Industrial Gas Turbines

"Illustration 3.1.1-1": The compressor characteristic diagram shows the abscissa, representing the mass flow rate and the ordinate, representing the total pressure relationship.

The so called speed lines (characteristic revolution lines) belong to the operation behavior of the compressor at a constant rotation speed. It can be seen how by flow restriction the mass flow rate with the pressure rise changed at the discharge of the compressor. The rotation speed of the characteristic correlates with the ratio to the 100 % desigen speed. The steeper the trend and the less the line is bendt towards the horizontal (abszissa) the more problematic the operation behaviour (handling) of the engine is to appreciate.

The diagram is divided into two areas by the socalled surge line. Underneath this curve is the stable compressor operation (stable area, marked light gray), above (unstable area, darker marked area) there is surge through stall. The compressor is normally operated on the working line (operation line). This should be applied for most frequent steady state operations, in the characteristic diagram through the area of optimum efficiency. This implies minimum fuel consumption for the gas turbine. During acceleration of the engine, the working line (rise of pressure in the combustor because the addition of fuel), approaches the surge line usually with enough surge margin. While decelerating, the working line moves away from the surge limit. It is desirable to reach a possibly larger surge margin for secure operation.

"Illustration 3.1.1-2": This picture shows the influences which result in decreasing stability of the compressor. They can narrow down undesirably the surge margin and are therefore, to avoid as far as possible.

One can divide the influences in such a manner that the working line is lifted or so that the surge line is lowered. An influence, such as the enlargement of the radial clearances ( "Ill. 3.1.1-3"), can work thoroughly negatively in both ways. The working line is raised, in the first place, through influences outside the compressor. These demand, e.g., a higher compressor end pressure or a bigger mass flow rate. If this is not possible it reduces the available output performance of the compressor. Is there the desire to adjust this, the hot gas temperature must rise. With this, the maitenance costs rise also and clearly ( "Ill. 2.3-4" and "Ill. 3.3.3-5").

The lowering of the surge line is, in contrast, self actuated through effects in the compressor

Raising of the working line results from ( "Ill. 3.1.1-3" and "Ill. 3.3-8"):

• Increased cooling air,
• extraction of power,
• acceleration,
• deterioration of the compressor and the of efficiency of the turbine,
• higher pressure drop in the combustor,
• smaller turbine cross sections (e.g., because of inconvenient blade tolerances).

Smaller turbine cross sections:

The most narrow cross section of a gas turbine can be found at the HPT nozzle guide vanes (NGV, "Ill. 3.3-8"). The narrower the cross section, the more the flow rate of the engine is restricted, whereby the pressure increases and the working line is raised, increasing the surge tendency. Apparently, small changes of the cross section of the flow can have big unexpected consequences on the operation performance of the engine. Thus, the production tolerances, e.g., form tolerances of the castings, the gaps at the sealings, can so strongly influence the efficiency of the engine that often the component combinations of each engine must be put together individually. Understandably, the change of individual nozzle guide vanes on site, in connection with a repair, will be problematic. Operation dependent alterations like bulges or distortion of the blades ( "Ill. 3.3-9") or noticeable deposits in the cross section of the flow, can also have the described effect.

Extraction of power and air in the compressor:

If the extraction of power is not scheduled in the design and has taken place mechanically, e.g., via the radial drive shaft to the auxiliaries, there is a lack of power at the compressor for compression work and it will be higher aerodynamically loaded.

The same effect emerges by an additional extraction of compressor air, which demands a higher flow of mass.

The extraction of air takes place in the compressor for different reasons: - As cooling air for the hot parts, e.g. turbine blades ( "Ill. 3.3-3" and "Ill. 3.3-6"). The hot parts, like combustor, turbine blades, discs and casings, necessitate a substantial portion of the entire air stream, in order to lower the operation temperatures to such an extent that the desired life is reached. Changes on the hot parts and/ or the air distribution system can raise the air consumption and with it the working line. This can be an important indication of the engine condition

• as air pressure for the periphery aggregates,
• as air pressure to keep the clearances at the blade tips (ACC =“active clearance control“),
• as air pressure for periphery units and production processes (e.g., external machines),
• as process medium.

