Table of Contents
3.7.1 Inlet air and the exhaust area
The efficiency of a gas turbine is finally dependent on the condition of the units up- and downstream. ( "Ill. 3.1.2.2-6" and "Ill. 3.7.1-1"). Among the up- and downstream units are:
- Sound absorbers at intake and exhaust.
- Air inlet filter (Lit. 3.7-7).
- Evaporation cooler for the inlet air.
- Bypass valves for intake and exhaust.
- Heat exchanger and steam generator (GaS units, "Ill. 2.1-3.3").
The inlet duct/channel and its installations.
Noise suppressors ( "Ill. 3.7.1-1") are difficult to control from the outside. During corrosion or through vibrations, unnoticed metal parts can break out, causing extensive secondary failures in the compressors. If substances from the inner suppressor break loose, combustor fins or cooling air passages in the hot parts of the turbine are in danger of blocking. Especially susceptible are the gills of combustion chambers or cooling channels in turbine blades ( "Ill. 3.3-3"). Outwardly, the filters are often covered by a sheet metal comprising low alloy steel, which rusts only lightly. This shell can then only be thoroughly visually examined for cracks (e.g., at weldings) when the rust is removed. The inner walls are to be checked for paint damages (flaking) and corrosion. They must, if necessary, be stripped of paint and rust, in addition to being provided with a suitable paint afterwards.
Also at the first sight non hazardous seeming flaking rust and paint particles have a high damage potential. According to experience they can block the cooling channels of the hot parts and so cause a considerably reduction of the lifetime of these expensive components ( "Ill. 3.3-12").
The most diverse filter systems ( "Ill. 3.7.1-3") are employed for the filtering of the inlet air, relative to the kind of particle and size. The operator must make sure that the filter effect is ( "Ill. 3.1.2.2-7") maintained over the entire time of operation ( "Ill. 3.1.1-2").
It’s, therefore, very important that the operator examines the inlet duct between filter and engine for the smallest leakages and, if required, immediately seal them. In order to operate it in an aimed fashion, he should understand the function of the filter ( "Ill. 3.7.1-2" and "Ill. 3.7.1-3"), paying heed that the pressure drop in the filter (i.e. the filter resistance) does not get prohibitively huge through blockages. For this purpose, he must monitor the instruments in their indication and function. If the filter resistance, however, is unusually low, one may assume that the filter is indeed so blocked that the bypass valve has already opened, deleting the filter effect and the engine proceeds to suck in the particles directly. Bypass valves are usually rectangular, weight-loaded flaps that release an opening during filter blockage, through which the inlet air can flow in. Such a bypass flow carries the danger of turbulences of the inlet flow, inducing fatigue failures in the compressor.
When filtering according to the band principle, one has to pay attention to the function of the drive and the guidances. On demand, it should be lubricated with a lubricant suggested by the manufacturer. Simultaneously, an examination of the loosening corrosion products which could block the hot parts, is to be carried out (chapter.3.3.2-4). Of course, one has to guarantee that the filter band does not discontinue, so that the compressor takes in unfiltered air.
Filters that work according to the principle of inertia, i.e., through deflection or rotating the flow, should be periodically checked for rust formation. If needed, paint touch up should be undertaken.
An obvious aspect of the filter is the possibility to examine residues, hinting at the sort and root of the air contamination, which could have influenced the engine. Those investigations are the precondition for specific actions against blocking and/or damaging reactions of the hot parts.
Surprisingly, there is the danger of icing at temperatures above freezing point for the filter and the inlet area of the compressor. This is decisively dependent on the atmospheric humidity and air velocity ( "Ill. 1.1-2" and "Ill. 3.7.1-5").
Evaporation coolers are used at hot sites, in order to cool down the inlet air, inducing a rise in the average mass flow to stimulate adequate engine performance. Transported humidity from the air flow can lead to corrosion, erosion and deposits (compressor fouling, "Ill. 3.7.1-6"). One should ensure that the components of this system work without hindrance. It is vital that the evaporator elements are arranged in the proper sequence and are not damaged. The spray nozzles for the water must show the prescribed spray distribution and a sufficient flow through. Naturally, one should pay attention to the maintenance of the water level in the reservoir, i.e., one may have to recheck the function of the level switch.
