GER-3419A

RL. Loud and AA SlaterpryceEngineer, Gas Turbine .wer, Gas Turbine

Power Plant SystemsGeneral Elect& Compasiy

Schenectady, NY

CONTENTSPage

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Air Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-..... 1Anti-Icing Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Inlet Cooling Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22A Table Conversion Factors is included at theend of this publication.

INTRODUCTIONGas turbines manufactured by General Electric

Company are operating successfully in rural areasand heavy industrial zones, in polar regions andthe tropics, in deserts and at sea In order to adaptmachines to a variety of environments while realiz-ing their full potential in performance and relia-bility, it is often necessary to treat the air whichthey consume. Even in relatively clean environ-ments, a gas turbine may ingest hundreds ofpounds of foreign matter each year. Whether ornot this will cause a problem depends on theamount of this material, its mechanical properties,and its chemical composition. The hazards ofnonremoval include erosion of compressor andturbine components, fouling of compressor air-foils, and corrosion. Solid particles are removedby appropriate filters, while potentially corrosiveliquids are removed by moisture separators.

In warmer climates, the power available froma gas turbine may be increased by using an evaporative cooler or a chilled water cooling system.Conversely, in very cold environments it is nec-essary to avoid icing of components such as inletfiltration and the trash screen, silencers, andinlet guide vanes.

This paper will discuss in detail the environ-mental conditions that indicate need for inletair treatment, the specific equipment utilized byGeneral Electric to perform these functions,and their impact on gas turbine operation.

1

AIRFILTRATION

Need for Filtration

Any gas turbine, due to its inherent designand the enormous amount of air consumed(e.g. 1296 lb/s or 587 kg/s for the MS9001F), issensitive to air quality. Filtration is applied toprovide protection against the effects of contam-inated air that may degrade gas turbine perfor-mance and life: erosion, fouling, corrosion, andcooling passage plugging. The need for properfiltration has increased in significance due tothe complex designs of the advanced technology7F and 9F machines.

E r o s i o n

Both the axial compressor and the hot-pathparts can be affected by erosion from hard, abra-sive particles, such as sand and mineral dusts.

As these particles impact upon the compres-sor blades, they cut away a small amount ofmetal. The net rate of erosion, although ‘notprecisely quantifiable, depends on the kineticenergy change as the particles impinge, on thenumber of particles impinging per unit time,the angle of impingement, and on the mechani-cal properties of both the particles and thematerial being eroded.

In general, our gas turbine experience indi-cates that particles below 10pm do not causeerosion, whereas particles 20pm and abovenormally cause erosion when present in suffi-cient quantities.

Two examples of eroded parts, a compressorblade and a first-stage nozzle, are shown inFigs. 1 and 2. Not only does erosion reduceaerodynamic performance, but the reductionin cross-sectional area of the compressor bladecould lead to serious turbine damage if,because of increased local stresses, it shouldbreak loose during operation. Air filtrationmethods are available which can easily and veryefficiently remove airborne particles of 1Opmand above.

COMPRESSOR

Figure 1

Compressor Fouling

The efficiency of an axial compressor is depen-dent on, among other considerations, thesmoothness of the rotating and stationary bladesurfaces. These surfaces can be roughened byerosion, but more frequently roughening iscaused by the ingestion of substances whichadhere to the surfaces. These include oil vapors,smoke, and sea salt. Figure 3 shows depositsformed on the leading edge of rotating blades.The output of a turbine can be reduced as muchas 20 percent in cases of extreme compressorfouling. The rate at which this fouling takes placeis difficult to quantify because it depends not onlyon the types and quantities of materials ingested,but also on the peculiar properties of the sub-stances that cause them to stick. Filtration canremove some, but not all, of these substances.Certain vapors that are adhesive when they con-dense can pass through filters. Other vapors mayoriginate between the filter and the compressor,such as occasional lubricating oil leaks.

Fortunately, today there are ways of removingthese adhesive deposits from the compressor

Figure 3

GTO 4151

ERODED FIRST-STAGE NOZZLE AIR FOIL

TC 197421

Figure 2

blades. One of these is the use of mild abrasivecleaning materials such as crushed nutshellswhich, when ingested at a controlled rate intothe compressor during operation, remove someor all of these deposits (depending on theirnature). Recent experience has shown that theuse of abrasive cleaning can damage compressorblade coatings and compressor blade surface fin-ish. Abrasive cleaning compounds are also apotential cause of plugged cooling passageswhich will be descussed shortly. For these rea-sons, abrasive cleaning is generally not recom-mended on any size turbine, and definitely notrecommended on the advanced technology tur-bines. General Electric Instruction GEL41042should be consulted for the use of any abrasivecompressor cleaning agents on GE gas turbines.

More recent work within the industry hasshown that certain ash-free detergents are veryeffective in removing compressor blade deposits.Both offline and online water wash systems areavailable. Offline involves injecting the cleaningsolution into the compressor while it is turningat cranking speed. Online is not as effective asoffline but has the advantage that it can be

EFFECT OF CLEANING ON GT OUTPUT

OUTPUTLOSS

FIRED HOURS

GTO 415;

Figure 4

2

applied during turbine operation. Specific waterwash chemical recommendations are containedin GEI41042.

Figure 4 shows the effect of compressor clean-ing on gas turbine output. It shows that outputdeteriorates along a decaying exponential curveas the compressor continues to foul. Removal ofthe fouling deposits restores gas turbine outputto nearly the original value.

Compressor CorrosionCorrosion of compressor components can be

caused by wet deposits of sea salt, acids, and otherdeleterious materials. In addition to rusting ofcompressor wheels, such corrosion is also manifest-ed as pitting of the compressor blading. Pittingcauses a roughening of the airfoils with conse-quent reduction in the aerodynamic performanceof the compressor. These pits also cause local stressrisers and may diminish the fatigue life of theblades. In addition to filtration, protective coatingsfor both blading and wheels have been very effec-tive where environments are known to contain cor-rosive compounds.

Hot-Section CorrosionPossibly the single most important and fre-

quently encountered consequence of inadequateair filtration has to do with the ingestion of cer-tain metals which, after combining with sulfurand/or oxygen during the combustion process,deposit on the surfaces of the hot gas path parts.These parts include combustion liners, transitionpieces, nozzle partitions and turbine buckets.There are four such metals which are of primaryconcern: sodium (Na), potassium (K), vanadium(V) and lead (Pb). These metals, either as sul-fates or oxides, cause the normally protectiveoxide film on hotgaz+path parts to be disruptedso that the parts oxidize several times faster thanin the presence of gases free of them.lJ They maybe found in fuels and in water or steam, as wellas in the inlet air. Allowable limits are set forth inGEI-41047. The effects of these contaminants onthe turbine are also discussed in this reference.The following relationship may be used to calcu-late the limits in the inlet air:

(A)&+ 6)(F) (F)*+*

Equivalent contaminants in fuel alone

where

++ = Air-to-fuel massflow ratio

= Steam-to-fuel mass flow ratio

x, = Contaminant concentration(weight) in fuel (ppm>

x, =‘ Contaminant concentration(weight) in inlet air (ppm)

Xs = Contaminant concentration(weight) in injectedsteam/water (ppm)

When concentrations of trace metals in fuel,water, or steam are not precisely known, a limit forthese contaminants in the inlet air of 0.005 ppmwill normally be set This limit, based on experi-ence, would cause an insignificant contribution tothe overall contaminant level and have a minoreffect on parts lives. Reference should be made tothe appropriate fuel specification for guidance.

