Tuesday, February 23, 2010

Understanding Vinyl Chloride Monomer

This "Chemical of the Quarter" excerpt is from the U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine, by LT Morgan Armstrong, U.S. Coast Guard Hazardous Materials Standards Division.

Vinyl Chloride Monomer

What is it?
Vinyl chloride monomer (VCM) is considered one of the world’s most important commodity chemicals. In 2007, the global production and consumption of VCM was roughly 79 billion pounds. That is more than 10 billion gallons. The majority of vinyl chloride monomer is used in the production of polyvinyl chloride, also known as PVC. PVC is the largest chlorine-containing end product in the world, and is used in a wide variety of products such as pipes, cars, bottles, lifejackets, wiring insulation, and credit cards.

How is it shipped?
The United States is one of the world’s top exporters, with the majority of manufacturing conducted on the coasts of Louisiana and Texas. It is typically sold directly from the manufacturer to the user. At room temperature and atmospheric pressure, vinyl chloride monomer is a colorless gas; however, it is shipped and stored as a liquefied gas under pressure. VCM is liquefied by moderately increasing pressure or reducing temperature.

VCM is typically shipped in liquid petroleum gas (LPG) ships. It may either be carried in pressurized tanks at ambient temperature or in fully refrigerated tanks at a temperature of 7ºF.

Why should I care?
Shipping concerns. There are several concerns when shipping VCM at low temperatures. These include but are not limited to brittle fracture and ice formation. “Brittle fracture” occurs when metal is rapidly cooled and loses its ductility (or “give”) and impact strength. The metal is then prone to cracking. This is common with steel. Other metals, such as aluminum and special alloy steels and nickels, have improved ductility and impact resistance at low temperatures. However, VCM is not compatible with aluminum and aluminum-bearing alloys. Due to low temperatures, ice can form from moisture in the tank system and block pumps, valves, and lines, causing damage.

When VCM is shipped in pressurized tanks, several issues can arise that are common among all pressurized cargoes, including pressure surges, condensation of trapped vapors, and a liquid free-surface “sloshing” effect, which can decrease stability.

Health concerns. VCM is designated as a human carcinogen. The OSHA permissible exposure limit is one part per million. VCM gas is heavier than air, and replaces air necessary to breathe in confined spaces, causing suffocation hazards. Inhalation of VCM can cause many symptoms, including dizziness, lung irritation, or death, even in a short period of time. Exposure to liquefied VCM can cause frostbite.

Fire or explosion concerns. Due to its highly volatile nature and tendency to form polymeric peroxides, VCM presents a significant fire and explosion hazard. Polymerization occurs when a chemical monomer undergoes a reaction, causing the formation of three-dimensional polymer chains. When polymerization occurs at an uncontrolled rate, it can cause explosions and fire. VCM has a very high evaporation rate and quickly vaporizes and spreads over great distances. It also has a very low flash point of -110°F.

What’s the Coast Guard doing about it?
VCM is regulated under U.S. and international shipping regulations. Stringent regulations are in place for the construction of LPG/gas carriers to ensure compatibility with cargo and maintain cargo and crew safety. Regulations also require that appropriate means be taken to stabilize VCM to prevent polymerization.

Recently there have been several incidents involving VCM leaks aboard foreign LPG carriers in U.S. ports, including one aboard the T/V Venusgas, a Type 2G LPG/gas carrier, at the Port of Corpus Christi, Texas. During the response to an ongoing leak of VCM in the compressor room, several Coast Guard marine inspectors, local law enforcement personnel, facility workers, and ship crewmembers were exposed to vapors and required medical attention and decontamination. The leak was caused by fractured stainless steel indicator lines.

It is crucial in these situations that the ship’s crew is familiar with and immediately implements safety and response procedures. It is equally important for Coast Guard personnel and marine inspectors to maintain awareness of the hazards associated with cargoes while conducting casualty and routine inspections and while responding to cargo spills.

For more information:
Full article is available at http://www.uscg.mil/proceedings/Spring2009/articles/72_Armstrong_Chemical%20of%20the%20Quarter.pdf.

Subscribe online at http://www.uscg.mil/proceedings/subscribe.asp.

Online survey available at: http://www.uscg.mil/proceedings/survey.asp.

Direct requests for print copies of this edition to: HQS-DG-NMCProceedings@uscg.mil.