WW&F No. 9 - Official Work Thread

[Stephen Piwowarski]

I'm not positive, but at that level on the boiler barrel, instead of emitting steam, wouldn't the valve emit hot water?

Steve

[Steve Smith]

Somebody correct me if I'm wrong, but I think a considerable fraction of the water would flash to steam, perhaps even in the pipe on the way to the pocket, owing to pressure drop.

[Keith Taylor]

Somebody correct me if I'm wrong, but I think a considerable fraction of the water would flash to steam, perhaps even in the pipe on the way to the pocket, owing to pressure drop.

Steve and Steve, Steve S. Is correct, as water boils at a higher temperatureunder pressure, the water will instantly flash off into steam when at atmospheric pressure.
Keith

[Stephen Piwowarski]

That makes more sense. I neglected to consider the pressure change! Thanks!

[John McNamara]

That brings up a question about try cocks. In normal operation, does the output of an under-water try cock instantly flash to steam? If so, that would be very similar to the result of opening an above-water (i.e. steam) try cock. Does enough water come out of an under-water try cock before the instant-flash to give the crew an indication of the real water level?

-John

[James Patten]

Let's see if I can explain the physics.

For a given pressure, when exposed to atmosphere a certain percentage of the water wants to flash to steam, but not all of it.  So with the try cocks, the bottom cock (if submerged in water) will blow out a mixture of steam and water.  The top one, if all steam, blows out just steam.  The trick is hearing the difference. 

[John McNamara]

Indeed, PV=nRT.

[Stephen Piwowarski]

Interesting info guys. Thanks! I had been doing a bit of reading since my last post and wound up finding that, according to the source I was reading, it is quite difficult to actually ascertain the level of water using the try cocks, hence the introduction of the glass as a standard safety feature. Interestingly, the same source mentioned that on the Gettysburg, when the fireman were interviewed after "the incident" none of them could accurately describe the procedure for testing the function of a water glass. Uh oh!

[Jason M Lamontagne]

James is dead on- only a percentage flashes.  At one time I heard oy 17%, but the actual percentage depends on the enthalpy of the water at a given pressure and temperature.  I never bothered to run the numbers at our operating levels but am content to understand the concept and let it explain why, amongst other things, try cocks work.  That's also why the ground is very wet where the boiler blowdown strikes it.  If all the water flashed, the ground would only be a bit wet from condensate.

I've never had trouble hearing the difference between water and steam out a try cock- I think that's just an excuse for poorly trained crews.  The real problem with try cocks is the artificial rise in water level at the back end of the boiler due to steam rising through the body of water and normal water circulation (which is ferociously upward at the backhead).  Thus "riding the edge" on the bottom try cock when under heavy load might actually be "over the edge," so to speak.

See ya
Jason

[Jason M Lamontagne]

Another way to view it:  a unit quantity of water at boiling point contains so much energy.  It takes energy to create a unit quantity of steam- far more energy than that held by the same unit quantity of water at the boiling point of that pressure.

  If the pressure is suddenly reduced, that water necessarily boils, as a way of releasing the excess heat, but becasue it doesn't contain enough heat to create an equivalent amount of steam, only a portion turns to steam.  The remaining water gives up excess heat by reducing its temperature to a point just below the boiling point of the new pressure.

Thus the governing factor in how much water flashes to steam is the difference in pressure, or, more to the point, the difference in boiling point and the diference in heat-of-evaporation at those two pressures.

See ya
Jason

[Greg Klein]

Jason,  Is this the same as latent heat?  When steam gives up heat energy before it condenses?

[Jason M Lamontagne]

Latent heat is that which is added to change the phase of a substance and, consequently, doesn't change its temperature.  With regard to this discussion- the total heat of the water prior to depressurizing doesn't include any latent evaporation heat.  The final combination of some flash steam and some water represents the same total mass of h2o and same total heat- it's simply distributed differently- including a portion which is latent evaporative heat in the flash steam.  It didn't keep going and all flash because the temperature of the remaining water fell below the boiling point of the new pressure.

See ya
Jason

[Greg Klein]

Ahh.  That makes sense. Thanks for clarifying.
 

[Brendan Barry]

Castings for number 9's whistle and number plate arrived from the foundry tonight.

[Bernie Perch]

Wow, those castings look great even in their rough state.  You should cast a second set and put them for sale in the gift shop.  It will be great to see a locomotive with an original style whistle.  Hats off to Alan.

Bernie