A brief discussion on issues related to the placement of the floating roof in internal floating roof storage tanks

This article mainly specifies the requirements for the liquid level in internal floating roof storage tanks. The float landing on the bottom refers to the situation where, due to an excessively low liquid level in the tank, the float ends up on the support legs. During normal operation, once the floating disk settles to the bottom, a gas space is formed between the disk and the oil surface. When the flow rate of the material is too high, static electricity tends to accumulate at the outlet of the material pipeline, which can very easily lead to fires and explosions. Typical accident case: On August 29, 2011, an explosion and fire occurred in a diesel tank at CNPC Dalian Petrochemical. The main cause of the accident was the low liquid level resulting from fuel being supplied to the tank; this created a gas space between the floating disk and the surface of the diesel, allowing air to enter ； At the same time, fluctuations in the operation of the upstream units led to an increase in the content of light components in the diesel that entered the accident storage tank, resulting in the formation of an explosive mixture beneath the floating disk ； Furthermore, the rapid flow rate of oil generates a large amount of static electricity that cannot be discharged in time, leading to discharges and subsequent explosions and fires.

Solution: A nitrogen sealing system is installed in internal floating roof storage tanks, thereby filling the gas space above the tank’s floating roof with nitrogen. When the tank dispenses oil, the pressure in its gas space decreases; at this point, a self-acting control valve automatically starts to supply nitrogen to the gas space. When the floating roof descends, a gas space is formed between the oil surface and the floating roof. Since the gas in this space is nitrogen, there is no oxygen available as a fuel, so fires and explosions cannot occur.

Storage tanks equipped with internal floating roofs and fitted with nitrogen sealing can indeed address the explosion prevention issues associated with the gas space that forms above such roofs, thereby enhancing safety. The nitrogen sealing also helps to replace the gas present in the upper part of the internal floating roof, as well as to manage the gases discharged from that area; it also helps to deal with VOCs problems

Due to commercial requirements, the material in the tank was lowered to the bottom of the floating tray. During the inspection conducted by experts from the Emergency Management Bureau, it was found that the floating tray was in its lowered position; however, the nitrogen sealing system was functioning properly. Will the Emergency Management Bureau reject this finding when their experts conduct another inspection?

You mentioned Order No. 84 at the beginning: floating platforms in operation are prohibited from sinking to the bottom. If tank cleaning is definitely not included in this range :)

When Order No. 84 was issued, internal floating roof tanks still used ear-window types; nowadays, they mostly employ nitrogen sealing combined with VOC vapor recovery. With proper nitrogen sealing in place and the oxygen level inside the tank kept below 8%, what safety issues could there be if the liquid level reaches the bottom? Moreover, today’s floating pads are all made of steel with dual discs, and their strength is incomparably higher than that of the original single-disc structures with skinning. In my opinion, regulations and rules should keep up with the times, and experts conducting inspections shouldn’t rely on outdated methods.

The risk with a properly sealed nitrogen system should be very low