API 650 Tank Safe Service Check: Cone roof or floating roof?

The Standard API-650 Welded Steel Tanks for Oil Storage covers the majority of large storage tanks constructed for the American petroleum industry. As a process engineer, I don’t have to design the tank myself, but I need to make sure that the products being sent to storage are in a safe condition.

In addition to worrying about tank overpressure and vacuum, and about corrosion, here are a few other safety items to consider. I use these factors to determine whether I can use a closed cone or dome roof tank, or a floating roof tank, or if I need to go up to pressurized storage.

Note that this is not an exhaustive list, just the first three criteria that I personally first look for.

Is the tank governed by API 650?

First of all, API 650 states that the tank contents must be less than 200°F and 2.5 psig under standard operating conditions. Otherwise, it doesn’t fall under API 650. Tanks are low pressure and low temperature, and are distinct from/different from pressure vessels.

(There are some exceptions in the appendix that can extend the temperature of API 650 tanks up to 260°F, but then you’ve got to worry about the boil-up¹ of water).

Check the flashpoint

The flashpoint of a liquid is the lowest temperature at which a liquid gives off enough vapour to form a flammable air-liquid mixture near its surface. It is a chemical property you can look up (e.g. in a MSDS) or predict with equations or process simulation programs. If you leave liquid in a storage tank at a temperature above the flashpoint, then you will have a flammable vapor-space on your hands. Consequently, it is desirable to operate any storage tanks below the flashpoint to avoid the risk.

How far below? This margin of safety depends on the company you work for. I’ve looked at several standards and there is a range: some say operate 15°F below the flash point, some say 30°F.

Note that you may have some trouble getting to this low temperature with kerosense or diesel in hot climates. If you cannot meet the standard you’ve got to do something: the most common solution is to use a floating roof tank to minimize the vapour space.

Some standards are written in a “backwards” way to save the user the need to calculate the flash point, by just flat out telling you to use floating roofs in certain circumstances. For example, a company standard may state “always use a floating roof for jet fuel.” I believe that these standards were written using the following logic: “our company spec says that kerosene has a flashpoint as low as X°F. We operate in a country that gets as hot as Y°F. Since Y > X, and tanks can easily approach the ambient temperature, the fluid will get hotter than X. Therefore, I’m going to write down that we always put kerosene in floating roof tanks.”

Emissions Issues

In the U.S., I look to USA EPA, 2007. “Title 40–Protection of Environment. Chapter I–Environmental Protection Agency. Chapter C–Air Programs. Part 60, subparts k, ka, kb“

To take an example from one of those subparts, for tanks that contain 40000 gallons or more of volatile organic product, I have to look at the True Vapour Pressure (TVP):

• If TVP at storage is <1.5 psia, a closed roof (such as a cone or dome) may be used
• If TVP at storage is 1.5 to 11.1 psia, a floating roof (external or internal) should be used to reduce vapour losses to the atmosphere. Or a vapour recovery system can be used, assuming that >95% of tank emissions are captured
• If TVP at storage is >11.1 psia, pressurized storage should be used.

Anyway, mechanically speaking it is difficult to build floating roofs above 11.1 psia:

http://bulktransporter.com/mag/transportation_environmental_regulations_drive/

Product true vapor pressure (TVP) is the single most critical design parameter when selecting the type of floating roof. Most current environmental regulations limit the product TVP to less than 11.1 psia. As the TVP increases above 11 to 12 psia, daily heating of the product under a center deck will produce enough vapors to balloon the deck. It is common for these vapors to condense during the cooler evening hours, allowing the roof to resume a normal flat shape.

“It must be emphasized that if the tank is in an area of significant rainfall, ballooning of a single-deck roof may not permit normal water drainage to the primary roof drain,” Gallagher says. “An unbalanced load can quickly be developed that can sink the floating roof.”

A secondary consideration with the high vapor pressure products is that with increasing TVP, the overall effectiveness of any floating roof design is reduced. More evaporation will occur, and this vapor will escape to the atmosphere above the floating roof. Air pollution and the risk for a fire are increased under these conditions.

“If the TVP is going to be high, approaching 11.1 psia or greater, consideration should be given to using a double-deck floating roof,” he says. “A double deck will maintain its ability to drain water while containing some amount of product vapor. The double-deck roof can help reduce vapors from product heating due to the insulating effect the design provides to the product surface. Properly designed, the double-deck floating roof can be the most stable design available.”

Using these three criteria will provide a good first check on which tank type to employ.