Cding Passage PluggingFlow of cooling air through passages in the

combustion liner, nozzles, and buckets is neces-sary to control metal temperatures of theseparts. Since the cooling flow is extracted fromthe compressor of the gas turbine, contaminantsin the inlet air may also be present in the cool-ing air. If these contaminants cause a buildup inthe cooling passages, heat transfer is degradedand temperatures may increase to levels whichgive rise to cracking. This is especially critical inthe advanced technology “F” machines which,because of their higher firing temperatures,require a very complex system of cooling pas-sages. Coal dust, cement dust, and fly ash areparticularly bad, since they tend to sinter.

Environments

Ambient air can be contaminated by solids,liquids, or gases. Of these three, contaminationby solids is the most common, and usually themost serious situation. The quantity of solids canbe defined in many ways, such as milligrams percubic meter of air or grains per 1000 cubic feet.A measure General Electric finds convenient isparts per million (ppm), i.e., the mass of con-taminants per million units mass of air. The factthat this is a convenient measure immediatelydemonstrates that the quantity of dust is gener-ally quite small compared to the mass of air..However, when account is taken of’the large:flow rates of gas turbines, it is evident that thetotal quantity of dust which is ingested can beappreciable when summed over hundreds orthousands of fired hours.

3

In the United States, the EnvironmentalProtection Agency samples airborne particu-lates periodically at some 4000 locations.Results of the annual surveys are published, giv-ing an excellent idea of the statistical variationof dust loading at the test sites.4 The typicalrange of values is shown in Fig. 5. Curve A givesthe percentage of U.S. sites exceeding a givendust load 50 percent of the time; curve B showsthe percentage of test locations exceeding aparticular dust load 10 percent of the time.These curves show that the typical dust concen-tration in most locations is from about 0.03 to0.06 ppm, with occasional excursions at somesites to 0.2 or 0.3 ppm. It must be understoodthat particular locations may deviate significant-ly from these typical values. While the curvesare derived only from U.S. data, similar valuesare to be expected in other developed countrieshaving temperate climates.

Dust loading in desert regions, particularlythose subject to sand and dust storms, is muchhigher than those usually experienced in theUnited States. Concentrations in sand stormsmay reach several hundred ppm for periods of

PERCENTAGE OF U.S. SITESEXCEEDING GIVEN DUST LOAD

. ..I.* AZ ,OlH PCRClllllLCm IOTH PC”CCWm.L9,

10 BPERCENT ,D A

lho

20

3

0.10.01 0.1 1.0

DUST LOAD. PPMTO 767W

Figure 5

TYPICAL VARIATION OF DUSTCONCENTRATION WITH ELEVATION

2

RELATIVEOUST t

COI(CENTRAlION

Figure 6

several hours, while long-term levels may aver-age one to five ppm. When the wind blows inthese regions, the larger soil particles becomeairborne first, smaller particles being moreadherent. When the large particles fall back toearth, they disturb the surface and “splash out”fine particles. By Stokes’ law, fine particles settlemore slowly; so they remain airborne longer.The results are that the dust concentration ishighest close to the ground, and that the parti-cles there tend to be coarser than at higher ele-vations. There is no exact relationship betweendust concentration and elevation above ground,but available data generally tend to fall withinthe range of Fig. 6. This shows that elevating afilter compartment some 20 ft. in the air approx-imately halves the dust load, compared to aground-mounted compartment.

The size distributions of airborne dusts arevariable with respect to time and place. In gen-eral, high values of dust concentration tend tobe associated with coarse dust and low valueswith fine dust. Large dust particles tend to fallout quickly, while smaller particles are morelikely to stay airborne. Consequently, dust sam-ples taken near the source of contaminationtend to be coarser than those samples taken ata distance.

Some idea of the size distributions experi-enced in practice can be had by reference to thestandardized dusts, Arizona Coarse and ArizonaFine, which are widely used in the testing of airfiltration devices. Table 1 shows their mass distri-bution as a function of particle size. Since the

Table 1Components of Arizona Road Dust

Particle Size

=%e(mkrons)

O-5

5-10

lo-20

20-40

40-80

80-200

Nominal Percentage ofTotal Mass of Particles

Course FineDust Dust

12 39

12 18

14 16

23 18

30 9

9

4

mass of a particle is proportional to the cube ofits diameter, it will be recognized that typicaldusts have many fine particles, and relatively fewparticles of large diameter.

The chemical composition of airborne dustsis significant, particularly with regard to sodi-um and potassium, which contribute to hotcorrosion. General Electric has analyzed manydust samples from around the world. Typically,it is found that airborne dust has about twice asmuch of these elements as do local soils. Thisapparent anomaly arises because the fine parti-cles of soil tend to be richest in sodium andpotassium, and these particles stay suspendedin the air while larger particles fall back toearth. Airborne dusts from different locationsvary widely in their composition, dependingpartly on local soils and partly on industrialpollutants released into the air. In general, themost corrosive dusts come from desert regionsin which the soil is a former seabed. Sodiumand potassium may make up as much as 5 per-cent of the weight in extreme cases. Values of 1to 3 percent are more typical. As an example,suppose that the long-term average dust load ata desert site is 3 ppm, and that the dust is 2percent sodium plus potassium. The corrosivecontent of the dust is therefore 3 x 0.02 = 0.06ppm. Assuming a typical air/fuel mass ratio of50:1, this is equivalent to 50 x 0.06 = 3 ppmsodium plus potassium in the fuel. This isexcessive and indicates that inlet air filtrationwould be required.

Equipment DescriptionEquipment designed. by General Electric to fil-

ter the inlet air can be divided into two classes,conventional and self cleaning. Conventional fil-ters include inertial separators and media-type fil-ters; the latter are normally replaced when theybecome dirty. Selfcleaning filters, introduced inthe 197Os, have become well accepted and nowaccount for 80 to 90% of the new systems sold byGeneral Electric. These are media-type filterswhich have the ability to renew themselves byautomatically shedding accumulated dust

An important characteristic of an air filter isits collection efficiency, calculated from theweight of dust entering and leaving:

WEfficiency = entering -Wleaving

W enteringx 100 (percent)

Collection efficiency varies with particlesize, typically being lower for small particlesthan for large.

High-Efficiency FiltersHighefficiency filters use a special filter medi-

um of fiberglass or treated paper to achieve goodcollection efficiency for all particles, includingthose as smaller than lpm. Figure 7 shows typicalefficiency as a function of particle size. Becausethe collectiori efficiency is very high, the air qual-ity downstream is also high, even when the ambi-ent air is badly contaminated,

High efficiency filters generally take the formof either a rectangular panel filter or a cylindri-cal cartridge filter. Other than their shape, thetwo major difference2 are in the type of mediaused and the design of the seal where the ele-ment is attached.

The panel filters typically contain a depthloading media. Particles are actually trappedwithin the body of the media itself. Depth load-ing media has a billowy texture which allows theparticles to penetrate. Cylindrical cartridge fil-ters contain a surface loading media. This trapsthe particles on the outside face where theyform a dust layer. This dust layer actuallyenhances the collection efficiency of the filtercausing it to become more efficient with time. Itis the surface loading characteristic that allowsthe dust to be dislodged during cleaning.

The seal on the filter element is another areaof prime importance. There is no point in provid-ing filters of such high efficiency if contaminatedair is continually leaking past them. It is here thatthe cylindrical cartridge has a distinct advantageover the rectangular panel. Cylindrical cartridgeshave a continuous, circular, neoprene gasket per-manently affixed to each element. This gasket iscapable of making up for variations in the matingsurface. Panel filters have various types of sealingmechanisms available, but none have shown to be.as reliable as the cylindrical cartridge seal.

