A “time-fill” CNG station refers to one that dispenses CNG to a group of off-duty vehicles, typically over night in a 4-12 hour timeframe. Most time-fill CNG stations fuel light-duty or medium-duty service vehicles, delivery trucks, vans, and automobiles that arrive at a centralized hub after their day’s service is complete, and after maintenance or restocking is complete the vehicle sits in a lot waiting for the next shift. Time-fill is very popular in the refuse and recycle industry as well as municipalities and other fleets that have sufficient down time. Time-fill stations can also work for lower volume applications such as a home fueling appliance or small business appliance.
Unlike fast-fill stations, time-fill stations typically fill vehicles directly from the compressor, not from CNG stored in vessels at the station. Compressor size depends on the size and daily fuel requirements of the fleet. There is typically a small buffer storage vessel with time-fill stations but this vessel’s purpose is not to fill vehicle tanks, but rather serve to prevent the compressor from turning on and off more frequently thus wasting electricity and causing unnecessary wear and tear on the compressor. Another advantage of a time-fill CNG station is that the heat of recompression is less so you usually get a fuller tank then from a fast-fill station. Also, with vehicles fueling over night the station will be using electricity a lower off-peak rate. The equipment configuration and gas flow through time-fill equipment consists of the following:
Time-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).
A time-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 nominal 3600 psi (in today’s stations that fuel 3600 psi vehicle tanks). During a relatively hot day, the compressor may be allowed to reach higher pressures > 3,600 psi and on a relatively cold day, the compressor would be limited to < 3600 psi. Natural gas does not need to be compressed to a higher level as in fast-fill stations since equalization takes place over hours instead of minutes. This nominal 3600 psi CNG then enters a small buffer vessel to prevent the compressor from turning on and off more frequently thus wasting electricity and causing unnecessary wear and tear on the compressor.
To illustrate this think of blowing up a balloon. To get a full (3600 psi) fill on the vehicle tank (the empty balloon) you simply run the station’s 3600 psi compressor until the balloon (the vehicle’s tank) has reached the desired fill pressure and volume.
Dried and compressed gas then moves directly to a hose post and into the vehicle’s tank(s). Time-fill stations require only a set of posts each of which holds 1 to 4 hoses with nozzles to fuel parked vehicles.
As drivers connect to a time-fill posts to refuel, the lower pressure in the vehicle tank vs. higher pressure in the CNG supply lines allows equalization and fueling to begin. As additional vehicles hook up to their hose posts, CNG flow will gravitate to the lower pressure vehicle(s) in a way that all vehicles reach full fill at about the same time.
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