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Old 19-06-2007, 17:05   #1
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Compress Gas to Propane Conversion

Has anyone converted from compressed gas to propane? Do you just change the fittings. Is there a conversion kit avaiable?

Any suggestions?
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Old 19-06-2007, 22:19   #2
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I don't understand the question. Propane IS compressed gas.....( when stored in the gas bottle).
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Old 19-06-2007, 23:39   #3
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I think it's referring to what we would know as CNG vs LPG. CNG is in gas form LPG is liquid.
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Old 20-06-2007, 03:53   #4
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A qualified gas fitter can convert from CNG to LPG by exchanging the regulator, appliance orifice(s), and a valve assembly.

CNG is natural gas liquefied under high pressure.(approx. 2250 psi - 155 bar).

LPG (Liquefied Petroleum Gas) is really two different gases that are generally classed together and are interchangeable. Propane and butane are both used in LPG appliances and have some advantages over CNG.

LPG is a gas at normal temperatures and pressures (the boiling point of propane at atmospheric pressure is about -45 deg. C). When subjected to modest pressure or cooling it becomes a liquid (116 psi - 8 Bar). The pressure in an LPG storage tank keeps LPG liquid, and it becomes a gas only when released from the tank.

CNG (Compressed Natural Gas)
LPG has a much higher heat output (21,000 BTUs/lb. for LPG vs. 9,000 BTUs/lb. for CNG) LPG also operates at much lower pressures.
A cubic foot of LPG (propane) contains approximately 2,500 BTU, whereas a cubic foot of LNG (natural gas) contains roughly 1,000 BTU of energy.
(BTU = British Thermal Units)

One thing to note, if you boat in cold weather, propane is preferable to butane, as butane has difficulty vaporizing in extremely cold weather and might not allow an appliance to work properly.

One drawback to LPG is that fumes are heavier than air (and can build up to dangerous levels in bilge compartments), whereas CNG is lighter than air.
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Old 20-06-2007, 05:22   #5
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The conversion of your equipment as noted is not that difficult. Much as you can convert your home appliances from "natural gas" to propane the process is similar from CNG to LPG.

The real difficult chore in the conversion is to design and construct a propane locker. This needs to be a compartment that vents and drains out of the boat with no connection to your space below deck. Propane is heavier than air and would settle in the bilge of your boat if it ever should leak. This may be unlikely to happen but if it did the consequences would exceptionally explosive and deadly. That is why it needs it's own locker. You then must run a solenoid control valve so you can electrically shut the propane on and off in addition to the shutoff valve like you see on your back yard grill. Companies like Sintex sell a kit that also includes sensors to detect a leak below and shut off the solenoid as well.

As you can see the retrofit includes a lot more safety items than you might think, but these are and have been accepted as standard method for using propane aboard for many years. You don't hear much about propane explosions on boats and this is why you don't. The explosion from such a leak would make very small pieces of any boat and send them all flying in all directions in a ball of flames. I doubt that the bodies would be recovered.
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Old 21-06-2007, 07:29   #6
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I've given some thought to the propane locker and agree it is a good idea. However the hose or line that routes to the stove is a weak point in isolating the propane from the cabin, isn't it? Also the stove valves themselves could leak.

What does this isolated system look like or am I missing something?

What are most people using? Propane makes sense as it is readily available anywhere in the world and thermal qualities are great as has been pointed out.
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Old 21-06-2007, 08:33   #7
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Propane Systems ~ by Don Casey
Propane Systems by Don Casey

Marine Propane Systems: Recommended Installation Instructions ~ by Xintex
http://www.fireboy-xintex.com/manuals/PropaneManual.pdf

Propane Installations:

Propane Tanks
The best place for propane tanks is on deck. If you want them out of sight, the locker you put them in must be vapor tight and separated from the boat's interior. It must open only above deck and have a drain at the bottom to let leaking gas escape over the side. Be sure the drain outlet is not located near any other hull opening and that it is always above the water, even when the boat is heeled.
Aluminum tanks are more suitable for the marine environment. As a rule of thumb, expect to use about one pound of propane per person per week cooking three meals a day. So a full 10-pound tank should last a crew of two at least 5 weeks.

Pressure Gauge
A pressure gauge is an essential safety feature, and it must be the first thing connected to the valve on top of the bottle. It can be mounted directly on the bottle or installed on the locker wall and connected to the bottle with a high-pressure pigtail.
The gauge does not tell you the level of the gas in the tank; you weigh the tank to determine that. What the pressure gauge does do is allow you to easily-and frequently-leak test the LPG system.

Regulator and Solenoid
The regulator (to reduce pressure) is next, and that must be followed immediately with an electric solenoid shut-off valve wired to a convenient switch in the cabin. The switch panel should include a bright warning light to tell you when the solenoid is on. The solenoid should always be off except when the stove is in use.

