Thanks for the questions. Let me start however by saying that I do not want to be seen as some sort of expert in these things. There are a number of members here who know more about model steam plant that I will ever know and they are more than welcome to add their thoughts and views as and when they feel. All I am happy to do here though is share my own experiences with those who would like to know a bit more about the subject and perhaps offer a bit of guidance as to how to put together and operate a model steam plant. They can be very frustrating but they can also be tremendously rewarding and well worth getting to know a few things about the basics.
If I don’t know something I will do a bit of research to support an answer and if I can’t find the answer I’ll tell you. Also be aware that in steam plant as well as with most areas of our hobby there are more ways than one to skin the cat. Consequently there are quite often many solutions and all I will try to do are offer my own views on their individual merits. It remains up to the modeler which path he chooses to take.
Lets have a look at the questions:
1. Do you use tap water in model steam engines?I have heard a number of sources say that they use tap water but to me, and I think it is probably as a result of my marine engineering background, I always use distilled water. I know we use the boiler comparatively infrequently so scale shouldn’t be a big problem but if you live in an area of hard water you will generate some scale build up. For me the fiver a gallon is worth it to know that you will never generate any scale. Remember that the very first steam engines actually used raw sea water as feed but they had to de scale the boiler every couple of days!
2. In the boiler, what the best ratio of water to air for making steam in a horizontal non tubed boiler?A difficult question and no hard and fast rule that I know of. Most boilers I suspect will be manually fed so the level will vary between a high and a low level as judged by the modeler so in the case of those I would suggest that you operate between 25%-33% full and 66%-75% full. Any lower is getting dangerously close to drying out and serious overheating and any more and you are getting too close to hydraulically locking (completely full) and then run a very serious risk of over pressure and rupture. Don’t forget the pressure relief valve is designed to vent off steam, it cannot vent off water fast enough to prevent over pressure if it is full. If you have a level control system I would go for a set point of about 66% full. Too low and an ingress of water will bring the temperature down too quickly and too high reduces the steam volume so you run out quicker when you use it fast.
3. Is the same for a vertical boiler?I would say yes, but only based on marine vertical auxiliary boiler experience!!
4. Is the ratio different for a tubed boiler?If there are any members out there with more experience please feel free to add your comments but my own views would be the same as long as the tubes remain flooded, which may modify the lower level a little.
5. Is brazing or silver soldering usually used in the construction of a boiler?It is a bit of a misnomer that brazing and silver soldering are different techniques. They are actually both the same and vary only really in the temperatures involved. In both cases the technique only melts the filler material and so the filler does not actually fuse to the parent metal. The differences being that silver solder melts at around 600 deg C but brazing rods, consisting of mainly bronze or brass melt at around 800-1000 deg C. A very good description of the whole process is here:
http://en.wikipedia.org/wiki/BrazingI thin k in most model boilers you will find that they have been brazed because A) The melting point is higher therefore the joints are less susceptible to heat issues and B) Braze material is a lot cheaper than silver solder. For home use silver solder is that bit easier to achieve as Oxy-Acetylene gear is not required to reach the higher temperatures needed with brazing.
6. Pressure testing, I understand you use a hydraulic method, what are the rules?Prior to January 1st 2006 all relevant bodies concerned with model steam plants had their own set of rules. This was obviously very difficult to understand and comply with and put a lot of people off. As of the 1st January 2006 these organizations all got their heads together and declared a set of rules and regulations for model boilers to comply with. These rules have been accepted by the Health and Safety Executive, most of the big insurance providers and most modeling organizations. The unfortunate bottom line with these rules was the one point five Bar-Litre requirement. If you find the capacity of your boiler as the total internal volume in litres and multiply it by the working pressure in Bars you will find you own Bar-Litre figure. A limit of one point five unfortunately meant that basically only Mamod and Wilesco type boilers were exempt so the vast majority of model boat boilers required testing.
