Design LNG Facilities to Minimize Risks from Cryogenic Exposure56e
Michael Livingston, P. E. and Richard Gustafson, P. E., C. S. P. WS Atkins, Houston, TX 77079 Phillip Guy, P. E. and Luis J. Padilla, P. E. AMG Engineering, Inc., Houston, TX 77084 Craig Bloom, Kamal Shah, P. E., and Victor H. Edwards, Ph. D., P. E. Aker Solutions US Inc. 3600 Briarpark Drive Houston, TX 77042-5206 Correspondence: email@example.com AIChE Spring National Meeting 2009 9th Topical Conference on Natural Gas Utilization Tampa, FL April 26-30, 2009 The cryogenic nature of LNG facilities poses the risk of potentially injurious low temperature exposure of personnel, structural steel, equipment, and instrumentation, control and power cabling. The probability of cryogenic exposure due to loss of containment of LNG is inherently greater than the probability of fire exposure due to loss of containment because of the many precautions taken to eliminate ignition sources in LNG facilities. This paper contrasts the hazards of cryogenic and fire exposure to personnel and facilities using examples of consequence modeling of pool and jet releases. Practical measures to eliminate or mitigate risk from cryogenic and fire exposure will be presented. Strategies for both the onshore and offshore LNG facilities will be discussed.
IntroductionLiquefied Natural Gas (LNG) is a safe and practical way to transport natural gas by sea from remote locations to user distribution systems. LNG is also an effective means for storing natural gas at peak-shaving plants during periods of low demand. Aker Solutions has designed and built state-of-the art onshore and gravity-based offshore LNG receiving, storage, and regasification terminals. As with any hydrocarbon processing facility, fire prevention and protection are important considerations in LNG facilities. Because of its cryogenic nature Page 1 of 17
Design LNG Facilities to Minimize Cryogenic Risks(atmospheric boiling point approximately 260oF), LNG also poses hazards of personnel injury and damage of structures and equipment from cryogenic exposure. The design and operation of LNG terminals minimizes ignition sources, resulting in cryogenic exposure being more likely than fire exposure. This is particularly true in the high pressure processing areas where the inventory of fluid is lower but where the higher pressure creates greater potential for cryogenic exposure to personnel and the surrounding area. Cryogenic exposure of personnel causes freeze burns; cryogenic exposure of carbon steel causes embrittlement, possibly resulting in structural failure. Consequently, protection from cryogenic exposure, as well as from fire exposure, is needed. Protective measures should be chosen that are effective for both fire exposure and cryogenic exposure. Protective measures add cost; thus, they should only be applied to those parts of facilities where the possibility of harm exists. Consequence modeling can be used to predict the extent of potential fire and cryogenic exposure so that protection can be applied where necessary. For the most part, onshore LNG facilities have generous spacing of equipment, so significant cost savings in fire and cryogenic protection can be achieved without compromising safety. In addition, relocation of personnel to a safe area is usually not an issue and the decision to provide facility thermal protection becomes an asset protection/capital investment question. Offshore LNG facilities have comparatively close spacing because of the high cost of building offshore, so fire and cryogenic protection must be applied to a much higher proportion of equipment and structural steel. Egress and relocation to safe refuge areas are also significant factors in this evaluation. If the structure of the offshore platform is compromised, it would have to be abandoned using egress chutes, davit boats, freefall boats, life rafts, etc. Two philosophies can be applied to fire and cryogenic protection. One philosophy is to protect all structural steel and equipment supports that could be exposed to fire and/or cryogenic temperatures. A second philosophy is to protect structural steel and equipment supports only where failure could lead to escalation of the incident. This paper presents an overview of the integration of fire and cryogenic protection for onshore and offshore LNG receiving and regasification facilities. The principles illustrated here can also be applied to liquefaction and peakshaving facilities.
