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LP or natural gas delivery pressures in tall buildings - NYC (C) Daniel Friedman 2015Tall Building LP Gas, Propane Gas, & Natural Gas Pressures & Pressure Settings

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Effects of Building Height on LP or Propane Gas Pressures & Natural Gas Pressure:

This article describes the effects on LPG, propane, or natural gas delivery pressures & pressure regulation in very tall buildings.

This article series describes the common operating pressures of natural gas and LP or "liquid petroleum" gas in the building gas piping and at the appliance? Since there several ways that people express gas pressures we include more than on description of common LP gas or natural gas system operating pressures in this article.



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Effects of Building Height or Piping Length & Diameter on Gas Delivery Flow Rates & Pressures

Rooftop gas tank installation in San Miguel de Allende, Guanajuato (C) Daniel FriedmanJust below we'll do some simple calculations to show that the impact of gravity on an LP gas vapor line on even tall buildings is beneath the level of detection, that air pressure is irrelevant, and that weight of the gas is so low as to not be much of a problem.

Photo: lifting a rootop LP gas tank into place in San Miguel de Allende, Guanajuato, Mexico in 2016. [Click to enlarge any image]

Article Contents

Reader Question: what are the effects of gravity at tall buildings on LP gas delivery pressures in the piping system?

27 January 2015 Charlie said:

RE: The height that a piped LPG gas system can extend vertically up a building before atmospheric pressure (gravity) kicks in and stops the gas flowing.

My question is specific to the height that a piped LPG gas system can extend vertically up a building before atmospheric pressure (gravity) kicks in and stops the gas flowing.

Is the advice/response issued below correct??

I would have thought that the LPG gas would be depended on the temperature adjacent to the gas tank. Too cold less pressure less height up the building??

Not sure the calculation used for water would be the same as gas. If for instance we had a 500m tall building would the LGP gas with .4Psi set at the regulator/tank still give .4psi at the 500m mark (less friction loss)being that the gas is heaver denser than air? and wouldn't the temperature adjacent to the tank effect it also.

Tough questions but would really like to understand the theory behind it.

Thanks again for any input.

Reply:

Interesting question, Charlie.

Bottom line, a U.S. gallon of liquid LPG weighing 4.4 pounds that was changed into vapour form at 1 atm pressure would fill a mile high pipe (if the pipe gave us one cubic inch of volume per inch of pipe length) and would still weigh 4.4 pounds if we measured weight at the bottom of that cylinder of gas. Of course you can use a fatter pipe and build a shorter building.

What are the effects of gravity or height on LP gas or natural gas pressure, density, & flow rates at tall buildings?

Executive summary:

Questions about the adequacy of gas piping in tall buildings has been a topic of discussion since at least the construction of the Empire State Building in New York City. (Gas engineer 1915).

Some online discussions we reviewed among engineers note that plumbing sources suggest that allowance be made for reduction in gas pressure (and density) when piping natural gas up in tall buildings. That may be because natural gas pressure in a vertical column (say a vertical gas pipe) will be less at the top of a very tall pipe than at the bottom, and the density of the gas will be less at the top of the pipe than at the bottom.

In a typical residential structure of 50 feet or less in height, no compensation is necessary and we assume the density and pressure of the gas remain constant. The model gas piping codes do not explicitly give special pressure compensating requirements for gas piping in tall buildings but model codes do require that piping be sized such that there is very little pressure drop under maximum flow conditions.

ICC model gas code 402.5 Allowable pressure drop must meet the appliance input requirements - between 0.1 in wc and 3.5 psi and typical implementation local codes specify a gas piping pressure drop limit of 0.5 in w.c. .(abuot 0.02 psi) Pressure is also not allowed to exceed 5 psi inside buildings except where special installation methods have been used (402.6 Maximum design operating pressure). Australian gas piping guidelines and most-likely those in other countries raise other critical concerns in high-rise building gas piping, attending to safety such as allowing for expansion, contraction, and proper use of pipe chases vs. fire safety.

Really? What follows is nauseating detail that you can skip but for readers who can offer clarifying editorial suggestions, please use our page top or bottom CONTACT link to send those along.

