Fire Protection for Liquefied Petroleum Gas (LPG) Installations: The Basics

In developing fire protection methods and guidelines for liquefied petroleum gas (LPG) storage facilities, the chief concern is a massive failure of a vessel containing a full inventory of LPG.


INTRODUCTION

In developing fire protection methods and guidelines for liquefied petroleum gas (LPG) storage facilities, the chief concern is a massive failure of a vessel containing a full inventory of LPG. The probability of this type of failure occurring can be mitigated or at least controlled to a reasonable and tolerable figure with appropriately designed and operated facilities. Since most LPG fires originate as smaller fires that become increasingly more dangerous, this article will focus on fire protection methods and guidelines in relation to small leaks and fires in LPG spheres. Of greater importance to the fire protection engineer is the more likely event of a leak from a pipe, valve, or other attached component leading to ignition, flash fire, pool fire, and eventually to a pressure fire at the source.

DEFINITION and PROPERTIES

LPG was first discovered in the 1900s. The applications and uses of LPG, which range from cooking and refrigeration to transportation, heating, and power generation, make it an all-purpose, portable, and efficient energy source. LPG consists of light hydrocarbons (propane, butane, propylene, or a mixture) with a vapor pressure of more than 40 psi at 100 oF. At standard temperature and pressure, LPG is in a gaseous state. LPG is liquefied by moderate changes in pressure (i.e. in a process vessel) or a drop in temperature below its atmospheric boiling point. The unique properties of LPG allow for it to be stored or transported in a liquid form and used in a vapor form. LPG vapors are heavier than air and tend to collect on the ground and in low spots. After LPG is released, it readily mixes with air and could form a flammable mixture. As a release occurs, there will be an area closest to the release that is above the flammable range, an intermediate area that may be in the flammable range, and areas that will be below the flammable range. Mixing, natural currents, and diffusion of LPG vapors affect the size and extent of these areas. If these processes continue, eventually the mixture is diluted to below the lower flammable limits (LFL).

Properties of Two Common LPG's
PROPERTY
PROPANE
n-BUTANE
Specific Gravity 1.5 2.p
Vapor Pressure (at 60 degrees F) 105psia 26 psia
Boiling Point -44 degrees F +31 o F
Cubic feet of gas/gallon of LPG at 60 degrees F 36.4 ft 3 31.8 ft 3
Lower flammable limit (LFL) % in air 2.0 1.5

Upper flammable limit (UFL) % in air

9.5 9.0

Gross Btu/ft 3 of gas at 60 degrees F

2,516 Btu/ft 3 3,262 Btu/ft 3
Table 1 from 1996 edition of API 2510A

Tank Pressures for Two Common LPG's

LIQUID TEMPERATURE
PROPANE
n-BUTANE
31 degrees F
50 psig
0 psig
60 degrees F
90 psig
110 psig
100 degrees F
175 psig
370 psig
130 degrees F
250 psig
650 psig
140 degrees F
290 psig
800 psig
Table 2 from 1996 edition of API 2510A

Vapor Volumes Obtained for Two Common LPG's

LIQUID
QUANTITY
VAPOR VOLUME (gal.)
VAPOR VOLUME (ft3 )
VOLUME of GAS/AIR MIXTURE at LFL (ft 3 )
Propane
1 gal.
270
36
1680
n-Butane
1 gal.
230
32
1630
Table 3 from 1996 edition of API 2510A

Other characteristics of LPG include:

  • LPG exerts a cooling effect as a result of vaporization due to releases at low pressure (as called autorefrigeration).

PRODUCTION AND OPERATIONS

LPG is derived from two main energy sources: natural gas processing and crude oil refining. When natural gas wells are drilled into the earth, the gas released is a mixture of several components. For example, a typical natural gas mixture may be (90%) methane or "natural gas", while the remaining percentage of components (10%) is a mixture of propane (5%) and other gases such as butane and ethane (5%). From there the gas is shipped in tankers or via pipeline to secondary production facilities for further treatment and stabilization. From these facilities it is sent by bulk carrier or pipeline to various industrial plants and gas filling facilities or used for power generation.

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