high-pressure cylinders, while the remainder returns to the steam generator to facilitate further evaporation.
Steam Circuit
Exhaust from the high-pressure cylinders passes through a low-pressure feed heater and the tubes of a low-pressure boiler, achieving comparable efficiency to the low-pressure boiler of the Schmidt system. The primary distinction is that the Schwarzkopff-Löffler system is heated by high-pressure exhaust steam instead of combustion gases.
When steam in the low-pressure boiler reaches 225 psi, it is directed to the low-pressure superheater before being delivered to the low-pressure cylinder. The exhaust from the low-pressure cylinder feeds into the smokebox blastpipe. After condensing in the low-pressure boiler heating tubes, the exhaust is pumped back to the generator.
Example
The only locomotive constructed using the Schwarzkopff-Löffler system was the German DRG H 02 1001 of 1930, which unfortunately proved to be highly unreliable.
The Straightforward Approach
The Baldwin 60000 prototype operated at a moderate 350 psi without employing complex systems, utilizing a standard water-tube firebox and fire-tube boiler.
High maintenance costs and unreliable performance negated the fuel savings promised by high-pressure and compounding systems. The United States produced other conventional high-pressure locomotives, such as the L F Loree locomotive in 1933, but none were successful.
In 1908, H. W. Bell Locomotive Works created a successful series of high-pressure locomotives that continued into the 1920s, leveraging Stanley Steamer technology. This compact, narrow-gauge machine weighed just 5,000 pounds and featured a 5-foot wheelbase, operating at 500 psi with boilers tested at 1,200 psi.
The fire-tube boiler was reinforced with piano wire, and the connecting rods and cranks were entirely enclosed and geared to a single axle. Following this, Bell Locomotive Works produced models with either 325 or 350 psi.
Water-Tube Boiler
In Great Britain, the LNER Class W1 employed a marine-type water-tube boiler operating at 450 psi (3.10 MPa), but it was not particularly successful and was eventually converted to a conventional fire-tube boiler.
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