Sight Glasses and Boiler Safety

[SL Ethel]

The excellent discussion of boiler safety going on right now prompts me to ask the experts out there on the list something about sight glasses that I have been wondering:

I know that it is considered proper practice to install the sight glass at a level where the lowest permissible and safe water level corresponds to the bottom of the glass, so that no water in the glass clearly indicates immediate danger.

BUT... I wonder why it would not be good practice to put the bottom nut lower than this and install some sort of larger, permanent bright colored marking on the sight glass to indicate the lowest safe level? On hears plenty of stories about losing water in the glass (it's happened to me once too), and what I always wonder when I hear these stories is - how low did the water actually go? It seems like it would be good information to have in order to assess if a pressure vessel has likely been damaged by the low water condition. Especially in small volume watertubes, 1/2" below the nut and 6" below the nut might not represent much difference in time, but would make a huge difference in thermal stress and overheating in the boiler.

Cheers,
Scott

[Spanky]

I also see that this would be good practice and would like to include it in my boiler design.

Josh

[DetroiTug]

I'm thinking the way a properly designed boiler has it, the lowest valve on the sight glass or the lowest trycock, would be the lowest allowable safe level.

The Rule I've adopted is: If the water level is not known, shut it down.

Probably the most dangerous condition firing/tending a boiler is adding water to it while empty or near empty. This has been the cause of almost all boiler explosions from my reading. The bare and very hot exposed interior surface flash boils the incoming feedwater. The relief valve can not relieve it and if the vessel can't take it....

Assuming the water is just a bit below the bottom of the glass could be a mistake. It's possible a blow down is partly open and the boiler has accidentally drained completely. No water in the glass? Pull the fire, throw out the anchor, fish for a while and then figure out where it went. Or at the very least, pull the fire, wait 10-15 minutes before adding any water.

-Ron

[Lopez Mike]

More harping from Mike:

One more reason, on a condensing system, to use a hot well float to regulate the levels. (And yet another reason to run a full condensing system even on fresh water. Ahem.

I would bet a six pack that a majority of our low water situations are due to inattention on a manually regulated setup. I'm no better than the rest us goof balls and I get distracted fairly regularly. In my car or around any modern mechanism, automatic systems, are, on balance, a pretty good thing. Yes, they can fail. But they are more reliable than people.

Most us are the skipper, the deck hand, navigator, watch standing officer and the purser as well as possibly our first love, engineer and fireman. Boating is an very busy, complex and distracting environment. Fooling around with yet another damned valve every time I change speed or have to deal with any number of seamanship issues is not a wonderful idea.

Speaking of sight glass issues, don't forget to have some sort of protective shield around tubular glasses. It's usually a matter of time before they shatter due to accumulated stresses. It will startle the rat poop out of you when it happens anyway. No point in having bits of glass in your face.

It has happened to me. I was just bringing an electrically heated sterilizer up to pressure after a routine shut down for cleaning when there was a bang and the room was full of steam. After I put on my big gloves and shut off the sight glass valves, mostly by feel, my co-workers started acting weird and staring at me. The needle like slivers of glass had stuck all over my chest through a white T-shirt and it looked as though I was bleeding to death. I was wearing plastic safety glasses! First aid took ten seconds and I threw away the shirt but it did make any subsequent safety speeches from me, well listened to.

[barts]

hear hear... I've run plants with manual bypass, and I'll vote for float-regulated hotwells every single time. It lets me pay attention to where we are, what other traffic on the water is doing, and watching out for underwater hazards, waves. etc. In addition, the slight hiss of the bypass back into the hotwell lets me keep track of what is happening with the pump(s).

It's a bit of work (scary, too, drilling big holes in the hull for the first time) but a hot well w/ float bypass will make steamboating more fun and safer.

- Bart

[fredrosse]

Back to the ASME Boiler & Pressure Vessel Code "The lowest visible water level in a gage glass shall be at least 2 in. (50 mm) above the lowest permissible water level". I often question ASME rules, but on thought it soon becomes obvious that the ASME Code is on the proper side of the issue.

This rule is to assure that someone who can see no water level will immediately know to shutoff the fire. Hopefully any operator of a boiler knows to kill the fire immediately if there is no water showing in the boiler. If the gauge glass shows levels into the danger zone, then there is a chance that the operator might think all is well, because he (or she) can see a water level. If the glass is somehow marked to show the minimum safe level, that would be OK, ..... BUT...... the glass will be replaced some day, and this marking might very well not be replaced on the new glass.

It is sometimes easy for us to assume that we will be the only ones ever operating our boilers, and that we will always be aware and remember all relevant parameters. Anyone with experience operating these types of plants knows that the level of attention to watch over everything all the time just does not always happen. As Bart says, you are not just running the steam plant, you are running a boat, with plenty of things to draw your attention.

[87gn@tahoe]

x3...

I bet one would be very hard pressed to find someone who could honestly say they haven't experienced a low water situation at least once due to inattention or equipment failure (stuck check valve anyone?...)

[barts]

Back to the ASME Boiler & Pressure Vessel Code "The lowest visible water level in a gage glass shall be at least 2 in. (50 mm) above the lowest permissible water level". I often question ASME rules, but on thought it soon becomes obvious that the ASME Code is on the proper side of the issue.

For our smaller boilers, I don't take the 2" literally. I ran an old porcupine a while back in the the Otter - two 4" water drums. It was steel, so low water was of little import, actually... but in any case, the idea is sound - we want some margin of error, so as soon as we see the water disappear in the glass we shut things down.

- Bart

[steamboatjack]

Folks
I had an experience of check valve failure in 2010, its in this forum some where. Since then I have come across three instances of loco type boilers (2 x boats 1 x 4inch scale traction engine) where the gauge glass, for all intents is fitted too low, in so much as the crown sheet is exposed before the water gets down to the bottom nut.
The first was a new boiler built by a reputable firm in UK where the drawings were correct, but the boiler maker had decided to raise the crown a bit to space the tubes more to his liking. Luckily the boiler has a fusible plug! The gauge glass assembly has now been raised.
The second boiler is having the gauges raised as to allow a bit more water above the crown sheet. The third is just plain bad design. Bad design is very common on “model engineer” type stuff including most “model” derived boat engines. Don't get me started!!
I believe from a friend who makes commercial model boilers for large T.E.s than the latest CE directive requires the position of the crown sheet (and I expect the tube plate in verticals) to be clearly marked on the outside of the boiler.
Having said all that, if you run a steam plant of any size, your first consideration is to keep half an eye on the gauge glass (assuming there is not an automatic system with back up alarms). A float system in the hot well is NOT an automatic system in this context, meerly a convenience. If you let the water level wander too much you should not be in charge of the plant.


Regards
Jack

[artemis]

In any boiler room of any steam powered vessel in the world, the operator's manual since the 1880s specifically states that the FIRST duty of the watch officer is the ascertain the level of the water in the boiler. Period. Exclamation Mark. Do not pass GO, do not collect $ 200