In 2007 however the pressure exerted on the organizations resulted in a reassessment of the requirements and the Bar-Litre rule was changed to three, which now includes about 80-90% of model boat boilers. Consequently if your boiler is less than the three Bar-Litre limit you are exempt from the testing regulations, however, it is recommended that safety valves are tested at least once every 12 months and the boiler and pipework installation should also be checked at least every 12 months. Exempt also means from the old steam test requirements. This is not to say however that all insurance companies accept these standards and even some clubs have their own rules for members to comply with so it is still not quite as cut and dried as it may seem. It is however the responsibility of the owner to ensure that any regulations that apply to the model in any situation it is in are adhered to.
7. Is the pipework tested too, if not, why not?As above
8. I often read the ships boilers have to be regularly cleaned, do modeller have to do the same?Marine boilers under Lloyds are surveyed every 30 months which is an inspection of all internal surfaces for signs of cracks and surface corrosion/erosion as well as a visual inspection of all fittings and shell penetrations. On reassembly the safety valves have to be tested and all safety shut downs demonstrated. If the water treatment has been looked after and maintained within limits and the combustion equipment has been kept in good order there should be very little cleaning required.
Model boilers are a little different in so far as you can’t actually crawl around inside them but similar things apply. If you keep your burners in good order and combustion clean and efficient you will not generate any soot deposits and if you use distilled water you should not generate any scale on the water side. If soot has built up a complete strip down and a soak in warm soapy water should shift most soot deposits and if scale has formed in the water spaces a descaler such as Calgon or any other kettle descaler could be used in the water side.
9. What does 'blowdown" mean?Blowing Down a marine boiler is done to remove impurities in the water that the water treatment has conditioned into sludge. You simply open a valve on the bottom of the boiler and then an over board valve and blow out the sludge. You will normally blow down about half the gauge glass of water to ensure the deposits are removed. If subsequent testing shows high levels of suspended solids them you may increase the treatment and blow down until the impurities return to normal levels. In conjunction with this process is the scumming of the boiler and we usually blow and scum at the same time. Scumming is where other impurities, also conditioned by the water treatment, float to the top of the water and remain there. A large funnel, just below the normal surface of the water is connected to the blow down line and a valve, when opened, scums off the floating impurities and blows them down the blow down line. Normal routine watch keeping practice would be to scum first, then blow down and the blow down the gauge glasses to ensure that they are clear and reading correctly.
As far as model boilers are concerned the same expression is used if you blow down the boiler i.e. open up a valve below the water level to remove the water from the boiler. Sometimes modelers will do this as part of the shutting down process to completely remove the water from the boiler and you might also hear it in reference to the lubricator when you would blow it down to remove the condensed water collected in the bottom of it.
10. I sort of understand 'supereated steam', what sort of percentage gain do you get over non 'superheated steam'?VERY basically steam is said to be “Wet” or “Saturated” when it still contains moisture which is what we produce in a model boiler and many marine auxiliary boilers. To get to “Superheated” steam you need to have to heat this steam in the absence of water to a temperature above the evaporation temperature. Hence if the saturated steam produced in a boiler is exposed to a surface with a higher temperature, its temperature will increase above the evaporating temperature.
The steam is then described as superheated by the number of temperature degrees through which it has been heated above saturation temperature.
Superheat cannot be imparted to the steam whilst it is still in the presence of water, as any additional heat simply evaporates more water. The saturated steam must be passed through an additional heat exchanger. This may be a second heat exchange stage in the boiler, or a separate superheater unit. The primary heating medium may be either the hot flue gas from the boiler, or may be separately fired.
Some model boilers are fitted with an additional coil that passes the steam discharge through the exhaust gas of the boiler and thus takes the steam into the superheated range but it is a bit primitive as the steam does not spend a lot of time in the coil and so does not get much chance to take on much more heat from the exhaust gas. How many degrees of superheat are achieved is anybody’s guess but any additional heat energy that you can get into the steam is an advantage as it is additional energy available to be used to drive the engine. It also helps to keep the supply steam that bit drier which then helps to prevent the steam from washing away the lubricating oil in the engine.