The water composition is not permitted to contain any dangerous impurities. This includes gypsum dust that can stimulate sulfidation ( "Ill. 3.4-2") in the hot parts. Here, one must be aware that there could be a gathering of impurities with time, if the evaporated water is exchanged occasionally. These impurities can collect in the evaporator and hinder evaporation, damaging the circulating pumps for the water, which must be renewed or cleaned at regular intervals.
Note:
A filter that is bypassed by even the tinies leak is ineffective.
The exhaust duct and the built in components
Different units such as the filter, heat exchanger, and sound absorbers are to be found in the area of the exhaust unit ( "Ill. 3.7.1-1"). These stay in a changing relationship to the gas turbine, i.e., changes in the gas turbine can influence the components in the exhaust ducts. Vice versa, alterations on the components can also have an effect on the gas turbine.
On the grounds of a noticeable flow resistance at the turbine outlet, the gas turbine can perform weakly. This can raise the hot part temperatures and deteriorate compressor behavior ( "Ill. 3.1.1-2"). An increase of the flow resistance could, e.g., be the result of failures on the exhaust noise suppressor. Pressure losses in the entire system are to be considered when acquiring an engine and by means of inspection and maintenance it is to be seen that this does not rise to a prohibited extent.
Noticeable gas vibration in the exhaust duct could also have an effect on the turbine.
More frequently, there are damages on the exhaust duct and the exhaust sound absorber. Plate vibrations on the flat walls of the exhaust ducts with rectangular cross sections can lead to fatigue cracks and to breakaways. Especially difficult to control are the low frequencies under 100 Hz, typical for the exhaust flow, also in relation to sound absorbing measures. Ducts with rounded cross sections are stiff in themselves and, consequently, less sensitive to fatigue failures. Therefore, their thermal expansion behavior (e.g., during the start) is more favorable, e.g., at start. The operator should thus choose tubular exhaust ducts during an acquisition, even if the acoustic behavior is not as good as that of flat walled ducts. Ducts where the exhaust flow hits the walls directly (elbow, bend), are especially vulnerable. Unfavorable pressure and velocity distribution of the exhaust flow can be avoided with the help of the baffles. A further problem is the emergence of hot strands, creating hot spots on the inner walls, leading to distortion through hindered thermal expansion and cracking due to thermal fatigue ( "Ill. 3.3-16") as well as through local heating up of the outer wall, threatening the maintenance personnel with burns.
High exhaust temperatures up to 600° C demand sufficient heat and oxidation resistant materials at the inner walls. Corrosion resistant steels are especially reliable. The outer walls can be made out of low alloy steel. Since one has to reckon with condensation formation during static, a noticeable amount of corrosion has to be considered. The exhaust sound absorber is particularly affected. Here, because of the vibration load, the type of packaging and covering of the absorber mats are decisive for operation behavior and long term durability. Inappropriate mat material can crumble and even fully disappear.
A vulnerable area of the exhaust duct is that of elongation compensating connections and compensators, which, as elastic connecting links, are meant to balance the most diverse thermal expansions of components. If mistakes of choice are made here, frequent repair and exchange is fore programmed. The choice of especially cheap compensators with limited specific aptitude is not cost saving in the long run.
Near the mechanical uncertainties, the acoustical demands of a component are to be considered. In this connection, it is especially important that the surroundings of the location should be observed. Are noise sensitive units (e.g., hospital) nearby or are there other sources of noise that could induce stronger effects? If several neigboring engines are to be used, they should be housed, for acoustical reasons, in one building.
Note:
Own „Improvements“ in the peripherals of an engine can trigger unexpected problems at the engine itself!
"Illustration 3.7.1-1": Typical built in units influencing the flow ( "Ill. 3.7.1-5") in front of and behind a gas turbine: (A) intake filter, (B) intake sound absorber, (C) bypass, (D) Exhaust sound absorber, (E) exhaust duct, (F) exhaust heat exchanger, (H) power output and driven system (e.g., generator, pump, compressor).
