Propane problem?

Lost Parent · Jun 24, 2013

Stephen Trapani said:
I have a propane tank on the stern rail, a cook stove/oven roughly amid ship, and a older Force 10 heat stove just aft of the V-berth. It seems the Force 10 only runs well when the propane tank is full. When it gets half empty the Force 10 won't light. At least so it seems that is what's going on. Meanwhile the cook stove still lights and runs fine. Plenty of propane left.So the Force 10 which is about twice as far from the tank won't run when the closer stove will. If I fill the tank, the Force 10 runs again. Does this make sense? Is there anything I can do about it without re-plumbing?

It should not matter , I have the exact same config and anyways LP runs by pressure which is constant at a half tank as well as full tank

MrUnix · Jun 24, 2013

Lost Parent said:
LP runs by pressure which is constant at a half tank as well as full tank

Not entirely true.. tank pressure will vary depending on ambient temperature and the amount of liquid in the tank. However, the regulator on the tank is designed to allow for the variable tank pressure and provide a fixed output pressure, usually around 11" W.C. The problem is that propane appliances operate on gas, not liquid.. so how fast the tank can provide that gas (rate of vaporization) is critical. This rate is dependent upon the wetted surface area in the tank (ie: the amount of liquid in the tank) and temperature (which also effects pressure). As temperatures and/or liquid levels drop, so does the rate of vaporization that the tank can provide.

As for the specific problem.. it sounds as though the rate of vaporization your tank can provide at half full is not sufficient for your Force 10 stove, but is for your cook stove. What are the BTU/Hr requirements for the two? A simple fix for that situation would be to increase the tank size, which will increase your rate of vaporization.

There are other possibilities depending on how your appliances are piped and regulated.. but bumping the tank size up is the easiest to do without any real modifications. You might find a slip neighbor with a larger tank they will let you borrow just to test

Cheers,
Brad

PS: I am by no means an expert on this subject, and hope that others who are will correct anything I've said that isn't accurate. I have however just gone through a fairly extensive crash course on propane due to a completely unrelated incident, and now know way more than I ever wanted to know about this topic.

Benny17441 · Jun 26, 2013

How big is your propane tank? How often do you use the Force 10 heater? Go ahead and conduct a system pressure check to test the lines and appliances for any leaks. The system should hold pressure for at least two hours to be deemed a passing grade. A good working regulator will accomodate pressure differences within the tank and after pressure testing the system if the problem persists then it can be a good indication that the regulator may need replacement. A really small tank like 5 lbs or less may cause problems and an appliance that is not frequently used may have air that needs to be purged from the line.

Bob J. · Jun 26, 2013

How is your tank mounted & is it in the proper position for the type of tank it is?

Mr. Unix has explained this quite well & has offered some real good advice for tracking down your issue.

tkanzler · Jun 26, 2013

MrUnix said:
.. tank pressure will vary depending on ambient temperature and the amount of liquid in the tank.

This part I'm not understanding. The vapor pressure of LPG is dependent on temperature only, and a tank with a drop of LPG in it, at equilibrium, will have the same pressure as one that's full, assuming there's some non-liquid space in there (vapor only), and that it's at equilibrium.

When the temperature goes up, the pressure goes up and the liquid expands, but the gas will condense back to liquid and equilibrium will be reestablished after some amount of time. When gas is drawn off, the liquid will 'boil', and the pressure will drop by an amount related to the rate heat enters the system, which will vary with ambient conditions and rate the gas is drawn off and how much liquid is in the tank, as more liquid means faster heat transfer and a higher fluid temperature and therefore a higher gas pressure. But when you stop drawing off gas, the pressure will build to the vapor pressure at the temperature of the tank, which will also increase as heat transfers into the tank, eventually settling at the ambient temperature.

The danger in filling to over 80% of tank capacity is in liquid getting into the the line where only gas should be (not counting horizontal tanks, like forklift tanks, which feed liquid to the evaporator/carburetor), and/or liquid being relieved through the relief valve, which will flash to gas at something like 175:1 volume ratio and fill the area in a jiffy with explosive gas. Or freeze your body parts if they're in the stream. Or blind you. All all three.

I suppose another danger of overfilling is that with an increase of temperature, if it expands and builds pressure faster than the LPG can condense, you could get a temporary overpressure condition, and again, blow down through the relief valve.

Unless I'm missing something, which is entirely possible.

Ross · Jun 26, 2013

MrUnix said:
Not entirely true.. tank pressure will vary depending on ambient temperature and the amount of liquid in the tank. However, the regulator on the tank is designed to allow for the variable tank pressure and provide a fixed output pressure, usually around 11" W.C. The problem is that propane appliances operate on gas, not liquid.. so how fast the tank can provide that gas (rate of vaporization) is critical. This rate is dependent upon the wetted surface area in the tank (ie: the amount of liquid in the tank) and temperature (which also effects pressure). As temperatures and/or liquid levels drop, so does the rate of vaporization that the tank can provide.

