On Fri, 13 Mar 2009 16:42:57 -0400, "Ed Huntress"
wrote:


"Mark Rand" wrote in message
.. .
On Wed, 11 Mar 2009 11:19:50 -0700 (PDT), wrote:


The impulse turbine is kind of dead,
the electric motor took over for running the stuff they used to use it
for, haven't seen mention of them except as historical objects in any
recent texts.

GE turbines are/were impulse rather than reaction based for the HP and IP
cylinders as were GEC in the UK and Alcatel-Alstom in France. That's
something
like 50% of the installed base in the world. Impulse turbines are
certainly
not kind of dead, I walked past two impulse rotors this afternoon in the
erecting shop at work!


Mark Rand
RTFM

I don't know about now, but I was under the impression that large steam
turbines for years were hybrids, with one or two impulse stages followed by
a series of reaction stages.


It tends to be a graduation rather than completely one or the other. It tends
to be more impulse at the HP end and more reaction at the LP end, but there
are/were major differences in proportion. Causes problems when major companies
merge, you don't always end up with an optimum design :-|


The major difference in philosophy is that in an impulse design all the
pressure drop is across the diaphragms (fixed stages). So you can make the
rotor similar to a series of disks on a thinner shaft. This way, the
interstage glands where all the pressure drop is (diaphragm to rotor) are
shorter, due to being on a smaller diameter. The blade tip seals are far less
significant doe to the token pressure drop across the moving stage. Because
there's no pressure drop across the moving blades, there's no end-thrust
issue. The disks even have (pressure) balance holes to ensure this.

Reaction design turbines tend to be made with a barrel type rotor since the
pressure drop across fixed and moving blades is similar, so you can't benefit
from a shorter length of gland and you don't want a great big pneumatic piston
effect generating end thrust on each stage. Obviously on a double flow
cylinder, the end thrusts should balance. but on a single flow cylinder you've
got problems.

You can also play games like 100% impulse near the centre and 100% reaction
near the tips for long blades. CFD is wonderful :-)


regards
Mark Rand
RTFM