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Sizing natural gas piping for a building and incorporating elevation changes

Sizing natural gas piping for a building and incorporating elevation changes

Sizing natural gas piping for a building and incorporating elevation changes

(OP)
I am assisting in mentoring younger engineers (and trying to learn new things myself) on natural gas pipe sizing for buildings and the issue of elevation changes has come up.

Traditional sizing methods do not address it and as has been stated in other threads, the gas codes, i.e. IFGC/NFPA 54 do not address elevation changes.

The equation, P(bot) = P(top)*exp(0.1875*SG*h/T(avg)/Z(avg)) has been suggested to be used. It was stated that absolute units are to be used.

So I did a sample calculation to see the order of magnitude and I want to make sure I am doing it correctly.

Elevation change is 50 feet.
Bottom pressure is 7 in w.c. so that would be 14.95 psia (14.7 + 7/27.7)
SG = 0.6
T(avg) = 70 F or 529.7 K
Z(avg) is assumed to be 1

I calculated the top pressure to be 14.93 psia (I am assuming atmospheric pressure change due to elevation to not be consequential since the difference is only 50 ft).

So my pressure drop is 0.02 psi or 0.55 in w.c.

I need to know if I am doing this correctly.

If I am, then we cannot neglect building elevation for our natural gas pipe sizing. While the pressure drop is not very significant, most gas providers we deal with require using 0.3 in w.c. as a maximum pressure drop over the entire system due to friction.

I need verification of my calcs and/or holes poked in my method.

RE: Sizing natural gas piping for a building and incorporating elevation changes

Your answer is correct. With a height of only 50 ft you can safely assume that the density of the natural gas is constant as you go up the building and this allows you to use the standard statics formula for the pressure at the base of a column of fluid,
i.e. Pressure = density x (acceleration of gravity) x height

If density = 0.048 lb/ft3 then
Pressure = 0.048 x 32.2 x 50 = 77.3 poundals/ft2

To get your answer in psi use 1 pound force = 32.2 poundals and 1 ft2 = 144 inch2
Pressure = 77.3 / (32.2 x 144) = 0.017 psi

Your formula would take into account the change in density as you go up the column, but that is negligible in this case.

Katmar Software - Uconeer 3.0
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Sizing natural gas piping for a building and incorporating elevation changes

For wells that are 5,000 ft deep (with pressures under 1,000 psig) we disregard the hydrostatic head contribution from gas. I think it is OK to ignore it for 50 ft.

I didn't check your math, but the order of magnitude of your answer is in the ballpark. Your compressibility is a touch high for natural gas, but inside the exponent it doesn't matter at these pressures (at high pressures it can matter a lot).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

(OP)
Thank you for your comments.

katmar According to Crane TP 410, as long as the pressure drop is less than 10% of the initial pressure, the initial density can be used. Since we are constrained to pressure drops much less than 10% (by code) - I wasn't too concerned with density changes.

I am not familiar with poundals - but I recognize what it is signifying. Where I was taught, they were referred to as "slugs"

But since the equation uses specific gravity, which is relative to air, wouldn't that term remain constant because as the density of natural gas gets smaller due to elevation, would not the density of air get smaller as well? Or is the change not the same for both? Again I do not deal with high elevations too much. We have correction factors we use for cities at different elevations, but we do not deal a lot with buildings that the elevation difference is significant.

zdas04: I don't deal a lot with compressibility factors since the highest pressure I usually see for natural gas is 5 psig. Most of the tables I have have pressure in increments of 1000 psia and seem to approach 1 at the lower end so that is what I used.

RE: Sizing natural gas piping for a building and incorporating elevation changes

I wasn't concerned about the density changes affecting the calculation of pressure drop due to friction (I agree with Crane on this). I was only pointing out that if the density is constant you do not need the (slightly) more complicated formula that you have quoted. I have not checked it, but I would imagine that your equation is simply the integrated (over height) version of mine and allowing the implied density to be a function of height and temperature.

You are correct that the SG (relative to air) would remain constant becaue the density of NG or air would decrease at the same rate with elevation.

