Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Steam => Topic started by: Martin (Admin) on July 12, 2008, 02:25:12 pm
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Can I ask a stupid question (another one!)?
I presume the the big ships, ocean liners, battleships etc (and I suppose and all steam vessels) use
fresh water for the boilers..... therefore, in real ships, how do they carry enough water for the
boilers, engines, turbines?
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hi there
To answer your question; basically, there is a condenser on board the life-size ships; this converts the exhaust steam back into water. This is then returned to a holding tank to where the boiler takes its water supply from. Obviously it is a lot more complicated than this, but, this is how it works. I have included a scan from a book to try and explain.
aye
john e
bluebird
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Not one you made earlier then John ? ;)
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Boilers dont use fresh water as we know it - from the taps. If they did, the heat exchange surfaces would scale up, reduce the heat exchange rate and ultimately fail.
One way is to uses de mineralised water, that is, all the salts have been removed by an ion exchange process, leaving the water extremely pure with a conductivity of well under 0.01 micro seimens. This pure water also tastes foul!
Another way is to use reverse osmosis units, these are basically an ultra fine filter that acheives the same result as ion exhange.
Clearly a ship cannot carry enough water to top up the steam/water cycle over a voyage - there are always losses and boiler blowdowns that remove water from the system.
Shipboard systems are designed to use sea water as the water source and purify it to boiuler quality.
On the ships I worked on, we used evaporators, under vacuum, boiliing the water and collecting the steam, condensing it and using it in the boilers. the problem with these was that the salt removed caked up the elements and and had to be cleaned on a regular basis.
The system Bluebird described is the basic system used in all steam/water cycles - ships, power stations etc.
Ian
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One of the biggest problems with flash evaporators over the years has been the build up of scale in the evaporating chambers but nowadays we dose chemicals which do an amazing job of preventing this scale from forming.
A couple of interesting points to add to the conversation, evaporators nowadays use "waste heat" to power them such as the cooling systems from the main engines and supported by steam generated in exhaust gas boilers, that's why they work under a partial vacuum, to enable boiling at a much lower temperature than at atmospheric pressure. The evaporators not only produce pure distillate for feed but also go through mineralisers to produce potable water and typical cruise ships evaporators could be capable of producing 1000-1500 tons of water a day.
If the vessel is steaming slowly and not producing much waste heat then auxilliary boilers have to be used to support the evaporators. Then the water produced instantly changes from becoming almost free to extreemly expensive.
As for the boiler feed the evaporated water is the best water you can use for feed as it has been distilled three times, however for high pressure boiler use, i.e. main propulsion this feed water would need to be of an even higher quality.
I always use distilled water in my model boilers, which everyone tells me is not necessary, but there is no doubt in my own mind that it is better for the long life and efficiency of the boiler.
Back to Martins first comments though it is interesting to compare modern practise with the very early coastal steamers. They used raw sea water as feed and had to go into the boilers every few days to scrape the salt out. This was usually in conjunction with scraping the furnace chamber out as well while the plant was still hot.
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I would always advocate de mineralised water in model boilers as well.
The problems associated with poor quality water will arise in model boilers just as in full size. The only difference being in time scale - models get used a fraction of the time of full size ones.
Ian
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A respected colleague....Sandy Campbell from ACS once mentioned in the PD forum that de-mineralised water was OK with copper but <:( should never be used with brass tubing as the fluid has the natural potential [through the electro chemical + - series] to leach the zinc component from the tubing substrate ....& hence over time reduce the actual wall thickness through micro porosity & resultant loss of strength in the tubing
As bunkerbarge notes above........the same as the recomendation from Sandy was......distilled water is the best option....... - Derek
PS..... nice to see many PD members also here :-))
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Hi Derek and welcome to Mayhem. Yes there are a few PD's on here as well.
Someone recently gave an excellent description of the potential dangers of using de-ionised water in model boilers, which should really be avoided. This is usually the water you buy in a car spares shop for battery use so read the label carefully.