Rectangular panel filters are available as areplaceable element which is held in shape by a

TYPICALFILTRATION

EFFICIENCY

EFFICIEKY

Figure 75

I HIGH-EFFICIENCY FILTER I

HIGH-EFFICIENCY

FILTERS

Figure 8 Figure 9

wire frame, Fig. S, or as an integral assemblywith the media bonded to a metal frame, Fig. 9.Cylindrical cartridge filters are also integralassemblies incorporating the frame and mediainto a single unit, Fig. 10.

High-efficiency filters have an initial pressuredrop which depends upon their construction,installation, and on the quantity of air passedthrough each filter element. Filters normally usepleated media in order to increase the availablesurface area; this decreases pressure drop andincreases dust-holding capacity. As dust is accu-mulated, pressure drop rises. The rise is relative-ly slow at first, but increases more rapidly as thefilter nears the end of its useful life. A typicaldesign would have a new-and-clean pressuredrop of about an inch of water; the final pres-sure drop depends upon a trade-off between fil-ter life and gas turbine performance. GeneralElectric recommends a final pressure drop of2.5 in. as a good compromise for panel filtersand 4.0 in. for cylindrical cartridges.

The dust-holding capacity of a filter is animportant characteristic, since it relates to filterlife. Dust holding capacity is not easy to define,

CROSS-SECTION QU,CX.NUT

CYLINDRICALCARTRIDGE

EXPANDED METALINNER LINER

SEALINGWASHER

PERFORPlTED METALOUTER LINER

GTZI 131

Figure 10

G T O 7680

since it depends upon the particle size distribu-tion of the incident dust. For a given pressuredrop, a filter can hold a greater mass of largeparticles than of small particles. Therefore, a fil-ter will “load up” more quickly with fine dustthan with the same amount of sand. Filter manu-facturers commonly use Arizona Fine test dustto rate high-efficiency filters. This is reasonable,since it provides a conservative rating which isvalid even in environments where larger parti-cles may not be present in the ambient air.

Self-Cleaning Filters

Self-cleaning inlet filtration, developed in the197Os, combines the effectiveness of the high-efficiency filter with low maintenance. This com-bination of characteristics is realized by using abarrier-type filter element which accumulatesdust on the surface which is exposed to theambient air. The collection efficiency is typicalof high-efficiency filters. When pressure dropbuilds up to a predetermined level, the filter iscleaned by a brief back-pulse of air, eitherextracted from the gas turbine compressor, or

GT?114

Figure 11

derived from an auxiliary source. A filter com-partment includes many filter elements, only afew of which are cleaned at any given time; sothe airflow to the gas turbine is not disturbed bythe cleaning process.

The action of a selfcleaning filter is illustratedin Fig. 11. Air flows through the filter elementsinto a clean-air plenum. Dust in the air is trappedon the surface of the lilter media, which is formedfrom specially treated cellulose, synthetic, or com-bination cellulose/synthetic lilter paper. The filterelements are typically in the form of cylindricalcartridges. The paper is pleated in order toincrease the available sutice area. Many filter ele-ments are used so that the velocity of the airthrough the filter media is very low, in the rangeof 2.5 to 3 ft/min. This low velocity decreases pres-sure drop, increases dust-holding capacity, and isessential to the cleanability of the filters.

As the filters accumulate dust, the pressure dropgradually rises. When the pressure in the clean-airplenum reaches a particular value, usually set at 3to 4 in. of water, gauge, cleaning is initiated. Acleaning manifold is pressurized nominally to 100psig with compressed air, extracted from the gasturbine compressor or some other suitable source.Upon command from the automatic sequencingcontrol, a solenoid-operated air valve directs abrief (about 0.1 s) pulse of air into the filters. Thisshocks and causes a momentary backflow throughthe filters, dislodging accumulated dust from theoutside of the elements and allowing it to disperse.Re-entrainment of dust is minimized by the low-velocity design. The updraft and between filtervelocities are kept at or below 320 and 580 feet perminute respectively. The process continues, clean-ing a few filters at a time, until all elements havebeen cleaned and pressure drop reduced to anacceptable level. A single cleaning cycle is usuallycompleted in 20 to 30 minutes. This ensures thatthe compartment can handle heavy dust loads,such as those associated with sandstorms, withoutexcessive rise in pressure drop.

The filter elements are replaced when theybegin to show signs of deterioration caused byheat and ultraviolet rays from the sun, or whenthe cleaning cycle can no longer restore pres-sure drop. While this period cannot be quanti-fied for all environments, experience indicatesabout a two-year life in Middle-East deserts.However, filter life may be sustantially lower inextremely harsh environments such as thoseheavily laden with airborne cement dust About100 man-hours are required for complete filterchangeout for an MS7001 gas turbine.

Fipure 12 shows a typical self-cleaning com-partment installed on 4 MS6001 gas turbine. It

SELF-CLEANINGINLET FILTER

Figure 12

includes several hundred filter elements, mount-ed in modules which feed into a tapered clean-air plenum. Each module has the filter elementsenclosed by a metal skirt, which protects themfrom damage. The upward velocity of air intoself-cleaning modules is low, so the module actsas its own weather hood. In order to reduce thecompartment footprint, the modules may bearranged in two or more tiers. The lower tier ofmodules acts as the platform for access to theupper tier. Walkways, ladders, and railings areprovided as necessary for safe access. Sinceaccess to the clean air plenum is infrequentlyrequired, a bolt-on hatch is provided instead ofa door. There is a convenience outlet to aid inte-rior inspections and maintanence. A differentialpressure gauge/pressure switch is supplied toread plenum pressure, and control the opera-tion of the self cleaning system. Alarms are pro-vided for excessive differential pressure in theplenum and for low pressure in the pulse clean-ing air supply. Pressure switches are also provid-ed to initiate a controlled shutdown in the eventthat differential pressure in the plenumbecomes dangerously high.

Cartridge Type,Non-Self Cleaning Filters

In environments where the concentration ofairborne contaminants or other considerationsmake it impractical to pulse clean inlet filters,the high efficiency selfcleaning type cylindricalcartridges may still be used. Such a system canbe identical in configuration to the self-cleaningcompartment with the exceptioning hardware is omitted.

that the puls-.,

This ‘system acts as a static barrier filter whilemaintaining many of the advantages of the selfcleaning system. Such advantages include highdust holding capacity, positive sealing mecha-

7

TYPICAL PRESSURE-DROP HISTORY

PRESSUREDROP

INCHES OFWATER

l 13x5FILTER CHANGEOUT

l * COMPLETE CHANGEOUT

Figure 13

nism, inherent low velocities and low pressuredrop. The dust holding capacity of a single cylin-drical self-cleaning we cartridge filter is on theorder of 2500 grams for Arizona Fine dust. Dustholding capacities for high efficiency panel fil-ters are in the range of 400 to 700 grams of dust.

PrefdtersIf the ambient dust load is fairly high, as in

some industrial areas, it may be economical tofurther protect the high-efficiency filters bymeans of inexpensive, disposable media-type pre-filters. The mounting frames for the prefilters aretypically placed directly in front of the high-effi-ciency filters which they protect. Figure 13 illus-trates how the pressure drop of the entire filtersystem typically varies with time. Three prefilterchangeouts are shown for each change of high-efficiency filters, which is a common experience.

A typical two-stage filter system (prefilters andhigh-efficiency filters) can remove several hun-dred pounds of dust from the gas turbine inletair before the high-efficiency filters must bereplaced. It is not possible to predict specificresults based on average filter life because eachsite is unique; however, 5000 to 7000 fired hours

-is typical of two-stage systems in the UnitedStates. Without prefilters, the life would bedecreased by a factor of two or more.

