The introduction of the high-pressure superheated steam boilers took place in connection with the first new ship builds by the Reichsmarine after WW 1. The major expectations of these power plants were:
- Reduction of the weight of the propulsion plants in order to free up tonnage for other items.
- Smaller space requirement
- Lowering of the fuel consumption rate in favor of a higher operating range.
- Reduction of the time necessary for raising steam pressure so as to increase combat readiness
- Faster power adjustments so as to increase maneuverability.
All in all, the combat effectiveness of the surface units was to be increased directly or indirectly by these characteristics.
The main purpose of these developments was to replace the old, but time-prooved, naval water-tube boilers. These operated with a nominal positive pressure between 15 to 20 atmospheres. Additionally, they usually operated without superheaters, flue gas feed water preheaters or air preheaters. The new propulsion plants were to have the following operational characteristics:
- Increase of the operating pressure to 60 to 125 atmospheres.
- Increase of the steam temperature by superheating to 450° to 475° C.
- Reduction of the required quantities of steam and water actually inside the boilers.
- Preferred use of radiation heating surfaces as opposed to contact heating surfaces.
- Higher heating space load.
- Built-in flue gas feed water preheaters and air preheaters.
The most extreme solution within this development here was the Benson-type boiler, which operated with the forced-feedwater circulation.
Hand in hand with the introduction of these high performance boilers was the start of the transition to high-speed, usually turbo-driven, auxiliary machines as well as the replacement of the piston-type pumps used for water, lubricating oil, fuel oil, etc., with new high-speed, multi-level centrifugal pumps and by screw- and spindle-pumps, the type selected depending upon the media to be driven. For auxiliary machines which were more frequently switched on and off, electric drive was preferred pump type.
The high-pressure boilers and the desire for improved maneuverability led to the expansion of automatic control. This was already partially established in connection with some wet-steam systems, but now its use was greatly expanded to include the regulation of air, water and fuel for the boilers. These systems became still more complicated by the increased "quality-requirements" of the feed-water in the boilers, regarding the maximum salinity, maximum density and oxygen content. For this reason, "closed" feed water cycles were installed everywhere, which were provided with degassing systems of different types, various regulating tanks and compensator cells. If this wasn't complicated enough, the boilers still needed to be able to be parallel connected for damage control purposes. All these new and complex machinery arrangements exceeded human reaction time and thus created an even greater need for automatic control systems.
The propulsion plants themselves became buried by all of the additional auxiliary machines required for normal operation and by all of the special electrically operated auxiliary facilities required to start them up plus their associated piping and stop valves. The geared steam turbines themselves were not subject to large modification, and the small modifications made to them improved their efficiency and operation.
The biggest error made during the prototype phase of these new power plants was a lack of testing. Due to the fast change in size and structure of the KM, especially after 1933, the responsibility for the required long-term, continuous testing of the propulsion plants was shifted to the crews of the warships themselves. What facilities for shore-based testing that were set up were of a rudimentary nature and of little use for finding systematic problems. What little testing was performed was on the prototype plants installed into the merchant vessels UCKERMARK, POTSDAM, GNEISENAU and SCHARNHORST. However, the test results of these early units came so late in the process that they had no influence on the first warship operational units.
A further complication was the substantial personnel expansion taking place in the KM. This led to large turnovers in warship complements, so qualified engineers, leading seamen and machinists moved up faster on the rank list or were transferred to new construction. Very frequently, a leading engineer would lose his best machinist just when a satisfactory state of training had been achieved. Generally speaking, the number of personnel with the proper technical knowledge was no longer adequate for the rapid pace of technological advance taking place nor for the increasing number of ships that needed to be staffed. There were simply too few experienced "old timers" available that had enough understanding and experience with these new power plants to pass on their knowledge to the newly commissioned engineering officers and enlisted machinists. In many cases, engine-room crews who had been trained on the older, lower-pressure steam systems tried to maintain the new plants in the same manner. The end result of all of this was that maintenance problems with the new power plants went unrecognized and uncorrected until a breakdown occurred.
Apart from the direct damage and disturbances caused by the new machinery required for high-pressure superheated steam propulsion, there were also associated problems in other regions of the power plants. For example, the greatly increased length of vacuum piping required for the new systems also increased the danger of air or salt leaks. Unwanted air getting into the cycle caused a power loss from the main turbines, which could result in a total failure as a result of too-wet steam. Salinity failures could result in the shutdown of entire groups of boilers. Small salinity leaks usually caused only short disturbances, but they also led to creeping damage, the full effect of which might cause catastrophic damage some time later.
The great maze of piping also now meant that steam leaks were much harder to find. Finding escaping super-heated steam by eye was difficult, although acoustically locating it was relatively easy. This meant consequential damage (e.g., cables, wires) if the leakage was not found quickly. In addition, there was an increased risk of fire if the high temperature steam came into contact with oil from leaky lines or valves.
After World War II had broken out, further problems showed up. Perhaps it is understandable that the new systems had operational as well as teething troubles. There were several reasons for this. First, these new systems were very sensitive even under peacetime conditions and secondly the spare-part supply situation for the new bases now being established in France and Norway was very bad. Except for the necessary fuels, the components needed for the power plants were rarely available, which made preventative maintenance nonexistent. The supply situation became better only after 1942. In addition, many cases of operator error were not detected as such. Due to the past experiences with teething problems, the crews tended to blame any damage on the design and quality of the propulsion plants and not on improper operating procedures. Since the faulty operating procedures were not corrected, the same problems kept reappearing, resulting in the ships being out of service on a regular basis.
These problems were also apparent for certain ship types. In particular, the smaller units such as torpedo boats and destroyers were very unreliable. This was partially because of the fact that these were the first ships to get these new power plants. In addition, it is also apparent that the larger the plant, the more reliably it operated. Compared with the destroyers, the heavy cruisers fared well and the battleships were had the best reliability of all. The BISMARCK and TIRPITZ also profited from a strengthened construction of the system. This was possible because these ships were to originally have had a turbo-electrical drive installed. When the turbo-electric drive was dropped, the larger spaces and weights that were necessary for the T-E plants were then used to build much stronger steam plants.
However, the reliability of the different Diesel propulsion systems remained unsurpassed. Naturally, this comparision is not totally fair, because the diesel systems had substantially lower performance for the same weight. Nevertheless, compared to the new high-pressure superheated steam systems used on most warships, the diesel engines used on the pocket-battleships, S-Boats, minesweepers and numerous other units experienced very few or no difficulties.
- 19 February 2000