Grandmother Goes Down at the Pole

On Mon, 22 Dec 2003 23:15:49 GMT, (Bruce
Hamilton) wrote:

I hope this post doesn't sound impolite or abusive to you, as you
obviously wanted to add some data, but the storage constraints on
Avgas mean it has to be regularly retested before it can be accepted
as fuel for aircraft. Avgas is usually more stable than Mogas, but it
still has to be within a current test period.

I though gas turbines (as used in aircraft engines) would run on just
about any old liquid, as long as it has a hint of hydrocarbon in it?

David

On Tue, 23 Dec 2003 16:51:25 +0930, David Pears
wrote:

On Mon, 22 Dec 2003 23:15:49 GMT, (Bruce
Hamilton) wrote:

I hope this post doesn't sound impolite or abusive to you, as you
obviously wanted to add some data, but the storage constraints on
Avgas mean it has to be regularly retested before it can be accepted
as fuel for aircraft. Avgas is usually more stable than Mogas, but it
still has to be within a current test period.

I though gas turbines (as used in aircraft engines) would run on just
about any old liquid, as long as it has a hint of hydrocarbon in it?


Yes and no, as Air New Zealand discovered. Many gas turbines have
other components like fuel pumps that turn out to be quite sensitive
to the lubricating properties of the fuel. The lubrication properties
of Avgas/Mogas are near nil. Jet A is actually pretty slippery stuff.
So while you can burn just about anything in the turbine, you have to
make sure that doing so doesn't damage something else...
David

"matt weber" wrote in message reply:::

I though gas turbines (as used in aircraft engines) would run on just
about any old liquid, as long as it has a hint of hydrocarbon in it?


Yes and no, as Air New Zealand discovered. Many gas turbines have
other components like fuel pumps that turn out to be quite sensitive
to the lubricating properties of the fuel. The lubrication properties
of Avgas/Mogas are near nil. Jet A is actually pretty slippery stuff.
So while you can burn just about anything in the turbine, you have to
make sure that doing so doesn't damage something else...
David



We had Garrett turbines powering 60Hz alternators at the Cooby Creek
Tracking Station at Toowoomba in the late 60's and found the FCU's chewing
up their internal bearings like crazy until we got an anti-static additive
put in the ATK. Electrostatic corrosion.

--
JW²
Norton AntiVirus 2003 installed
************************************

David Pears wrote:

I though gas turbines (as used in aircraft engines) would run on just
about any old liquid, as long as it has a hint of hydrocarbon in it?

The problem is that turbine fuel ( effectively an aviation kerosine ) has to
also satisfy several requirements, both with regard to engine performance and
safety. The fundamental ones are flash point ( temperature at which the fuel
will give off sufficient vapours to ignite if a flame is applied ) and freezing
point ( engines tend to stop when fuel turns to sludge ).

Avgas has suitably low freezing point, but the flash point is far lower than
the fuel systems on a commercial Jet A1 fuelled airliner are designed to cope
with. Military airliners often use a wide cut kerosine with a flash point well
below airfield ambients.

If a plane is carrying passengers, it's usually running on Jet A1, a narrow cut
fuel with a flash point above most airfield ambient temperatures ( 38C ).
Flash point is also the easiest method of detecting whether a kerosine has been
contaminated by a gasoline. The 1997 explosion of the "empty" centre tank in
TWA Flight 800, has reignited interest in fuel flammability issues.

Kerosines tend to have more lubricity than gasolines ( higher sulphur and
viscosity ) but high pressure hydrotreating and hydrocracking have made fuels
"harder" ( less lubricity ), and NZ aviation authorities have convinced other
countries to include a lubricity specification in DefStan 91-91 but, AFAIK,
it's not yet in the more common ASTM D1655.

Note that refractory elements ( silicon, calcium, potassium, vanadium etc )
will erode-deposit on turbine blades, and they are present in higher boiling
fractions such as diesel ( which wouldn't pass the freezing point requirement -
but is used on ship and power-station gas turbines ). Fuels for gas turbines
tend to ensure such elements aren't present, even though falling out of the sky
isn't an option, as overhauls are very expensive.

There are a whole heap of other criteria for Jet fuels associated with
cleanliness, corrosivity, combustion properties, stability and approved
additives that mean than commercial aircraft tend to use a well defined narrow
cut kerosine fraction.

Bruce Hamilton