"Illustration 3.7.1-2": The air inlet system (according to Lit.3.7-13 and Lit. 3.1.2.2-2) of a gas turbine ( "Ill. 3.7.1-1") tends to be very complex. Its components are specialized to perform the most varied tasks (Lit. 3.7-7 and Lit. 3.7-13). Due to the local requirements and the operators wishes, all possible built in devices are not necessarily present. The picture presents the individual components and their arrangement in relation to each other schematically. The surrounding air enters at the left.
- (A) weather protection,
- (B) wire mesh guard (against animals as well as birds and cats,
- (C) self purification of the filter system of older assembly styles (see "Ill. 3.6.1-3"),
- (D) second filter,
- (E) bypass during filter blockage,
- (F) sound absorber (shielding of the surrounding against compressor noise),
- (G) air pre heater (against icing, see "Ill. 3.1.1-1" and "Ill. 3.7.1-5"),
- (H) casings.
"Illustration 3.7.1-3": This figure (Lit. 1.1-7) shows the historical development of filter systems. It demonstrates the variety in filter systems (Lit.3.1.2.2- 2), capable of being combined so as to be in a position to fulfill every users specific demand. At first, systems were introduced that solely prevented clear geometric alterations and the roughening of blades through erosion and impacts from small foreign objects ( "Ill. 3.1.1-2"). So failure relevant geometric profile changings could be avoided. Wet or dry roll filters are typical for this purpose. The filter resistance of these units are remarkable.
In order to eject bigger particles with possibly less filter resistance at higher air flow rates, inertial filters were introduced. Today, there are two basic types: those that effect a deflection of flow with flat blades and those that alter the flow into a rotation. In both cases, the flow is so deflected that the particles in the outer zone of the flow are centrifuged and separated by an additional split off flow. These filter systems have a bad degree of precipitation, especially with regard to small particles ( "Ill. 3.1.2.2-7" and "Ill. 3.7.1-3"), during extreme dust falls (as in desert areas) thoroughly dangerous amounts of dust can pervade the engine.
Small particles under 0,01 mm can induce so called fouling. The presence of additional corrosive media (e.g., dirty condensation) enhances the danger of corrosion. With the purpose of preventing these failures, highly effective filter systems were introduced, which can be divided into two main groups: Filters with in depth effectiveness (deep bed filters) and surface filters. As the name suggests, in surface filters particles gather on the surface and form a ‘filter cake’, increasing the effectiveness of the filter. The deep-bed filter, through its very structure, deflects the air flow in such a manner that the particle of the material can be retained in the interior. The disadvantage of these filter systems is their relatively short overhaul, respectively, exchange intervals. These highly effective filter systems for small particles are the choice today, possessed, if need be, with upstreamed inertial filters that do not have any blockage problem or rougher plate filters that are, quickly and cheaply, exchangeable. Especially exacting conditions are at hand, should the inlet air be saturated with abundant humidity. Such conditions are typical for oil bore platforms.
A further stage of evolution lies in the self purification systems that should make a minimum repair effort for the filter system possible. Highly effective, tubular, surface filters are vertically arranged in these units. If the filter resistance has reached a borderline value, the filter process for the affected element is stopped temporarily. On the exit side of the filter an air pressure surge is released (Lit.3.7.1-2), detaching the deposited particles. These fall to the floor and can be removed from there. Such filter systems are also trustworthy in cold surroundings, in conditions of snow and ice ( "Ill. 3.7.1-4.1" and 3.7.1-5).
Valid for the choice of a filter system (Lit. 3.7.1- 7): sufficient long time effectiveness is to be paid attention to particularly. Filter effectiveness in a new state is no adequate criterion (Lit.3.1.2.3- 1).
"Illustration 3.7.1-4.1": Basically according to Lit 1-1 there exist two main forms of icing ( "Ill. 3.7.1-4.1" and "Ill. 3.7.1-5"). They appear in the stationary operation also as in test benches: An icing of the compressor inlet or an icing of the whole rig. Those icing types differ in the formation temperature.
„A“ Icing of the compressor intake can also at ambient temperatures markedly above the freezing point occur, depending on the prevalent air humidity ( "Ill. 3.7.1-4.1").