Pressure Change vs. Height Calculations

For tall buildings plumbing engineers may calculate the difference in gas pressure between the ground level and the top of the highest end of the gas piping:

P(bottom) = P(top)*exp (0.1875*SG*h/T(avg)/Z(avg))

Where

SG = 0.6

T (average temperature) = 70F or 529.7K

Z (average) = 1

In a typical gas installation with P(bottom) set by the gas regulator to 7 inches of water column (w.c.) we calculate the bottom pressure as the sum of [1 ATM (air pressure at sea level also = 14.7 psi) + gas regulator set pressure (7" w.c. = 0.25 psi) ] pr about 14.95 psi.

For gases and common building heights, the effects of gravity on the pipe pressure should be small. We discuss the pressure of water at the bottom of a vertical column separately at DEFINE WATER PRESSURE per FOOT of HEIGHT.

The change in pressure of a fluid between two heights can be calculated by this formula:

Δ P = - p g \ Δ h 

where

P = pressure

p = density (not very significant for water but possibly significant for gases)

g = accleration force of gravity

h = height

The minus sign is pointing out that the accleration force of gravity (g) is negative, meaning that if we increase the height we'll see a decrease in pressure.

Since water is almost in-compressible its density won't vary over height and calculating water pressure at the bottom of a vertical column is simplifed - see DEFINE WATER PRESSURE per FOOT of HEIGHT. You might want to digress to read WATER PRESSURE TANK LAW but I'd skip it for now.

But this may not be the case when considering gases.

The density of a gas such as LP gas or natural gas will decrease as pressure decreases, and the pressure of a fixed quantity of gas in a closed vertical cylinder (say a vertical pipe) will decrease exponentially as height increases.

You can stop right there if that's enough or you can read on some details adapted from a Wikipedia article we cite below.

For this discussion we're holding the volume of the gas container (a vertical pipe) constant as any arbitrary volume V per unit h of height.

The pressure of natural gas (which is lighter than air) at the top of a vertical gas pipe will be less than the pressure of the same gas in the same pipe at the bottom of the pipe as described by this formula:

Ph = P0 e (-mgh) / (kT)

where

Ph = pressure at the top of the column of height h

P0 = pressure at the bottom of the column of gas

e = Euler's number or roughly 2.71828 This number forms the base of natural logarithms (also called Napier's constant) discovered by Bernoulli

m = mass of the gas per molecule

g = gravity

h = height (in some unit) above the starting point or 0 representing ground level (or the bottom of our vertical gas pipe)

k = Boltzmann constant that can be given as different numbers depending on the units of measurement, such as 1.380 ergs K-1 This constant describs the energy of an individual (gas) particle consistent with temperature. It is also defined as the gas constant R divided by the Avogadro constant NA - the number particles (atoms or in our case molecules of a gas) in one gram molecular weight or mole of a substance.

T = Temperature of the gas in Kelvin

The Gas Laws (Boyles Ideal Gas Law, Charles Law) are discussed separately at BOYLE's LAW and at CHARLES' LAW and a complete set of gas laws is organized at GAS LAWS & CONSTANTS

Gravity Force Calculations Show Very Small Effects on Gas Pressures

In estimating the effects of the force of gravity on gas pressure in the distribution of gas in tall buildings, the text and simple calculations below conclude that you can ignore it. The effects of gravity on natural gas or LP gas distribution up to the top floor of even a 10,000 foot tall building would be so small as to be insignificant.

The force of gravity obeys the inverse square law (unlike my daughters who sometimes disobeyed certain laws) to describe the effects of distance above the center of mass (or from the center of the earth in our case).

G = 1 / r squared

where r = distance from the center of the earth

r (earth) = 1/2 d (earth) or r(earth) = 3959 miles.

That's 20,903,520 feet (5280 feet per mile x 3959 miles)

We can compute the effects of change in height (radius above center of the earth) as

g1/g2 = r2 / r1 (note that the radiuses are inverted with respect to the gravities)

Let's build a building that is 10,000 feet tall. Wow! Imagine dashing up those stairs or waiting for that elevator!