In the FPS (foot-pound-second) system of units the unit of mass is the pound and the unit of force is the poundal. In the British Engineering System the unit of mass is the slug and the unit of force is the pound-force. If you want to mix the two systems (as always happens in the US Customary system) then you have to introduce gc - which is really just the conversion factor between poundal and pound force that I used above. Plus of course if we measure density in cubic feet but we want the pressure per square inch then we need more conversion factors. When I do calcs like this for myself then I go straight to SI - but we have debated that whole story enough so I will stop there.

Katmar Software - Uconeer 3.0
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Sizing natural gas piping for a building and incorporating elevation changes

The only thing I have add to Katmar's post is that "going straight to SI" can be risky with most empirical equations--it can be really hard to know for sure which unit-conversions are and are not included in the equation constants. Sometimes you just have to get your hands dirty and work in FPS. Also, if you have kg/cm^2 for pressure then you need a "metric" gc.

[Note to Katmar: I'm using Uconeer 3.0 for a project where I'm converting a hundred numbers a day. I LOVE the ability to define my own units and it is really straightforward. This is a very good upgrade and I'm very glad you've started charging for it, it is worth much more than you're charging.]

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

(OP)
What originally brought on my question was a portion of a continuing education article (not for PE) that is offered of engineers and/or designers who work with natural gas piping design in buildings. It has to do with determining pressure drop in the piping system.

The portions just seemed to not make sense.

The source says it is common practice to not use the vertical portion of the piping system because NG is lighter than air. It expands at the rate of 1 in. w.c. for each 15 ft of elevation and the increase in pressure due to the height will offset any friction loss in the piping (vertical portion).

Another part which gives guidance for high rise buildings states that consideration must be given to the rise effect in available gas pressure as gas rises in the building through a high rise. You could have a kitchen on a lower floor and a boiler on a top (say 50th) floor and you may need to boost the gas pressure to the kitchen but not the boilers because the gas rises to the upper floor through the piping system because of the density differential.

These assertions just did not sound right, but I had never looked at the engineering behind it, having mostly dealt with incompressible fluids. So there is a bit of egg on my face for not looking in to it more closely.

Now, granted the change in elevation is not great, there is no temperature change, being in a building, and the change in atmospheric pressure is not going to be that great either.

Thanks for all the help.

RE: Sizing natural gas piping for a building and incorporating elevation changes

People often have plausible hypotheses that folks can just grab onto and feel good about using, they sound logical. [Anthroprogenic Global Warming comes to mind but that is another discussion.]. While natural gas is buoyant in air, there isn't any air within the pipe and steel (or even plastic) pipe is not buoyant in air.

If you filled a vertical pipe with pressurized natural gas then took VERY precise pressure measurements at regular intervals up the pipe you would find that at any internal pressure and temperature, the pressure indicated by the lower instruments will always be higher than the pressure at the higher instruments. Always. The pressure gradient may not be a very big number, but it will not be zero.

The first-floor stove vs. 50th floor boiler certainly has a lot more to do with pipe size than with the molecular weight of natural gas.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

A similar problem was discussed in thread378-231839: Calculating Natural Gas Pressure Drop in a 3" Riser

Applying this to your problem stated above: The density of the air will be about 0.075 lb/ft3, compared with the density of 0.048 lb/ft3 for the natural gas. This means that the air pressure over the 50 ft rise will change by 0.027 psi, compared with 0.017 for the natural gas. Or in NG units, the air pressure changes by 0.73 inch WC while the NG pressure changes by 0.47 inch WC.

If the gas pressure at the base of the building was 7 inch WC (remember this is a gauge pressure) then the pressure at the 50 ft level is 7.00 - 0.47 + 0.73 = 7.26 inch WC (gauge!). This does not agree with your figure of 1 inch WC per 15 ft of elevation - perhaps someone with more experience of LP NG reticulation can comment on that.

I think that rather than make the assumption that this pressure rise will offset the friction loss it is better to calculate them both and know properly what you gauge pressure is at the top of the building.

