The Steamboating Forum

From Mike: "What I didn't understand was why the subsequent rise is so lethargic at first. And stays fairly slow for a relatively long time. And then all hell breaks loose."

ANS: The reason for rapid pressure rise when approaching operating pressure is the steam saturation pressure/temperature curve.

As an example: The dominant materials relevant to raising steam is heating of the boiler metal mass, and the contained water mass in the boiler. Both of these components have nearly constant specific heats, so with constant heat input, temperature rise is linear in time, say 10C per minute.

Starting with the boiler (and its contained water) at 100C, the pressure is just atmospheric, steam is starting to be produced.

After 1 minute the boiler is at 210C, and steam pressure is now 0.42 Barg, pressure rise is 0.42 Bar per minute, sluggish.

After 7 minutes of warming, the boiler metal and its contained water are 170C, saturation pressure is 6.9 Barg, and during the next minute the temperature rises to 180C, with 9.0 Barg pressure, pressure rise is 2.1 Bar during the one minute interval. That pressure rise rate is almost 5x faster than when the pressure gauge just started to move off the peg.

If the heat input from the fire is also getting bigger during the interval, then pressure accelerates even more rapidly, and that can often happen with solid fuel firing, especially if the blower is being used to liven the fire.

Again from Mike: "I steam up when still on the trailer due to launch ramp congestion and have always had difficulty timing things so that, on one hand, I have enough pressure to back off of the trailer and, on the other hand, don't have to deal with the relief valve roaring away while I'm trying to avoid stray dogs and children while backing down the ramp. Non-linear pressure rise doesn't help!"

I hear you on that one! Timing that pressure rise vs. launching the boat, getting the boat tied to the other side of the dock (so others can use the ramp), driving away with the tow vehicle, parking the trailer, all the time with the fire going, is a challenge.

My boiler is Propane fired, but I can only modulate the fire from about 70% to 100% heat output, and I should arrange the burners to allow a firing range from about 20% to 100%, will do that as soon as I can get a "round tuit". However I can turn the burners completely off in an instant, and re-lighting the fire is quick too, so for me it is much less of a problem than steamboaters burning solid fuels.

Just to make it more difficult here, I live on an island and all three of the launching locations have the associated dock far away enough from the ramp that I have to have steam up before launching. Ridiculous!!

Fred,
The there is about a full page of text that further explains the ifs and buts of the tube wall graph I posted. You're examples and graphs say the same thing in and easier to understand. Now that said, I think there is a miss type with 210c and 0.42bar, looking at the steam tables reveals 110c at 0.42bar or am I missing something.

Mike,
I guess there are advantages to dreaming of but not having a boat, like anticipating all the fun you will have without having to hassle with launching it.

Yes, my mistake, miss-typed 210C vs 110C, which corresponds to 0.42 Barg

I usually say to the reviewer "I put that mistake in the calculations just to be sure you were paying attention"
In calculations I have often done exactly that, but I correct the numbers shortly later with another "mistake"
Interesting to note that the calculation checkers, required to check every number, for all safety related nuclear plant calculations, catch this about 50% of the time.