Inertial SeparatorAir containing dust, dirt, and chemical con-

taminants enter the open end of a V-shapedpocket (Fig. 14). The ends of the pocket aresolid, with both sides made up of louvered slots.The dirt is separated from the air as the airturns to pass through the open slots in the sidesand the larger dirt particles continue in astraight line to a collection chute aided by ableed fan. The bleed rate is approximately 10

8

INERTIALTYPE FILTER

VANE TYPE

TOTAL AIR IN1

AIR OUTLET

1924611

Figure 14

percent of the primary flow. This system must bein operation while the turbine is running toensure the collection of particles.

Another type of inertial separator is the spin-type, shown in Fig. 15. The incoming air/dustmixture is given a spin by the stationary inlet cen-trifugal blades, thereby causing the heavier parti-cles to collect near the surface of the outer tube,where they are scavenged by a bleed fan system.The clean air is drawn through the center tubeand then on into the gas turbine. Spin tubes iypi-tally require more frontal area than van-type separators, which may sometimes preclude their use.

While the arrangements of vane-type andspin-type inertial separators are different, opera-tionally they have much in common. Separationefficiency rises rapidly at first as flow throughthe separator increases; it then reaches a broadplateau where higher flow has relatively smalleffect on efficiency. Since increasing flow leadsto higher pressure drop, which lowers the per-formance of the gas turbine, the design engi-neer must balance separator cost and perfor-mance against gas turbine performance whendeciding the appropriate design point. The

INERTIAL TYPE FILTER

SPIN TYPE

F i i e 1 5

usual choice is the point at which high collec-tion efficiency is combined with acceptable pres-sure drop, typically 1.0 to 1.5 in. of water.

The bottom curve of Fig. 7 shows typical sepa-ration efficiency as a function of particle size.Since the performance of an inertial separator isexcellent for particles larger than lOpm, thisprovides a defense against the compressor ero-sion described previously. It is also effectiveagainst corrosion if the corrosive particles are inthe greater-than-1Ollm size range.

Generally, inertial separators which, in a senseare selfcleaning by design, are used as the firststage of filtration, preceding high-efficiency fil-ters. In this case, they extend the life of thehigh-efficiency filters by removing some of thedust which would otherwise cause them to foul.

Weather ProtectionIn cold climates, ingestion of large quantities

of snow or fkeezing rain can cause icing of inletcomponents. This can adversely affect perfor-mance of filtration equipment, and may result inphysical damage to the inlet duct or the gas tur-bine compressor. In warmer climates, prolongeddownpours may overload inertial separators,allowing water to be transmitted downstream. Ifthere are no high-efficiency filters, this will not beharmful. If there are highefficiency filters, prolonged wetting will increase the pressure dropand weaken the filter media structure. .For thesereasons, certain applications may require that theair filters be preceded by weather protection.

Weather protection, when required, is usuallyprovided by means of an inlet hood, such asshown in Fig. 16, or weather louver%. Louversmay be subject to icing under winter conditions.Hoods do not have this problem, and havedemonstrated their suitability in both tropicaland arctic climates. Weather hoods achieverejection of precipitation by drawing inlet air

INLET COMPARTMENT WEATHER HOOD

PRECIPITATION TERMINAL VELOCITY

Figure 17

upward at low velocity, thereby discriminatingagainst the snow or rain which is falling down-ward at some terminal velocity. The terminalvelocities of different forms of precipitation varywidely (Fig. 17) ,s and it is intuitively evident thata given hood design will be more effective inrejecting fast-falling raindrops than slow-fallingsnowflakes. In order to quantify this, a modelingstudy was conducted on the computer, takinginto account not only the geometry of the hoodand its associated flow field, but also such factorsas the drop-size distributions in rainstorms ofvarious intensities, Figure 18 gives the rejectionefficiency of single hoods as a function of facevelocity and rainfall rate. A curve is also givenfor typical snow rejection. It is clear from theseresults that a high degree of rain rejection isavailable from hoods of moderate size, but thatlarge hoods are required to reject snow.

Inlet Filter Compartments

Inlet filter compartments are normally ele-vated in General Electric designs. If only iner-tial separators are used, this improves air quah-

WEATHER HOOD PENETRATION .

Figure 16

9Figure 18

MULTI-STAGE INLET FILTER

PREFn.lER ma” LF”ClCHCl

GT21lJ

Figure 19

ty entering the gas turbine; if the compart-ment uses high-efficiency filters, eIevation pro-longs the filter life. A typical arrangement ofan inlet filter compartment using conventionalcomponents is shown in Fig. 19. The enteringair first encounters a bird screen, then theweather louvres. The access door is just down-stream of the weather louvres. Entry is via acaged ladder and service platform. The com-partment includes interior lighting and conve-nience outlets, and a junction box for electri-

cal power. An alarm is provided to indicatethat the pressure drop of the inlet filters isexcessive. If corrective action is not taken andpressure drop rises further, pressure switcheswill automatically initiate an orderly shutdownof the gas turbine. The alarm is a signal to stopand service wornout filters.

Inlet compartments are typically fabricatedfrom 3/16 in. steel plate to provide a rigidstructure capable of withstanding severe envi-ronmental loadings. Guides for these loadingsare the Uniform Building Code and ANSIA58.1. A wind force contour of 40 lb./ft.*,Zone 4 seismic loads, and 40 lb./ft.* snowloads (as defined by these authorities) areused as design criteria. Allowable stress levelsare taken from the AISC Steel ConstructionManual. The standard finish is inorganic zincprimer with a high build epoxy intermediatecoat inside and out. The epoxy serves as a finalcoat for the inside while the external surfacewill require a final coat after installation.

Because of their size, inlet compartments areusually shipped in several subassemblies. Theseare seal welded in the field to form the completestructure. Inlet compartments are normally ele-vated in order to avoid ground-level polhta~~t~.

Table 2Classification of Ambient Air Quality

Air Quality Location Iand Use Description

Clean Rural Recreation Undeveloped landModerate rainNo extended dry seasonContinuous ground coverLight traffic

Suburban/Urban Residential Mainly single or multiple dwellingsPaved roads, Iight to moderate traffic

Dusty Rural Agriculture Dust-producing activities such asplowing and harvesting

Urban Commercialor Industrial

Stores, warehouses, trucking, miningconstruction, manufacturing

Contaminated Seacoast hY Less than 10 miles from salt waterDry Lake hY Former seabed in area of low rainfall

Urban Industrial Corrosive elements in dust, such as chemicals,cement, and coal-fired boilersAlso includes areas subject to smoke and fumes

Rural Agriculture Dust from chemical fertilizersDesert Arid Lands hY Little or no rain, extended dry periods “

Winds sometimes cause sand or dust stormswith limited (cl mile) visibility

10

Recommended Inlet Air FiltrationDesign of a system to adequately protect the

gas turbine requires knowledge of ambient airquality and establishment of criteria for inlet airquality; the ratio between them defines therequired filter efficiency. Trade-offs can bemade between first cost, maintenance cost, andgas turbine performance. The following guidehas taken these concerns into account, and givesrecommendations for heavy-duty gas turbines innonmarine applications.

Table 2 can be used to classify ambient airquality at a particular gas turbine site as eitherclean, dusty, contaminated, or desert When mak-ing judgments, consideration should be taken ofpossible future land use as well as current condi-tions. The gas turbine itself will have negligibleimpact on the quality of the air at the site, butother associated site developments may have aneffect. This is particularly true when the gas tur-bine is to be part of a larger process or system.

Recommended inlet air filtration can be relat-ed to air quality through the use of Table 3.

Self cleaning filters are the standard filtrationmeans for General Electric heavy-duty gas tur-bines. If passive filters are required, prefiltersare also recommended if the ambient dust loadis moderately high (> 0.1 ppm by weight), basedupon economic studies which show that this isto the user’s benefit.

