Fuel is drawn into it through a metal strainer and flows from the turbine around the outside of the motor casing and through a section of pipe to the discharge manifold (head), which usually sits on top of the tank. The system check valve and the air eliminator are located inside the discharge head (manifold). The air eliminator functions in much the same way as its counterpart in a self-contained unit: air and vapor, along with a small quantity of fuel, are separated from the main body of fuel and drawn through an orifice. But in this type of system, the gases and fuel drain directly back into the storage tank; the tank itself functions as the atmospheric chamber or sump does in a self-contained unit. In remote systems, several dispensers are often served by the same pump. When the on-off switch at any of these dispensers is placed in the on position, the pumping unit is activated, and all pipelines connected to the pump are pressurized (to 24-28 psi for a typical unit). To prevent the discharge nozzles of all the dispensers from being pressurized, each dispenser is equipped with its own control valve. This control valve must not be confused with the control valve in a self-contained system: its design and function are quite different. The control valve in a remote system is not automatic: it is actuated by the dispenser's on-off switch, or is controlled by the electronic computer system. Figure 3-6 illustrates how a typical control valve works. The control valve itself usually consists of a piston and cylinder (or diaphragm) and a spring, as shown in the cutaway drawing. A removable filter/strainer at the inlet to the valve (or at the fuel inlet to the dispenser) traps solid contaminants in the fuel flow before they enter the valve. When the piston is retracted, the valve is open and product flows through it toward the meter; when the piston is seated, product cannot pass through the valve.
In older systems, the control valve may be operated mechanically, by means of a linkage between the valve and the dispenser on-off switch. Today, however, a more sophisticated electrical operator, including a solenoid and pilot valve, has been incorporated in most designs, as depicted in Figure 3-6. In some systems both the main valve and the pilot valve are incorporated into the same valve body. The device operates the valve by regulating the fluid pressure in the cylinder behind the piston or diaphragm (the area marked “B” in the drawing). When the dispenser is not being used (that is, the dispenser switch is in the “off” position), the solenoid is not energized and the pilot valve rests in such a position as to connect fluid lines (1) and (2). This maintains equal pressure on all sides of the valve piston, allowing the spring to keep the valve tightly closed.
When the dispenser switch is turned “on” or when the electronic computer sends power to the valve, the solenoid is energized, opening the pilot valve. This has the effect of opening a passageway between lines (2) and (3), and simultaneously closing off line (1). As long as the discharge nozzle remains closed, pressure remains the same in regions (A), (B), and (C), and the control valve remains closed. However, when the nozzle is opened, pressure at the outlet (C) falls, bleeding pressure from the space behind the cylinder (B). Pressure is now higher at (A) than at (B), and pressure on the head of the valve piston pushes it off its seat. When the dispenser is shut off, the solenoid is once again de-energized, closing the pilot valve and thereby shutting off the connection between (2) and (3) and opening the connection between (1) and (2). When pressure is once again equal at (A), (B), and (C), the spring (or diaphragm) forces the piston onto its seat, closing the valve.
The control valve prevents a remote dispenser from delivering product unless it has been switched on, and thus prevents accidental discharge under normal conditions. However, if a remote dispenser were to be struck with sufficient force, as might happen if a moving vehicle collided with it, even at relatively slow speed, piping in the dispenser could quite easily be ruptured. Since the fuel entering the dispenser is pressurized, this situation could cause fuel to flow uncontrollably from the ruptured pipe, creating an extreme safety hazard. To prevent this, every remote dispenser is equipped with an automatic emergency shut-off valve, also called an impact valve or shear valve because of its function, or a fire valve (as shown in Figure 3-7).
FIGURE 3-7. REMOTE SYSTEM EMERGENCY SHUT-OFF (IMPACT OR FIRE) VALVE
This valve is located at the bottom of the dispenser, connected to the pipeline at the point where it enters the unit. The operation of this valve is very simple. In the event of damaging impact, the top portion of the valve shears away, isolating the damaged dispenser, and a spring valve automatically closes off the pipeline, preventing any further flow of fuel from that source. If it has not been too severely damaged by the collision, the control valve in the dispenser should then respond to the sudden drop in system pressure and close automatically, keeping fuel loss from the dispenser also to a minimum.
Self-contained systems do not require an emergency shut-off valve because the fuel dispensers are pressurized inside the dispenser. So, a damaging collision will not result in uncontrolled flow from the storage tank pipeline, since suction pressure will cease immediately when a severe rupture occurs.