Katmar Software - Uconeer 3.0
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Sizing natural gas piping for a building and incorporating elevation changes

David, it is important here to distinguish between absolute and gauge pressures. Of course I agree that the absolute pressure will always be higher at the bottom of the building than at the top (assuming no flow), but the trend for gauge pressure can be opposite for the very low densities used in building reticulation systems. Since the nozzles in stoves etc see gauge pressure it can be important. In this particular case nobody is going to be affected by a change of 0.26 inch WC, but in a taller building it may be necessary to design around it.

I totally agree that the pipe sizing is more important - that is why I recommended a proper analysis of the friction and the bouyancy affects. It would be negligent (in my opinion) to simply assume that they offset each other.

Katmar Software - Uconeer 3.0
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Sizing natural gas piping for a building and incorporating elevation changes

I have not found the equations for changing atmospheric pressure with elevation to be correct for small changes--the heat sink is just too big for a locally unconstrained hydrostatic column to actually exhibit an homogeneous density gradient over short distances. The uncertainty is on the order of +/-0.25 psi. I would bet my own money that the experiment I described above would turn out with an increasing gauge pressure from top to bottom of the column (the difference is that the natural gas is constrained and the air is not locally constrained).

Given that, I use the equations for big changes all the time, but I really question them for changes less than about 500 ft.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

David, I think your intuition is flawed, in this case, and you might want to give more detailed consideration to the issue. The containment of the pipe is completely irrelavent to the density effect.

RE: Sizing natural gas piping for a building and incorporating elevation changes

These are things that I've measured, not much intuition involved here. The containment of the pipe contributes to the near-effect homogeneity of the fluid. The local atmosphere is very much non-contained and can have considerable temperature variation from any control volume to any other control volume. You can measure the difference between the "atmospheric pressure" between the inside of a heated/air conditioned house and the back yard or between a location on concrete in the sun and one on grass in the shade. The difference that I've measured is greater than the differences that we're talking about here.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

P.S. compressibility at 1 Atmos, 60F for both air and gas is 1.0000, so can be completely ignored at these pressures and typ ambient temperatures.

This next spreadsheet is using a rough average for density in each segment.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Sizing natural gas piping for a building and incorporating elevation changes

(OP)
BigInch

I gave you a star for a very well done spreadsheet.

It took me a bit to realize you were indicating flow at different exterior elevations and not different building elevations, ie. not many buildings are 12,000 feet tall.

It also appeared to be only considering horizontal flow, not any vertical flow, i.e. the Spitglass formula you used did not have the potential energy term.

It also seemed the some of the constants were different than the form I have seen, maybe due to a different friction factor and different units than I am familiar with.

In my example, there would be changes in atmospheric pressure due to elevation, there would be changes in atmospheric pressure due to "stack effect" of the air in the building (~0.5" w.c. per a HVAC engineer). There would be changes in the absolute pressure of the gas in the pipe due to the density changes of the gas due to elevation changes and once flowing, there will be pipe/fitting/equipment friction to account for.

I want to be able to get my head around all this.

Thanks for all the help.

RE: Sizing natural gas piping for a building and incorporating elevation changes

Ya. I went all the way up, even passing zdas' elevation.

It also appeared to be only considering horizontal flow, not any vertical flow, i.e. the Spitglass formula you used did not have the potential energy term.
Potential energy, change in pressure due to elevation (column K) is handled apart from the frictional drop calculated in the Spitzglass formula. Flow in a no gravity situation does not care if it is up, down, left or right and friction loss is the same in all directions. If your formula calculates flowrate and does include elevation & gravity to drive the flow, then a pressure differential due to elevation is present and then it should have that term. If I calculate remaining pressure due to friction and gravity and whatever else, magnetic plasma force, etc., then I calculate the density the gas must be at that new end of segement pressure, then all necessary expansion or contraction to accomplish that pressure is also automatically considered and no additional term is needed.


It also seemed the some of the constants were different than the form I have seen, maybe due to a different friction factor and different units than I am familiar with. Maybe.