In addition to the filters described in Table 3,other devices are needed to supplement conven-tional filters in special situations. Insect screensmay be required in subarctic, subtropical, andtropical locations subject to insect swarms. Theuser often provides this equipment. Weather

Table 3Recommended Filtration

I Filtration 1

Environment 1 1 HighEffIciency 1Prefdters

Passive SelfCleaning

Clean I 1x1 I

Desert I I IX

I

protection is recommended on all units withhigh efficiency filtration. Protection can be inthe form of weather hoods and/or rain louvres.The self cleaning system is inherently protectedby the skirts that extend to a few inches belowthe bottom of the filter cartridges.

For several reasons, General Electric does notrecommend that standard, conventional inlet fil-ter compartments be entered when the gas tur-bine is operating, or that high-efficiency filtersbe serviced under this condition. First, the pres-sure drop of the weather louvres appears acrossthe compartment access door, holding it closedwith a force of about 120 lbs., although becauseof the relative positions of the ‘hinges and thelatch, only about 60 lbs. pull is required to breakthe door free. Second, the pressure drop of thedirty high- efficiency filters tends to hold themin place against their mounts with a force ofabout 50 lbs., which must be overcome by theservice crew, who may be working several feetabove the floor. Third, the filters should bereleased and removed in such a way that they donot dump dust back into the airstream, and thismay not be possible if the crew does not havespecial training and supervision.

Compartments with conventional media-typefilters can be designed with special features toalleviate these problems. However, in most casesself-cleaning filter compartments offer a solu-tion that is technically and operationally moresatisfactory, and less expensive. General Electricrecommends the use of self-cleaning filters forall applications where site conditions requirethe use of high-efficiency filters and where thelife of these filters is projected to be shorterthan the interval during which continuous oper-ation is required.

In regions subject to sand and dust storms,such as deserts of North Africa and the MiddleEast, most users find it extremely burdensometo properly maintain conventional media-typefilters because dust loads may become high atunpredictable times, resulting in very rapid fil-ter consumption. Unless large supplies of sparesand a changeout crew are available at the site,the frequent result is that the gas turbine oper-ates for a period with the implosion door open,ingesting unfiltered air during a time when fil-tration is needed the most or the protectivepressure switches simply initiate a shutdown andthe turbine is taken out of service. Selfcleaningfilters should be used in this type of environ-“mm because they overcome these problems.The self-cleaning function permits continuousoperation during storm conditions withoutoperator attention. Filter life is relatively inde-

11

pendent of ambient dust loading; experienceindicates that two years is typical. Consequently,filter maintenance can be combined with otherscheduled maintenance in order to increaseavailability. The fact that self-cleaning filtershave longer life in the desert than conventionaldesigns reduces overall maintenance costs. Amajor user of gas turbines in Saudi Arabia hasreported that conventional systems cost threetimes as much as selfcleaning for replacementfilters and associated labor for changeouts

In cases where there is a question as to themost appropriate inlet air filtration, GeneralElectric should be consulted. Site informationand operating experience are available for manyspecific locations, worldwide. Where data arelacking, various services are available to acquireand analyze ambient air quality and then to pre-pare recommendations.

AIRF’ILTRA~ON IN MAARINEE N V I R O N M E N T S

The Marine EnvironmentCoastal, marine, and off-shore platform instal-

lations present unique problems of inlet air con-tamination, as salt from seawater can becomeairborne in significant quantity due to wind andwave action. As discussed earlier, this can giverise to corrosion. The sodium concentration atany given time and place is a function of manyfactors, including elevation, wave height, windvelocity, temperature, humidity, and the previ-ous history of the local air mass.

The salt content of air above or near the seacan be thought of as being from two sources: thefine droplets ejected from bursting bubbles, andthe relatively coarse spray from whitecaps andbreaking waves. These two effects are to a degree

SALT LEVELS VERSUS WIND SPEED

Figure 20

L

LOCATION OFSAMPLING STATIONS

FOR AIR-BORNENATURAL SEA SALT

GT0407c

Figure21

SALT LEVELS INLAND FROM SURF

WIND SPEED. l

SALT-rcu

.mr0 2 4 s 8 (0 12

MILES FROM SUM

Fiie 22

separable; they can be evaluated by comparingthe salt content of aerosols in which spray wasexcluded, and those in which it was not

Figure 20 contains data from many differentinvestigators showing salt concentration as afunction of wind speed.’ The data within thedotted lines were recorded during test condi-tions in which there was some effort to eliminatespray effects. Some of the uncertainty arisesfrom the fact that salt particles can be transmit-ted over long distances by the wind; local windspeeds may not be typical of those at the loca-tion where the particles were generated.

If the gas turbine intake is subject to salt spray,as from surf, whitecaps, or the bow-wave of aship, the situation can become much moresevere. The measurements by National GasTurbine Establishment (NGTE), Jacobs, andGPU Service Co. are in poor agreement, demon-strating that salt ingestion from spray can varyover wide limits depending upon wind speed,wind direction, elevation, distance from surf, andselfgeneration (as by a ship).

The measurements by GPU Service Co. areinteresting in that they include data at a number

12

I-

SALT CONCENTRATION VARIATION

Figure 23

of stations at different distances from the surfline. As a result, this information can be used toestimate the dropoff in salt concentration in on-shore applications in coastal regions. Fig. 21 is amap showing the measurement locations.Station 1 was on the seaward side of a sandy bar-rier beach. (It is Station 1 data which are shownin Fig. 20). Station 2 was on the land side of thebarrier beach or the seaward side of the bay,about a quarter mile from the surf Station 3 wason the land side of the bay, while subsequentmeasuring points were farther inland.

Figure 22 shows data taken during on-shorewinds, plotted in such a way as to emphasize therate of decay of salt level with distance. It is obvi-ous that an order-of-magnitude drop is experi-enced in going from the surf line to the leewardside of the barrier beach; it can be assumed thatthis is due to the fall-out of spray generated bythe waves. If the data from the lee side of the bar-rier beach are compared to the salt levels shownin Fig. 20, it is seen that they fall very close to thedashed line representing the upper limit of thenonspray data, confirming the assumption thatthese data are essentially free of spray effects.The next measuring station, some 4.25 milesfrom the surf, on the land side of the bay, has saltlevels falling well within the limit lines of Fig. 20.Salt levels continue to decay with distance up to arange of eight miles; the data at eleven milesshow a small increase for reasons which are notexplained. It is probable that the data at eight toeleven miles are approaching the natural back-ground level due to non-marine sources, Thissupposition is supported by the fact that themedian sodium level in the ambient air as mea-sured by General Electric at 41 domestic gas tur-bine sites-mostly far inland-was 0.0026 ppm,which is equivalent to 0.008 ppm sea salt.

If the statistical variation of wind velocity at asite is known, Fig. 20 can be used to estimate the

MASS DISTRIEIUTION VERSUS WIND SPEED

*rn” fi.a*.I,

Figure 24

salt content of the ambient air. The hours per yearthat a given salt level is likely to be exceeded inthe North Sea and the Gulf of Mexico has beenpredicted (Fig. 23). The dashed upper limit of saltconcentration (without spray effects) was used inthis calculation. Figure 23 also includes and inter-pretation of data on sodium levels measured at 30ft. elevation during winter months in TheNarrows, at the entrance to New York Harbor.*Salt levels have been deduced by assuming thatthe contaminants were 32 percent sodium, as istypical of sea salt. The measured salt levels areabout half those predicted for the Gulf of Mexico,which as a similar wind velocity distribution.When one considers that winds in this protectedharbor will often come from over land rather thanwater, this seems reasonable agreement.