I have actually started to use the water my missis throws away from our condensing tumble drier. It is basically distilled and a plentifull supply for nothing.
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Snap Bunkerbarge, thats where I get my water from, unending supply in our house. :-))
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hi..
i fund this
What is the difference between distilled and deionized water? Can I use deionized water in my boiler? Do not use deionized water in your boiler! Use only distilled water in your small-scale steam boiler. Your engine will run on deionized water or even ordinary tap water. But there are problems with each of these which we should avoid.
Tap water contains minerals which will stay behind when you boil the water out of the boiler. When you run your steam engine, you are in essence distilling the water in the boiler. What comes out the regulator is pure water vapor, and the minerals are left behind in your boiler as scale. When you use distilled water in the boiler, there is nothing left when you're finished.
Scale reduces the boiler's ability to transfer heat to the water, and thus reduces the efficiency of your engine. Eventually, the lines may become so clogged with scale that you can't get any steam out of the boiler at all. Operators of larger engines can sometimes remove the scale physically by scraping or brushing. We have no such small scrapers and brushes, though. Scale can sometimes be removed by boiling vinegar or some other very weak acid in the boiler. This should not be considered a desirable alternative to using distilled water, however. Stick with distilled water and you won't have to worry about mineral scale.
Deionized water contains no ions, which means it has had the chemically reactive molecules removed. If put into contact with metals, however, it will happily take on new ions, with disastrous results. Mike Chaney wrote about the effect of using deionized water at a UK exhibition:
After about a weeks running some of the loco boilers started to "weep", although they had been properly tested and certified. An investigation showed that the silver soldered joints were failing because the water was trying to grab back ions from any metal with which it came into contact. Copper, zinc and silver were found to be particularly susceptible.
Moral: for the long-term health of your boiler, avoid deionized water!
Bob Stiegler wrote
I bought two measuring devices at the local wholesale nursery supply co. One measures pH, the other total water hardness. I tried them out on collected rainwater, and measured 7.5 pH (neutral) and approx 5 ppm total hardness (verrrrrry soft). Not having the facilities for collecting large amounts of rainwater (and having no way to collect any rainwater whatsoever when its snowing), I bought some bottled water. It was also neutral and sufficiently soft, but expensive. I then saw something called an RO (reverse osmosis) watermaker in a tropical fish supply catalog. Not cheap ($200.00, approx), but very effective (7.5 pH, 1 ppm total hardness). It takes a couple of hours to fill a 7.5 gallon water bottle. My plants thrived, and I didn't go bankrupt buying soft neutral water.
While this is a bit too expensive to be justified by the the average small-scale live steamer, it's something which could be attractive for a club to buy as a group. Or, if you keep tropical plants or fish, you might well get enough use of it to justify one yourself.
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I think the loco boys have to be a lot more careful than we do, as they tend to have lot longer running times. I have several boilers (scott-semi flash) that were built pre-war (WW2 as opposed to any other war we may have got involved in) and although I can't see inside, I still use them regularly and they are tested every 2 years to twice working pressure (around 220 psi) as if I'm going to test why bother with the 1 1/2 times, and they have both been using tap / pond water . Now when I add up total running time in a year it amounts to around 6 hrs. OK so its straight running, so running times are not as high as say a radio steam boat, but I've not had any problems (so far), but then these days there are a lot more "things" in our water, so I tend to use the water from the tumble drier---after all, if it helps prolong the life another 60 odd years :-))
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Not knowing very much about steam,i do know that full size steam vehicles need a boiler certificate.
Does that apply to models as well,as Steamboatphil mentioned above tested at 220psi,thats a lot of pressure.