In my example, there would be changes in atmospheric pressure due to elevation, there would be changes in atmospheric pressure due to "stack effect" of the air in the building (~0.5" w.c. per a HVAC engineer). Changes in atmospheric pressure are included in column F. A new atmospheric pressure is calculated at each end of pipe segment, by subtracting atmospheric air density x segment length (weight of air in that column) from the previous pressure. Stack Effect??? Open or close the windows, or include it if the HVAC guys say to do it.

There would be changes in the absolute pressure of the gas in the pipe due to the density changes of the gas That is again column K due to elevation changes and once flowing, there will be pipe/fitting/equipment friction to account for. All you mentioned is some imaginary pipe as far as I'm concerned. You are responsible for all extra pipe you need to get between floors (I assumed straight pipe), plus any fittings/equipment/valve losses AND LEAKS.

Thanks a lot for the star!


If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Sizing natural gas piping for a building and incorporating elevation changes

(OP)
The Spitzglass formula I referenced is found in "Considerations about Equations for Steady State Flow in Natural Gas Pipelines, which I found at http://www.scielo.br/pdf/jbsmse/v29n3/a05v29n3.pdf.

The equation is indicated on page 269. It appears the coefficients being referenced are due to the friction factor being used

The potential energy component, which is indicated in equation 34 of the document appears to be a variant of the P/T/SG/z/elevation relationship.

RE: Sizing natural gas piping for a building and incorporating elevation changes

I have a version without the potential energy term. Spitzglass is for very low pressure flows, so I wouldn't use it where z was anything but 1.0000 As your document says, most all forms of this equation do that plus ignore potential. Since mine did ignore potential, I subtracted the density * height of the column separately from the frictional flow loss. I do believe it is an equally proper way to consider it. I have been known to be wrong .... on occasion. Let me know if your spreadsheet yields comparable results using the potential term directly without subtracting the weight of column in a separate calculation.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Sizing natural gas piping for a building and incorporating elevation changes

I can't see your spreadsheets. So what are the conclusions? I was simple disputing the following assertion:

"I have not found the equations for changing atmospheric pressure with elevation to be correct for small changes--the heat sink is just too big for a locally unconstrained hydrostatic column to actually exhibit an homogeneous density gradient over short distances. The uncertainty is on the order of +/-0.25 psi. I would bet my own money that the experiment I described above would turn out with an increasing gauge pressure from top to bottom of the column (the difference is that the natural gas is constrained and the air is not locally constrained)."

If the gas density in the pipe is less than air then the gauge pressure in the pipe must rise with altitude (pressure drop due to flow is a separate issue). The gauge pressure in the gas pipe is 7"wc. That is about 0.25 psi. Respectfully, the above statement just doesn't make sense.

RE: Sizing natural gas piping for a building and incorporating elevation changes

First one note. My version of the Spitzglass equation is for 0.6 SG natural gas at very low pressures and z = 1.

Compositepro, your theory matches my results, which conform to those prediced by the LP gas distribution literature, assuming constant temperatures, or temperatures outside the building decrease slightly faster than gas temperatures inside, and for low flow gas distribution applications where friction is slight.

In low flow, LP gas systems where there is little friction, the absolute pressure of the gas can become higher than the local atmospheric pressure (absolute) with each stepped increase in elevation. The gas absolute pressure within the tube is not affected by outside air pressure, but gauge pressures are. It is gauge pressure which drives the flow across outlets.

Starting at 0 ft elevation and 0.07645 pcf for air and 0.4644 for natgas at 1 atm + 5" WC, the ever slightly decreasing densities of air on the outside and gas in the inside, air density is calculated from 2.7 * P_psia / T_degR for small increments of height, each weight of air in the height increment being subtracted from the pressure at its bottom. Temperature is held constant with elevation. That might be improved by assuming a typical 3.5F cooling with each 1000 ft of altitude for the outside air, but the pipe, being inside a building, would tend to maintain its temperature constant. That obviously results in the pressure of the atmospheric air decreasing faster than the gas inside the pipe, and faster still if atmos cooling rate is assumed.

