Steam pressure guages

[Peter VennParticipant @petervenn53369]

I have just been fitting the steam pipe which leads from the boiler to the pressure gauge on my 5 inch gauge O2 Loco. It occurs to me that steam may never reach the bourdon gauge in service because when it is fitted together there is presumably air trapped inside the pipe and gauge in what is essentially a blind ended tube. This air presumably gets compressed by steam pressure but the steam itself does not actually reach the gauge. Are we therefore actually measuring the compressed air pressure above the head of steam rather than the steam pressure itself? It’s of academic interest only but I thought it might stimulate some discussion because colleagues I’ve spoken to so far say they have never really considered it.

[PeteParticipant @pete41194]

In a closed system, pressure will always be the same. Any air has to be at the exact same pressure as the steam that’s compressing it. Compressed air obviously doesn’t expand like steam, but your only trying to measure line pressure with your gauge.

[duncan webster 1Participant @duncanwebster1]

Gauges used on steam are usually fed via a syphon tube to prevent steam getting to the gauge (by condensation). This is to keep the gauge cool I think

[Martin Johnson 1Participant @martinjohnson1]

Pete is incorrect.  In a tall tank, the pressure at the bottom is greater than at the surface.  If it was as Pete suggests submarines would not need any strengrh in the hull.

So the gravitation element in fluid statics needs accounting for.  That being so the OP is in theory right although the density difference twixt air and steam over the heights involved would be negligible as the OP infers.

We did have to account for the height differences when testing pumps where tapping points could be at different heights and instrument lines could be air or water filled and might need flushing through so we could be sure what they were filled with.  In one fairly extreme case on low pressures we had a gauge with a means of bleeding right through the Bourdon tube, otherwise the weight difference between air and water in the tube affected  the calibration.

Martin

[Nigel Graham 2Participant @nigelgraham2]

Hydrostatic head, which Martin describes, is not relevant to a pressure-gauge on a pressure-value such as a boiler or compressor reservoir.

So the pressure transferred from boiler to gauge is the same in both, and the gauge is indeed full of air not steam.

As Duncan says, the pressure gauge itself is isolated from the hot steam by a syphon or a coil that soon fills with water (the initial vapour into it, condensing); to keep it cool.

The “syphon” is usually just a U-tube but can be a single-turn coil, and in some full-size applications is a neat “halo” right round the gauge’s cylindrical case.

The gauge is normally on a vertical plane, or at least steeply inclined, so the Bourdon tube itself stays empty of water- though the lengths Martin encountered in a test-laboratory are as he says, extreme!

[noel shelleyParticipant @noelshelley55608]

One figure seen mentioned for the gauges I use , stainless case and brass internals is the temperature rating of 65*c as the upper limit. These are high quality industrial gauges often of 100mm dia from  40psi to 4000psi or more. I have never seen the temperature rating of the small gauges supplied to the model trade mentioned ? That the above mentioned siphon will stop steam reaching the gauge is true,  but that the gauge is much below the boiler temperature I question and suspect it is well above the figure of 65*c. Comments ?   Noel

[old martParticipant @oldmart]

The pressure will be very close to exact whether there is condensed water or air trapped in the bourdon tube. That said, there are exceptions caused by the weight of the fluid in gauges which run at very low pressures. I used to do gauge testing when I was an instrument mechanic and had to rig up an air only coupling when testing a bourdon gauge which read only to about 1psi because the weight of any liquid adversely affected the ultra thin tube in the gauge. It would have been easier to have used one of our Texas boxes used for aircraft instruments such as ASI’s but they were both in use at the time. Another rare mishap was when we got a hydrogen cylinder to fill up balloons at the local fete and the boss fitted a gauge that had been used with oxygen. It exploded when the hydrogen reached the bourden tube, nobody was hurt, but we did have a good laugh while looking for a replacement gauge which had been used with nitrogen.

[Nigel Graham 2]

On Nigel Graham 2 Said:

Hydrostatic head, which Martin describes, is not relevant to a pressure-gauge on a pressure-value such as a boiler or compressor reservoir.

So the pressure transferred from boiler to gauge is the same in both, and the gauge is indeed full of air not steam.

It is relevant insofar as Pete claimed it did not exist.  The OP was quite correct in realising that a gauge full of air is measuring the air pressure.  He also realised that any height corrections on the density difference between steam and air would be miniscule.

I also pointed out such corrections are usually only relevant in low pressure work.  But as Scotty pointed out “Ye cannae change laws of physics”.  So it might not be relevant to the case of a highly inaccurate model loco gauge on 80 lb of steam, but that wasn’t the OP’s question.

Martin

[elarahongParticipant @elarahong]

For steam pressure gauges, I’d definitely make sure the gauge is rated comfortably above the maximum working pressure and is suitable for steam service. It’s also worth checking the siphon/pigtail and keeping it filled with condensate, as that helps protect the gauge from direct steam heat.

[Nigel Graham 2Participant @nigelgraham2]

To amplify that:

The pressure-gauge selected is usually about twice the working-pressure.

That pressure should be marked indelibly on the dial or the case.

Those sold by the established model-engineering retailers can be trusted as suitable for purpose; but yes, they should still be fitted with a siphon or coil that places a water-barrier between the steam and the air in the gauge itself.

The connection to the boiler should be a bit away from any steam outlets so those do not artificially depress the pressure in the gauge itself. Normally the gauge has its own bush in the shell.

 

The form of the siphon is not important as long as it provides that barrier, and is out in the open air to help keep the gauge moderately cool.

The usual shape is a U-tube like a sink-trap. Sometimes it is a single-turn coil encircling the gauge, or ditto but in a vertical pipe, keeping the gauge vertically above its connection to the boiler.

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