Water Gauge

Hi People, I have been reading up on boiler making and testing and something crossed my mind about the boiler water gauge. Am I right to assume that the full boiler pressure will at times be applied to the little glass tube. Can they really take up to 100 psi? David.

Certainly, and a lot more.

Its a special Borosilicate glass which is incrdibly strong, although having broke one once when I was steaming up (acciently smacked it with the shovel) it is a good idea to fit a guard around it.

Regards

Lofty

Thanks for that Lofty. David.

Full boiler pressure is applied to the glass tube. Thick walled glass as intended for gauge glass use, from a model engineering or industrial supplier, will take the pressure. I like the stuff with the blue stripe, it helps to see the level. Never had one burst on its' own. It's also important to have the glass touching only the rubber seals, and not have the metal gland nuts or fittings contacting the glass applying stress to it at a small area. Best to follow a proven design for your gauge glass and fittings arrangement, preferably with shutoff valves above and below the glass, and a blowdown valve for the glass, with a drain below the cab.

Cheap glass tube trinkets from the dollar store or ex neon sign glass tube or any other glass tube not specifically rated for gauge glass use likely will not take the pressure.

As usual, I expect 15 people on the forum will disagree. The above is just my $0.02 worth based on my own experience.

No, I don't think they will.

Excellent point Jeff, its also worth noting that the gauge glass nuts are only tightened finger tight.

Regards

Lofty

It does raise a point - the irony that we go through all the rigorous actions of boiler design and testing and then stick on a vulnerable glass tube with iffy seals on it.

There's probably no other way though?

In the past, I've bought gauge glass with a blue line embedded in it, Schellbach I think Reeves listed it IIRC. However, I found that it wasn't truly round, at least the pieces that I received, which made sealing difficult.

I found that the plain borosilicate tube was better in this respect.

Some folk also reckon that lined tube is also more easily broken.

Just looked, Reeves don't seem to list the lined tube any more, just the plain.

Edited By Clive Brown 1 on 01/02/2018 10:42:28

Could go back to ye olde method of 'try cocks'

I looked up some numbers

Ordinary glass has a tensile strength of about 30-40 MPa, but toughened glass is more like 200Mpa

Annealed copper has a tensile strength of about 200 MPa but a yield stress of only about 70 MPa.

So you could argue a toughened glass tube is stronger than the boiler shell... but obviously a bit more fragile!

Neil

Great stuff chaps, thank you. Just a little side question, I have never driven a 5" loco, or any model come to that so how easy is it to see the water level in the gauge glasses.

David.

It all depends on the gauge location/aspect, loco, track and experience, but all said I have never had any problems. There are usually a lot of things to think about and do 'on the hoof' but you will soon get the hang of things. Probably!

Hi David,

Not sure why you raised this issue.

If properly made and fitted, a miniature water gauge is as easy to read as in fullsize. I have never used a commercial fitting, making all of mine. In 35 years I have never had a gauge glass break, and on the 4 yearly boiler hydraulic test they withstand a pressure of 1.5 x working pressure. I have followed a series of articles for many of mine from EIM many years ago.

Some miniature locos give a poor reading in the glass because of restricted water legs in the backhead and protrusion of the lower bushes and water gauge fitting. There is much turbulance of the water and foaming in this area of the boiler.

Reading the gauge itself is simplified by sticking a bit of white card behind with diagonal parallel lines written upon it in black ball point. A gauge glass against a black painted backhead is not easy to read.

I have never used schellbach glass (though I have a stock of it) because it is more brittle. Discard any ordinary gauge glass that shows greenish ends.

Cheers,

Julian

Edited By julian atkins on 01/02/2018 21:04:05

Hi Julian,

You say discard ordinary gauge glass with greenish ends. May I ask why and what does the greenish colour indicate?

John

Hi John,

It was told to me by Don Young after his Eastleigh apprentice days, and also related in his writings. Try greenish ended water gauge glass if you dare! I haven't tested it myself, as I have a large tin of gauge glass of various sizes, and I am quite happy with non- greenish glass ends that have never caused a problem!

Cheers,

Julian

I don't actually agree with Jeff re separate shut off cocks top and bottom of the gauge as in 3.5"g and 5"g these restrict the passageways, and in any event I have never found them necessary.

Cheers,

Julian (first of Jeff's 1 in 15)

If you are working in larger scales with a 3 cock gauge glass set, I was taught to do the hydraulic test with the glass isolated and the glass drain cock open so it cannot be pressurised. Obviously not an option with only drain cock.

I have 2 full size glasses break in service, one when I was in the cab. Not much fun, fumbling around to find the isolating cocks in a cloud of steam. At the time we couldn't get (afford) the proper glass seals so were "improvising" - hence the breakages.

Martin

Agree, and there is another reason....

Having had a breakage, and when it happens believe me you don't forget it, it was near-enough impossible to get to the valves.
In retrospect, the time would have been better spent getting rid of the fire!

Good maintenance in this area pays off - I change the glass and seals each season - actually over the top but i feel better.

I agree Julian has a good point re fitting of shut-off cocks to model steam boilers, as they are seldom in steam for long periods and size and space can be a restriction in model use and glass failure is unlikely to result in a major drama.

The fitting of shut-off cocks comes from full size boiler use of course. It allows the passage ways through to the boiler to be rodded through even with the boiler in use should their ways between boiler and gauge become blocked in use while the boiler remains in steam, with the help of a blanking plug opposite and in-line with the shut-off cock fitted to the gauge of course, and allows for the renewal of the gauge glass to be renewed also while the boiler remains in steam too, both important considerations where boilers remain in steam for considerabl periods at a time like on ships.

Chris

Dimly remembered from School Chemistry, I believe a greenish tint at the ends is a strong hint that a tube is made of soda glass. This is the stuff that melts in a bunsen flame. It's inexpensive, easily worked, and very handy for making simple glassware in a lab. Whilst brittle, it withstands high pressures provided they are applied slowly. It expands and contracts more than other glasses when heated. Not ideal in a locomotive sight gauge, where the temperature goes up and down and a tap might shatter the glass.

Although soda glass tubing could used at a pinch there are better alternatives.

Dave