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Design LNG Facilities to Minimize Cryogenic RisksHazards of Cryogenic ExposureHazards to Personnel Exposure of personnel to LNG and cold gas can cause severe cryogenic burns, resulting in tissue damage similar to frost bite or thermal burns. Contact with noninsulated and even insulated parts of equipment or vessels containing cryogenic fluids can also result in frost bite. Unprotected skin may stick to low-temperature surfaces and flesh may be torn upon removal. These hazards should be controlled by separation, guarding, insulation, and personal protective equipment such as gloves, safety glasses, and face shields. Inhalation of cold vapor can damage the lungs and may trigger an asthma attack in susceptible individuals. Asphyxiation is a serious hazard because vaporized LNG is usually odorless. Air contains 21 percent oxygen. If the oxygen content falls below 18 percent, adverse effects such as loss of mental alertness and performance may result. At six to ten percent oxygen or less, exertion is impossible; collapse and unconsciousness occurs. At six percent oxygen or below death would occur in six to eight minutes. Personnel in the vicinity of an LNG release can quickly be enveloped by cold hydrocarbon vapors resulting in oxygen deficient zones. The expansion ratio of LNG is approximately 600:1. Therefore the release of 1 m3 of LNG will produce 600 m3 of 100 percent natural gas. Hazards to Structures and Equipment Carbon steel, which is widely used in process plant structures and in the hulls of LNG carriers, loses its ductility and becomes brittle when exposed to LNG or cold natural gas. Figure 1 shows that AISI 4130 steel loses half of its impact resistance at 60oF below zero. Some other carbon steels become brittle at temperatures of 20oF below zero. LNG has a boiling point of 260oF below zero or 200oR
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Design LNG Facilities to Minimize Cryogenic Risks-360 -260 Temperature oF -160 -60 40
Boiling Point of LNG
Figure 1 Low Temperature Impact Strength of Metals (Flynn, 2005).
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Design LNG Facilities to Minimize Cryogenic RisksSince beginning LNG tanker trade in 1969, there have been eight marine incidents resulting in spillage of LNG with some hull damage due to cold fracture. However, to date there have been no cargo fires. Figure 2 shows a 2 m crack in the deck of an LNG carrier exposed to a 30 liter LNG spill.
Figure 2 Damage to Deck of LNG Carrier by LNG Spill. Direct contact of LNG with structural steel can rapidly cool the steel to below embrittlement temperature. Experiments have demonstrated that immersion of 1/2 in and 1 in pieces of painted steel in LNG can completely cool the steel to LNG temperatures in less than two minutes. When combined with suggested failure criteria for structural steel sections due to embrittlement, these high heat transfer fluxes predict steel section failure in as little as one to five seconds. Vapor heat transfer due to contact with cold natural gas velocities is predicted to be much slower. The cooling rate of structural steel depends on the amount of LNG available for cooling the steel per surface area, i.e. the LNG liquid flux in the jet. The LNG liquid flux is controlled by the flow rate and the location of the steel relative to the origin of the LNG release. Because cooling rates are so rapid, early leak detection and system isolation and shutdown have little effect on managing cryogenic LNG hazards in the immediate area of the release. By the time the detection and shutdown system has activated, the cryogenic damage is complete within the LNG exposure hazard envelope. Thus cryogenic protection requires changing position, changing material of construction, or adding protection such as cryogenic insulation or shielding. It should be noted that rapid detection and process isolation will serve to limit the total volume of LNG released, lowering the potential for the LNG to spread over an even greater area, thereby reducing the exposure of even more equipment and structures to cryogenic conditions. Polymeric materials, such as plastics and elastomers, are also subject to rapid brittle fracture on exposure to LNG, compromising some equipment components and electrical insulation. In the United States, NFPA 59A Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG) is one of the key design documents for Page 5 of 17
Design LNG Facilities to Minimize Cryogenic Risksthe design of LNG facilities. In Europe, EN 1473 Installation and Equipment for Liquefied Natural Gas Design of Onshore Installations is normally used. Both NFPA 59A and EN 1473 require that equipment, controls, and structures whose failure would result in escalation of the incident must be protected from cryogenic embrittlement.
Hazards of Fire ExposureIn contrast to cryogenic hazards, fire hazards associated with vaporization of LNG releases can be substantially reduced by rapid detection of releases, followed by shutdown and isolation of equipment. Experience has shown that fire impinging upon structural steel takes a few minutes of exposure to threaten the steels integrity. Figure 3 illustrates the rate of temperature rise of steel plates exposed to a gasoline pool fire (API 521, 2007). Heating rate would be more rapid for direct impingement of jet fires. The heat flux associated with large pool fires would be app