Mark
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Hi Craftysod, yes we do test our boilers as required by Model Power Boat Assocation rules and guidelines, I know that other clubs which may not be members of the Assoc also have boiler testing guidelines, its two fold, 1) it checks the boiler for any failings 2) a lot of public parks may insist on 3 rd party insurance, and boiler testing is usally a requirement of the insurance company. There is also a bar/litre chart which some clubs have adopted. In a nut shell, if your boiler contains a small amount of water (when full) and works on a low pressure you bo not require a boiler test (you x one by the other)
For testing, new boilers are tested to twice their working pressure, and then to 1 1/2 x every two years after, although I tend to test my own a little higher. Hope this all helps.
Phil
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Thanks for that,i get a little wiser everyday
Mark
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I would always advocate de mineralised water in model boilers as well.
The problems associated with poor quality water will arise in model boilers just as in full size. The only difference being in time scale - models get used a fraction of the time of full size ones.
Ian
Apparently demineralized and deionized water is the same thing:
http://en.wikipedia.org/wiki/Distilled_water
So if I understand correctly, it's destilled water which is okay to use?
Cheers, A!H.
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Just to throw a spanner in this thread- up until condensing technology- after the freshwater tanks had been used up the boilers were fed with salt water- making salt stallactites on every leak in the engineroom!!! Not reccomended. :D
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Hi,
Another thing bad for boilers was oxygen in the water.
When I was an apprentice, one of my tasks was to check the boiler hydrazine injector was at the correct flow rate. Yes, as we all know hydrazine is used as a rocket fuel, but it also scavenges up any oxygen in the water.
The station chemist, in those days, had a very good sideline in wine making (and brandy!) and one day asked me to find an empty hydrazine barrel, i.e. the thick steel one with the skull and cross-bones label on it. Enquiring as to its use, he said it was for the next batch of wine production!#$%!!!
Noting my total disbelief, he then gave me a quick chemistry lesson on how water breaks down hydrazine - oh what faith!.
Ian.
(I did test the wine on an uncle (he survived), who had been a wine waiter, and he thought it was a German Hock - perhaps it was still useful as rocket fuel!)
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Hi,
Very old vessels ran seawater in their boilers and if you google a "salinometer" you'll find out how they determined when to "blow down" the brine and refill with with "fresh" seawater.......Not the best thing for a boiler, but at 20 psig, it wasn't such a big deal.
In the modern era....1870's forward....and I speak to reciprocating engines only.
For ocean travel, ships used evaporators to create fresh water from sea water.
Boiler steam was run through coils in large tanks. The water would boil off and be collected. Periodically , the tanks would be opened and the salt cleared.
Of course these ships, at least the modern era, ran condensers...which also helped engine performance. The steam was condensed and brought to a tank called a "hotwell". This tank was used to dearate the condensed water brought there by the wet air pump and to remove any lubricating oils that might have been in the steam....A Wet air pump is an appropriate name as it took condensate and air out of the condenser.
Boiler water chemistry was and still is very important to the life and well being of a boiler. This analysis was done by the "Chief" by performing various chemical analyses usually by titration reactions. From there, The Ph , dissolved solids, and sulfates among other things could be determined and altered with various chemicals added to the feedwater usually in the hotwell. Keeping the Ph slightly alkaline was ideal......now in small scale....starting out with distilled water is easier!
Dave
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I had read the publicity about de-ionized water and was avoiding that but having trouble getting distilled water when another member told me that for many years he has just used tap water but that once a year he does a couple of boil-ups and flushes using a de-scaling chemical made for cleaning commercial water heaters and kettles. Is there anything wrong with doing this? Ian.
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Difficult to say right or wrong but for me I would rather avoid the build up in the first place. As long as this is occurring the boiler is loosing it's ability to transfer heat and there is even a slight danger of a hot area occurring due to poor heat transfer. I don't actually think it is an issue in a model boiler but I'd rather keep my plant in the best possible condition rather than alow it to get dirty and then clean it.
As I said though no right or wrong, only personal preferences.
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I had read the publicity about de-ionized water and was avoiding that but having trouble getting distilled water when another member told me that for many years he has just used tap water but that once a year he does a couple of boil-ups and flushes using a de-scaling chemical made for cleaning commercial water heaters and kettles. Is there anything wrong with doing this? Ian.
