Dispenser Machine Service Details



What the motorist sees of a modern fuel-dispensing system is very impressive—a metal cabinet, extending perhaps eight feet in height, lighted price and volume indicators, a length of hose or hoses, with nozzles on the end, push buttons, possibly a card reader, maybe even a small TV screen and instructions for operating the device. While the outward appearance may be impressive, the reality of the dispenser hydraulic, mechanical and electronic complexity is hardly recognizable. Opening the cabinet reveals a profusion of components, tubing, wiring, gears, and linkages, enough to daunt even the mechanically or electronically minded amateur. In this chapter, you will learn to recognize these parts, and gain a basic knowledge of how they work together. The fuel-dispensing system performs several interrelated functions. The purpose of some components is to maintain hydraulic continuity, to regulate the direction of flow and fluid pressure. Others are responsible for pressurizing the fuel and moving it through the system. Still others, of course, are involved in metering the liquid fuel, registering accurately the quantity delivered, and computing the price of the delivery. Finally, some components serve to control the operation of the system, switching it on and off, resetting the volume and price indicators, regulating the delivery, and so on. We will proceed to look at the major components in each of these functional areas.


Fuel-dispensing systems are designed to measure, register, and deliver accurately the desired quantities of fuel product. The customer is paying for motor fuel, not air or fuel vapor. So these gases must be prevented from entering the dispenser's metering device. In addition, in order to be able to lift the fuel efficiently from an underground storage tank to the dispensers—especially in a self-contained system—the pipelines must be essentially free of air and vapor. RMFD (Rev-7-03) Chapter 3 Page 3-1 The most practical means of eliminating these gasesisto keep the entire system—from the storage tank to the delivery nozzle—filled with liquid fuel at all times, even when the system is temporarily idle, as it is between deliveries, or when the station is shut down overnight. This could be accomplished in a number of ways, but the most practical and efficient design incorporates a simple automatic valve, called a check valve. In the cutaway drawing shown in


In a fuel-dispensing system, a check valve permits liquid fuel to flow toward the delivery nozzle, but never back toward the storage tank. A check valve is located between the storage tank and the dispenser (as shown in Figure 3-2), usually close to the outlet of the storage tank, either at the angle joint where the pipeline drops vertically into the tank (self-contained systems), or inside the discharge manifold that sits atop a submerged pumping unit. (In some self-contained systems, a foot valve, located at the bottom of the intake pipe, performs the function of the check valve.)


As we proceed through the system, you will see other automatic valves that operate in much the same way as these system check valves, functioning to regulate the direction of flow or fluid pressure of the product as it makes its way toward the meter and discharge hose. You will also see how a similar valve, located at the opposite end of the system, prevents fuel from draining from the discharge hose, assuring delivery of the full metered amount of fuel.


In retail fuel-dispensing systems, a motor-driven pump furnishes the hydraulic pressure that movesfuel from the storage tank to the dispenser, through the metering device, and to the discharge hose and nozzle. As you know, two basic types of systems are in common use—self-contained and remote pump systems. Most of the systems you will encounter today will be remote pump systems. These lend themselvesto larger multiple dispenser installations. However, let us take a look at each type. Self-contained Systems In self-contained systems, the pumping unit is located inside the dispenser. Its major components are: • a pump • an electric motor • an air eliminator • flow regulating valves Several manufacturers produce a self-contained unit, which includes the pump, air eliminator, valves, and flow passages connecting them, all enclosed in a single casing, as in Figure 3-3. The electric motor is separate, and drives the pump by means of a belt and pulleys. FIGURE 3-3. SELF-CONTAINED PUMPING UNIT AND MOTOR RMFD (Rev-7-03) Chapter 3 Page 3-3 The operation of a typical self-contained pumping unit is illustrated in Figure 3-4. When the dispenser's onoff switch is placed in the “on” position, the electric motor is activated, and begins to turn the rotary-vane pump (other pump types may be used). As the pump turns (clockwise in Figure 3-4), it propels fuel forward from its outlet. This displacement of the liquid creates a partial vacuum at the pump inlet.