AEROSTAT December 2002

The Pressure to fly by Glen Everett

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Often the winter gives perfect conditions to fly all day. Cold crispy mornings can give spectacular flights and the hard ground will keep your fabric from getting muddy.

The problem, though, is low burner pressure. You may think that having bags of spare lift because of the low temperatures will negate the low pressure. It’s not true; a small reduction in burner pressure can make a light balloon fly like a brick. Low burner pressure can give a woolly flame, overheat your coils and the radiant heat can scorch your Nomex as well as your head.

Minimum burner pressures vary slightly by the manufacturers but caution should be used below 75-80psi.

The pressure of propane varies approximately 2.4 pounds per square inch (psi) per degree of ambient temperature. At 10¢ªC, the cylinder pressure will be 80psi. Therefore it is wise to pressurise your cylinders at anything below 10¢ªC (my Sky burners are markedly more woolly below 100psi). It is also worth remembering that tanks stored overnight in the cold may be a lot colder than the ambient temperature as the sun comes up.

There are a few ways of pressurising cylinders and the methods are described below.

Advantages — free. Does not affect pilot light operation.

Disadvantages — will cool, especially if lengthy time before take off (and remember the cylinders will cool and reduce pressure during the flight too); also lots of effort lugging cylinders about. Please bear in mind the safety precautions and implications of gas indoors.

Heated pads of low wattage, as used in home brewing or reptile tanks, seem to be popular. It is recommended that the temperature of the tank be kept below 25C — use a thermometer or better still have a pressure gauge manufactured to screw on to your liquid take off. You could always check using your burner, but what do you do with all that unburnt liquid in the hose? Do also be careful of propane leaks or venting bleed gas, as any spark from electrical connections could be more than embarrassing.

Advantage — relatively cheap, especially for the infrequent flyer.

Does not affect pilot light operation.

Disadvantage — Not always portable with the requirement of an electrical source within range, and the need for a heater per cylinder.

Be aware that the temperature will drop if you have a lengthy drive to your launch site, and the cylinders will cool and lose pressure during the flight too.

Often known as tank spiking, nitrogen is more portable, easier and more controllable, but more expensive, not in terms of equipment such as the regulator or even the gas, but purely in terms of the rates hire companies charge for bottle rental. To pressurise with nitrogen, connect the hose, and adjust the regulator to give your desired pressure. Opening the tank valve will cause the regulator outlet pressure gauge to quickly drop, and slowly rise as the pressure builds. Shut the valve when your desired pressure is reached. My recommended pressure for small balloons, would be 100psi, with up to 150psi for those flying larger balloons. Pressurisation should normally be done just prior to the flight. Too high a pressure can lead to liquid being thrown out of the main jets and a yellow flame.

Advantages — portable, quick and exact pressures throughout the flight.

Disadvantages — expensive bottle hire. Pilot light operation requires management (see below).

Pilot lights — if you pressurise this will not affect liquid pilot lights, but could extinguish vapour ones as the nitrogen tends to sit on top of the propane, from where the gas for vapour pilot lights is drawn. The solution is to pressurise only your slave tanks and not your masters, and save the slaves for takeoff, descents or landings. Problems with pilot lights can be solved using a mini vapour cylinder. It is also recommended that you have two to comply with the standard aviation practice of dual redundancy. The major manufacturers were approached to see if more readily available forms of gas can be used other than nitrogen, gases which perhaps could be borrowed from, say, work. Argon perhaps (as used in welding) or CO2 (as used in welding or from pubs).

Camerons said

‘Pressurising with CO2 is a possibility but it should be noted that the CO2 dissolves into the propane more readily than nitrogen. This has the positive effect of maintaining the pressure throughout the cylinder but does tend to give a mixture which tries to extinguish the pilot lights. It is also possible to pressurise with other inert gases but again this has not been approved due to lack of demand’.

Lindstrands said

‘CO2 is absorbed into the propane to a greater extent than nitrogen. This can result in a drop in pressurisation level if there is a time delay between introducing the gas and using it. The Jetstream burner liquid pilot light is quite tolerant of both of these gases.

‘The most common reported difficulty is on initial lighting of the pilot light. This is normally due to an amount of gas being left in the tank dip tube after pressurisation is complete. This means that when the burner is first connected up, the inert gas goes straight into the regulator and comes out of the pilot light and will not light. The best way to overcome this is to open the main blast valve until liquid is feeding through the burner. If the pilot light is then turned on it lights normally.’

Check what gas your bottle supplier gives you — nitrogen and oxygen bottles are virtually identical. A British pilot was killed in Africa when tanks were spiked with oxygen and a flashback ignited the contents. I have had an oxygen cylinder delivered by mistake instead of nitrogen and only noticed when about to open the valve… phew!

Should you pressurise in the cold, especially the extreme cold of alpine flying, and temperatures return to normal, you should be aware of excessive tank pressures. At –10¢ªC the pressure is around 30psi and you might add another 90psi to bring it up to 120psi. If you return to a temperature of 20¢ª the tanks could now be approaching 200psi. It is recommended that cylinders do not exceed 100psi if stored in a pressurised state.

Label any pressurised and part pressurised cylinders

Bleed heavily to remove excess pressure or use the burners. It should be noted that if you part-use and refill a pressurised cylinder, it will still be part-pressurised because the gas partly goes into the liquid in solution, so merely removing the excess pressure may not be a sufficient procedure, especially for vapour pilot light burners. Even with the pressure returned to normal, there may be a significant amount of the inert gas remaining in mixture with the propane vapour. This may cause continuing problems with vapour pilot light stability until the tank has been significantly refilled. Enjoy your winter flying, but as the usual disclaimer says, read your flight manual for procedures relating specifically to your balloon.

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