Something to ponder 01

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

A large steam locomotive hauling a heavy train at speed is typically generating between 600 and 1200 HP . When the engine is brought to a halt the boiler continues to generate steam at the same rate as when running and all this steam blows out of the safety valves until the engine moves off again or the crew reduce the steaming rate of the boiler .

 

The same steam that was generating all that power when running the engine only results in a bit of loud hissing , some kettle steam and a cloud of tepid rain drops when it is discharging from the safety valves .

 

Question 1 : Where does all that power go when the engine is blowing off ?

 

Question 2 : What has the answer to question 1 got to do with the efficiency of injectors and turbines ?j

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

[blowlampParticipant @blowlamp]

Never having had more than a passing interest, let alone built a steam engine. my knowledge is limited to say the least.

 

However, I believe that the engine does not make the same quantity of steam when at rest, because no draught is created over the fire by the exhaust steam.

 

So my answer to Question 1 is that the energy is dissipated by the act of heating and moving the surrounding atmosphere from the release of the steam. It’s only necessary to dump the steam at the rate it’s being produced at that instant in time to prevent a constant increase in boiler pressure.

 

As a stab in the dark for Question 2, I’d say that any heat loss would be detrimental to it’s efficiency, particularly if condensation occurs somewhere in the system.

 

 

Martin.

[Roderick JenkinsParticipant @roderickjenkins93242]

As a man who has absoulutely zero experience of steam engines large or small I feel eminently qualified to join in the discussion

 

I guess that the 1200HP comes from the rate at which the steam is generated. When the engine is standing in the station and the safety valves are gently hissing then the fire is only just big enough to keep it at that pressure. When the train starts then the pressure will rapidly drop as the steam is consumed in the cyclinders until the fire is built up again in order, at full chat, for that 1200HP to be available continuously. And thereby lies the skill of the fireman, ensuring that the safety valves are sizzling in the station but wailing like banshees at the bottom of a steep bank, feeding the coal and water at the appropriate rates and adjusting the blast to keep the fire just right. Has this got anything to do with question2?

 

Rod

Edited By Roderick Jenkins on 24/06/2011 12:33:45

[Peter G. ShawParticipant @peterg-shaw75338]

Here are a few wild guesses!

 

At rest the engine is not doing anything, so it is not developing any power. True it has what I would call latent power in that the boiler is acting as an energy store, which is only called upon as and when required.

 

The steam issuing from the safety valves is an indication that the boiler is at maximum working pressure, ie has the maximum amount of energy stored so all that is happening is that the excess energy is being dissipated through them.

 

When running, the power generated is dependant on the work required, hence some of the stored energy is used up and replaced by the fireman.

 

Don’t know anything about injectors or turbines (other than the basic idea).

 

I do remember many years ago reading about a technique known as mortgaging the boiler (I think). What I understand happens is that if the engine has to attack a hill, then the aim is to have the boiler at full pressure and the fire going flat out at the bottom of the hill. The driver then takes more steam than the boiler and fire can provide with the result that the boiler pressure gradually drops as the engine climbs the slope with the result that the engine arrives at the top of the hill with the boiler at minimum pressure. By using this technique, the driver can effectively get more work out of the engine than it can theoretically provide.

 

Right, I’ll now stand by for a full blasting as everyone “shouts” at me.

 

Regards,

 

Peter G. Shaw

 

(Who has now proved the old maxim: “It is better to keep one’s mouth shut and be thought a fool than to open it and prove it!”)

[John HaineParticipant @johnhaine32865]

Assuming that heat input to the boiler continued at the same rate (so fireman kept shovelling, blast switched on) the steam coming out of the safety valve would have to carry all the surplus energy no longer going into the engine. It will very quickly condense and give up its latent heat of vapourisation to the air and the water droplets that will form. In practice of course shovelling and blast stop, rate of firing reduces drastically and therefore also rate of steam generation and energy loss. It all comes down to conservation of energy.

[Hugh GilhespieParticipant @hughgilhespie56163]

I don’t know anything about steam locos but in a previous incarnation I used to run a small company that did boiler steam blowing. The idea was to blow all the crap out of the lines to the steam turbine to stop the blades getting damaged. To be effective the steam pressure was as low as possible to give the highest velocity in the pipes. The steam was finally dumped to waste through large knock-out pots. The numbers were fairly impressive – for a typical power generating boiler running at about 80% of full load, we were discharging steam at up to 100 tonnes per hour with a power equivalent of around 180,000 HP. Quite a bit of the energy was dissapated in noise, hence the rather dodgy state of my hearing now!

[Sub MandrelParticipant @submandrel]

People seem to be close to the target. When the engine is in a ‘steady state’ the boiler produces steam with enough energy to propel the rain. If it stops, what happens?

 

The exhaust blower will stop as soon as the engine halts this will reduce both the intensity of the fire and the effciency of heat transfer through the tubes immediately. I think this latter effect will be the most immediate. But let’s assume that more energy still enters the boiler than it loses through radiation and conduction.

 

1 The temperature in the boiler rises. The energy from the fire goes into raising the temperature and pressure of the water in the boiler. the amount of steam reduces, but some is still produced as this allows the pressure of the steam to rise in proportion to the rising temperature.

 

2 When the pressure rises enough the safety valves blow. This carries away heat in the steam and also (as a result of the reduced pressure) allows the water to boil more freely, using more energy.

 

3 If the safety valves can’t pass enough steam, the temperature rises making heat transfer from the flues to the water even less efficent. Only if this doesn’t happen at a low enough pressure/temperature wil the boiler.

 

So, in short, the systems is designed to turn heat from the fire efficiently into energy in steam only when it is working; that is why an inefficient blower is needed to keep the fire in when stationary. I imagine that for a large stationary plant the fireman had to be a lot quicker with the dampers to manage the fire if the load dropped suddenly.

 

Neil

 

A little extra musing, I recall that Tom Walshaw tested one of his engines with a condenser and it ran quite happily with the boiler running at below atmospheric pressure. More proof that the laws of thermodynamics trump common sense

 

Edited By Stub Mandrel on 24/06/2011 20:41:10

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