Steam & Excursion > Steam question

OK, I just got all frustrated trying unsuccessfully to install an updated driver on my smart TV. I give up. I'll go back to something I can understand. Like steam. So, for discussion, consider this.

You've got a closed end pipe sticking out the top of a firebox roof sheet, the bottom end of the pipe is open below the water line.

What will be in the pipe, and why? The usual cast of Super Power guru's are temporarily barred from answering, so the young'uns can have time to think this through.

-John



Edited 1 time(s). Last edit at 09/30/14 12:57 by LarryDoyle.

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This should be good. Great idea to post this.

Some sort of fusible plug arrangement to protect against low water on crown sheet

Posted from Android

Marty Harrison
Knob Noster, MO

No, there is no plug in the pipe, except to close off the top end.

In household plumbing, something like that is installed to prevent water hammers. I don't believe that pipe will prevent such from happening, though, due to the volume of water in the boiler. The pipe should be mostly dry on the inside, like plugging a straw then inserting it into a drink.

Looking forward to what this is for and how it effects steam boilers!

Jim Montague
IRVINE, CA
Train and Nature photo Art

Put a valve on the top of it and use it as an air bleed when you're first filling the
boiler (nyuk-nuk).

Out

callum_out Wrote:
-------------------------------------------------------
> Put a valve on the top of it and use it as an air
> bleed when you're first filling the
> boiler (nyuk-nuk).
>
> Out

Well it's not an all bad idea to get oxygen out while filling. But this would only work in the described arrangement until the water level reached the bottom of the pipe, trapping the remaining air, even with that valve open. Nope. That's not it.

3751_loony Wrote:
-------------------------------------------------------
> In household plumbing, something like that is
> installed to prevent water hammers. I don't
> believe that pipe will prevent such from
> happening, though, due to the volume of water in
> the boiler.

Nope, doesn't prevent water hammer.

> The pipe should be mostly dry on the
> inside, like plugging a straw then inserting it
> into a drink.

Think this through a little more.

-John

Is it an early warning device? Seems like if the water level got too low, you would see steam escaping.

Jason Rose
Spring, TX
Rio Grande Explorations

You didn't mention whether or not the boiler is under pressure. If it isn't wouldn't the water find it's level in the pipe?

Frisco1522 Wrote:
-------------------------------------------------------
> You didn't mention whether or not the boiler is
> under pressure. If it isn't wouldn't the water
> find it's level in the pipe?

Good point - let's say the boiler is in service.

My guess would be that it would blow hot water until it fell below the pipe end and then steam. It's under pressure.

Frisco1522 Wrote:
-------------------------------------------------------
> My guess would be that it would blow hot water
> until it fell below the pipe end and then steam.
> It's under pressure.

Well, I think your thinking along the right lines, but what do you mean by "blow hot water", since the pipe is capped. Where would the water blow, or how high up in the pipe would it rise, and what would happen to any steam that might be trapped?

-John

LarryDoyle Wrote:
-------------------------------------------------------
> Frisco1522 Wrote:
> --------------------------------------------------
> -----
> > My guess would be that it would blow hot water
> > until it fell below the pipe end and then steam.
>
> > It's under pressure.
>
> Well, I think your thinking along the right lines,
> but what do you mean by "blow hot water", since
> the pipe is capped. Where would the water blow,
> or how high up in the pipe would it rise, and what
> would happen to any steam that might be trapped?
>
> -John

Would there be nothing in the pipe because the fusible plug melted and the steam and water is being shot downward onto the fire?

Mayfair Tower Wrote:
-------------------------------------------------------
> LarryDoyle Wrote:
> --------------------------------------------------
> -----
> > Frisco1522 Wrote:
> >
> --------------------------------------------------
>
> > -----
> > > My guess would be that it would blow hot
> water
> > > until it fell below the pipe end and then
> steam.
> >
> > > It's under pressure.
> >
> > Well, I think your thinking along the right
> lines,
> > but what do you mean by "blow hot water", since
> > the pipe is capped. Where would the water
> blow,
> > or how high up in the pipe would it rise, and
> what
> > would happen to any steam that might be
> trapped?
> >
> > -John
>
> Would there be nothing in the pipe because the
> fusible plug melted and the steam and water is
> being shot downward onto the fire?

