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

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