Gaseous oxygen measurement with amperometric sensors is the most direct and easiest solution for protection against oxidation and explosion.
Unwelcome oxygen
Tank blanketing is the process of filling the headspace in storage vessels and reactors with an inert gas, usually to protect its contents from exploding, degrading or polymerizing due to the presence of oxygen but also to protect equipment from corrosion. A blanketing system is normally designed such that it operates under higher than atmospheric pressures, therefore preventing outside air from entering the vessel. As the presence of oxygen and moisture in the air is undesired in numerous processes and applications, blanketing is performed in a wide range of industries, varying from (petro)chemical to food and beverage, pharmaceutical to pure water.
Inerting is done for similar reasons but is not limited to storage tanks and reactors. Any confined space can be sparged with an inert gas to create the desired atmosphere. This ranges from packing food under protective atmosphere to increase shelf life to lowering the oxygen concentration in rooms where welding takes place to reduce risk of fire. Typical also is complete or partial inertization in process equipment and unit operations such as:
• Centrifuges;
• Mills;
• Mixers;
• Fluid bed dryers;
• Silos;
• Pneumatic transport;
• Incinerator / flare feedstock supply.
For economic reasons and because of its availability the most common type of inert gas used is nitrogen.
The blanketing process
The simplest way to protect a vessel, for example a storage tank, against either overpressure or negative pressure is to have an opening at the top of the tank. In that situation any excess air or gas can freely leave the tank when product is pumped into the tank and, conversely, air can flow into the tank when product is drained. Such a system also allows “breathing” of the tank due to temperature fluctuations that normally can lead to significant volume changes.
For a number of reasons this method is not suitable for all products. Air entering the tank might contaminate the product and, especially when storing organic solvents and hydrocarbons, an explosive gas/air mixture will form above the product. Also, undesired gases and vapors may be emitted into the atmosphere. As these situations must be avoided, the tank needs to be sealed. The tank does however need to be kept under constant pressure in order to avoid overpressure when it is filled or when temperature rises, and much more importantly, to avoid vacuum when product is drained. Large storage tanks especially are not compatible with low pressures.
The blanketing system is there to guarantee that the tank headspace is kept both under inert atmosphere and at constant pressure. One way of achieving this is through continuous purging with nitrogen, which is a relatively easy and safe alternative. Though this method requires low capital investment, it involves high operating costs as it consumes nitrogen continuously.
Slightly more sophisticated is pressure-based blanketing. In a traditional set-up, such a blanketing system consists of:
• A blanketing valve or regulator allowing the inert gas to enter the tank whenever required;
• A breather valve or vapor recovery valve to allow headspacegas to escape from the tank;
• A safety pressure / vacuum relief valve to prevent tank overpressure or vacuum (the latter could lead to implosion of the tank, a risk that grows with tank size)
• and of course piping and inert gas supply
In this operation the breathing valve opens when the headspace volume decreases and lets headspace gases leave the tank. In case the product is pumped out of the tank or when temperature decreases, the blanketing regulator shall open and fill the tank headspace with nitrogen, avoiding underpressure. Maintaining a constant gauge pressure makes sure that air, and thus oxygen, does not enter the tank. Changes in temperature and weather conditions mean that the tank breathes continuously.
Wrongly engineered or poorly maintained blanketing systems may lead to serious incidents. The statement that all blanketing systems leak one way or another is probably true. Their complexity; valves with moving parts, packings and sealings, etc. are prone to failure. A malfunctioning pressure transmitter may register the wrong headspace pressure causing too high a nitrogen consumption. When a blanketing valve does not open far enough the inflow of nitrogen will be too low which may result in sufficiently reduced headspace pressure as to cause the tank to implode or the leaking of air into the tank, which, as mentioned, can have consequences with regard to product quality and, depending on the stored product, can seriously increase the risk of explosion.