CNG operators are familiar with “fast-fill” CNG stations (fills vehicles in minutes) vs. “time-fill” CNG stations (fills vehicles in hours). A relatively new term, “ultra-fast-fill”, refers to stations that dispense CNG at a rate similar to what truckers experience when they refuel at an ultra-fast Diesel at a truck stop. These flow rates usually range between 10-15 GGE per minute with peak flow (during the first minute filling an empty vehicle tank) of 20-40 GGE per minute. At the higher end of the peak flow rate of ultra-fast-fill CNG stations, filling time is restricted by the receptacle, piping and components on the truck’s fueling system.
Ultra-fast-fill CNG stations are preferred for fueling heavy-duty class 7 and class 8 tractor-trailers that arrive at random times and need to fill up very quickly to get back on the road to make deliveries. Ultra-fast-fill equipment configuration and gas flow through this equipment consists of the following:
Ultra-fast-fill CNG stations receive natural gas via pipeline from the local distribution company (LDC) to the CNG station site at a low pressure (typically 5-100 psi).
An ultra-fast-fill station will typically include a dryer to remove moisture from the natural gas. You don’t want to compress a “wet” gas because this moisture can find its way downstream within the station piping network or even into the vehicle as the high-pressure gas condenses as it enters to equalize with the lower-pressure storage tank on the vehicle which could then enter the fuel lines leading to the engine.
Once dried, the natural gas then enters one or more compressors where the gas is compressed to a typical value of ~ 4,200 psi (in today’s stations that fuel 3600 psi vehicle tanks). Natural gas is compressed to a higher level than the vehicle tanks due to the act of equalization during the filling process. To illustrate this think of two balloons, one full of air and the other one empty, and you seal the mouths of the balloons together. The full balloon will deflate and the empty balloon will inflate until both balloons are equalized in pressure. To get a full (3600 psi) fill on the vehicle tank (the empty balloon) you need to have the pressure of the full balloon (the station’s CNG supply pressure) higher than the vehicle’s tank pressure.
Dried and compressed gas then moves through a “priority panel” which is the mechanical control center for the station. This panel senses the pressure in the station’s storage vessels (see below) and uses solenoid valves to switch and direct the dried and compressed gas from the compressor(s) into each station storage vessel as gas is depleted during fueling operations. The priority panel then switches and directs gas from each station storage tube into the lines that supply each CNG dispenser depending on the pressure of the vehicle being refueled.
As mentioned above, following the priority panel will be a series of storage vessels typically configured as “low bank”, “medium bank” and “high bank”. CNG at ultra-fast-fill stations is stored in each of these vessels, all at a high pressure (~ 4,000 to 4,500 psi), to supply CNG fuel to the dispenser and vehicle. Flow rates utilizing these station storage vessels is much faster than if the CNG was coming directly from the compressor which is limited to compressor size and flow rate. From the storage vessels, CNG flow is only limited by the size of the piping from the vessels to the dispenser, fueling hose and vehicle’s fuel receptacle.
As truckers connect to an ultra-fast-fill CNG dispenser to refuel, sensors in the dispenser determine pressure in the vehicle tank vs. pressure in the CNG supply lines to begin equalization and fueling. As the vehicle’s tank nears equilibration with the station’s low-bank storage vessel (let’s say ~ 1,500 psi in each), flow begins to slow down. This slow down in flow rate is sensed by the dispenser which sends a signal to the priority panel to shut off supply from the low-bank and direct CNG from the medium bank to the dispenser. The medium bank is still 4,000-4,500 psi so it equalizes with the vehicle tank which is ~ 1,500 psi resulting in a new equilibrated pressure of ~ 2,700 psi when flow begins to slow down again. The priority panel shuts off supply from the medium-bank and directs CNG from the high bank to the dispenser where equalization to 3600 psi in the vehicle’s tank is completed. Public-access fast-fill dispensers are temperature- and pressure-compensated to DOT standards to make sure the GGE dispensed is accurate to within .01 pounds (1 GGE of CNG weighs 5.66 pounds). Midwest Energy Solutions provides equipment and Technicians to certify CNG dispensers for our stations as well as stations owned by others.
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