Liquified natural gas (LNG) and liquified petroleum gas (LPG) are energy-dense fuel products that can be used for heating, cooking, power generation and numerous other tasks. LNG is mostly methane with a few other light hydrocarbons that are not worth extracting. LPG is primarily propane and butane, with a few other gases mixed in.

Based on the phase diagrams of both materials, they can either be cooled cryogenically, causing them to condense, or they can be placed under high pressure, which will achieve the same purpose. In either case, the game is to get the gas molecules from moving slowly enough that they behave as a liquid through either the removal of thermal energy or by pressurizing them to force them closer together.

Chemical Safety Board’s rendering of the BLEVE incident in Philadelphia, Penssylvania.

LNG and LPG physical properties

Both fuel sources are used in the vapor state, and so much of the planning and calculations around them deal with a vapor rather than a liquid. Also, by their nature, they are both flammable vapors. If allowed to leak, they quickly turn into a Boiling Liquid Expanding Vapor Explosion (BLEVE), which is one of the most catastrophic disasters that can occur in the chemical processing industry. In order to understand how to avoid BLEVE incidents, the properties of the materials must be well understood.

LNG boils at around -162° C, meaning it must be stored cryogenically or pressurized to remain a liquid. If the temperature exceeds -162° C, the LNG will begin to boil (convert to a gaseous state), rapidly expanding. This expansion can rupture pressure vessels. Once the LNG escapes the pressure vessel, it will rise, as it is less dense than the air around it, making it slightly safer than LPG, but it is still a hazard.

LPG boils at a higher temperature, -42° C, which means it is a little easier to keep in the liquid state. However, it is also denser than air, meaning any leaks will allow flammable vapor to sink to ground level. This can lead to easier ignition, as many sources of ignition are closer to ground level, such as vehicle engines, static discharges, arc flashes from mechanical relays and others.

If either of these substances vaporize, they can overpressurize vessels, rupture them and find an ignition source. As the saying goes, “light hydrocarbons are going to light hydrocarbon,” meaning if they are roaming free, they will find an ignition source.

Storage mechanisms

Safely and efficiently storing either of these fuels is an engineering feat. Large quantities are typically stored outdoors in tanks of various styles. Regardless of the weather conditions, these tanks must keep the fuel as a liquid; if the liquid is allowed to vaporize at the wrong time, it can expand to many times its initial volume, leading to a BLEVE, which has catastrophic results.

For smaller operations, spherical, above ground tanks are used. These spherical tanks are efficient in terms of their volume to surface area ratio. They can be pressurized or cryogenically controlled, though pressurized tanks are vastly more common. This is because it is less efficient to constantly cool an outdoor structure to cryogenic temperatures. Consider a tank at a refinery in Houston, Texas, on a hot, sunny day. The cryogenic system would be working hard, using lots of energy. A pressurized system is more energy efficient in this case.

For larger storage solutions, underground tanks are used. Once again, either mechanism (pressurization or cryogenic temperature control) can be implemented. With adequate soil surrounding the tank, the ambient temperature around it remains constant. In this case, a cryogenic system does not work as hard as it would in an above ground tank.

Inside an LNG storage tank.

Process safety and instrumentation

The fear of a BLEVE dominates LNG storage design. The object is to keep the LNG a liquid under a variety of circumstances, including during a main power failure. For cryogenic cooling systems, the cooling loop must be maintained and operational at all times. This means having a backup power source, such as a diesel generator to maintain the low pressure. For pressurized systems, backup power sources are used to operate compressors as needed. In both cases, the backup power source continues to run instrumentation systems.

Even with the fear of BLEVE, both fuels use tanks equipped with pressure relief valves. These valves should be vented as far away from ignition sources as possible. These will be separate from a system of plumbing and pressure valves that vent to the flare. Should a tank become overpressurized, material can be first vented to the flare and consumed in a safer manner. If the pressure is still building too quickly, it can be vented through a regular pressure relief valve. In either case, there is still a risk of BLEVE, so instrumentation is in place to hopefully alleviate problems before these final backup measures.

Pressure and temperature sensors are the obvious hardware used to monitor the tanks. If pressure rises faster than expected, they should be automatically tied to upstream control valves to limit flow into the tank. Each tank should be equipped with liquid level gauges, and it is a good idea to have both a manual gauge with a sight glass and an electronic gauge. The electronic gauge can be used for automated process control, while the sight glass can be used to verify operating conditions during a sensor anomaly.

Pressurized tanks should also have a control mechanism to monitor outlet flow. As the tank is drained during use, the pressure in the tank will drop. At some threshold, the outlet valve should close, the inlet valve open and the compressor start. This way, the high pressure of the storage vessel can be maintained.

Final thoughts

While there is a push to do away with fossil fuels as an energy source, they will be used for decades to come. The process of phasing them out in favor of renewable energy sources will be a generational challenge. Furthermore, Not In My Backyard (NIMBY) attitudes tend to block new LNG or LPG storage facilities for the fear of a BLEVE.

While BLEVEs do happen at storage facilities, it should be pointed out that such instances are rare. Liquefied gas storage is not built like a water tower. They have numerous safety devices and constant process monitoring to ensure that they will be operating safely for many years.