It depends on where you live and thus the hardness of your tap water. Doon in the Deep, Deep, Sooth I recall kettles built up a thick layer of limescale. Not nice at all for boilers. However, this side of the Border, kettles very rarely show any kind of build up because the water is very soft and was often used (still is?) neat for topping-up car batteries without any obvious ill-effects. Thus, I don't have a problem with using tap water but, if your soap is hard to lather, I wouldn't risk it.
Regards
Barry M
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Hi Guys
When de-ionised water is used in a boiler it always has oxygen scavenger and, more importantly, a corrosion inhibitor added.
As previously posted de-inoised water will very rapidly attack metals because it doesn't like to be de-ionised,
it wants some mates to play with so goes looking for ions.
If you read your car manual most specify that de-ionised water must not be used 'cos it causes rapid corrosion.
Has anybody experimentedwith de-ionised water with car antifreeze added? since most are also corrosion inhibitors.
Cheers
Mark
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Hi,
I do recall an apprentice (not me!) thinking it might be a good idea to use water from the boiler water treatment plant in his cars cooling system. Within days his radiator sprung leaks from numerous hard to repair joints - a lesson not to be forgotten.
Ian.
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Actually did a bit of reading in my library on evaporators. It would seem that the supply steam could come from the boiler or from one of the engine Intermediate receivers. This steam was run through the coils and then to the condensor. The salt water side was run to the condensor or to the LP exhaust receiver. It was important that the evaporators not be forced or left highly scaled as the salt would carry over into the fresh feed water.
Dave
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Dave,
Early vaps did use fairly high pressure steam but anything fron at least the '50s on used live steam at about 30psi max or a turbine bleed if the vessel was a turbine proppelled vessel. Motorships commonly use waste heat from the main engine CW or an exhaust boiler. The condensate from the heating element drained to the main condenser.
I'm not at all sure what you mean by the "salt water side was run to the condenser or the LP exhaust receiver etc...." - makes no sense to me. The SW feed was boiled off under a vacuum by the heating element, passed through a demister and subsequently condensed in the vaps own heat exchanger and then pumped to the feed tanks if the salinity of the output was low enough. If the salinity of the output was too high, it was dumped to bilge. The direction of output was controlled by a salinometer which continuously monitored the output and opened a dump valve if the salinity rose too high.
To keep the salinity of the boiling SW feed in the vap shell to a reasonable level, a proportion was continuously pumped out via a wier and brine pump to sea.
To try and keep scaling on the heating element to a reasonable level, the vap was shut down and the element 'cold shocked' daily. This involved heating the element with steam and then shutting off the steam and rapidly cooling it with a cold seawater spray in an attempt to crack-off the scale. It only worked to a point and eventually the element would have to be extracted (not the most favoured job) and soaked in a bath of 'Safacid' (spelling?) or similar. Even this was often only partially successful. While the element was out, that useful item found in most engine rooms and known as an Apprentice was introduced to the inside of the evaporator shell with a chipping hammer. There he would discover the delights of sitting in a pool of hot seawater on sharp fragments of scale while leaking steam valves dripped boiling hot water on him and, if he was unlucky, the lead lamp might drop in the water and smash; before - hopefully - the fuse or breaker shut off the current he could enjoy the thrill imparted by 110Volts. This was known as "Good Experience" and heartily endorsed by more senior engineers who believed in tradition.
Hope this helps,
Barry M
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Hi,
Wasn't there a part of the old vaps called the "ebulation chamber". Why it wasn't called the boiling chamber I never did discover.
Ian.
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Barry,
Laughed like a drain at the last paragraph - been there, done that both as apprentice and more senior engineer :-))
Ian
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Ian,
I also recall another tradition which found Apprentices working on deck in January in northern Sweden in inadequate clothing while the Engineers always found work in the Engine Room. In the Tropics the reverse ocurred. This was known as 'Character Building' and always supported without question by all Engineers as being in the best traditions of the Merchant Navy.