Higher cooling air amounts for the hot parts:

The cooling air amount can increase if, e.g., leakages occur ( "Ill. 3.1.2.4-1") through enlargement of the seals (labyrinths) in the cooling air ducts from the compressor to the hot parts. Failures like crack formation on cooled parts, e.g., turbine nozzle guide vanes ( "Ill. 3.3-9" and "Ill. 3.3-12"), can also result in a clear loss of cooling air

Lowering of the surge line:

This is caused mainly by effects in the compressor itself ( "Ill. 3.1.1-3", "Ill. 3.1.1-4" and "Ill. 3.1.1-5"):

• Design tolerance of the blades,
• damaged or rough blade surfaces,
• enlarged clearances (enlarged radial blade tip clearances, oval casing),
• unequal air intake (pressure, angle, temperature).

Enlarged clearances:

Radial tip clearances of the blades have the biggest influence on the surge line. Especially those of the rotor blades.

Enlarged radial tip clearances, above 1% of the blade length lead to a dramatic deterioration of the aerodynamic performance parameters like flow rate, degree of efficiency, and surge margin. The surge line is lowered (flow disturbance in the tip region), and a rise of the working line follows simultaneously, (deteriorated degree of efficiency), whereby the surge margin narrows from both sides. The high compression ratio of modern engines lead in the rear (high pressure) compressor part to very short blades and, on account of the relatively high compression temperatures, to correspondingly large thermal expansions. Thus, this compressor area is especially sensitive to air flow disturbances.

For this reason the compressor region is especially prone to flow disturbances. This effect is specifically pronounced in small engines with equal short blades and/or in large engines in the region of the high pressure compressor ( "Ill. 3.1.2.4-1").

Labyrinths between guide vanes and rotor, especially the compressor exit labyrinth, influence the operation performance of the compressor noticeably, through clearance enlargement and increase of leakage.

Irregular air inlet:

Irregular pressure and temperature distribution at the inner circumference of the compressor inlet have a damaging influence on the compressor performance. In circumference direction, such irregularities can be only poorly balanced out, as the compressor performs as though it consists of parallels to the peripheral arranged conduits. In the conduit with the least inlet pressure, the highest pressure ratio must be produced by the given outlet pressure. In this conduit, the surge line is reached first ( "Ill. 3.1.1-4"). From there the entire air flow system is destabilized ( "Ill. 3.1.1-5"). In this process, the number of disturbances at the periphery does not necessarily have a noticeable influence on the deterioration of the surge line.

The areas with inlet pressure disturbances and airflow separations receive a higher energy supply with temperature increase. The consequence is a lower aerodynamic speed, at the same pressure ratio, which narrows the surge margin. Inconveniently formed inlets, missing engine inlet ducts (bell mouth), bigger leakages in the inlet duct and an opened bypass after the filter are to be avoided. Turbulence of the inlet flow can lead to strong, vibrational excitation of the blades. It is not to be forgotten that an icing of the inlet area or the false functioning of the de-icing unit can result in those kind of disturbances.

Higher cooling air amounts for the hot parts:

The cooling air amount can increase if, e.g. leakages occur ( "Ill. 3.1.2.4-1"), in the cooling air ducts from the compressor to the hot parts through enlargement of the seals (labyrinths). Failures like crack formation on cooled parts, e.g., turbine nozzle guide vanes, ( "Ill. 3.3-9" and "Ill. 3.3-12"), can also result in a clear loss of cooling air.

The acceleration of the engine:

The increase of the rotor speed, demands an increase in the fuel amount. Thus, the combustor pressure that was used up by the compressor clearly raises the working line. Only when a deterioration of the compressor efficiency outside the frame of the lay out is present, the surge line can be crossed. Such a surge incident is also to be evaluated as a hint that the engine, especially the compressor, should be inspected for unpermitted divergences (e.g., borescope inspection of FOD, "Ill. 4.1-8").