The size distribution of marine aerosols as afunction of wind velocity can be estimated fromFig. 24, which is based upon data published byTobag and by the National Gas TurbineEstablishment.io The two references show varia-tions in detail, but similar trends. Part of thedifference lies in the difIiculty of the measure-ments, but there are also differences in the phi-losophy of the studies. Toba combined theoryand experiment to define the environment overthe open ocean, while the NGTE information isbased upon shipboard studies. In each case aswind velocity increases, The droplet size distri-bution skews toward the larger sizes. Therefore,wind speed will generally determine the concen-tration and size distribution of the particles.

Whether salt particles will exist as dry crystals orsaturated droplets will depend primarily on therelative humidity. If the salt particles start out asdroplet.% as they will at the high humidity present ‘:at the water/air boundary, they will remain assupersaturated droplets until the relative humidityfalls to 45 percent or less.11 This is probably due tothe presence of highly soluble magnesium and

13

TEMPERATURE RISE TO OIVE 45% R.H.

*Y-

-*I I.“. .

GT04OQ(

Figure 25

MOISTURE SEPARATORS

SINGLE STAGE

+@%%aAIR FLOW

TRIPLE STAGE /COALESCER PA0

@@fig,,,

MIST EXTRACTOR 12265551

Figure 26

calcium chlorides. Conversely, once the salt is incrystalline form, it will not deliquesce until the rel-ative humidity rises to about 73 percent,

Since relative humidity in maritime air veryrarely falls below 45 percent, salt will almostalways be present in droplet form. The exceptionto this could be gas turbine installations usinganti-icing systems to heat the inlet air. Under theassumption that the inlet heating system addsnegligible moisture to the air, Fig. 25 shows thetemperature rise required to decrease relativehumidity to 45 percent as a function of ambientconditions. If the inlet heating schedule has atemperature rise equal to or greater than thatdefined by the appropriate curve, the relativehumidity of the heated air will drop to such lev-els that salt will exist as dry crystals.

Equipment for Salt Removal

Several manufacturers offer equipment suit-able for the removal of liquid salt from theincoming air stream. The majority of these sys-tems operate on the same physical mechanismand differ only in materials and design details.

Most are available either as single- or three-stage systems, the use of which depends on theenvironment expected (Fig. 26). Single-stage sys-tems typically consist of a series of vertically ori-ented hooked vanes mounted parallel to the air-flow. These vanes impose several directionchanges to the incoming air. Entrained dropletsimpinge upon the sides of the vanes, beingunable to follow the air path due to their greatermass. The effectiveness of such a system is pro-portional to the velocity of the air stream andthus to the momentum of the droplets. Afterimpingement the droplet migrates along the sur-face of the vane until a hook is encountered. Thesolution migrating to the hooks flows downalong the hooks to a catch through and drainlocated below the separator.

A three-stage system typically consists of a first-stage vane as described above, followed by a coa-lescer pad and a second vane stage. The coalescerpad is typically a nonwoven pad approximately 1in. thick, made from polyester or similar material.This pad functions to capture smaller dropletsthat were not removed in the first stage of theseparator. After capture these droplets may eitherdrain down through the pad to the catch throughbelow or agglomerate with other droplets to formlarger droplets which become re-entrained in theair stream. These agglomerated droplets are captured by the last stage of vanes, which is typicallyidentical to the first stage.

Figure 27 shows salt penetration through typi-cal single-stage and three-stage moisture separa-tors as a function of windspeed. These penetra-tion curves can be computed by combiningmoisture separator collection efficiencies with theparticle size distribution curves of Fig. 24 and theoverall salt content as a function of wind velocityfrom Fig. 20. Toba’s particle size distributioncurves and the dashed upper limit curve from Fig.20 were used for these particular calculations.

SALT PENETRATIONSPRAY EXCLUDED

SALT?ENETRATlON.

020400000WIN0 EFEEO. NW GTO408

Figure 2714

Table 4Experience with High-Velocity Moisture

Separators on GEHeavy-Duty Gas Turbines

MoistureSeparator

Application1 z&gi 1 Eg- 1

Ships 2 12

Platforms, North Sea 45

Platforms, Gulf of Mexico I 2 I 5 I

Platforms, Arabian Gulf 1 9 1 - 1

Others I I 2 I

ExperienceOver 75 General Electric heavy-duty gas tur-

bines equipped with single- or three-stage high-velocity moisture separators have been installedon platforms, ships, or coastal sites.12 Theseapplications are summarized in Table 4.

Several of the platform. and ship turbineshave accumulated over 40,000 fired hours withthe original buckets and nozzles. Successfuloperation is based on the following conditions:

l The number of stages in the moisture separa-tor is a function of the allowable salt inges-tion criteria and the expected wind velocitiesat the site. North Sea sites have statisticallyhigher wind velocities than Gulf of Mexicosites and therefore require three stagesinstead of one.

l The moisture separators, particularly thecoalescer pads, must be protected fromdrilling mud and cement, sandblasting mate-rial, and, in some locations, duststorms. Thiswill prevent frequent changeout of the coa-lescers due to plugging from these materials.Prefilters upstream of the separators may benecessary to remove these contaminants.

l Liquid seals on the separator drains must bemaintained so that drainage water will notbe drawn into the compartment down-stream of the separators._.

Turbines at coastal locations have generallybeen far enough from the surf line that salt

spray and droplets are not encountered. High-efficiency filters were used to remove salt crys-tals from the inlet air.

ANTI-ICING SYSTEMS

Introduction

The operation of gas turbines in cold climatespresents certain unique problems, one of whichis inlet icing. Icing can block inlet filtrationequipment, causing the gas turbine to ingestunfiltered air or shut down. It can increase thepressure drop across trash screens and otherinlet components, leading to performance lossand possible damage to ductwork from implo-sive forces. In extreme cases, ice can build up oninlet bellmouths, with hazard of foreign-objectdamage and compressor surge. Anti-icing sys-tems are designed to inhibit ice formation oninlet components in order to protect the gas tur-bine from these effects and to allow it to operatereliably in the icing environment.

Icing Phenomena

Precipitate icing occurs when water is ingestedas a liquid or solid at temperatures near or belowfreezing, with wet snow and freezing rain beingobvious examples. If the precipitation remainssuspended in the air-stream, it causes no specialproblems. However, ice will adhere strongly tomost surfaces, and buildup can be a particular

problem if the temperature is near freezing.13If a body of air cools at relatively constant

moisture content, a point is reached at which thevapor condenses, forming water droplets. This isthe dew point. Further depression of the temper-ature results in super-cooling of the droplets.This condition is unstable, so that when dropletscontact an inlet surface, rapid buildup of hoar-frost results. In typical air masses, with many con-densation nuclei present, suspended dropletsremain liquid until about -22OF.14

When fuels are burned, both heat and watervapor are released into the atmosphere. Theheat tends to reduce relative humidity (RH),while the water vapor tends to increase it.Typically, the burning of fuel tends to reduceRH when ambient temperatures are warmerthan about -20°F. When colder than about -30°Fthe burning of fuel increases RH. Between theselimits, a calculation (which includes initial RH)is required to predict the effect. If weather con-ditions inhibit mixing of the air, an increase inRH can give rise to the arctic phenomenon

15

known as ice fog, in which the atmosphere issupersaturated with respect to ice.14

While icing generally occurs on the firstobstacle encountered by air entering an inletsystem, typically a trash screen or a filter, it ispossible for ice to form on the inlet bellmouthof the gas turbine without forming on otherupstream components. This phenomenon isexplained by the isentropic acceleration of theair as it enters the bellmouth, which results incooling. While much of this temperature depres-sion is recovered at surfaces such as the bearingsupport struts or inlet guide vanes, these sur-faces can still be several degrees cooler thanambient. The calculated wall temperaturedepression at the inlet guide vanes of theLM2500 is 3.5OF, while the corresponding valuefor heavy-duty machines is 2.4”F. This tempera-ture depression can cause local icing in the bell-mouth if the air is sufficiently moist. Ice forma-tion in the bellmouth could reduce the surgemargin of the compressor, and if chunks of icebreak off and enter the engine, foreign objectdamage may occur. Heavy-duty gas turbines aremore tolerant of the bellmouth icing than areaircraft-derivative engines due to their lessertemperature depression, heavier compressorblading, and greater surge margin.

