The gas pump
Road Master
Joined: Aug 2006
Posts: 1,189
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From: Massachusetts
The Blend Valve
Safety Measures
As the customer pumps the gas mixture of his or her choice into the car's gas tank, any number of accidents could happen -- most of them attributable to human error.
If an absent-minded customer drives away with the nozzle still inserted in the tank, the hose is designed to break into two pieces. One remains with the car and the other with the dispenser. Check valves on both sides of the breaking point prevent fuel from leaking out of either half.
One of the first things that a customer will notice at the pump is the variety of choices offered. In most cases, a dispenser will offer several grades of gas -- sometimes as many as five -- each with a different octane rating. The desired octane rating is usually chosen simply by pushing a button. Does this mean that there are five different underground tanks feeding into that dispenser, each offering a different grade of gas? That's not usually the case. In fact, the dispenser can produce as many grades as it wants from as few as two underground tanks, as long as one tank contains the highest grade of octane available at that station and the other contains the lowest. The grades are blended together at the pump -- not unlike the way you'd blend gin and vermouth to make a martini -- producing a kind of octane cocktail. The precise proportion in which the grades are blended determines the octane of the gas that enters the customer's tank.
This feat of gas pump bartending is performed by something called a blend valve. This valve has inputs consisting of two grades of gasoline, each from different tanks. A single, moveable barrier called a shoe is connected to both in such a way that it can be moved across the inputs with a single motor-driven ratchet. As the ratchet opens one valve, it closes the other valve in precise but opposite proportion. This means that when one valve is, for example, 90 percent open, the other valve is 10 percent open, creating a mixture that consists of 90 percent of one octane and 10 percent of the other. By shifting the ratchet back and forth, the blend valve can produce any octane of gas, ranging from the highest to the lowest grades stored in the tanks -- and all octanes in between
Safety Measures
As the customer pumps the gas mixture of his or her choice into the car's gas tank, any number of accidents could happen -- most of them attributable to human error.
If an absent-minded customer drives away with the nozzle still inserted in the tank, the hose is designed to break into two pieces. One remains with the car and the other with the dispenser. Check valves on both sides of the breaking point prevent fuel from leaking out of either half.
One of the first things that a customer will notice at the pump is the variety of choices offered. In most cases, a dispenser will offer several grades of gas -- sometimes as many as five -- each with a different octane rating. The desired octane rating is usually chosen simply by pushing a button. Does this mean that there are five different underground tanks feeding into that dispenser, each offering a different grade of gas? That's not usually the case. In fact, the dispenser can produce as many grades as it wants from as few as two underground tanks, as long as one tank contains the highest grade of octane available at that station and the other contains the lowest. The grades are blended together at the pump -- not unlike the way you'd blend gin and vermouth to make a martini -- producing a kind of octane cocktail. The precise proportion in which the grades are blended determines the octane of the gas that enters the customer's tank.
This feat of gas pump bartending is performed by something called a blend valve. This valve has inputs consisting of two grades of gasoline, each from different tanks. A single, moveable barrier called a shoe is connected to both in such a way that it can be moved across the inputs with a single motor-driven ratchet. As the ratchet opens one valve, it closes the other valve in precise but opposite proportion. This means that when one valve is, for example, 90 percent open, the other valve is 10 percent open, creating a mixture that consists of 90 percent of one octane and 10 percent of the other. By shifting the ratchet back and forth, the blend valve can produce any octane of gas, ranging from the highest to the lowest grades stored in the tanks -- and all octanes in between
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