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.


Figure 1 considers the radiant heat flux from a pool fire, assuming a 20-mile-per-hour wind. To illustrate this procedure, first locate the diameter of the pool fire along the x-axis. Using an imaginary line from the designated point along the x-axis, locate the corresponding point of intersection on the 7,000 BTU/hr-ft2 line. Next, extend an imaginary horizontal line to the y-axis. The corresponding point of intersection on the y-axis is the distance between the vessel and the pool fire at which cooling water must be applied. For example, if a pool fire is 30 feet in diameter, it is necessary to apply cooling water when the distance between the vessel and the center of the pool fire is approximately 120 feet.

In general, there are three primary methods that can be used to apply water for cooling or extinguishment to LPG vessels exposed to fire. The three methods are: water deluge, fixed monitors, and water spray. Additionally, portable equipment such as ground and trailer mounted monitors can be utilized, but should not be considered a primary means of water delivery. This is mainly due to the potentially extended set-up times, logistics, and requirement of human intervention that is not necessarily reliable.

The table below describes some of the advantages and disadvantages of the 3 primary water application methods and the use of portable equipment.

Application Method
Advantages
Disadvantages
Water Deluge
  1. Rapid activation
  2. Can be automatic
  3. Lack of plugging
  1. Problems with wettability
  2. Possible water spray supplement for legs
  3. Effectiveness with jet fires
Fixed Monitors
  1. Ease of activation
  2. Can be automatic
  3. Effective for jet fires
  1. Exposure to operators
  2. Wind
  3. Large water demand
  4. Monitors may be changed unknowingly
Water Spray
  1. Rapid activation
  2. Wettability and run down
  3. Can be Automatic
  1. VCE damage
  2. Plugging
  3. Effectiveness with jet fires
Portable Equipment
  1. VCE damage not an issue
  2. Specific application to area
  3. Portability for multiple hazards
  1. Prolong set-up times
  2. Manual
  3. Exposure to operators

The first method involves the use of a water deluge system and some form of water distributor. This could include high-volume spray heads, perforated pipe or a distribution weir. An underflow or overflow weir is a form of distribution weir that allows water to be evenly distributed over the surface area of a sphere by water flowing up the piping network, over the top of the sphere, and out of the weir. This type of water distributor is commonly used but is prone to corrosion from standing water and clogging and requires increased preventative maintenance. Additionally, weirs may not be as effective on bullets and are often greatly affected by wind. The remaining components of this method are similar to other deluge installations. The typical deluge system contains a supply piping network, deluge valve and trim, and a branchline-piping network near the top of the sphere. Newer installations are usually activated automatically, whereas older installations are commonly activated manually. The decision as to which activation method to use requires evaluation of spacing, available protection, exposures, and other factors.

The principle behind the use of a deluge or weir system for LPG sphere protection is that the geometric shape of the sphere and gravity work together as an advantage. As water is applied to the top of the vessel, the shape of the sphere and the force of gravity facilitate the flow of the water as it covers the surface area of the vessel. This type of protection is very effective to facilitate an even distribution of water over the surface area. Caution should be exercised, however, because paint, corrosion, dust, and other environmental influences can cause changes in the surface of the sphere, resulting in uneven water distribution. Additionally, settling and other conditions inside the weir can also cause uneven water flow over the sphere's surface.