General questions (refilling, tank capacity, etc...)

[cngf250ut]

When I bought my truck I was in quite a rush. I was at an extremely busy dealership that specializes in CNG vehicles. People were buying them left and right and literally buying them after I had test drove the vehicle.

I bought a dedicated F250. I can't remember the tank capacity the salesman told me it had, but he said that I should easily get 225 miles -- as in that was his conservative guess. I can only get 200 miles on the highway. How do I find out how far I can go?

I don't know how to find out how much fuel I can put in there. How do I find out the capacity of the tank?

I noticed that my tank pressure is 3000 PSI, and sometimes the pump delivers more than that. To be safe, I always stop fueling at like 7 or 8 gallons. Even if I know the exact tank capacity, I have to wait until I'm 100% empty to fill to that amount. How do you top off the tank without damaging it?

I seem to get a disproportionate amount of milage per quarter of a tank. The first quarter of a tank is almost always gone after like 20 miles, the next after about 60-80, and so forth until it's empty. Why would that first quarter of a tank be consumed so quickly? Is it not really 'full' ?

I know there's tons of information on replacing tanks, but has anyone here done it successfully? Just a yes or no is what I'm looking for.

Thanks in advance.

[jetboatjohnny]

Re: General questions (refilling, tank capacity, etc...)

DONT ever stop the fueling. Let it go until it is finished. Your 3,000psi tank can handle the extra pressure. Just drive it until your fuel light comes on then fill it up and that should be about one gallon less than your capacity.
Always reset the trip meter to keep track of your miles. The gauge will always act the same, you will get used to it. I can tell how far I can go on the highway by how many miles until it hits the full mark, then about 80 till half, then 50 till the light comes on.
Tank replacement can be done by a qualified installer, extra tanks can be added for greater range, but it is not cheap.

[afvman]

Re: General questions (refilling, tank capacity, etc...)

cng250ut,

Lot of questions, all good ones.

Here's some answers:

1. All CNG cylinders are required to list the water volume capacity, usually in cubic inches. Look for the cylinder label. It'll give you a lot of information, (like the expiration date).

2. If you can find the water volume then use this formula to calculate the gasoline gallon equivalent:
Volume of Water Volume (V/V) divided by 5.861 (for 3600 psi) = std. cubic ft.
" " " " " " 6.657 (for 3000 psi) = " " "

3. scf divided by 123.2 cu. ft = gasoline gallon equivalent

Jetboat is absolutely right. Cylinders are designed to handle overpressure due to temperature variaition, but should never go over the Maximum Allowable Working Pressure (MAWP) which is currently 1.1 x 1.25 x the service pressure (or 1.38 x the service pressure), regardless of the temperature.

Ususally fuel gages are linear...gasoline gages aren't. So there might be problem with the pressure transducer that convert pressure to voltage for the gage.

Adding or replacing tanks are both doable, like John says, by a qualifited CNG technician.

Hope this helps,

afvman/Bill

[Jennifer]

Re: General questions (refilling, tank capacity, etc...)

I noticed that my tank pressure is 3000 PSI, and sometimes the pump delivers more than that. To be safe, I always stop fueling at like 7 or 8 gallons. Even if I know the exact tank capacity, I have to wait until I'm 100% empty to fill to that amount. How do you top off the tank without damaging it?
.

CNG is delivered at temperature compensated pressures. If you are filling on a 3000 psi hose, you may get more or less depending on the temperature outside. You tanks take 3000 psi at 70 degrees. When its cold, you'll get less pressure. When its hot, you'll get more pressure. It will be the same amount of gas either way, because CNG is measured by mass.
I wouldn't worry about starting and stopping the pumps manually, the station controls should do it. IF your tank should somehow overfill (which is EXTREMELY UNLIKELY) there's also a Pressure Relief Device that will open to vent the excess pressure.

[cngf250ut]

Re: General questions (refilling, tank capacity, etc...)

So wait a minute...

You guys are telling me that even if my tank pressure is 3000 PSI, and I'm getting 3500 at the pump, just keep going until it stops? Does it ever stop on its own the way gasoline does? I've filled when it was 2500 PSI and it just starts to taper down infinitesimally slow so I just get impatient and think that's all I'm going to get that day.

