Fire Protection for Floating Roof Tanks

There are hundreds of thousands of API storage tanks in use across the United States. Relatively few of them catch fire or have anything catastrophic happen to them. But it’s always best to be prepared.

Owners should keep a list of tank specifications available for routine maintenance and inspections, as well as emergency situations. It’s important to know the dimensions for height and width of a tank, along with the product capacity, what type of product is stored, whether the tank is insulated, what type of insulation it has, what kind of instruments it has and roof type among other things.

Owners should also be well versed in their tank storage product, including how much is stored, how much can be stored, the product temperature, whether it is a gas or a liquid, as well as what are the flashpoint and explosive limits for the product.

API 653 is the standard for above ground storage tanks that were constructed under the standards of either API 650 or API 12C. The first edition was published in 1991 and it has since been updated five times, most recently in 2014. According to an Environmental Protection Agency archived document, a series of catastrophic tank failures in the late 1980s inspired API to come up with three new standards - API 650, API 651 and API 653.

In one instance, a leakage at an above ground storage tank led to a South Dakota school closing. In another instance, 100,000 gallons of diesel fuel spilled into a California waterway following a tank failure. About one million gallons spilled into the Monongahela River in Pennsylvania in 1988 after a tank failed. 

Storage tanks can be designed and built to hold anywhere between thousands of gallons to several million gallons. As technology has advanced, so too has the ability to make larger storage tanks. Due to their sheer size, larger tanks that catch fire can be more difficult to put out. Property damage costs are also higher for larger tanks, given that larger tanks cost more to make.

Storage tanks can hold an abundance of flammable and combustible contents. These tanks may be concentrated on tank farms along the Gulf Shore, or they may be found anywhere across the country at such varied places as manufacturing facilities, airports or power plants.

Flammable liquids like crude oil will generally be stored in storage tanks with floating roofs. The roofs help minimalize evaporation losses as the liquid levels change inside the tank. This, in turn, helps prevent fires. Lightning strikes are the culprits for most fires at storage tanks with floating roofs. Most external floating roof tank fires are rim seal fires caused by lightning. The lightning ignites the vapors which then catch fire. Usually reducing the pressure in the tank or using a dry chemical fire extinguisher will douse the flames. 

Fires at outside storage tanks have actually been on the decline over the past 40 years. According to a 2014 NFPA report, there were, on average, 1,142 fires and 28 injuries per year caused by storage tank fires. As of 2011, the number of fires had dropped 76 percent to 275 fires and one civilian injury. Even though there have been fewer fires in the past decade, the NFPA reported that the fires at outside storage tanks still caused roughly $3 million in direct property damage.

The peak period for outside storage tanks fires is between May and August. This makes sense as lightning strikes are the ignition source for about a third of all the fires while storms contributed to another third. Late spring and summer conditions are usually ripe for thunderstorms, which, of course, produce lightning.

Nearly half of storage tank fires take place between noon and 8 p.m. Thunderstorms are also most prevalent during the afternoon and evening hours, according to the National Severe Storms Laboratory. Lightning caused just over a third of the ignitions. Storms were considered the ignition source for about 1/3 of all storage tank fires.

Quick actions can help prevent small fires from spreading. News reports stated that an oilfield worker in Texas put out a fire caused by a possible lightning strike by acting fast and dousing the blaze with a fire extinguisher. His actions helped prevent any leaks or spills. 

Following a bad storm and definitely following a fire, full inspections should be made of storage tanks to assess any damage. Inspections should include thorough evaluations of the roof, shell, bottom, and foundation of a tank. If a floating roof is damaged, any repairs made should be consistent with the original construction drawings. Any mechanical damage to a tank should be immediately addressed with either repairs or a total replacement, depending on the severity of the damage and what part or parts were damaged.

For instance, torn seal fabric on a floating roof should be replaced, not repaired. Rim-mounted and shoe-mounted secondary seals may be readily installed, repaired, or replaced while the tank is in service, according to API 653 7.13.2. To minimize evaporation losses and to reduce the potential hazard to workers, no more than one-fourth of the roof seal system should be out of service on an in-service tank at one time, according to API 653 7.13.1.                                      

Erin Schmitt is the media director/technical writer for Pittsburg Tank & Tower Group, which is celebrating its 100^th year in business in 2019. She has a bachelor’s degree in journalism from the University of Kentucky.