The Steamboating Forum

All,

I am finalizing my boiler design and have a question about reserve capacity. I am going to have a B&W style watertube boiler welded up. I am assuming 50% fill on the Steram drum and calculating my reserve as the the amount of water in the steam drum at a given time, (50 % of the volume), divided by the gpm consumed at full steam. Clearly this is worst case since if one encountered a water injection issue it would make sense to slow down while you add water by hand, pull the fire etc.

Is this a correct way to look at it ? Using this method gives me about 7 minutes at full steam before I have exposed the tops of the tubes in the steam drum.

jon

Steam boilers experience "shrink and swell" in their levels. When steaming at full load, there are many steam bubbles below the water level, mostly in the steam generating tubes. These bubbles take up space, giving the sight glass an indicated level which is high. When steaming is reduced, less bubbles are present, and the water level indicated goes down. Many small watertube boilers will show a mid-drum level when underway, but if the steaming is stopped, the water level completely disappears below the bottom of the sight glass.

This kind of condition can be daunting for the operator, carrying a mid-drum level when starting up that turns into an excessively high level when steaming, possibly leading to water carryover to the engine. This condition has damaged many a steam engine. Damage to the boiler can result if firing with the water level below the drum on startup.

The proper generous sizing of the drum to allow for the "shrink and swell" properties of all boilers is highly recommended. I have used firetube boilers which generally have much less of a problem with "shrink and swell", although even this type of boiler can have excessive level swings, especially when generating steam at low pressures.

Perhaps some of the forum members with various sized watertube boilers could provide some experience & guidance here

One way to think about this is what percentage of the volume the glass covers.... with a B & W style boiler, if there's water in the glass the tubes are all covered properly... So:

Calculate the total amount of water in the boiler, drum & tubes. Assume it expands by 20%.... if you carry the water level at the center of the drum, after expanding all the water in the boiler by 20%, where is it now? Perhaps the right thing to do is to begin firing w/ the water near the bottom of the glass so there's room for expansion.

Otter's boiler water level jumps several inches, esp. when cold.... so some "feel" is required. It steams best with high water, so I walk a bit of a tightrope on the water level - and sometimes I know not to blow the whistle.

- Bart

I will be building an Ofeldt soon for another project. Seems these water tube boilers all have a bit of an issue with sight glass accuracy. In the case of an Ofelft, what about two baffles just above the water surface? Imagine two plates with hole patterns one above the other and staggered in rotation. They could just be tacked in place internally in the center drum. Is this something that is feasible?

-Ron

Feasible, yes... useful, probably not. The issue is that bubbles form everywhere in the liquid as the pressure drops due to a sudden steam demand, such as whacking open the throttle at the bang and go back race or blowing that 2" chime whistle; this increases the volume of water significantly; if insufficient space is available, the boiler will prime. Alternatively, if the gage glass covers too small a working range of the boiler volume, the level may go from almost at the bottom of the glass to disappearing at the top.

These conditions can be dealt with in the steady state; it's the transient case that is tricky as the engine primes just after blowing the whistle, or the steam atomizing burner takes a slug and the fire goes out.

- Bart

These baffles would possibly create a differential in pressure in the steam space, here's how: If the holes in these baffles were sized accordingly, when there is a sudden drop in pressure in the steam space above, water would surge in to these baffles and the restriction in flow will cause a higher pressure in the water space below and decrease steam generation for the brief period. It may not eliminate the problem, but it may serve to regulate it. Similar to a lid on a pot of boiling water.

-Ron

All,

THanks for all the input. Another question, I am going to have a 8" steam drum. Allowing for the space taken up by the welds of the endcaps, my glass length is likely to only be 4 to 4.5 ". Is that adequate. If it isnt, I dont know that I have much recourse unless I take the gauge glass down onto one of the downcomers.

jon

JonRiley56 wrote:All,

THanks for all the input. Another question, I am going to have a 8" steam drum. Allowing for the space taken up by the welds of the endcaps, my glass length is likely to only be 4 to 4.5 ". Is that adequate. If it isnt, I dont know that I have much recourse unless I take the gauge glass down onto one of the downcomers.

jon

What percentage of the overall volume of the boiler does that 4.5" represent?

= Bart

Try to avoid plumbing the glass into a downcomer; In the examples I've seen, including a roberts watertube my father built, the water level in the glass is extremely unstable due to the circulation through the downcomer. ALSO, don't plumb it from the steam drum to the mud drum, as it will act like a downcomer itself, causing the same issue.

Try to use a few elbows and nipples to make the connections to the glass further apart.




One could also make a stilling column with tri-cocks plumbed into it rather than the limited space of the drum.

DetroiTug wrote:These baffles would possibly create a differential in pressure in the steam space, here's how: If the holes in these baffles were sized accordingly, when there is a sudden drop in pressure in the steam space above, water would surge in to these baffles and the restriction in flow will cause a higher pressure in the water space below and decrease steam generation for the brief period. It may not eliminate the problem, but it may serve to regulate it. Similar to a lid on a pot of boiling water.

-Ron

This restriction in flow may limit your ability to utilize wide open throttle (WOT). Stilling column may be the way to go