Good one! No, the lower end of the pipe is in the water above the crown sheet.

Fusible plugs are uncommon in North American railroad service. We can talk about that in a future thread.

-John

The pipe would be full of water up to a point that the air above the water is compressed to the pressure in the boiler. Air compresses whereas fluid doesn't. So the fluid level is quite high in the tube, by the drawing. Remember, no measurements were given so the answer cannot be in inches or any other measurement, weren't asked for either..

LarryDoyle Wrote:
-------------------------------------------------------
> Mayfair Tower Wrote:
> --------------------------------------------------
> -----
> > LarryDoyle Wrote:
> >
> --------------------------------------------------
>
> > -----
> > > Frisco1522 Wrote:
> > >
> >
> --------------------------------------------------
>
> >
> > > -----
> > > > My guess would be that it would blow hot
> > water
> > > > until it fell below the pipe end and then
> > steam.
> > >
> > > > It's under pressure.
> > >
> > > Well, I think your thinking along the right
> > lines,
> > > but what do you mean by "blow hot water",
> since
> > > the pipe is capped. Where would the water
> > blow,
> > > or how high up in the pipe would it rise, and
> > what
> > > would happen to any steam that might be
> > trapped?
> > >
> > > -John
> >
> > Would there be nothing in the pipe because the
> > fusible plug melted and the steam and water is
> > being shot downward onto the fire?
>
> Good one! No, the lower end of the pipe is in the
> water above the crown sheet.
>
> Fusible plugs are uncommon in North American
> railroad service. We can talk about that in a
> future thread.
>
> -John

Worth a shot and I might miss my train waiting on the answer but maybe Hotwater can come out and hoop up the answer in an hour.

Thanks for teaching!

Ooops, I missed the fact that the end is closed on the outside.

How about a sensor to warn the crew of low water? If the water level gets below the open end of the pipe, steam will enter the pipe. Steam sets off the sensor, an alarm in the cab.

dcfbalcoS1 Wrote:
-------------------------------------------------------
> The pipe would be full of water up to a point that
> the air above the water is compressed to the
> pressure in the boiler. Air compresses whereas
> fluid doesn't. So the fluid level is quite high in
> the tube, by the drawing. Remember, no
> measurements were given so the answer cannot be in
> inches or any other measurement, weren't asked for
> either..

OK. I think you've pretty well nailed it. Any air bubble trapped will actually get absorbed by the water and would become insignificant.

So, what happens to any bubble of steam that might rise into the tube? Well, the end of the tube is outside of the boiler, surrounded by ambient temperature air, so the tube will run slightly cooler than the rest of the boiler. For any given pressure in the boiler there is an EXACT temperature for both the steam and the water in the boiler. But even though the steam and the water have the same temperature, the steam contains more energy, called latent heat (it is this latent heat that actually powers a locomotive). If that steam looses even the tiniest part of a degree of heat, part of that steam will condense back into water.

The temperature of the water and steam in the boiler is, roughly, 300 F degrees higher than the air surrounding the pipe, so some heat will be lost from any steam (or water) in the pipe. Any steam in the pipe will condense, and the pipe will become filled completely with water, which because it does not circulate well through the pipe causes the pipe to fill completely wit water at a temperature slightly below that of the rest of the boiler.

Got that?

Next question (the answer is so obvious I won't bore you and will give you the answer right away): What happens if the water level drops below the bottom of the pipe? (Though the crown sheet of the firebox is still covered).

Duh! The water runs out of the bottom of the pipe and the pipe fills with steam.

Right answer!

Now you're thinking (aren't you?) that maybe, somehow, this information could be useful to tell enginemen whether the water level might have dropped to such a low level that it might start to become dangerous?

Ahah! You're right. It can. And is.

Next Question: HOW can apparatus be set up to use this knowledge as a device to warn of low water hazard? There are two correct answers - one obvious, one not obvious. I've given you all the information you need to solve both. Go at it.

-John