As for the specific problem.. it sounds as though the rate of vaporization your tank can provide at half full is not sufficient for your Force 10 stove, but is for your cook stove. What are the BTU/Hr requirements for the two? A simple fix for that situation would be to increase the tank size, which will increase your rate of vaporization.

There are other possibilities depending on how your appliances are piped and regulated.. but bumping the tank size up is the easiest to do without any real modifications. You might find a slip neighbor with a larger tank they will let you borrow just to test

Cheers,
Brad

PS: I am by no means an expert on this subject, and hope that others who are will correct anything I've said that isn't accurate. I have however just gone through a fairly extensive crash course on propane due to a completely unrelated incident, and now know way more than I ever wanted to know about this topic.

The problem with this theory is the burner in the Force 10 heater is the same size as the burner on the stove. I have noticed that the fuel line to either the stove or the heater needs to be purged if the appliance hasn't been used for several months. This just involves holding the valve open for a few minutes and trying to light it periodicly. I use a standard 20 pound tank

Ross · Jun 26, 2013

Mine is about ten years old with a thermocouple safety, standing pilot and spark ignition. As others have said the tank pressure remains quite constant until the last few ounces of fuel. Have you considered disconnecting the heater supply from both ends and blowing compressed air through to ensure that there are no contaminates in the line. A tank of fuel lasts me for two or three years.

Lost Parent · Jun 26, 2013

looks like 20lb

centerline · Jun 26, 2013

it sounds like the regulator isnt doing its job... it is a simple single stage regulator so as the tank pressure drops, you will also see a small drop in output pressure.... and they are designed to be fail safe, so when there is any kind of problem they will begin to shut down, rather than allow any excess gas thru...
more than likely it isnt putting out as much pressure as it should when the tank is full and when the tank gets used down a bit it just cant maintain enough pressure in the line to carry the fuel that far fast enough to maintain ignition at the appliance.....

gas lines, like any other type, have friction in them as the gas moves thru them, and the longer the line, the more "head" pressure you need to maintain the working pressure at the down stream end. OR... the larger the line must be to carry the required flow... but either way, if the regulator isnt working properly, either by a pressure malfunction, or by a flow malfunction (they arent both the same), it wont deliver what is needed to do the job.... and, as this type of system supplies such a small amount of pressure, a very small variation can be enough to cause a problem like the one you are having.

Bill Roosa · Jun 27, 2013

I'm inclined to agree with MrUnix (love that name). The tank can't boil the liquid fast enough due to the reduced surface area that is "wet" when she is 1/2 full. And we thought thermodynamics would never enter into sailing a sailboat, what where we thinking.

Ross · Jun 27, 2013

The detail being forgotten here is that these same tanks will fuel a BBQ grill summer or winter and will expend all of the fuel in the tank. The burner(s) on a grill are several times larger than the burners in a marine stove or heater. The orifice in the burner for the heater is a number 59 drill (about .015 inches) and it is supplied by a 3/8 inch tube so the velocity of the gas in the supply line is very low. I am certain that there is a factor that is being overlooked that can be determined only by careful on site inspection.

centerline · Jun 27, 2013

Ross said:
The detail being forgotten here is that these same tanks will fuel a BBQ grill summer or winter and will expend all of the fuel in the tank. The burner(s) on a grill are several times larger than the burners in a marine stove or heater. The orifice in the burner for the heater is a number 59 drill (about .015 inches) and it is supplied by a 3/8 inch tube so the velocity of the gas in the supply line is very low. I am certain that there is a factor that is being overlooked that can be determined only by careful on site inspection.

as in the regulator... it may deliver the pressure at "no flow", which is how they are normally checked, but if its doesnt supply enough volume of fuel at a higher flow rate, its a problem.
its like a pinched off garden hose... it will develop full standing pressure, but as soon as the flow starts, the down stream pressure drops, with hardly any flow, even though the "tank" has plenty of product in it...

Ross · Jun 27, 2013

One of the simple tests would be to install a new regulator.

Ross · Jun 27, 2013

The make two types. They are labeled for outlet pressure. you will want one that is marked for about 11 inches WC(water column). The other type is high pressure for things like turkey fryers.

Steve Dion · Jun 27, 2013

You may want to take the tank to a LPG gas facility and have them check out the OPD. Those damn things are a POS. I have had full tanks that would not work. I normally had to pound them on the ground to release the propane.

Steve Dion · Jun 27, 2013

I would try someone else tank that is low on gas to see if your stove will work, or even a small portable stove tank.