Gas Stations

A typical gasoline station has a storage capacity of 30,000 to 40,000 gallons in underground tanks. In the past, these tanks were sometimes subject to spills from overfilling and to leaks caused by corrosion. Today, station owners have taken several important steps to reduce these risks.

Overfill Protection Devices

Sensors and alarms let the operator know when the tank is getting full, and automatic shut-off switches stop fuel from being pumped into the tank before the tank completely fills.
An electronic alarm, triggered by a floating sensor within the tank, activates a warning light and/or sound to tell the operator when the tank reaches 90 percent capacity – the target fill level.
As a backup, a so-called flapper valve attached to the delivery pipe uses a floating arm to trigger a shut-off valve when the petroleum product in the tank reaches a certain level, similar to the shutoff device of a toilet.
A ball float valve consists of a ball floating on top of the petroleum product while inside of a cage, which is attached to the end of a ventilation pipe. As the product level rises, so too does the ball, until it is raised to the bottom of the vent, blocking the vent and restricting outward vapor flow before the tank is full. When closed, this valve can create enough pressure to stop the flow of product into the tank.

Cathodic Corrosion Prevention

When in contact with moisture in the air or ground, steel slowly rusts, causing corrosion of metal storage tanks and pipelines. When the moisture combines with the carbon dioxide in the air, a weak carbonic acid is formed, which dissolves the steel tanks or pipelines, appearing as rust. The application of a small positive electrical charge to the tank helps prevent this corrosion process.

New Tank Materials/Coatings

New fiberglass tanks and steel tanks coated with fiberglass or other durable casings help prevent corrosion caused by underground moisture. The same high-tech coatings and linings also protect the nation's pipelines and above-ground storage tanks.

Catchment Basins

All tanks are equipped with large "buckets" located around the fill pipe, which catch any motor fuel that may spill when the delivery hose is disconnected from the fill pipe.

Leak Detection Devices

Sensors can detect even small leaks in underground storage tanks and piping. An automatic tank gauging system monitors the volume of petroleum product within a storage tank versus the amount of product dispensed to consumers.

Double-walled tanks provide an additional measure of spill protection – the space between the inner and outer walls is filled with brine. A float sensor can detect any increase in the brine level that results from product escaping the inner wall.
Line leak detectors use a spring-loaded arm to test the pressure in the pipes carrying petroleum products from the tank to the dispenser. If the line pressure is low, which could possibly be due to a leak, the testing arm makes contact with the sensor pin below, triggering an alarm.