That's not correct but it is also inconsequential considering the elevation differences here.

This topic comes up in the design of pressurized water piping for high-rise buildings. With no
water flowing at all, i.e. the system is pressurized and capped, you read 50 psig on the first floor of the riser pipe. Go to the top of the riser 100 feet up. You read (approx) 50-(100)(.43) = 7 psig. That's why you need pumps or water tanks on the roof for these tall buildings.

Now, I'm not a steam guy, I'm a juice jock. So forgive me, I may be ignorant of typical loco boiler pressures but I'm thinking 150-200 psi? So if you are hydro testing at 200+ psi, the inaccuracy introduced by a few feet of elevation difference hardly matters.

That is one of the points I was making to Robby about his concern with the difference in precision of a typical, say, 1% accuracy class Bourdon gauge between the precision close to center and the precision near scale extreme (but still guaranteed within the limits Jeff indicated). I think that simply doesn't matter substantially in a make-or-break sense in a practical hydro test, even more so if the issue of technical MAWP specification becomes involved.

It's a little like designing a boiler to have a 5.05 factor of safety instead of the 5 times required by present code. The difference in practical terms is not worth engineering for when maximum pressure at the extreme of testing is only 125%. Much more of a 'concern' would be assuring you reached the statutory actual 125% regardless of gauge reading that might be a couple of psi low -- I know it's not directly comparable, but a very expensive railcar development effort came to naught for a comparable percentage of 'miss' under the nominal buff test requirements, and EMD very famously went to great lengths to 'prove' that the original Siemens PRIIA 125mph locomotive could not quite make the full statutory 125mph and therefore couldn't be purchased by any subsidized agency...

This depends on the accuracy class of the gauge. ASME B.2 is the standard governing this. For example, ASME class A gauges gives an accuracy of +/- 1 percent of the total span in the central 50% of the span, and 2% outside of this. But grade 1A and better have uniform accuracy from 0 to full scale.

Any Bourdon type gauge should be able to tolerate being pressurized to full scale without any fear of damage. Excursions beyond full scale may cause the bourdon tube to take a set if the metal in the tube is stressed beyond the elastic range. More commonly, the "movement" can be damaged if the extreme overtravel causes the sector gear to crash against another internal component.

Gauges can be damaged by violent rapid changes in pressure which mechanically shock them. The most frequent effect is that the pointer just gets knocked off zero and this is easy to correct. Long term operation under these conditions can cause premature wear-out of the mechanical parts. There is also a long-term tendency for the bourdon tube to work-harden when the pressure is oscillating in a narrow range. I don't think these details are important for hydrotesting, where the increase in pressure is controlled and gradual.

Thank you, Jeff. All this information is helpful and useful.

"This topic comes up in the design of pressurized water piping for high-rise buildings. With no
water flowing at all, i.e. the system is pressurized and capped, you read 50 psig on the first floor of the riser pipe. Go to the top of the riser 100 feet up. You read (approx) 50-(100)(.43) = 7 psig. That's why you need pumps or water tanks on the roof for these tall buildings."

That is correct, but water piping in a high rise building is not exactly the same as a heat fired pressure vessel (like a steam locomotive boiler). A heat fired steam boiler will not show the elevation effects of a pressurized liquid system like pipes in a high rise building. If a 5000 foot high boiler was ever built and sufficient heat was applied to generate steam then the water would circulate from the bottom (higher pressure) to the top (lower pressure) where it would more easily become steam. And the pressure would be the same from top to bottom. This is of course a first order explanation, nobody ever built a 5000 foot high boiler so there are secondary effects nobody understands.

A "pressure tight" piping system (like high rise water pipes) is not the same as a "pressure vessel" (like a boiler). If sufficient pressure (say 500 psi) was applied then the water pressure at all heights of a high rise would be the same. There is just not a practical / cost effective way to do that.

Pressure and circulation of mass are two very different things. That is why natural circulation works in powerplant boilers, and why steam rises in water legs, even though the nominal overpressure throughout the saturated volume of both the steam and the water is nearly the same.

This is one of the effects in the modern version of a Lamont boiler, where water is force-circulated through a waterwall passage at about 6x delivered steam mass flow, then ejected into centrifugal steam separators where the water runs down to an effective hot well from which the circulation pump draws. A BFP on a big once-through boiler has to be rated to develop a large amount of differential pressure -- whereas the unit on a good Lamont might only see a pressure difference of a few psi, mostly related to flow resistance effects. But the steam still goes quickly and positively 'up' in the separators, and the momentum of the water can become substantial as it spirals 'down' -- and it is the gravitational attraction effect on the water mass, regardless of the imposed pressure, that produces the gauge deflection we were discussing for the hydro test setup, at just about any range of pressure for a practical locomotive hydro.