Protective FeaturesInlet systems for cold climates are designed to

protect the gas turbine from damage due toicing and to keep the machine running withminimum effect on performance. Typicaldesigns include self cleaning filters which canremove ice in much the same way that theyremove dust, an inlet heating system to inhibitice formation downstream of the filters, andprotective devices to prevent damage in theevent of system malfunction or operation out-side the normal design envelope. These featuresare illustrated in Fig. 28 which shows the sideelevation of an inlet system.

The anti-icing module contains the inlet heat-ing manifold, which introduces warm air down-stream of the self cleaning filters in the inletduct. If there is icing on the inlet filters, a pres-sure switch which senses increasing pressuredrop, initiates the self cleaning system. An alarmis signalled if pressure drop continues toincrease. If no action is taken by the operator, agas turbine shutdown is signalled by the inletprotective pressure switches. The split trashscreen in the inlet duct protects against inges-tion of ice as well as trash. Its design is such thatit can pass air without excessive pressure drop,

16

INLET SYSTEM WITH INLET HEATING

Figure 28

even if the screen ices because of cold air beingdrawn through any leaks in the ductwork.Finally, another pressure alarm can be locatedin the inlet plenum. If desired, this switch canbe used to initiate a controlled shutdown whenthe pressure drop of the total inlet system reach-es a predetermined level.

Ingestion of snow and freezing rain into theinlet should be minimized in order to make thejob of the anti-icing system easier, particularly inthe near-freezing temperature range. One waythat this is done is by elevating the inlet filtercompartment. Studies show that the flux ofblowing snow drops by a factor of about 5 whengoing from an elevation of 5 to 25 ft.15 There islittle benefit in further elevation because wind-driven snow tends to be concentrated nearground level. Ingestion of rain and snow is alsominimized by use of a properly designed weath-er hood if conventional filtration is to be used.

Self-Cleaning FiltersExperience shows that selfcleaning filter car-

tridges can remove hoarfrost in much the sameway that they clean themselves of dust. Tests

SELF-CLEANING INLET FILTER

. I

GTO6807

Figure 29

have been conducted by filter vendors to simu-late precipitate icing by spraying water from fognozzles under winter conditions. These testshave demonstrated that, although ice can bebuilt up on filters, the porosity of this ice is typi-cally so high that the drop in filter pressure stayswithin acceptable limits. A frost point detectormay be used to signal the selfcleaning system tobegin pulsing when icing conditions are pre-sent. This helps alleviate any potential problemsassociated with a buildup of ice on the filters byremoving it as soon as it begins to form.

The unanswered key question was whetherthere would be ice formation in the gas turbineinlet bellmouth due to the temperature depres-sion which occurs there. To study this, extensivetests were run on an LM2500 gas turbine inwestern Canada during the 1981-82 winter (Fig.29). This machine, which has a self-cleaninginlet air filter with no inlet heating, had alreadycompleted a full year of successful operationbefore the test.

During the test, which covered an additional2700 fired hours of winter operation, a data log-ger recorded ambient and inlet temperatureand humidity, pressure drop, and engine perfor-mance parameters at lo-minute intervals. Inaddition, time-lapse video tape recordings weremade through viewing ports in the inlet plenumin order to visually identify any ice in the bell-mouth. During the test period, frequent inter-vals of high humidity at below-freezing tempera-tures were recorded. Frost was visible on theinlet guide vanes for one period of less than aminute, but there was no ice build-up, and noicing problems were experienced.

Two trends can be seen which help to explainthese favorable results. First, when ambient air issupersaturated with respect to ice, air down-stream of the filters is found to be just at thefrost point, indicating that frost is forming onthe outside of the filter elements. Moisturewhich freezes on the filters is obviously nolonger available to cause problems at the bell-mouth. If too much frost builds up on the fil-ters, it is removed by the self-cleaning action.Second, temperatures in the inlet bellmouthrun about 2 to 3°F warmer than air leaving thefilter compartment, even though there is noinlet heating system. This heating, which tendsto counteract temperature depression, is appar-ently due to a combination of radiation and con-duction from hot parts of the engine.

Numerous gas turbine installations now useself-cleaning filters as an anti-icing system. Theself-cleaning filter has become the standardinlet filtration system at Prudhoe Bay, Alaska

because of its anti-icing capability. It has alsobecome the favored system throughout Canadafor the same reason.

Inlet Heating SystemsMost General Electric inlet heating systems

are operated by mixing hot gas from somesource with cold, ambient air at the entrance tothe inlet system. The hot gas has always been air(or air plus some combustion products); so thetemperature rise can be calculated by a simpleheat balance:

W hot = W mixed Tmixed - Tambien tThot - Tambient

wherew = Weight flow per unit timeT= Temperature

Early inlet heating systems included exhaustrecirculation, exhaust heat recovery, and corn-pressor bleed recirculation designs. These sys-terns were used to prevent ice buildup both onthe inlet filters and in the ducting and compres-sor. With the advent of the self-cleaning filter,and its inherent anti-icing ability, inlet heating isused today mainly in areas where compressoricing is of potential concern.

Of the previously mentioned inlet heatingsystems, only the compressor bleed system istypically used today. This is due to the relativesimplicity of the system and its less costly effectson turbine performance.

Compressor Bleed Inlet HeatingA compressor bleed inlet heating system uses

a portion of the compressor discharge air forheating the inlet air (Fig. 30). As a result of com-. ,. _.presslve forces, this air typically has a tempera-ture of 500 to 75O”F, depending upon the ambi-

COMPRESSOR BLEED INLET HEATING

GT2113j

Figure 30

17

FROST POINT SENSOR

LED PHOTO TRANSISTOR

TEMP SENSOR

Figure 31

ent temperature and the gas turbine model. Thesystem is basically quite simple, since only onecontrol valve is required. Because of its simplici-ty, the reliability has been excellent in Alaskaand Canada, as well as the North Sea.

While the compressor bleed system is simpleand reliable, there is some performance degra-dation due to the compressor bleed require-ment. This can be minimized by heating onlythe minimum amount necessary to keep allparts of the inlet system at a relative humiditybelow the frost point. This determination can-not be made on the basis of temperature alone.What is needed is a device which can measurethe potential for icing, so that a control can bedesigned which causes the air to be heated justenough, but no more.

The key to the solution of this problem is a sen-sor which measures the moisture content of theair. Such a device is shown in Fig. 31. A beam froma lightemitting diode is reflected onto a photo-transistor from the polished surface of an electri-cally cooled plate. The plate is cooled to the pointwhere dew or frost just begins to deposit, inter-rupting the beam. Its temperature is measured,and the difference between this temperature andthe air temperature defines the potential for con-densate icing. This device measures absolutehumidity, avoiding the problems inherent in rela-tive humidity sensors and lending itself well tooptional inclusion in a control system.

