Natural Gas Vehicle Refueling Stations

NGVs store natural gas in high pressure storage tanks. These are often located in the trunk area of a car. Our service vehicles have storage tanks mounted in the bed of the trucks. Buses may have them located on the roof of the bus.
No matter where the tanks are located, their purpose is to hold compressed natural gas (CNG) at pressures up to 3,600 psi. The reason for this high pressure is to provide as much natural gas possible for added driving range. It's like blowing up a balloon. The more air put in, the higher the pressure within the balloon. Unlike a balloon, compressing natural gas is a bit more complicated!

Imagine being able to refuel your car at home! No more running to the local gas station and spending tons of money to fill the family chariot!
Today, thanks to companies like FuelMaker, you can refill your NGV right at home.
FuelMaker manufactures a home refueling appliance that will take the natural gas from your local distributor and compress it directly into your vehicle. It is simple to install, simple to operate and simply great for the average home owner that has had enough of paying at the pump.
Take a look at their newest product called the Phil. This little self-contained unit will fill your NGV overnight so you are ready to roll in the morning.
Want to learn more? Visit www.FuelMaker.com

The equipment required at a public refueling facility is much different than the home refueling option illustrated above. Public refueling requires fast fills and the ability to calculate how much natural gas is transfered into each individual vehicle.
A fast fill station has the following components:
- An Inlet Dryer
- Compressor Package
- Above Ground Storage
- Priority Sequential Controls
- A Dispenser and Card Reader
Gas is provided to the station from the local utility. It enters an inlet dryer to reduce the mositure content of the natural gas. It then is compressed to pressures up to 4,900 psi and stored in above ground vessels that maybe arranged in a three bank cascade. This storage cascade will allow a vehicle to fill first from the low bank, then the medium bank and finally the high bank. Conversely, the compressor will fill storage from the high bank to the low bank to assure that there is always enogh pressure available to completely fill a vehicle.
The size of all this equipment depends on the anticipated duty the station will see.

Think about it! Water comes into your home from the street and you have to heat it for use in the shower. The rate at which it is heated and the amount that you store will depend on your family's specific demand. My kids stand in the shower forever making it impossible for me to get a hot shower! I have a couple options:

1. Get a bigger burner for my water heater.
2. Get a bigger storage tank so I can hold more hot water or
3. Ask the kids to move out!

An NGV refueling station works off of the same principal of balancing delivery with storage. So how big a station do you need?

A good rule of thumb is 125 scf (standard cubic feet)of natural gas = 1 gallon of gasoline. So, if you need 10 equivalent gallons of gasoline, you will need 1,250 scf of natural gas. Need 20 equivalent gallons? Then you need 2,500 scf (20 X 125 = 2,500) Pretty simple!

Where it gets a little complicated is balncing storage with compression to satisfy the number of vehicles using the station in the time frame that they will be there to fill.

For example: if you know you have ten vehicles in your fleet that need 10 equivalent gallons each and they will be in to fill over an eight hour period, then we can calculate the compression size needed.
10 vehicles X 10 gallons X 125 scf/gallon = 12,500 scf needed.
12,500 scf/8 hours/60 minutes per hour = 26.04 scfm (standard cubic feet per minute)

Well, at 26 scfm delivery from a compressor (without storage) it would take about 48 minutes to fill each vehicle. At 52 scfm it would cut the refueling time in half; 24 minutes. Still too long. So we add storage.

If we have enouigh storage available, each vehicle can come in and fast fill without any waiting. When the pressure in storage drops, the compressor will kick on and fill up the storage vessels.

Storage for NGV Refueling Stations must meet ASME Code and are located at the fueling site according to NFPA-52.