The only persons on whom Engineer Apprentices could inflict minor atrocities were Deck Apprentices sent below for ER experience. They took every advantage then... O0
Cheers,
Barry M
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That was a funny paragraph Barry! {-)
The source I have....can't put a finger on it at the moment...was speaking to Reciprocating engines only, and old ones at that.
The IP Receiver pressure was approximately 60 psi, or in that neighborhood, so that part agrees with your comments. As to how to make sure salt was not carried into the system, I am sure the old reference outlines old ways of doing things. Far older than 1950, more like 1900. My reference stated that the evaporated vapor went to the condenser or LP exhaust receiver....now what they meant by LP Exhaust receiver I don't know, To me it means the exhaust line from the engine to the condenser. I can gladly say, that I have never run an evaporator and only saw one first hand on a Liberty Ship........and it was open being either scaled or repaired....and I didn't have do it either! O0
I am sure that as pressures went up over the years since the turn of the century, the necessity of keeping salt out of high pressure boilers become more and more critical, and far better ways and means of doing so were developed.
I'll report back the title in bit.....
Dave
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Hi,
Am I missing something here?!?
Didn't ships in the early 1900's carry fresh water in tanks that were inneficiently condensed- being gradually diluted with salt water- then de-scaled when back in port, by apprentices of course!?
As condensing machinery got more efficient less and less salt water was needed to top up the system, until the more 'ideal' plants of the 40's/50's which were true closed systems, only needing topping up when back in port to replenish the tanks from steam leaks, blowdowns and drain valves opening?
Greg
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"Carry fresh water in tanks that were inefficiently condensed"? - You've lost me there.
Early steam plants of the 19th C. used seawater for feed and regular blowdown to keep the salinity to a tolerable level. However, eventually the boilers had to be shut down for manual cleaning. The changeover to fresh (shore) water and then distilled seawater feed was gradual. Some ships would initially use water loaded in port and then, when that ran out, switch to a seawater feed with all that meant in terms of blowdown and scaling. Eventually, evaporation plants were introduced (1910 or therabouts?) became more efficient enabling ships to make-up feed losses and then the Weir's Closed Feed System (the great advance for steam in the 20th C. ) arrived in ca. 1950(??)
Incidentally, at one time new Scotch Boilers were run on seawater to form an eggshell thin coat of scale on the internal surfaces. This was intended to stop corrosion 'bleeding' when mill scale cracked off.
Barry M
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Hi BarryM,
"Carry fresh water in tanks that were inefficiently condensed"? - You've lost me there.
Some ships would initially use water loaded in port and then...switch to a seawater feed...Eventually, evaporation plants were introduced (1910 or therabouts?) became more efficient enabling ships to make-up feed losses and then the Weir's Closed Feed System (the great advance for steam in the 20th C. ) arrived in ca. 1950(??)
To say I lost you that was the answer I was looking for, thanks!
Greg
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Have I been helping you with your homework? :o
Cheers,
Barry M
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Hi BarryM,
:D Always ready to do a bit of homework on steam!
Greg
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Hi,
Very old vessels ran seawater in their boilers and if you google a "salinometer" you'll find out how they determined when to "blow down" the brine and refill with with "fresh" seawater.......Not the best thing for a boiler, but at 20 psig, it wasn't such a big deal.
In the modern era....1870's forward....and I speak to reciprocating engines only.
For ocean travel, ships used evaporators to create fresh water from sea water.
Boiler steam was run through coils in large tanks. The water would boil off and be collected. Periodically , the tanks would be opened and the salt cleared.
Of course these ships, at least the modern era, ran condensers...which also helped engine performance. The steam was condensed and brought to a tank called a "hotwell". This tank was used to dearate the condensed water brought there by the wet air pump and to remove any lubricating oils that might have been in the steam....A Wet air pump is an appropriate name as it took condensate and air out of the condenser.
