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A condition that sometimes occurs during hot-weather months in suction pumping systems, and which interferes with the flow of gasoline through the system.
In a suction system, the action of the dispenser pump creates a vacuum that “pulls” gasoline from an underground storage tank and through the piping to the dispenser. Between transactions, the system remains “primed” with gasoline in the piping between tank and dispenser. If the driveway above the piping heats up to 100ºF or more, heat radiating into the ground will affect the piping and may cause the gasoline in the piping to vaporize. Vaporization leads to expansion, and the expanding vapor can create a bubble of vapor in the pipe line.
The vapor bubble, called vapor lock, may make it impossible to move gasoline through the system. When the pump is turned on, it reduces the pressure in the line. This, in turn, lowers the boiling point of vapor in the line and causes the vapor lock problem to worsen. Suction systems are especially vulnerable to vapor lock if the piping runs are excessively long.
Vapor lock can be avoided by burying the piping at a proper depth or by cooling the piping so that the gasoline that has vaporized will revert to liquid form.
While vapor lock is more prevalent in the summer months due to the increase in temperature it can also occur at any time and with any liquid. since you are pulling more vacuum than what the liquid can handle to stay in a liquid form without boiling. A general rule of thumb was to keep the calculated vertical lift below 15 ft., however with some of the newer fuels this has been reduced and will be on a case by case basis with the determining factors being type of fuel, temperature, and elevation above sea level.
A pressurized system is not subject to vapor lock.