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11 Power Gas Turbines
be adapted better for mass flow rate lower than the design value. We can easily understand the need for adjustable inlet guide vanes. It is somewhat more difficult to
understand the benefit of a number of front stages with adjustable stator vanes for
reduced load operation of the gas turbine (reduced mass flow rate).
During start-up of a compressor with many stages, the density rise from the first
stage to the last stage is less than at design conditions. For a constant density fluid,
the shape of the velocity triangles of the stages does not change when the stages
maintain their flow coefficient and their work coefficient. So, at a reduced rotational
speed, the shape is maintained for an operating point that obeys kinematic similitude. With a compressor, due to equal mass flow rate in successive stages, the reduced density rise causes deviations with increased through-flow velocity in the rear
stages and decreased through-flow velocity in the front stages. Figure 11.3 shows
that by lowering the through-flow velocity, with unchanged flow direction at outlet
of the rotor ( w 2 ) and unchanged flow direction at outlet of the stator ( v 1 ), because
these directions are imposed by the blades and the vanes, the incidences at rotor and
stator inlet increase. Flow turnings in rotor and stator increase, which means that
the stage work increases with diminished flow rate. But, big incidence may lead
to flow separation. When this happens, the work diminishes, which then typically
leads to impossibility to bring the gas turbine to full speed. There are several remedies for the stall in the first stages of a compressor during start-up. First, part of
the flow after a few stages may be blown off through valves and led to the turbine
part. A second way is splitting the compressor in parts and so making several spools.
The objective is limiting the number of connected stages in a compressor part. The
compressor has to be split into two or three parts, depending on the pressure ratio.
Splitting the compressor is typically done with aero-derivative turbines, but is not
typical with heavy duty machines. With large gas turbines for electricity generation,
it is advantageous with respect to the precision of the rotational speed control to use
only one shaft. Mostly, stall is avoided by adjustable stator vanes. Figure 11.3 shows
that by turning the stator vanes more to the tangential direction (hatched profiles),
both the incidences at stator and rotor inlet decrease. This allows starting up of the
gas turbine.
With only the inlet guide vane ring adjustable, starting up of the gas turbine
becomes possible, because avoiding stall in the first stage obviously is the critical
action. When a compressor has multiple variable geometry front stages, it means
that part-load operation of the gas turbine has been optimised. The output power of
a gas turbine is diminished by reducing the fuel flow rate. In order to understand
the consequence for the mass flow rate, one has to know that the turbine normally
operates with a mass flow rate very near to the choked value or even on the choked
value (for a detailed discussion, see Chap. 15). The choking mass flow rate is proportional to the density at turbine inlet and the velocity of sound at turbine inlet.
The product of these quantities is proportional to the pressure and inversely proportional to the square root of the temperature. So, a reduced fuel flow rate causes,
in first instance, lowering of the turbine inlet temperature and thus increased mass
flow rate through the turbine. Increased mass flow rate means for the compressor,
at fixed rotational speed as in a single-shaft machine, reduced pressure ratio (the
11 Power Gas Turbines
be adapted better for mass flow rate lower than the design value. We can easily understand the need for adjustable inlet guide vanes. It is somewhat more difficult to
understand the benefit of a number of front stages with adjustable stator vanes for
reduced load operation of the gas turbine (reduced mass flow rate).
During start-up of a compressor with many stages, the density rise from the first
stage to the last stage is less than at design conditions. For a constant density fluid,
the shape of the velocity triangles of the stages does not change when the stages
maintain their flow coefficient and their work coefficient. So, at a reduced rotational
speed, the shape is maintained for an operating point that obeys kinematic similitude. With a compressor, due to equal mass flow rate in successive stages, the reduced density rise causes deviations with increased through-flow velocity in the rear
stages and decreased through-flow velocity in the front stages. Figure 11.3 shows
that by lowering the through-flow velocity, with unchanged flow direction at outlet
of the rotor ( w 2 ) and unchanged flow direction at outlet of the stator ( v 1 ), because
these directions are imposed by the blades and the vanes, the incidences at rotor and
stator inlet increase. Flow turnings in rotor and stator increase, which means that
the stage work increases with diminished flow rate. But, big incidence may lead
to flow separation. When this happens, the work diminishes, which then typically
leads to impossibility to bring the gas turbine to full speed. There are several remedies for the stall in the first stages of a compressor during start-up. First, part of
the flow after a few stages may be blown off through valves and led to the turbine
part. A second way is splitting the compressor in parts and so making several spools.
The objective is limiting the number of connected stages in a compressor part. The
compressor has to be split into two or three parts, depending on the pressure ratio.
Splitting the compressor is typically done with aero-derivative turbines, but is not
typical with heavy duty machines. With large gas turbines for electricity generation,
it is advantageous with respect to the precision of the rotational speed control to use
only one shaft. Mostly, stall is avoided by adjustable stator vanes. Figure 11.3 shows
that by turning the stator vanes more to the tangential direction (hatched profiles),
both the incidences at stator and rotor inlet decrease. This allows starting up of the
gas turbine.
With only the inlet guide vane ring adjustable, starting up of the gas turbine
becomes possible, because avoiding stall in the first stage obviously is the critical
action. When a compressor has multiple variable geometry front stages, it means
that part-load operation of the gas turbine has been optimised. The output power of
a gas turbine is diminished by reducing the fuel flow rate. In order to understand
the consequence for the mass flow rate, one has to know that the turbine normally
operates with a mass flow rate very near to the choked value or even on the choked
value (for a detailed discussion, see Chap. 15). The choking mass flow rate is proportional to the density at turbine inlet and the velocity of sound at turbine inlet.
The product of these quantities is proportional to the pressure and inversely proportional to the square root of the temperature. So, a reduced fuel flow rate causes,
in first instance, lowering of the turbine inlet temperature and thus increased mass
flow rate through the turbine. Increased mass flow rate means for the compressor,
at fixed rotational speed as in a single-shaft machine, reduced pressure ratio (the