Boiler water chemistry was and still is very important to the life and well being of a boiler. This analysis was done by the "Chief" by performing various chemical analyses usually by titration reactions. From there, The Ph , dissolved solids, and sulfates among other things could be determined and altered with various chemicals added to the feedwater usually in the hotwell. Keeping the Ph slightly alkaline was ideal......now in small scale....starting out with distilled water is easier!
Dave
This might answer your question.......
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Hi Steamer,
Thanks for your reply. That has clarified things further for me. My knowlege is of SY Gondola's plant and other smaller freshwater launches, some of which have condensing machinery, but no evaporation plants, so I hadn't realised the part this technology played in the latter years of reciprocating machinery- and unfortunately in this life i'll probably never get the chance to. :((
Greg
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Interestingly enough we still have the basic components of the steam system in evidence even in low pressure auxilliary boiler systems on ships today. On a modern cruise ship you will find exhaust gas boilers supporting two auxilliary oil fired boilers with the condensate coming back in two streams, one from clean tanks and one from dirty tanks to enable the seperation of any contaminants.
Feed is from the main fresh water evaporators with the normal path through the hardeners being by passed and pure condensate being stored in the feed tanks. In the time I have been with this ship we have had contamination of the feed tanks from a couple of different sources, both with disastrous results and one of which very nearly took the ship out of service.
It may be of interest to know that when we go to dry dock we change over one of the potable water storage tanks to hold distillate so that we have sufficient reserves of feed for the two weeks we are going to be without evaporators.
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Hi Greg,Barry
My experience is similar to yours Greg. My own launch is condensing, but I have never worked the "big stuff" other than an engine restoration team I was on for a 750 BHP compound....all 40 ton of her!
The name of the reference was "Heat Engines" by Ripper. Not the most technically advanced book of the turn of the century, but it reads well.... :-) If I can coral some time, I'll scan the pertinant page or two and post here if required..
I admire SY Gondola and salute you for your stewardship.....I trust she will sail for many more years. Check out "Sabino" when you get a chance. I was a volunteer in her engine room for 12 years.
Dave
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It makes me smile when I read of "Traditional Steamship Courses" on the Sheildhall at £420 for a 4-day course. http://www.ss-shieldhall.co.uk/
I spent many years on steamships where the owner paid me considerably less. :P
Barry M
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Hi Dave, Barry,
I like Sabino a lot, she's full of character, and a true steamboat- born through necessity rather than opulance or wealth- interesting that she was built with a watertube boiler- she must have been a dream to steam?!?!
Could we have some details of your launch, maybe some pics? %)
It makes me smile too Barry- the more you know about steam the more you realise there is to learn- however from Sheildhall Ltd.'s point of view it is a necessity to get punters in- something that we are fairly lucky with Gondola- being placed in the heart of the lakes- but we struggle enough- we normally only break even- so how hard SS Sheildhall, PS Kingswear Castle, PS Waverly etc find it I can't imagine!
Greg
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Greg,
Sent you an email...
Dave
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Hi Dave,
Thanks for the email- very pretty little boat you have there- is the hull design your own- it looks very similar to Glynn Lancaster Jone's Mariamne Class launch?
Greg
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Thanks Greg,
It's a 25' x 5' "Panatela"....I designed her, the engine and boiler.
Dave
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Another question, can you over speed a steam engine?
1 Run a model engine at full bore with no load?
2.Run a full size engine at full bore with no load?
...what could happen?
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Martin,
Re No. 2 - In this situation one usually takes steps - b***dy great big ones out of the Engine Room. :((
Barry M
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Hi Mayhemers,
Two cases come to my mind of steam turbines with failed overspeed protection system. One was at Uskmouth Power Station in Wales, where the hp turbine landed in the mud one mile away. A colleague of mine was a young engineer there and had only walked away from the turbine five minutes before it blew up taking a plant attendant with it. My colleague still carried a photo of the turbine in the mud in his wallet years after the event. The cause was esturine cooling water getting into the protection hydraulics.
The other case was at Woolwich Power Station when the turbine took off and landed on the other side of the River Thames.