G1 = 1 gravity or 1G
G2 = gravity at the top of our building which we made really really tall at 10,000 feet

R1 = distance from the center of the earth on the ground at sea level or the radius of the earth at sea level = 20,903,520 feet
R2 = distance from center of the earth on the roof of our new building = 20,913,520

1/G2 = 20,913,520 / 20,903,520

or flipping this over to calculate G2

G2 / 1 = 20,903,520 / 20,913,520

G2 = 0.9995218

So the reduction in gravity at 10,000 feet is .00047 G's or about 0.048% 

Frankly I don't think we could measure the actual impact of gravity on gas pressure in a vertical gas pipe as a function of the height above ground level on any actual building.

Articles relating height of a column of water or gas, building height, water pressure, chimney draft, gas pressure include

Gas delivery pressures drop in long or small diameter piping runs

I'm not sure the question is properly asked as you put it - because despite the un-measurable effects of gravity on gas flow in LP or natural gas piping systems, there can be gas flow rate or delivery rate problems due to the pressure drop in long gas piping runs especially with smaller diameter gas piping.

Indeed there are often LP or NG gas flow rate and pressure delivery rate problems in long gas piping runs especially of small-diameter piping systems carrying gases (not liquefied LPG or LNG).

If we are talking about the delivery of appliance pressure (0.4 psi) and whether or not adequate LP gas is being delivered, we would consider the length of piping, even horizontally, as well as the pipe diameter, as well as any gas regulators in that routing.

In that case we would use a primary LPG pressure regulator at the tank to delivery say 10 psi into the gas piping system and we'd step that pressure down to 0.4 psi at or close to the appliance.

See GAS REGLATORS, TWO STAGE

Separately at WATER PRESSURE MEASUREMENT- we discuss the problem of calculating (or measuring) the pressure exerted by a vertical column of a substance (we use water) focusing on pressure at the bottom of the column (a starting point).

In the article above we mention that LP gas operating pressure for typical residential gas fueled appliances is is 10" of water - which is equivalent to about 0.36 psi and which we round-up to 0.4 psi. That is to say, the 0.4 psi figure you cite from our article is the typical operating pressure of LP gas appliances, NOT the pressure of the LP gas (vapor above the liquid) in an LP gas storage tank.

In the storage tank the LP gas vapor pressure ranges from 0-200 psi LP gas pressure depending principally on temperature (and of course having some liquid LPG in the tank. Typical pressures in the tank are at 10 psi (freezing weather) up to about 200 psi (very hot weather and a sun-exposed LPG tank).

You'll see in that article that indeed the temperature effects on LP gas delivery pressure are important in potentially cold-climates.

Reader follow-up:

With natural gas being lighter/less dense than air, I never thought that getting gas up a pipe would be a problem – turn the appliance on and the gas would want to naturally rise and escape. But with LPG being heaver/more dense than air I always thought that there will be a limit that the gas will rise up the pipe before it stops rising.

Using water as an example (and not including all the different losses) if we had a pump delivering water at 0.4psi (about 2.75kpa) than the water would only rise up the pipe 26.9 meters. Would that not be the same with the LPG?

I’m starting to further understand the theory behind your calculations but am not yet totally convinced (maybe that’s just the plumber in me talking)

I guess the question should be asked differently.

At what height above the regulator will the gas no longer flow?

Keeping the number simple and assuming 50/50 propane to butane with an approximate weight of 2kg/m3 and the vapour pipe being 100mm (to overcome the system losses assumed) and a length of 400m I calculate the weight of the gas to be 628kg. being that its denser than air this weight would be exerted on the regulator. Would that much weight on the regulator diaphragm effect the regulator?

This is all new to me and I thank you in advance for any input.

Again keep up your good work with the website.

Regards,

LP gas (vapor) pressures in piping systems: at what height does gas weight stop the flow of gas for a given pressure?

Reply: Interim comments - as we're still reviewing this question and we invite critique from other readers:

1. Liquid vs gas vs pipe or riser height:

For the example we've been discussing we're not putting LPG (liquid) into the piping but rather LPG vapor or gas from liquid LPG that's in the storage tank, right? Otherwise 'd agree that immediately more stringent height restrictions would pertain to a liquid. And in fact for some applications we've discussed with readers who needed to deliver a huge LPG fuel volume to a device (high capacity kiln for example) the distribution was done as a liquid with conversion to vapor at the point of use.