„B“ General icing of the rig needs temperatures about or under the freezing point and condensed water. In this case the ice can build up as well on the bell mouth (intake cone) as at the compressor intake ( "Ill. 3.7.1-5"). Especially endangered are possible FOD, grills on the bellmouth. Here the icing can be so fast that the grill is even sucked into the engine due to the underpressure.
"Illustration 3.7.1-4.2": An icing of the intake region can trigger failurs of the gas turbine ( "Ill. 1.1-2" and "Ill. 3.7.1-5"). H.J.Willcocks describes the important conditions for icing in the compressor inlet in Lit.3.6.3-3. Of specific influence on the flow temperature is the air velocity and air humidity. TT (T-Total) is the temperature of dry air at rest (relative low air moisture), TS is the temperature of the dry air flow. Through the velocity of flow in the inlet, the temperature of the previous dry air at rest sinks many degrees. This drop (grey area) is dependent on the relative air humidity. The higher the atmospheric humidity, the greater the energy set free by the condensation of moisture and the lower the temperature drop. The latter is also connected to the inertia of the condensation process. The dwell time in the inlet is clearly too short, on account of the high speed of flow, to reach the balanced condition of condensation. Thus, the actual temperature rise through condensation is correspondingly lower. For air with a relative atmospheric humidity of nearly 100%, one has to reckon with a temperature drop in the gas flow of many degrees (with typical velocity values in the compressor inlet).
So in the intake region of a gas turbine the danger of icing exists, even when the temperature exceeds 0°C.
"Illustration 3.7.1-5": The icing of gas turbine units during extremely cold weather is described by J.Dickson (Lit.3.7-4 and 3.7-5). Depending on the origin of the humidity, there are two types of icing:
Through frozen precipitation and icing through condensation ice. For the former, the presence of water in an air flow under 0° C is required. Water can then exist in a variety of forms, snow, snow pellets, frozen rain, floating ice crystals, fog and undercooled water drops. The latter freezes immediately, as soon as it meets a surface. Thus, within seconds, centimeter thick milky ice layers are formed. Through such snow and ice deposits the inlet resistance can be increased to unduly heights and there is the danger of ice entering into the compressor. One can reckon with bigger foreign object damages in this case
In a certain type of derivate, with about 3000 KW performance, a hand formed snow ball of about 10 cm. diameter can evoke a total damage of the axial compressor. Problems can arise in different areas of the plant inlet. Icing problems can occur in different regions of the plant inlet:
“A”: Strong, hoarfrost, e.g., in front of the inlet filter. Hoarfrost originates when oversaturated air is whirled and comes into contact with cold surfaces.
“B”: Ice formation and ice cones at the inlet noise suppressor.
“C”: The icing of the bypass flap increases the risk of hindering inlet air flow. When using the flap, there is the danger that released ice gets into the compressor inlet.
“D”, ice in the plenum chamber: Ice and ice cones can originate on every projection in the inlet duct and are potential foreign objects (see also "Ill. 3.1.2.2-2"). Especially precarious are ice cones on the ceiling of the inlet space. They can break off and fall into the compressor inlet.
“E”: Icing of the compressor inlet, of the bellmouth, the inlet guide vanes or the front radial casing struts. This includes icing through condensed water at already relatively high temperatures ( "Ill. 3.7.1-4.1" and "Ill. 3.7.1-4.2"). Further problems: icing of external cooling air heat exchangers, such as, e.g., Dundas describes for a heavy frame engine. Lubrication oil problems, (high viscosity) and lubrication system problems (oil cooler).
Remedies: In technical literature a multiplicity of preventive measures are set down, in order to avoid problems concerning icing and low ambient temperatures.
"Illustration 3.7.1-6": The term exhaust fouling indicates the undesirable influence of the downstream periphery (Lit. 3.7.1-6). (“B”) (see "Ill. 3.7.1-1") through sedimentation. Corrosion and oxidation in contact with the exhaust flow (“A”). These changes are noticeable through an increase in the flow resistance, leading to a pressure rise behind the turbine which impairs the performance. The lower the velocity of the gas and wall temperature, the higher the fouling rate or rather the fouling speed. This shows the rise of the flow resistance during operation. Here, the gas velocity is the deciding parameter. The fouling rate is highest at the beginning and decreases over a period of operation to a low, almost constant value.