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The Steam Boat
Association of Great Britain |
Safety considerations raised in Funnel 101. |
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| An
engine simpling valve problem
It seems that a major steam leak occurred in the lagging of the engine while cruising in Stockholm harbour last year and the launch was disabled. A part of the simpling valve - see arrow on photo - the hex brass screw part of valve failed. The pictures were taken after the repair, so don't show exactly which bit broke. Ivor suggests anyone with this engine type should check the simpling valve component and maybe carry a spare. |
| The Use of Copper in Boilers
Some people may think that copper is the ideal material for boiler making. However a problem was highlighted last year because of its use as a boiler construction material. Copper has excellent anti-corrosion properties, however it has rather poor strength characteristics, with only about a third the tensile strength of steel. This is widely understood and allowances are made for this in copper boiler designs. Unlike steel however, copper also suffers from a severe loss of strength at quite modest elevated temperatures. BS 853 (a code for calorifiers - ie a pressure vessel not externally fired) gives the maximum allowable design stress for copper at about 48 N/ mm2 at room temperatures, reducing to 19 N/mm2 at 200°C. To illustrate the significance of this - the boiling point or water is 183°C at a pressure of 150 psi. Without external heating the actual temperature of the copper can easily be predicted from steam tables. The problem is that a steam boiler has heat applied externally, which is transferred through the metal to the water, thus the metal will always be hotter than the contained water.Fortunately copper is a good conductor of heat so you might conclude that the temperature of the copper in contact with the boiler water will only be slightly higher than that of the water. This however cannot he assumed if the heated copper is only in contact with steam, in these circumstances the temperature will rise significantly. Field tests carried out on a copper boiler last year showed that the temperature of copper in the steam part of the boiler could rise to 230 °C under forced firing conditions, even though the steam pressure in the boiler was only at 120 psi (water boils at 177°C at this pressure). So in this case, the copper was some 50°C hotter than might have been assumed from figures listed in steam tables. This has a serious implication on the safe working of any copper boiler, because of the great loss of strength with increasing temperatures.It would seem prudent where copper is used for boiler construction and where the metal is in direct contact with the hot flue gases, the copper should also he in direct contact with the boiler water at all times. The American Society of Mechanical Engineers (ASME) have a code covering the use of non-ferrous metals in Power Boilers (these are defined as externally fired pressure vessels). |
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These notes are provided for the public good.
However it is not possible to publish a comprehensive list of all safety concerns relating to steam boats.
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