Propane or LPG will form a gas or vapor above -44F - its boiling point at sea level.
Propane (C3H8) has a specific gravity of 1.5219 at 20C and 1 ATM (sea level). This is also referred to as it's vapor density.
MA govt. gives LPG specific gravity as .509. (hmmm)

2. Gravity is not a significant factor in LP gas distribution

For gases into a vertical pipe that's closed at both ends by regulators, gravity, as we saw in earlier reasoning in this article, is not a factor. Gas in the pipe to any plausible height will reach the pressure set by the input end regulator - we're using 10 psi as a working example.

3. Effects of air pressure on building mechanical systems for tall buildings

Ambient air pressure has no effect on the pressure of gas or liquid in a building piping system as long as that system is enclosed.

That's because the ambient pressures can have no effect on what's inside of a closed container except for very subtle effects of air pressure on container size - negligible in the case of gas or water piping. I also argue (in tables and calculations below) that the ambient air pressure effects on piping or mechanical systems are close to nil at tall buildings, if we are ignoring weather and wind.

But because it's cool we'll take a look at air pressures at different elevations anyway. Engineering toolbox and a zillion other sources give us the standard pressure at sea level and above - http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

Air pressure above sea level is about 14.696 psi (at about 60 degF) and can be calculated as

p = 101325 (1 - 2.25577 10-5 h)5.25588 (1)

where p = air pressure (Pa) and h = altitude above sea level (m)

Air pressure effects on building mechanical systems at higher elevations

The air pressure at various altitudes can be calculated as above or you can use a handy-dandy online calculator. For a more accurate assessment of the effects of altitude on a building's plumbing, gas piping, or HVAC system operations we'd of course need to use the true altitude:

Building top floor altitude = building height + building's first floor height above sea level (unless we're building in Miami)

IF we built a 1200 foot building on top of Mount Kosciuszko in Australia, the top floor (or maybe really the peak of the roof) of that building would be at 1200 ft + 7,310 ft (the elevation of the top of Mt. Kosciuszko) = 8510 ft. or about 2594 meters. I'm ignoring wind effects and variations in barometric pressure and temperature changes of course.

Table of Air Pressures at some Interesting Altitudes

Elevation in Feet / Meters Air Pressure in psi / Pa / atmospheres
0 = sea level 0ft / 0m 14.70 psi / 101,353 Pa / 1 atm
50 ft / 14.67 psi / 101142 Pa /
1200 ft / 380 m 14.07 psi / 97,009 Pa / 0.97 atm
8,510 ft / 2594 m (our kitchen atop Mt. Kosciuszko) 10.73 psi / 73826 Pa / 0.73 atm
10,000 ft / 3,048 m 10.14 psi /69,913 Pa / 0.69 atm

Notes:

We are holding temperature constant at 15°C or 59°F

At sea level (1 atm) propane C3H8 boils at -44°F so it's pretty easy to get it into vapour form. Don't play with matches. When changing from liquid form to vapour form propane increases in volume about 270 x. How do we keep propane as a liquid in the tank? It's under pressure. At 80°F or 27°C the vapor pressure in the tank of propane will be about 128 psi or 883kPa. Pressures in LPG tanks range widely as a function of temperature as we showed in an earlier table.

Remember that the earth's atmosphere is rather high; if we use the Kármán line, at 100 km (62 miles) to define the upper edge of the atmosphere, that's at 1.57 times the earth's radius. That's why at any building height likely to be built before rising seas make us re-think this analysis, the effects of atmospheric pressure on HVAC and plumbing systems are pretty small.

Use this handy altitude and air pressure calculator provided by MIDE Engineering: http://www.mide.com/products/slamstick/air-pressure-altitude-calculator.php

4. Weight of propane gas as a factor in gas pressure in tall buildings

I agree that the weight of propane vapor needs to be considered in propane piping and use just as does flow rate and piping diameters. We do see for example inadequate burner performance when a pipe at a given supply pressure can't deliver enough flow or volume.

Outside air pressure falls (slightly) at higher elevations (thousands of feet) but not enough to have much effect above high altitudes. (At high altitude locations one might adjust a gas appliance orifice, metering device, regulator but we're talking typically over 7000 ft.