If frictional losses are less than the 40% difference in the pressure decrement between air and gas with each change in elevation, gas pressure increases with elevation.

Since the local driving pressure across any outlet to a gas appliance, inside pressure - atmospheric pressure, or the gas' local pressure reading in psig, measured by WC or pressure gauge, becomes greater with elevation, higher gas flows to appliances at higher elevations will likely be experienced, especially if lower appliances are shut off.

At much higher gas pressures, where its higher density would match more closely with air, the inside and outside pressures would match more closely, or as more typically is the case, reverse and the above effects would diminish accordingly.

The same effects can be observed offshore, but to a much more exadurated extent. I neglect gas and seawater compressibility and assume constant temperature. A gas pipeline at 1000 foot seawater depth with 259 psiA inside, has an effective gauge pressure of 259 psiA-459 psiA outside = -200 psiG, seawater pressure outside being much greater than the gas pressure inside. The gas pipeline is not flowing to the pipeline junction/compression platform today, so flow losses are nil. As the pipeline runs up the riser to the platform deck, at a depth of 544 feet of sea water, the gas pipeline pressure reading is 0 psiG. Constant seawater density of 64 pcf. At 200 feet depth, the gas pressure is 151 psiG, 104 absolute in the sea and 254 psiA in the pipeline. At sea level the gas pipeline pressure is 253 psia -15 psia in the water/air splash zone = 239 psiG pipeline pressure. So, should there be a puncture in the pipeline at 1000 foot water depth, water would flow into the pipeline, whereas above 544 feet of water depth, gas would flow out from the pipeline. Since there is no flow today, gas pipeline gauge pressure increases constantly, from -1000 feet below the sea all the way up to the water surface, but falls slowly thereafter, gas at that pressure being heavier than the outside air.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Sizing natural gas piping for a building and incorporating elevation changes

I evaluated this as part of helping some Boy Scouts on a merit badge project 20 something years ago. I know it sounds lame, but the experiment was done very carefully and multiple times to confirm the results by people who REALLY wanted to understand the results.

We mounted several feet of clear plastic tubing to a board in a U shape and double sealed one end (i.e., put in a cork with a pipe clamp holding it in then bent the tube over and tied it off with wire). Then we checked that the board was level and plumb and added distilled water to the tube until both legs were about in the middle of the range. We carefully marked this zero point. When we took the device into the mountains (3000 ft elevation change), we were able to measure a water height change that was very consistent with the expected change in atmospheric pressure.

The problem was that before going up in the mountains the boys wanted to test the rig. We had built and zeroed it in my basement shop. We took it to the ground floor (about 11 ft elevation change, the boys measured it carefully). We expected the height of the free leg to go up about 1/8 inch. It didn't move. We went to the second floor (25 ft elevation change from basement) and should have seen about 1/4 inch increase in level and again there was no movement. So we took it onto the roof (32 ft elevation change, but now out of the air conditioning) and the change was a function of the temperature difference not the elevation difference (i.e., if it was hotter outside then the level in the free side of the tube went up and if it was colder it went down).

The boys looked to me for an explanation and I didn't have one. Still don't. I just know that there is a LOT more random behavior in low pressure gases than our arithmetic would indicate. An equation that gives good results with pipe lengths of hundreds of feet may not be as close to reality with a pipe length of dozens of feet.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

Water works great in a u-tube manometer where you are measuring the pressure difference between the two ends. In your case any temperature change would cause a volume change in the air in the closed end as well as a change in the humidity in the closed end. Your experiment could only work if the temperature stayed very constant to fractions of a degree at all times. A mercury barometer has no air in the closed end and mercury has a very low vapor pressure. Even then, sophisticated barometers have temperature compensation for the change in mercury density.

