Safe Fuel/Air Slow Compression

R. P. Hart and K. Hughes, Gilbarco/Veeder-Root

Veeder-Root is investigating solutions to an environmental air pollution control problem. The problem involves containing the fuel/air vapor mixture found in the ullage (vapor) space of liquid fuel storage tanks to prevent any escape of the mixture to the atmosphere under normal operating conditions. Under certain operating conditions, the potential exists for excess vapors to escape due to over pressurization of the containment system's ullage space. This is a low-pressure system that cannot hold any significant pressure. To remedy this, Veeder-Root has a patent pending on a method and apparatus that can be used to extract, compress and store the excess fuel/air vapor mix.

The problem for the workshop is to suggest and analyze a compressor device to show that safe vapor compression is possible. Since heat of compression can raise temperatures and thus increase the risk of reaching the ignition point of a fuel/air mix, a compressor must be designed and operated appropriately to prevent this from occurring. In the application, the need for the rate of extraction and compression of the vapor mix is very low, which works in our favor. As a result, a custom compressor design is required that operates at a very slow rate, thus minimizing temperature rise by providing ample time for thermal flow to cool the device and vapors within the device.

We can start with a simple compressor chamber, valving, piping, and materials concept, analyze the dynamic thermal operating conditions, use the pressure/temperature ignition curves for a worst-case vapor/air mix, and make recommendations based on the analysis for safe operation. Given a required volumetric or mass extraction rate, what is the proper tradeoff between compressor chamber size and cycle rate? Are cooling fins required, and if so what total area and configuration? Can the device be very small and inexpensive or is a larger, more massive (and more expensive) device needed to provide adequate thermal conduction to minimize temperatures? Can we stay with inexpensive, iron alloy based materials or are more exotic materials needed, again for better thermal flow properties? A successful result will describe a few alternative design concepts and show how much safety margin is attained for the given throughput and output pressure needs. Worst-case temperatures, hot spots, pressures, and associated ignition points will illustrate how well the design concepts can meet the required goals.

• Some Basic Parameters