And as the pipe is closed at both ends outside air pressure or barometric pressure won't have any impact on gas pressure or levels inside of the enclosed vertical pipe. Outside conditions only have effect when gas is being released to the atmosphere, say at the gas burner of the heating appliance.

So I think the remaining question is what's the effect on pressure of the weight of gas in a vertical column. If we fill a column with gas X at 10 psi, what will be the weight in PSI of that column if measured at its bottom. My water in a well pipe example still is probably the right starting point mathematically.

More pressure = more density = more weight of a gas in a fixed volume container.

Convert some LPG to gas, weigh it and figure its space or volume in both forms (liquid and gas) at ambient air pressure

We can look up the weight of LPG vapor at a given volume and pressure. If we are metering LPG vapour out of an LPG tank stationed at ground level using a regulator giving us 10 psi (because 10 is easy to use in calculations), when that weight of that vapor now stored in a vertical pipe or cylinder exceeds 10 psi we'd find that we couldn't push that gas out of the top of that pipe or cylinder. Right?

Liquid Propane Gas (in liquid form) LPG weighs in at about 4.4 pounds per gallon. By comparison, a U.S. gallon of water weighs about 8.345 pounds.

Liquid Propane volume expands from liquid to gas at 1:270.

Let's allow our gallon of LPG to expand to a vapor at sea level and at 60 degF or 15 degC

If we expand that gallon of LPG to a gas in an enclosed container it will still weigh 4.4 pounds but its volume will now be 270 gallons. How tall will our vertical pipe be if it has to hold 270 gallons? Well it depends of course on the pipe diameter.

1 U.S. gallon (of anything) = 231 cubic inches

270 US gallons x 231 inches per gallon = 62,370 cubic inches if we vaporize all of the gallon of LPG into a gas. Now we've got 62,370 cubic inches of a gas that weighs 4.4 pounds - because we didn't leak anything during all of this fooling around and we're using a closed container to hold the vaporized but previously liquid petroleum gas.

Let's make a LPG gas vapour pipe that gives us one cubic inch of volume per inch of pipe length. We'll use some space-age material that's not so heavy so we can also hold the pipe in a vertical position - for reasons to be explained in a moment.

So if we stay at sea level and manage not to strike a match while vaporizing all of that gallon of liquid propane, we could fill a vertical pipe with our propane vapor. If that pipe were 1.28" of internal diameter we'd have one cubic inch of gas per inch of pipe length (or height if we hold the pipe pointing up).

Our LPG vapour-filled pipe will be 62,370 inches tall at at 0 psi (at sea level it's really at 1 atm but our pressure gauge, measuring PSI at the bottom of the pipe will read 0 psi because ... well, that's life). That's a pipe that's 5,197 feet high.

Techie note: actually this is a funny pipe. A 1" I.D. diameter pipe actually has an area of 0.79 sq. in. So to get a round pipe whose cross-sectional area was really one square inch our pipe would have to be 1.28379 inches in I.D. and the ID circumference would be 2.5449" - nobody makes gas pipe nor any other kind of pipe in that size but then I admitted I was going to use numbers that made these calculations easy.

That's a pipe that's almost a mile high! About half as tall as my theoretical 10,000 foot building that I wish I never started discussing.

Conclusion: a vertical gas pipe with enough volume to hold 4.4 pounds of LPG all in vapor form at 1 atm or 0 psi relative to car tires would have to be about a mile tall.

On 29 Jan 2015 we asked an engineer girl to check our math. See http://www.engineergirl.org/ - we'll let readers know if engineer-girl can put up with reviewing and commenting about this question. Watch the engineers at work. We post the engineer's reply just below:

Thank you for submitting your question to EngineerGirl! We will do our best to make sure you receive a timely reply, but due to the number of questions asked we are unable to email you when an answer is posted. If/when your question is posted online it will appear at [the URL below]/ Please check this page over the next few weeks to see if an answer has been submitted.

http://www.engineergirl.org/default.aspx?id=23953

Or if you want to start all over in metric, 1 kg of LPG = about 1.81L or about 0.49 M3 at 30 degC at 1 atm.

...


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