RE: Sizing natural gas piping for a building and incorporating elevation changes

There was enough air trapped in the closed end to give reasonable results for a 3000 ft elevation change. The house had air conditioning, so in Denver in the summertime it was able to take the normal 5-10% RH down to very close to zero and the temperature gradient within the conditioned space was very small (the data sheets the boys filled out included temperature, and as I recall they found the same temperature at all three elevations within the house).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

"There was enough air trapped in the closed end to give reasonable results for a 3000 ft elevation change."

David, think about what you were actually measuring. What if there were 2" of air space in the closed end on the tube. What if you had 20" of air space in the closed end? What would a 0.25" change in column height indicate in either case?

RE: Sizing natural gas piping for a building and incorporating elevation changes

A 1/4 inch change in a liquid level that is trapping 2" or 20" or 200" of atmospheric air would indicate exactly the same thing. Remember this experiment started with the two legs at the same height. That represented a given atmospheric pressure (one leg was free range and the other was trapped). I don't remember all the details of the rig, but I remember that it was really big to accommodate a 3000 ft elevation change (about 36 inches of water-level change) without pulling all the water around the "U". It worked very well for big changes. Didn't move at all for small changes.

That is my point. All of the fluid arithmetic that we do started life as a statistical analysis of molecular-level theoretical data that some really sharp person saw patterns in and was able to define the pattern with an algorithm. The underlying behavior of the fluid still has a non-homogeneous component and a random component. If you have a gas relationship that works over hundreds of feet or hundreds of miles that is great, but don't assume that it also works over dozens of feet or inches without verifying it. It may scale up and down perfectly (liquid hydrostatic calcs do that), but then again it may not.

The relationship that this thread is talking about is one that really doesn't scale down to short distances very well and since we're relating it to an empirical equation (Spitzglass) it won't necessarily break at undefined points to show you that it is inappropriate.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

David, changing the headspace in the closed end from 2" to 1.75" requires an absolute pressure change of .25/2=0.125 or 12.5%. If the original absolute pressure in the closed end is 14.7 psi, then that is a change of 1.84 psi or (x24) 44" wc not 0.25" as you are assuming. (Actually 0.5" in this case since a 0.25" change in head space would be the result of a 0.5" change in column height).
Your experiment really only measured the volume change of the trapped gas with temperature. Also, the relative humidity in the closed end will be about 100%. Ambient relative humidity is irrelevant.

RE: Sizing natural gas piping for a building and incorporating elevation changes

Do what? If I have an air-filled u-shaped tube that has one end sealed and the other end open and I put enough water in the tube to seal the U, then the trapped air pressure will equal the current local air pressure. It doesn't matter if that trapped volume is 1 cc or 100 cf, it is a trapped volume at a given pressure. If I then move the rig to a different local atmospheric pressure the two surfaces of the U will move apart and the distance they move will indicate the change in local atmospheric pressure from the reference (or trapped) case.

I'm pretty sure that that is how a manometer works. I worked a couple of manometer problems for the P.E. exam and I think I got them right (at least I'm sure I got the 50 of them in the pre-test prep right).

Evaporation, air dissolving into the water, and humidity within the trapped gas make this a rig that you don't want to leave standing for weeks on end, but if it is zeroed when you start and will go back to zero when you finish then those effects are not a factor for the short term.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

I give up.

RE: Sizing natural gas piping for a building and incorporating elevation changes

Never, never, never give up.

David, you must FIRST fill the manometer THEN seal one end. If you seal one end first then any water added to the open side will compress the trapped air as both water levels come to a new equalization level. Beginning with 14.7 psia in the atmosphere and in the sealed end, then adding 3" of water to a sealed U will result in 14.7542 psia in the trapped air when the water level equalizes. That's roughly an error of 100 feet elevation before you even leave your basement workshop.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Sizing natural gas piping for a building and incorporating elevation changes

It was 20 years ago. Some of the details have blurred (e.g., I can't for the life of me remember what merit badge this experiment supported, but I rememeber that all the boys got it). The rig zeroed so we must have sealed it after filling.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Sizing natural gas piping for a building and incorporating elevation changes

It would zero in the basement again, if the temperature and atmospheric pressure remained the same.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

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