Asphalt Tank Gauging Applications
FuelsManager® and radar tank gauges provide an accurate, repeatable and reliable level measurement solution

Asphalt can be considered the most difficult level application in a refinery due to its properties and the nature of its production. The composition of asphalt is a mix of solids (aggregates) with a liquid additive, producing a sticky and highly viscous substance. To manage the product as a liquid, the tank must be maintained at high temperatures (350 to 400°F). This means the atmosphere inside an asphalt tank is composed of product vapor, air and water vapor, which condensates and solidifies around the tank, adhering to any instrumentation that penetrates the tank.

Traditionally, three types of technology are used to measure asphalt - mechanical float and tape, hydrostatic pressure and ultrasonic devices. Each has its own specific problems that must be considered for the application. The primary concern, regardless of the chosen technology, is the loss of measurement signal that may cause the tank to overflow. Radar technology addresses the problems associated with asphalt tank gauging if the correct antenna and tank seal material are selected.

Temperature
Product temperatures can reach over 350°F, which causes the ambient temperatures in the tank to approach 150°F. The use of a high temperature seal in conjunction with a specialized heat shield eliminates any effect high temperatures could have on the electronics of the radar gauge. The physics of the radar signal are not affected in any appreciable amount by temperatures in this range.

Viscosity, Adhesion and Coating
As the radar is a "non-contact" technology installed on the tank roof, the antenna is not in direct contact with the product. However, asphalt vapors tend to solidify on everything within the tank (see above), including gauging devices. Although coating of the antenna is a problem, it does not affect the transmitted or received signal of a radar gauge if the operating frequency is in the lower ranges. For example, devices operating at frequencies over 20 GHz are more susceptible than devices operating at 10 GHz or lower.

Condensation
The humidity level in a typical asphalt tank is very high and, as the vapor comes in contact with the cooler antenna, condensate forms, producing water droplets along the rim of the antenna. These droplets can cause severe reflections in the signals of both ultrasonic devices and radar gauges, which would give misleading measurements.

One of the key factors in any radar gauge application is the selection of the correct antenna and asphalt measurement is no different. A parabolic antenna overcomes the problems of condensation in two ways:

1. The ratio of the circumference to surface area of a parabolic dish is much smaller than other radar antennas. Therefore, the parabolic antenna produces a stronger signal with less noise for the radar's electronics to analyze.

2. The shape of the parabolic antenna is designed so that only a signal perpendicular to the dish will be reflected back to the focal point of the antenna. Any misleading signals reflecting around the tank due to the water droplets are unlikely to reflect perpendicular to the dish with any consistency.

Repeatability with Accuracy
The Varec Radar Tank Gauge with a parabolic antenna offers a unique solution to gauging asphalt tank level due to the design of a parabolic antenna and the 6 GHz operating frequency. The two-wire intrinsically safe design allows for simple installation with 10 mm accuracy. It meets the relevant inventory management requirements according to OIML R85 and API 3.1B. It has been in service and performing as required with no cleaning or maintenance since it was first introduced into asphalt applications by Varec in June of 2001.

Asphalt Calculations
Once the level is measured, FuelsManager® utilizes an industry standard ASTM calculation for asphalt applications ( ASTM D4311-04 °C, ASTM D4311-04 °F, ASTM-IP Table 7).