The no load situation for turbo-generators is when they get disconnected from the National Grid when on load. These days there are sophisticated electronic overspeed protection devices, but even they have a mechanical system as the final backup.
You can get a partial no-load incident when the generator pole-slips. This is the electrical equivalent of a bicycle chain jumping a tooth. A friend of mine was finishing shift and having a wash, when a pole-slip caused the water to jump right out of the washing bowl, hit him in the face before settling back in the bowl again - like a minor earth-quake.
Most steam engines require a speed governor to stop them over-speeding, although I have noticed that one of the smaller pumping engines at Kew doesn't have one because it always has a pumping load - until the shaft breaks!
Ian
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As long as there is sufficient lubrication to the main bearings of the right quality oil and the bearings are large enough and the pistons have just the right fit and enough lubrication and the gudgeon pins are strong enough then technically no you can't, but we don't live in an ideal world so I wouldn't like to try on a full size- but I regularly run the model cheddar puffin plant at full speed with no load- but a basic oscillator with 11mm Sq. cylinders doesn't bear any resemblance to say a stuart D10, perhaps some other members have experiences with larger model engines (bogstandard step forth...)?
Flashtwo, we must remember that steam turbines and steam engines really are two wholly different beasts here!!!!!
At full size pressure is a square, speed is a constant, and disaster is to the power of 10- unless you can guarentee the engine has everything it needs to run at it's fullest speed.
Greg
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Steam turbines and steam reciprocating engines are certainly different beasts although both rely on the expansive powers of steam. However, Martin does not make it clear what he means by his idea of "full bore". If he means simply opening the throttle of full-size plant without applying load then nothing untoward will happen as the overspeed governor will act to limit the revs to a safe figure. If the overspeed governor is faulty or disconnected for whatever reason then the engine will likely suffer catastrophic damage no matter whether it is rotary or reciprocating in principle. There will be no difference whatsoever in the result; large bits of metal spread over the geography.
As far as the effects of running a model engine at excessive speed and off load (and usually without a governor are) are concerned, the outcome (probably possibly?) will be less drastic as the scale effect comes into play. Nevertheless, with no forced lubrication it risks seizure of bearings and galling of sliding surfaces while any weakness in construction may well lead to mechanical failure. Not an experiment to be attempted.
Barry M
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As Barry M notes.....nevertheless, with no forced lubrication it risks seizure of bearings and galling of sliding surfaces while any weakness in construction may well lead to mechanical failure
mmmmmmm, many years ago we completed an upgrade to a large axial turbo compressor.......steam driven & everything is fail safe
So on start up ....duplex electric lube pumps provide lubrication for the turbine...after a number of permissives have been attained [speed & pressure] the gear driven duplex lube pumps take over & the electrics are shut down
Well yes.... >>:-( <*< the two tonne rotating turbine element also came to a screaming halt [3000 rpm to ZERO in about about 3 minutes] ......the :police: investigation found flap style GHH lube pipe check valves were installed back to front
It was fail safe ...excluding human error <:( naturally a revised set of inspection procedures were written..........Derek
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As Barry states, nothing good will come of it, and the bigger the plant, the worse the situation will get. This by way of the large increase in my by the 3rd power of the scale. The forces involved go through the roof....
Dave
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By way of an exception I recall a casualty which took place some years ago when a diesel in good condition with a working overspeed governor nevertheless took off and ran to destruction.
The details I cannot remember but it involved a running diesel located in a compartment which for whatever reason started to fill with flammable vapour. The vapour was sucked into the air intake and thence to the cylinders where it combusted and the engine speed increased. To counter this the overspeed governor gradually reduced the fuel until the point was reached where the fuel was shut off completely but the engine continued to run - and revs increase - because it was sucking in the flammable vapour/air mixture. With no effective governor, engine revs increased to the point where something gave way and everybody's day was thoroughly spoiled.
