Additional Math Pages & Resources

Tuesday, March 15, 2011

My name is Gas, Natural Gas - Part I

This post is not about food, or intestinal distress. Nor is it about gasoline (petrol).

It's about gas - a colorless, odorless fuel that we burn for cooking, heating, lighting, fireplaces, etc. There are many forms and names for gaseous fuels today - including natural gas, Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG), propane, etc.

The names methane, propane, butane, etc. describe varieties of hydro-carbon fuel gases. Natural gas as we know it in the United States is mostly methane and comes out of the ground like oil.

Gas can arrive in containers or be delivered by a truck, but most gas arrives through a pipe. Here's an image from a gas utility company, showing how it comes to us:


The gas company has compressors that push the gas through underground pipes to your house, through the meter and into the stove or heater.

Today I want to look at the units of measure for gas and see how they fit in with the elementary math on which we non-mathematicians depend. It's a troublesome substance to measure, due to the "temperature/pressure relationship".

Gas is metered out to us in various ways:
VOLUME 
  • the cubic foot
  • a therm (100 cubic feet)
  • the mcf (1000 cubic feet)
WEIGHT
  • pounds or kilograms (as with propane for your gas BBQ)
 ENERGY CONTENT
  • British Thermal Unit [BTU] or the energy needed to raise 1 pound of water 1 degree F. A quantity of 1000 BTUs ~ roughly 1 cubic foot
  • Gasoline Gallon Equivalent [GGE] or the energy equal to that in a gallon of  gasoline

Let's summarize -  1.15 therms  (volume)  ~ 5.66 pounds (weight) ~ 115,000 BTUs  (energy) ~ a gallon of gasoline

Confused? Do you wonder, Why all these different units of measure?

Natural gas is compressible. If you heat it, gas expands; if you cool it, gas contracts. At which point on a hot day is it most profitable to buy or sell? We had to set up rules. In order to sell it by volume, you need to specify the temperature and pressure at which you measure.

If you sell by weight, the same thing applies. You need to know the pressure to know how much gas is in the pipeline, or the container.

Since we almost always use gas for fuel, it seems useful to sell it by the energy contained in the gas. If we used it for filling up balloon or inflatable swimming pools, energy content wouldn't be a good standard of measure.

Does the energy content in natural gas vary? Yes. Our local utility says:

Natural gas is composed of methane, ethane, propane, butane, and nitrogen. Each gas has a different heating value, and their proportions in natural gas vary. The same volume of natural gas from two sources may have different heating values. We bill customers for the amount of energy contained within the gas. Natural gas is metered by volume (units of 100 cubic feet). We then apply a factor to reflect the heating value of the gas. Our Service Territory has been divided into six Thermal Zones. The heat content is measured by gas heating value measurement stations in each zone. The stations monitor the gas continuously; heat value is averaged in each area for each billing month.


Sheesh. That's enough complexity for today. I shouldn't have done this blog right after lunch. Now I DO have gas ...