|20 lb propane cylinder|
Propane is one substance that is present on almost every boat - large or small, power or sail. In its own way, it can be quite dangerous. But modern technology, techniques and regulations have taken the edge off of the dangers, almost to the point where people think it is a benign substance that is sold in grocery stores. Oh wait, it is sold in grocery stores.
I think everyone carrying propane onboard should have some basic knowledge about this substance. First, because knowledge is power. But also so that they are aware of the consequences of violating those regulations or short-cutting those techniques. Now those of you who know me know that I am no fan of regulations. But those controlling the storage and handling of propane are one of the things that make regulations worthwhile. Also, those cruisers voyaging outside the range of those regulations will find a different world - one where common sense is more critically relied upon - and here knowledge is crucial.
So what is propane? First, it is a member of that class of chemicals known as LPG, or Liquified Petroleum Gas. It's composition is CH3-CH2-CH3. It is a hydrocarbon - that is, it is made up solely of hydrogen and carbon. It is the third in the series of straight-chain hydrocarbons, after CH4 (methane, AKA natural gas) and CH3-CH3 (ethane), and preceding CH3-CH2-CH2-CH3 (butane, which you are probably familiar with as the fuel in, ahem, butane lighters).
Why is propane such a ubiquitous fuel? Because it is easily liquified. And liquid fuels under pressure do not require pumps for their distribution from a storage tank - they deliver themselves. Also, because it is a simple compound, propane burns quite cleanly, which is why fork trucks running inside factories are frequently propane-powered.
OK, those are the basics. But propane is a substance delivered in a manner that is unusual for consumer goods - it is a liquid, but it is under pressure. To better understand what this means, an analogy is in order. Everyone is familiar with another simple substance: dihydrogen monoxide - water. Under normal ambient conditions, water is a liquid. If you were to put water into a propane tank, and then take that tank to a hypothetical planet Hypethos where the ambient temperature is 350°F, you'd have our analogous situation. Water cannot not exist as a liquid on Hypethos, unless it is kept under pressure, just as propane cannot exist as a liquid on Earth unless it is kept under pressure. Once our Hypothetical tank full of liquid water reached equilibrium with the surroundings, it would be under pressure - about 135 PSI in fact. If you opened the valve on the tank, water vapor (AKA steam) would fly out.
Propane does this too, but at the temperatures we are comfortable with here on Earth. At 75°F, the pressure in a propane tank is about 150 PSI. If you leave that tank out in the sun, and it gets warmed up to say 110°F, the pressure climbs over 250 PSI.
FACT #1Pressure gauges are really completely useless in showing how much propane is left in the tank. At a given temperature, the pressure in the tank will be the same regardless of the amount of propane in the tank, as long is there is one tiny drop of liquid propane still present. Once the last of the liquid has evaporated, then the pressure gauge will tell you how much vapor is left. But by that point, you don't have enough propane left to cook dinner. Aside from those transparent fiberglass tanks where you can directly see the propane, the only reliable way to gauge the remaining propane is by weight. If you don't have a scale, you can get an idea of the amount of liquid remaining by sloshing it about in the tank.
OK, so the pressure rises as the temperature rises. Then you won't be surprised to find that the pressure falls as the temperature falls. At about -35°F, the pressure in the propane tank drops below 15 PSI. That is, if you opened the valve, nothing would come out (remember, ambient atmospheric pressure is 14.7 PSI). But actually it is worse than that. Propane accessories (stoves, BBQs, etc.) depend on the pressure in the propane tank to deliver propane gas to the accessory. A regulator in the circuit ensures that, regardless of the pressure in the tank, the appliance will see a constant pressure, and therefore flow rate to the burner. But regulators can only work by reducing the upstream pressure. If the pressure in the tank is below the regulator set point, the regulator cannot make up for it. Also, regulators are designed for a "reasonable" upstream pressure. As the propane is cooled, the pressure departs more and more from "reasonable", and the flow rate drops... Your stove flame gets smaller and your BBQ refuses to heat up.
Fact #2What might make the propane this cold? Well, if you sail in the high latitudes, you could see temperatures low enough to impede or stop propane flow. But even folks in the tropics can have problems. Why? OK, you have liquid propane in the tank, topped with a layer of vapor. As you withdraw the vapor, more liquid evaporates, replenishing the vapor. Ah, but evaporation (call it boiling, if that helps you to imagine what is happening) takes heat. Where does this heat come from? From the propane itself - it cools. Now as the propane cools, heat flows into it thru the tank walls from the surrounding air. You've all seen this... First there is condensation on the outside of the tank (serving, by the way, as a pretty reliable liquid propane level indicator). As the cooling continues, the tank wall could drop below 32°F, and the condensation turns to ice or frost. (Please note that the propane is still liquid inside the tank. For it to freeze, its temperature would have to drop to -306°F.) But the accumulation of frost on the outside of the tank only impedes heat flow, the propane cools, evaporation slows, the pressure falls, and the flow rate drops. And your Cajun burner goes out, ruining your crab boil even tho there is still propane in the tank.
- Liquids are virtually incompressible.
- Things grow larger as they are heated.
How to make this understandable? Try this. If a standard 20 lb propane tank were filled with compressed air at 150 PSI, in its roughly 1 cubic foot of volume it would contain 10 cubic feet of air that would want to rush out if the tank were ruptured. But if instead it contained liquid propane, that propane would instantaneously flash to something like 270 cubic feet of vapor at tank rupture.
This is on top of the obvious fuel-air explosion hazard. This is the reason that Overflow Protection Device valves were added to the regulations concerning handling liquid propane in consumer tanks back in 1988 - to ensure that there is always an adequate vapor space above the liquid to accommodate liquid expansion. An OPD is supposed to shut off the filling of the tank when it is about 80 - 85% full. Note that the little 1 lb cylinders do not have OPDs because they are not intended to be refilled.
OK - now you know:
- What propane is
- Why pressure gauges don't tell you how much propane you have left
- Why propane stops coming out even tho the tank is not empty
- Why you should have an OPD on your tank