Deteriorated compressor and turbine efficiency:

A deteriorated degree of efficiency of the compressor demands an increase of speed, in order to maintain the mass flow corresponding to a raising of the working line. Typical influences that deteriorated the degree of efficiency of the compressor are treated in the chapter regarding the drop of the surge line.

A deterioration in the efficiency of the turbine results in a poorer available performance for the propulsion of the compressor. Speed, which therefore invariably falls, is increased by an addition of the fuel amount, influencing the compressor end pressure and raising the working line. Influences, like roughness increase, gap leakage, tip clearance loss etc., that lessen turbine efficiency are handled in "Ill. 3.3-9" and "Ill. 3.3-10".

Increased pressure loss in the combustor:

A pressure loss in the combustor is effective as increased resistance of the air flow. With that the working line is raised. Unplanned pressure losses in the combustor can be caused by foreign objects, choking and distortions ( "Ill. 3.2.3-1").

Design tolerance of the compressor blades:

The profiles of the compressor blades naturally underlie permitted tolerances. On account of the large number of blades and their special geometrical deviation (tolerances!), for which the aerodynamic work principles are particularly sensitive, prohibitve effects can occur due to alterations in statistic distribution. Typical problem areas are the leading edges, radius, transitions of the angle of attack and transition to the root platform. Profile deviations also lead to problems.

Especially tricky is the scattering of measurements that have not been gathered through the demands of design and quality control, whose existence is first discovered during operation. Deviations can also show up, if especial operation demands necessitate coatings, like paint or erosion protection.

Damaged or rough blade surfaces:

The blades of the compressor can be deformed through the operation as a consequence of erosion and /or foreign object impacts, geometrically altered or roughened through abrasive wear (see Chapter 3.1.2.2). The roughening mainly occurs at the leading edge and pressure side. The deformation in the micro range also plays a role at the leading edge, especially at the transition to the suction side, where the separation of the airflow takes place. The higher the pressure niveau, the thinner the boundary layer. So a smaller roughness is therefore damaging. In the front compressor area bigger roughness, as at the compressor exit, can thus be accepted.

If the blade is strongly deformed as a consequence of foreign object damage (FOD), an airflow separation (rotating stall) follows here, first of all, which is mostly not recognizable from the outside, and the danger of vibration fatigue failures is increased.

"Illustration 3.1.1-3": This picture shows the typical tendency of the dependency between the compressor efficience and the blade and vane tip gap (Lit. 3.1.1-1). It is clearly to see, that the gap between vane and rotor influences the compressor performance relatively little (lower range). Radial tip clearances of the rotor blades against the casing have the biggest influence on the surge line (upper range). Tip gaps of the rotor blades are therefore of highest significance for the data of the whole engine ( "Ill. 3.1.2.4-1").

The diagram shows on the horizontal axis (abscissa) the relation between tip gap and blade hight (lenth). This becomes especially noticeable for the short blades of the rear compressor. Even small gaps lead to markedly more efficiency loss than long blades in the front region of the compressor. A s/h -relation (tip gap/blade hight) of 1% means for a blade with 20 mm a tip gap of merely 0,2 mm. How difficult the compliance of the gap is ( "Ill. 3.1.2.4-3"), highlights the consideration that for bigger compressor diameters the extensions of rotor and stator have to be matched very good to ensure an acceptable operation behavior of the engine. It is easy to understand that small engines, because of the necessary extrem small gaps, require special high demands on the accuracy of the components. Therefore small engines are particularly sensitive when a gap enlargement occurres.

