Every plant has a budget and every plant manager has to control cost. Cost and profit margin are what make the plant viable. In the tank maintenance industry, costs are looked at closely as they should be. When taking a tank out of service, the shortest duration that can do the job safely and in compliance is the path that must be chosen. The process begins with the tank be drained down until suction is lost. Once suction is lost, a high volume fan blower, sometimes called a “copus blower” is attached to the tank. The fan blower will take several days to 2 weeks to vaporize the several inches of liquid. To speed that up, sometimes a vacuum truck is used to suck liquid and men are used to water wash, using a fire hose on the opposite manway. That results in the possible personnel exposure unless fresh air is donned. There are also inherent hazards or running explosive vapors through a vacuum truck that will be running hot. The vacuum truck blower typically runs at 140 – 160 degrees F. To reduce the cost of having the tank out of service for several days or weeks and to eliminate the safety and exposure, many operators are turning to tank degassing using a control device.
The primary technology being today for controlled industrial vapor degassing is thermal oxidation. Some call it thermal combustion, but it is the same thing. The fire box is heated to a minimum of 1400 degrees Fahrenheit to ensure maximum DRE (and because that is often the minimum regulatory limit for BACT). Combustion is supplemented with small amounts of propane to ensure a stable flame. As the tank goes from rich to lean, the inlet valve is opened up and the flow rate increases. Some proprietors have units capable of up to 6000 CFM of tank vapors. The older technology is using an Internal Combustion Unit (ICE), however, this technology is typically 7-10 times slower than the thermal oxidizer and in some cases, the job doesn’t get done. The global vapor controls that one of the ICE units can handle is around 25 CFM. It is simple variation of the Ford 460 CC engine block. The do not generate useful energy to meet the exemptions of Subpart J nor are they mechanically capable of processing Hydrogen Sulfide (H2S) as this causes immediate and substantial corrosion and the H2S is not completely oxidized to Sulfur Dioxide (SOx) so you have personnel exposure concerns of H2S exposure.
Generally, some proprietary thermal oxidizers say they can have a 100 ft diameter gasoline tank degassed and ready for entry in 6-12 hours, at a cost of $3000-$8000. The lost opportunity cost in terms of waste time of that tank in the current market is estimated at $60,000 to $100,000 per month. If the tank is a critical crude tank and you don’t have a spare, the cost skyrockets. The unseen costs are exposure, safety and compliance – they show up when there is an incident.
There are other methods for tank degassing such as chemical treatment, “refrigeration”, biological (“bugs”) but we have studied these technologies and found them not to be viable in tank degassing. In general bugs don’t eat vapors or liquid, the chemicals are variations of surfactants (i.e., soap) and we the refrigeration method only addresses vapors that can condense and not re-volatilize at standard atmospheric conditions which means just water vapor. The act of circulating air from one side of the tank to the other is where some operators may actually see vapor reduction but the result of the circulation causes found these methods to result in air circulation that releases standing losses vapors out the roof hatches and fittings.