LPG is also collected in the crude oil drilling and refining process. LPG that is trapped inside the crude oil is called associated gas. The associated gas is further divided at primary separation sites or Gas Oil Separation Plants (GOSP's), Central Processing Facilities (CPF's) for offshore installations or Drilling, Production, and Quarter's Platforms (DPQ's). At these facilities, the produced fluids and gases from the wells are separated into individual streams and sent on for further treatment.
At refineries, LPG is collected in the first phase of refinement or crude distillation. The crude oil is then run through a distillation column where a furnace heats it at high temperatures. During this process, vapors will rise to the top and heavier crude oil components will fall to the bottom. As the vapors rise through the tower, cooling and liquefying occurs on "bubble trays," aided by the introduction of naptha. Naptha is straight run gasoline and generally unsuitable for blending with premium gasolines. Therefore, it is used as a feedstock in various refining processes. These liberated gases are recovered to manufacture LPG.
In commercial applications, LPG is usually stored in large horizontal vessels called "bullets." These bullets can range in volume size from 150 to 50,000 gallons. In industrial applications, LPG is typically stored in large vessels that are sphere or spheroid shaped. These are the large "golf ball" shaped and oval vessels commonly seen at refineries and other similar occupancies. In this article, we will deal primarily with the protection of LPG spheres.
Various sources of standards and codes exist for dealing with LPG facilities and proper fire protection. Some of these sources include:
- NFPA 54, National Fuel Gas Code.
- NFPA 58, Liquefied Petroleum Gas Code.
- NFPA 59, Utility LP-Gas Plant Code.
- American Petroleum Institute (API) 2510, Design and Construction of LPG Installations.
- American Petroleum Institute (API) 2510A, Fire-Protection Considerations for the Design and Operation of Liquefied Petroleum Gas (LPG) Storage Facilities.
- IP Code of Practice for LPG
Additional sources of information can be obtained from various organizations such as the British Standards Institute, the World LP Gas Association, The LP Gas Association, and industry producers and suppliers. For the purpose of this article, we will focus on some of the above-mentioned sources that are typically accepted as the industry standard.
FIRE PROTECTION DESIGN CONSIDERATIONS
In order to reduce the fire risk at LPG facilities, adherence to various design considerations and requirements such as layout, spacing, distance requirements for vessels, drainage, and containment control will help to limit the extent of fire damage. Additional considerations such as fireproofing, water draw systems, and relief systems are also important with respect to the integrity of the installation and the reduction of risk. These considerations address the various ways to prevent leaks or releases that may lead to a fire.
Equally as important to the prevention of a leak or release is a properly designed, installed, and maintained fire protection system. These systems attempt to minimize or limit the fire damage once a fire occurs. In the situation that a fire does occur, the levels of required fire protection are affected by several factors such as location and remoteness of the fire and the availability of water.
To determine if cooling water is required, the anticipated radiant heat flux from an adjacent tank, maximum tank shell temperatures if the vessel shell is not cooled, and other specific risk management guidelines must be analyzed. API 2510A contains a procedure to identify the point at which cooling water should be applied based on the size of the pool fire and the distance between the vessel and the center of the fire (Figure 1.) Additionally, an analysis of the relief valve parameters is necessary to maintain certain internal vessel pressures. Although computer models are available to more accurately anticipate the heat fluxes, this procedure helps to determine if a more detailed study is required.