NATURAL GAS—CHEMICAL AND PHYSICAL PROPERTIES
tems have pressure relief devices to prevent overpressure of the pipeline in the event of inadvertent pressure surges.
Boiling Point
Since natural gas has an extremely low boiling point it is difficult for it to exist as a liquid. It cannot be transformed into a liquid by using pressure alone, like propane or butane. However, under the right conditions and using a spe- cial process, operators can reduce the natural gas’ temperature low enough to liquefy the product, making liquid natural gas (LNG) which is a cryogenic.
To liquefy natural gas and reach its boiling point –259ºF (–162ºC)—the gas
must reach very cold temperatures. Liquefied gaseous products with boiling points below –151ºC or –254ºF are considered cryogenics. Contact with these super cold products can cause severe frostbite-type burns.
Expansion Rates or Liquid/Gas Ratios
When liquids, liquefied (gas) products, and cryogenics change to gas, large amounts of vapors/gas are given off. The amount of vapor or gas is usually given in the form of a ratio—liquid to gas volume (1:1). Gasoline expands 1:35. Liquefied petroleum gas (LPG) or propane expands 1:270. Liquefied nat- ural gas (LNG) expands at a rate of 1: 420. Under the right conditions, a small spill of a liquefied product vaporizing and producing enormous volumes of flammable gas could create asphyxiation hazards.
There are advantages and disadvantages of high expansion ratios. Due to
natural gas’ high vapor pressure and corresponding low boiling point, large amounts of natural gas can be stored for future use. When LNG is in the cryogenic state and is released it can expand at an enormous rate. This makes it a very good way to store natural gas for excessive demand during peak gas use.
Liquefied Natural Gas—Cryogenic LNG
Cryogenics are products that are cooled to a very cold tem- perature. The physical temperature of the gas can cause ther- mal harm and potentially severe injuries upon contact. Cryogenic LNG is an economical way to store large amounts of natural gas because of the enormous expansion ratios. It is usually stored as a cryogenic in large storage tanks for sup- plemental distribution to customers.
Any cryogenic will produce enormous vapor clouds and
large expansion ratios. When extremely cold gas contacts the water vapor in the air, it creates a visible cloud or fog. Remem- ber, the LNG-induced cloud you see is only air condensing. The invisible flammable gas could actually be a distance out from the visible cloud.
FIGURE 2.7 Cryogenic LNG storage tank. STREET SMART TIP
In an emergency a cryogenic release can cause large volumes of flammable gases to be released and can create oxygen
displacement scenarios.
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