Supply Hoses
The fuel line from the solenoid valve to the stove must be continuous-no connections inside the boat except at the stove. Solid copper fuel lines are used extensively in many parts of the world, but flexible, high-pressure hose is probably safer for boat use, and it is certainly easier to install.
When connecting the supply hose to the solenoid, an adapter is usually required. Wrap the threads of the solenoid end of the adapter with Teflon sealing tape. Do not use Teflon tape on the hose end. The rule is that when the threads form the seal, you need Teflon tape, but not when the seal is formed with a flare, O-ring, or other compression fitting.
The hose should exit the propane locker near the top, and it should pass through a vapor-tight fitting to prevent gas from leaking out of the locker into the interior of the boat. Use a similar fitting to protect the hose from abrasion everywhere it passes through a bulkhead.
Common straight-through fittings are too short to pass through a plywood bulkhead. You can thin the bulkhead by counterboring one side with a spade bit. Alternatively, cut an oversize hole through the bulkhead with a hole saw, screw a metal or fiberglass cover plate over the hole, and install the fitting through the cover plate.
Be sure to protect the hose from abrasion and excessive heat (140F). LPG hose should run through the top of lockers rather than through their bottoms. Support the hose every 18" with nylon cable clamps.

Accessories
With a simple T-connector between the solenoid and the main supply hose, you can connect an additional hose to supply a second propane appliance, a rail-mounted grill, for example. As with the main hose, the accessory hose must also exit the locker through a vapor-tight gland.
Propane barbecue grills designed for disposable propane canisters have their own regulators, so connecting the grill to the low-pressure side of the boat's propane system requires a low-pressure control valve-available from the grill manufacturer. It is possible to tee off the high-pressure side of the system, but this arrangement is not nearly as safe and should be avoided.
Here is a tip: a disposable canister can be used to supply all onboard propane appliances if you have a reverse cylinder adapter aboard. Disconnect the pigtail from the tank and fit the adapter to it, then screw on the canister and you are back in business. Having this adapter and a single propane canister aboard can get you through a weekend when you run out of gas unexpectedly.

Sniffers
Gas detectors add an additional level of safety to an LPG installation. So-called sniffers constantly monitor the air for the presence of LP gas, activating an alarm and shutting off the solenoid when they detect gas at about 10% of minimum explosive level.
Install the gas sensor where gas is likely to accumulate, i.e. as low as possible and near the range.

Use
To operate the system, the valve on the tank must be open. From inside the cabin, a flip of the solenoid switch allows the gas into the supply hose that feeds the stove. Open the burner valve and light the flame.
When you are through cooking, leave one burner ignited and turn off the solenoid. When the burner goes out, then close the burner valve. This empties the line of gas so that none will leak into the bilge should a burner valve fail to seal.
If you will leave the boat unattended for a while, it is a good practice to close the valve on the tank to eliminate all risk of undetected leakage.

Testing
If you are using the stove regularly, you should make a habit of testing the system for leaks. Once a week is not too often.
To test the system, operate the stove, then close all the burner valves, but leave the solenoid switch on. Read the pressure gauge, then turn off the manual valve on the tank. After three minutes, read the pressure again. If it is unchanged, wait 15 minutes and read it again. Any drop in pressure indicates a leak that must be located (with soapy water) and stopped. If the system is leak-free, reopen the tank valve, light a burner, then shut off the solenoid as normal.
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Old 21-06-2007, 08:34   #8
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Quote:
Originally Posted by Ex-Calif
I've given some thought to the propane locker and agree it is a good idea. However the hose or line that routes to the stove is a weak point in isolating the propane from the cabin, isn't it? Also the stove valves themselves could leak.

What does this isolated system look like or am I missing something?

What are most people using? Propane makes sense as it is readily available anywhere in the world and thermal qualities are great as has been pointed out.
The solenoid valve that is in Pblais's description is in the propane box. This is a valve that you are supposed to open, remotely from the galley, everytime you operate the stove, and turn off when you are done. So if any part of the system develops a leak outside of the box, only the propane in the hose downstream of the the valve gets in the bilge. A leak that develops upstream of the valve is directed off the boat by the drain in your propane box. You should also have a pressure gage on the high pressure side of the system for leak tests. You open the main tank valve, turn on the solenoid, make sure all stove and oven burners are off. Close the main tank valve, note the pressure reading. Wait 3 minutes (I think that's the ABYC number). If the pressure has changed you have a leak that you need chase down. I forget how often ABYC recommends doing this test, somehow monthly pops up in my head.

I presume you would notice any significant leaks between tests, while using the stove, by the stove not functioning or having low flame. I don't know if this part is standard, but my system has a maximum flow valve. Too much flow causes a valve similar to a check valve to close and stop the propane flow.

John
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Old 21-06-2007, 08:50   #9
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Thanks Gord and John. Makes complete sense now.
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Old 21-06-2007, 09:03   #10
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The new Sintex sensors have two interior sensors you can install. One would presumably be at a low point and another might be near the stove. These alarms would tell you if there was a leak in the line or the stove. A poor stove could blow you up as easy as a propane locker open to the cabin.
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