Storage can take the form of long, cylindrical vessels such as the ones made by CP Industries. CPI makes seamless vessels with forged ends. Take a look at their web site -

http://www.cp-industries.com/prod02.htm

Each one of these vessels holds 10,000 cubic feet of natural gas at 4,900 psi. So if we have a three bank cascade, we would store 30,000 cubic feet of natural gas. But keep in mind, that not all of that gas is ready to transfer into vehicles at the 3,600 psi required. In fact, a three bank cascade holding 30,000 cubic feet will have only about 40% (or 12,000 scf) avaialbe for immediate use.

Another storage option are storage spheres. We like to get ours from Allied Equipment in Odessa, Texas. Here is a link to their site -

http://www.alliedeq.com/home.htm

Allied manufactures 48" diameter spheres that will hold 11,650 scf of natural gas at 4,500 psi.

Let's look at compression and specifically the Ingersoll Rand compressor packages of which I am most familiar.

Both the IR20H40NG and the IR05H25NG are reciprocating , positive-displacement compressors. Gas is confined within a cylinder where the volume is reduced by the action of compression. A piston moving in a reciprocating motion within the cylinder compresses the gas. As the volume decreases the pressure increases. The gas is then pushed out of the cylinder to the next stage of compression or to the discharge connection.

Each compressor includes a LOL (low oil level) switch and crankcase heater.

The Ingersoll Rand natural gas compressors are four stage units, with the first and second stage sharing the same cylinder. For the 20H40NG compressor, interstage pressures are as follows:

Stage 1 - inlet 20 psig to 200 psig (1.4 kg/cm2 to 14 kg/cm2)
Stage 2 - 800 psig (56 kg/cm2)
Stage 3 - 2400 psig (169 kg/cm2)
Stage 4 - 5800 psig (408 kg/cm2)

Ingersoll Rand incorporates these bare blocks into pre-packgaed modular compressor systems engineered specically for natural gas. Pre-packaged design means that all the components needed to operate the compressor system are included on one common steel skid. There are only three connections to made in the field. Inlet Gas, Electric, and Outlet Gas.

The Ingersoll Rand modular design means more flexibility in the field. As station demand increases, additional compression modules can be added. This is the plan at the Oakford Storage Pool in Pennsylvania pictured here. The pad has been sized to accommodate three more packages should the demand warrant the increase. Common controls allow for non-complicated lead lagging of compressors.

Another advantage to the modular concept is the ease of dividing up a larger station into smaller stations. At one site, the owner found that there was not enough traffic to warrant a large facility. Both the compression and the storage was removed and placed at another site.

Ingersoll Rand offers Four Compressor Packages -

05H25NGSX (simplex) - 5 psi inlet, 28scfm capacity, up to 5,000 psi discharge, one (1) 25 HP electric motor drive

05H25NGDX (duplex) - 5 psi inlet, 56 scfm capacity, up to 5,000 psi discharge, two (2) 25 HP electric motor drives

20H40NGSX (simplex)- 20 psi inlet, 58 scfm capacity, up to 5,000 psi discharge, one (1) 40 HP electric motor drive

20H40NGDX (duplex) - 20 psi inlet, 116 scfm capacity, up to 5,000 psi discharge, two (2) 40 HP electric motors.

This offers a refueling range between 13.5 equivalent gallons per hour to 56 equivalent gallons per hour. When two 20H40NGDX packages are combined capacity increases to 112 equivalent gallons per hour.

Drawings and specifications for these packages can be found on our web site. Please visit -

http://www.mckenziecorp.com/compression_packages.htm

Once the gas has been compressed and stored, it must then be delivered to the vehicle. There are two ways to do this.
If you are not metering how much gas is put into the vehicle, then a fill post assembly is what you are looking for. These are exactly what the name implies. A post with a fill hose attached. They include a break-away incase a driver forgets to disconnect before they drive off. They also include an approved connector, such as a Staubli or Sherex, that allows you to connect to the NGV for fueling.

Dispensers are more involved since they actually meter how much natural gas is transfered into a vehicle. We like the dispensers made by Tulsa Gas Technologies. Here is a link to their site -

http://www.tulsagastech.com/