If I remember right, when all the bits were put back together, a modification was applied which resulted in unsafe speeds operating a damper on the air intake as well as closing the fuel rack.
Barry M
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Years back I did building maintenance, the one building I had just started at (17 floor Office building) had an Natural Gas powered emergency generator, 16 cylinder truck style engine. At the time I started this engine had nor been run for some time ( It was scheduled to be tested weekly), I posted in the logbook that the engine should be serviced and checked out by the manufacturer before we tested it. The fellow on the next shift disagreed and started the engine, It ran fine for a couple of minutes and he decided to let it run to charge the batteries and wont off to do something else. Five minutes later the lube pump ran dry, then the generator bearings and then as the engine tore loose from its mounting the engine seized. The Generator set being in the mechanical floor at the top of the building had to be taken out through the side of the building which was not fun.
Regards,
Gerald.
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Hi Guys
Having 40+ years on steam engines, mostly turbines, a couple of thoughts on overspeed.
Turbines have two sets of valves, throttles and emergency valves. The throttle should be able to control the
speed but if these fail the machine will activate the emergency valves. These are usually latched open by oil
pressure and when the oil is dumped the valves slam shut under springs. If this fails then the electrically driven
boiler stop valves are used.
I am surprised by comments from flashtwo, the rotor is a heavy beast and would have to smash its way through the casing to escape.
What usually happens is the blades begin to come off and these do sometimes go through the casing.
My only experience was debris under the valve seats that prevented the machine from overspeeding.
It shed LP blades that ended up in the condenser, they could not penetrate the casing. Boiler stop valves ended that exciting incident.
On reciprocating engines as they go faster the steam port is open for less time so perhaps they could be viewed as self limiting.
Just an idea, I've never seen a recip. overspeed.
On a point back on the original thread, even when using clean water and chemicals it is still required to operate the blow down
valves depending on water analysis to remove the solids from the boiler. Sometimes several times per shift.
Cheers
Mark
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Regarding your comment on blowdowns, Mark, it is also common practise to keep TDS (total dissolved solids) under control by use of continuous blowdown at a lower flow rate.
Ian
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Hi Guys
Having 40+ years on steam engines, mostly turbines, a couple of thoughts on overspeed.
Turbines have two sets of valves, throttles and emergency valves. The throttle should be able to control the
speed but if these fail the machine will activate the emergency valves. These are usually latched open by oil
pressure and when the oil is dumped the valves slam shut under springs. If this fails then the electrically driven
boiler stop valves are used.
[unquote]
My experience over a similar time at sea is that he who puts his trust in the emergency stop valve may well end up regretting it. As for "electrically driven boiler stop valves", most of my time was spent on vessels where shutting the boiler Main Stops involved big wheel keys and a lot of sweaty effort in awkward places. By the time the stops were shut the damage would be well past and the Chief already filling in the Casualty Report. As the manoeuvring platform was never left unmanned, a swing of the throttles was preferable.
Regards
Barry M
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Hi Mark,
Your surprise at my remarks regarding turbine overspeed made me refresh my memory regarding the Woolwich incident and I found the following website:-
http://homepage.ntlworld.com/chris.mansfield1/power.htm, which states it was only part of the turbine that was projected across the Thames into North Woolwich on the Essex side. I had forgotten about the poor "xxxxx" in the bus!
Certainly one of the turbine rotors (LP?) landed in the Uskmouth marsh - devoid of blades. It was the sort of incident that was drummed into young engineers when getting involved with turbine protection systems, as you most likely have experienced.
One of the problems with boiler feed pump turbines was demonstrating that the overspeed systems would work. Unlike a turbo-generator, which could be run unloaded, feed pumps always (or should have!) had a water load in the pump. The only way to overspeed was to uncouple the turbine from the pump and run it up (nominally 7000RPM). This exercise would take a week of removing and replacing the coupling bolts, and a week without the steam pump was a lot of money.
Barry..
I too remember the non-electric valves - the boiler interceptor being the worst for the number of turns it required and being a two man job.
Ian.