Seriously? Just fill it 'till it stops, regardless of the pressure my tank can take?

[jetboatjohnny]

Re: General questions (refilling, tank capacity, etc...)

YES, we alway fill our tanks until the pump stops. Like Jennifer said, the pressure is set to compensate for temperature. The fuel heats up when you are filling your tank, when the fuel cools down the pressure drops in your tank. When I need a little extra range I can hit my local fuel station on my way out of town and get 1-1.5 gge after driving 8-10 miles.

[Larryect]

Re: General questions (refilling, tank capacity, etc...)

Yes, as long as you are using a pump rated at 3000 psi. 3000 X 1.1 = 3300, 3000 x 1.25 = 3750.

The gas heats as it is compressed and then cools in the tank and with usage. That is why your first 1/4 tank seems to go faster. As the gas cools, pressure goes back down.

The pumps I am familiar with in CA typically have a percentage fill read out that will flash (blink) when it decides it has filled as much as it can.

[CNGfamily]

Re: General questions (refilling, tank capacity, etc...)

Seriously? Just fill it 'till it stops, regardless of the pressure my tank can take?

That's the consensus. Clearly a system designed for 3600 PSI would have been tested at higher pressures than a 3000 PSI system. What everyone is telling you is that considering the factors of safety involved, there is essentially no risk letting the pump shut off normally during the fill.

[Larryect]

Re: General questions (refilling, tank capacity, etc...)

Another point:

If the fill has slowed that much at 2500, it could be that is all that is available at the time you are filling.....

Sometimes the stations are not able to keep up with demand.

[ch4]

Re: General questions (refilling, tank capacity, etc...)

From your name I'm guessing that you're in UT.

Most Questar pumps are set at 3200 so either 3000 or 3600 cars can fill.

In reality, (please do not take this as advice,) most people fill their cars with either.
I've a friend w/ a 3000 Crown Vic and he always fills it full even in Park City which may have the highest pressure in UT.

[jetboatjohnny]

Re: General questions (refilling, tank capacity, etc...)

It's not unusual to get more than 3,000psi from a 3,000psi pump, I got 3,400 at the pump in Lancaster Saturday, and when my local pump was busy with a bus filling I got 3,500 on Wednesday.

[Devo]

Re: General questions (refilling, tank capacity, etc...)

UT does not have 3000 and 3600 psi pumps, they've decided to take an average approach and put ~3300 psi at just about all of their stations.

I won't tell you where to stop or keep going (that's your call), but I will tell you that I have a 2000 bi-fuel Ford Contour (3000 psi system) which I've filled to nearly 3800 psi at the Flying J in Springville, UT (back in the good ol' days). This was before I realized it was only a 3000 psi system, so I didn't stress being a couple hundred psi over what I thought the rating was. I was on the road right away burning it off and it didn't bother me much.

Now if I was filling with a slow-fill station that would be something different. I hear that there is more stress on the tank using a slow-fill station, so I personally wouldn't go much over 3000 psi. But that's just my personal opinion ...

[CraziFuzzy]

Re: General questions (refilling, tank capacity, etc...)

Now if I was filling with a slow-fill station that would be something different. I hear that there is more stress on the tank using a slow-fill station, so I personally wouldn't go much over 3000 psi. But that's just my personal opinion ...

I find it hard to believe that a slow fill would be harder on the tank than a fast fill, just the opposite i would guess. Only difference is in the end, if they both shut off at the same pressure, a slow fill will actually give you more fuel, as it has time for the heat of compression to dissipate.
I do know people that have replaced their NGV1-30 on their 3000# car with an NGV1-36 to allow using a Phill (Definately not recommended, but Fuelmaker's response to requesting a 3000# Phill is to buy a new civic gx). They still only use the 3000# side when at a fast-fill site though, because it seems that would be more stress.

[jetboatjohnny]

Re: General questions (refilling, tank capacity, etc...)

It makes sense that a tank that sees a life of slow fills will be under a lot less stress even though the pressures encountered may be more than a tank with a life of fast fills.