Well no, that is just factually incorrect from a basic mechanical engineering standpoint.

You could argue that at 500psig, the 43psi difference between the top and bottom floors, 100 feet apart, is a small enough percentage as to be irrelevant. But there is still an elevation head. Just google that term "elevation head" for an explanation.

You could also argue that if the fluid under consideration is steam in the vapor phase as opposed to water in the liquid phase, the elevation head effect is minimal because the density of steam is significantly lower. So that is why, as a practical matter, the height at which the steam pressure gauge is mounted in the locomotive has no practical effect. But again, the laws of physics are not suspended. There would be a measurable pressure difference between the top and the bottom of a 5000-foot high boiler, just as there is a measurable difference in atmospheric pressure between sea level and Pike's Peak.

Steam rises because its density is lower than water. If you took a steam boiler into a fighter jet and did a plunge with negative g's, the steam would wind up on the bottom of the boiler.

The density of water also decreases with temperature, so hot water will rise by convection and create a circulation.

You spoke of moving steam and water. That's a totally different ballgame, with velocity head and inertia. Think about "water hammer" which causes violent overpressurization of a system unless some compressible relief is introduced. But again I don't think this is a factor in hydrotesting. I'm not 100% positive because I've only ever hydro-tested pneumatic tanks, never a boiler. As I said, I'm not a steam guy. Boiling water for me is for making pasta.

This is why you have calibrated test gauges vs your running gauges and why in the days of steam there were calibrated test sets used to periodically, when service call in's were done, to pull the gauges and check calibrate them They wear in and wear out, loosing calibration. In the past couple of months I have seen exactly that, vintage steam gauge calibration test kits out there for sale. Called collector items by those who don't know what they are.

I spent quite a few years in test and research facilities and we tested all kinds of things. If you want to really be accurate, use electronic digital precision pressure gauges to parallel track your mechanical gauges. Those come with self calibration as well. But that is calibration you will probably never need unless you are doing hydro testing and in that case I would recommend them. That is what we have used and you would be surprised how far a new calibrated mechanical gauge of any kind is off. From personal experience, a 12inch column of water does not mean spit to a water compressed tank ( that is how water pressure is generated on a lot of farms
a combined compressed air - water tank ) water or steam at 160psi.

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Sure it does. Take your bucket example, If the full bucket is 1 foot tall, there will be 0 PSI on a gauge even with the water surface, and .43 PSI on a gauge even with the bottom. Gravity does not get suspended when you add pressure to the vessel. In a locomotive boiler, when a gauge at the steam space reads 200 psi, a gauge at the mud ring six feet below will read 202.6 PSI due to the weight of the water in the boiler. Note in my earlier post about a gauge mounted 10' above the boiler that I said that the tube leading to the gauge was filled with water (as it would be if connected to the boiler below the waterline, or if there was a syphon at the boiler). The weight of that water must be taken into account, and that high mounted gauge will read low as I said. Now if that tube was filled with steam *with a syphon next to the gauge), then no, that gauge would read virtually the same pressure as that in the steam space in the boiler proper.

That example is all academic in our context since no locomotive has a pressure gauge mounted at a significantly different altitude from the steam space of the boiler.

It would be an issue in a large power plant with 60' tall boilers, and the control room located at the base of the boiler, at least back in the days when power plants used mechanical gauges.

Without intent on derailing this thread, I offer one of my experiences as an inspector:
A new coal fired "supercritical" plant was under construction, and we were performing the hydro on the main boiler feedwater piping. Not the entire boiler, just from the pumps to the boiler proper. That pipe held just above 55k gallons of water, and we pumped to 7500psi. The MAWP of the boiler was just above 4k psi, and the hydro pump was mounted on a 53' flatbed truck trailer, powered by a diesel engine. The main test gauge was a mechanical gauge, and I was told it cost $10,000.00.

We had some drips and issues, but did squeeze out a successful test.

I had no concern about hydrostatic head pressure........

ASME Code

PG-22 LOADINGS

PG-22.1 Stresses due to hydrostatic head shall be taken
into account in determining the minimum thickness
required unless noted otherwise. Additional stresses
imposed by effects other than working pressure or static
head that increase the average stress by more than 10% of
the allowable working stress shall also be taken into
account. These effects include the weight of the component
and its contents, and the method of support.

here is a different hydro test.

Robby Peartree

OK, I'll bite -- how in the SAM HILL did that happen???

Hydro testing smokeboxes to find air leaks was a thing. I recall seeing it mentioned in old standard practices.