"Illustration 3.1.1-4": This picture should convey an understanding of airflow instability in the compressor: The airflow diversion through the blade produces a reaction force that corresponds to the aerodynamic lifting force and the profile drag. The rotor blade transmits the torque to be brought onto the shaft, as well as axial force to the front, against the direction of the flow. The lifting force on the airfoil originates through differences of the flow velocity between the upper side (suction side) and underside (pressure side). The“ aerodynamic load“ is proportionate to the difference in the velocity between the decelerated flow of the suction side and the exit velocity. If the stagger angle (deflection) of the blade profile is increased, the direction of the air flow towards the profile is constantly steeper. A separation of airflow (stall, surge) on the suction side,with a breakdown of the lift, follows. The pressure difference between suction and pressure side was, therefore, too big for the condition of the existing airflow. The drop in lift signifies that the blade does not deliver the air.

The angle under which the air leaves this stalling blade is altered opposite the blade with abut flow. The blade at the next stage, following the direction of the flow, receives likewise an inconvenient incidence of air that also causes stall. Thus, stall can spread in the compressor ( "Ill. 3.1.1-5").

"Illustration 3.1.1-5": Rotating stall (mild stall, cold stall) with the specially distinctive form of deep stall. In contrast to the idea that stall appeared nearly simultaneously on the entire periphery of the blade row, one should assume that stall first takes place on the blades of a limited peripheral area and spreads from there. Susceptible to stall are, e.g., blades with geometrical defects, as well as areas with bigger tip clearances or a locally disturbed incoming airflow. A stall released influence can be all the more minor, the closer the blade row works already in the area of the maximum angle of incidence ( "Ill. 3.1.1-4") of air

Local air separation works as a hindrance and diverts the air flow in such a way that the following blade in the same row is flown against inconveniently. The previously disturbed blade, however, is flown against more conveniently, so that behind the flow is abut again. The cell of the stall appears to rotate with 10% to 50% of the peripheral speed in the direction of revolution of the rotor, from the outsider’s perspective. Such cells can show up several times on the periphery in very different sizes, in radial direction and across the periphery.

"Illustration 3.1.1-6": A compressor surge ( "Ill. 3.1.1-5"), can lead to multiple failures in the entire engine. This picture shows the locations of typical problems.

In the compressor, strong deflection of the blades and radial, as well as axial, deflection of the entire rotor through pressure impact can occur. A contact between rotor blades and guide vanes is therefore possible. Additionally, vibration excitations can dynamically over load the blades. This holds the immediate danger of fatigue fractures ( "Ill. 3.1.1-7").

The radial and axial rotor deflection can also lead to contact of the rotor with the casings.

This can be applied besides for the rotor blade tips also for vane tips and labyrinth seals. Such a contact raises the danger of increased abrasion at the abradables (soft) and the damage of systems with hard surfaces/coatings by overheating.

Bearing overload and unpermitted labyrinth abrasion can emerge as a consequence of pressure impacts or insufficient thrust compensation. The pressure impacts in the casings can release obvious elastic deformations of the casing walls with brittle breakouts of abradable and thermal barrier coatings.

If there is, momentarily, no air flow during surge, the rotor emits it’s energy into the air and onto the blades as whirl loss. There is also the danger of overheating failures in the compressor. Added to that, there is the danger that the hot gases from the combustor expand in the rear area of the compressor

Surge also means a lack of cooling air during simultaneously increased fuel supply in the combustor. Cosequently, there is the danger of unpermitted excess temperatures on the hot parts.

"Illustration 3.1.1-7": With a gas turbine of an elder type test runs were apparently carried out for the determination of the operating behavior of the compressor (surge line, efficiency). The blading material seemed to be a martensitic steel and showed markedly roughness from erosion and corrosion. This may have lowered the fatigue strength considerably.

Between the runs the cross section of the exhaust area was noticeably reduced. Probably with the consequence of an distinct backpressure of the exhaust gas. This caused compressor surges. Some rotor blades of a stage to the front of the compressor were subject to such an intensive vibration that it came to fatigue cracks and the fracture of one blade.

The result was a catastrophic compressor damage which made a new blading inevitable. Besides the high costs the resuming of the operation was drastic deleted.