RecommendationsGood experience with self-cleaning filters for

both inlet air filtration and anti-icing has madeit the standard system in environments with ahigh icing potential. There is no question thatself-cleaning filters provide very high air quality;thousands of hours of desert operation havedemonstrated this conclusively. As a means of

anti-icing, the simplicity of the concept and itsminimal effect on performance make it particu-larly attractive to the user. For applicationswhere supplimental inlet heating is required toprevent icing in the compressor bel,lmouth,.Fompressor bleed inlet heating is recommend-ed. This is based on a balance of cost, reliability,and impact on performance.

INLET COOLING SYSTEMS

IntroductionAn inlet cooling system is a useful gas turbine

option for applications where significant opera-tion occurs in the warm months and where lowrelative humidities are common. The cooled air,being denser, gives the machine a higher mass-flow rate and pressure ratio, resulting in anincrease in turbine output and efficiency. This isa cost-effective way to add machine capacity dur-ing the period when peaking power periods areusually encountered on electric utility systems.

There are two basic systems currently availablefor inlet cooling. First, and perhaps the mostwidely accepted system is the evaporative cooler.Evaporative coolers make use of the evaporationof water to affect a reduction in inlet air temper-ature. Another system currently being studied isthe inlet chiller. This system is basically a heatexchanger through which the cooling medium(usually chilled water) flows and removes heatfrom the inlet air thereby reducing the inlet tem-perature and increasing gas turbine output.

In addition to the obvious advantage ofachieving extra power, the use of an evaporativecooler improves the environmental impact ofthe machine. Increasing water vapor in the inletair tends to lower the amount of oxides of nitro-gen produced in the combustion process and,therefore, lowers the emissions of the machine.

PSY

COOCINti

00 6 0 60 100 120

D R Y BUl.B T E M P E R A T U R E F TC235641732566

Figure 3218

Evaporative Cooler Theory

The actual temperature drop realized is afunction of both the equipment design andatmospheric conditions. The design controls theeffectiveness of the cooler, defined as follows:

Cooler effectiveness = TIDB - T2WB

TlDB-T2WB

Subscript 1 refers to entering conditionsand 2 to the exit; DB means dry-bulb tempera-ture and WB means wet-bulb. The effective-ness of General Electric evaporative coolers istypically 85 percent; so the temperature dropcan be calculated by:

ATDB = 0.85 (TIDB - TIWB)

As an example, assume that the ambienttemperature is lOOoF and the relative humidityis 32 percent. Referring to Fig. 32, which is asimplified psychrometric chart, the corre-sponding wet-bulb temperature is 75°F. Thetemperature drop through the cooler is then0.85 (100-75)) or 21°F. The cooling process fol-lows a line of constant enthalpy as sensible heatis traded for latent heat by evaporation.

The current self-cleaning filter/evaporativecooler design is shown in Fig. 33. Water ispumped from a tank at the bottom of the mod-ule to a header which distributes it over themedia blocks. These are made of corrugated lay-ers of fibrous material, with internal channelsformed between layers. There are two alternat-ing sets of channels, one for water and one forair. This separation of flows is the key to reduc-ing carry-over. However it is standard practice atGeneral Electric to provide a stage of drift elimi-nators downstream of the media to protectagainst the possibility of water carry-over. Thewater flows down by gravity through the water

1

MEDIA PACKCOOLERDESIGN

channels, and diffuses throughout the media bywicking action. any excess returns to the tank.The level of water in the tank is maintained by afloat valve which admits make-up water.

The ambient temperature setpoint (located atthe cooler controller) is adjustable. It is factorypreset to allow cooler operation at ambientsabove the setpoint, which must be 60°F or higher.If evaporation were permitted at too low a tem-perature, there would be a possibility of causingicing which is of course to be avoided. Whenthere is a possibility that the dry-bulb tempera-ture will fall below freezing, the whole systemmust be deactivated and drained to avoid damageto the tank and piping, and to avoid the possibili-ty that the porous media would plug with ice.

Water Requirements

Evaporative coolers find their greatest applica-tion in arid regions. In such areas it is not uncom-mon to find that available water has a significantpercentage of dissolved solids. If make-up water isonly added in sufficient quantity to replace thewater which has been evaporated, it is obviousthat the water in the tank (which is also the waterpumped to the media for evaporation) mustgradually become more laden with minerals. Intime, these will tend to precipitate out on themedia and reduce evaporation efficiency, and thehazard increases that some minerals will becomeentrained in the air and enter the gas turbine. Inorder to minimize this it is usual to continuallybleed some water from the tank to keep the min-eral content diluted. This is termed blowdown.

edThe total amount of water which must be provid-as make-up is the sum of evaporation and blow-

down. The rate at which water evaporates from acooler depends upon the ambient temperaturead humidity, the altitude, cooler effectiveness, andthe airflow requirement of the gas turbine. The

EVAPORATION RATE FOR MS600185% EFFECTIVENESS

EVAP~F~ATION. GPM20 ,

80 90 1W 110 120

TEMP - ‘FGT2114

Figure 33 Figure 3419

evaporative requirement of an 85 percent effectiveMS8001 gas turbine cooler at sea-level is shown inFig. 34. The corresponding value for an MS7001 orMS9001 machine may be estimated by respectivelydoubling or tripling the quantity shown.

One of the main concerns in determining theacceptability of water quality is its propensity todeposit scale. Scaling is influenced by the interac-tion of the water’s total hardness, total alkalinity(ALK), total dissolved solids (TDS), pH and watertemperature. To assist in determining whether theavailable water is suitable for use in General Electricevaporative coolers, a saturation index (SI) is used.

A standard laboratory analysis of the watercan determine the total hardness (ppm asCaCOs), total alkalinity (ppm as CaCOs), totaldissolved solids (ppm) and pH. The levels of thefirst three components are first modified by anadjustment factor W:

W=(%+l) (%+l)where

F = flood = water drain ratefrom media to tan

factor water evaporation rate

B = blowdown = water bleed ratefrom tank

factor water evaporation rateWater evaporation rate can be estimated from

Fig. 34.For most cases F and B are adjusted during

installation to be approximately uniform on atypical hot day so W = 4. However, to make low-quality water more suitable, an increased blow-down rate may be used to lower the adjustmentfactor. Flood factor should not be adjusted tocompensate for water quality since this couldresult in liquid water carry-over.

The ppm of TDS, ALK, and hardness are mul-tiplied by the adjustment factor to obtain ppm(adjusted). To evaluate the suitability of waterfor General Electric evaporative coolers, a modi-

LANGELIER SATURATION INDEX CHART

Figure 35

lied Langlier saturation index chart is used (seeFig. 35). The adjusted total alkalinity (ppm asCaCOs) is converted to PALE, and the adjustedtotal hardness (ppm as CaCOs) is converted toPCA by entering the chart on the righthandordinate and reading the appropriate quantityfrom the righthand abscissa. The adjusted totaldissolved solids are converted to PTDS by enter-ing the left ordinate, selecting the appropriatewater temperature (which may be taken to bethe wet bulb temperature), and reading theupper left abscissa.

Saturation Index (SI) = pH - PCA - PALK -PTDS

SI < 1.0 indicates no water treatment isrequired.

Water treatment may be used to control anyproperty or combination of properties to reduceSI to 1.0 or less. Initially, the blowdown rate isadjusted to be approximately the same as theevaporation rate on a typical hot day; this maylater be adjusted based on operational experi-ence and local water quality.

Despite care with water quality, the media willeventually have to be replaced as material precipi-tates out in sufficient quantity to impair its effec-tiveness. However, experience indicates that thismay take quite a long time. At one site there hasbeen operation for six seasons under very adverseconditions with insignificant performance degra-dation. It is expected that the media will continueto be used for at least two more years. While thisis believed typical, the estimate is subject tochange as more experience is gained.

Tests indicate that the feedwater may havehigh levels of sodium and potassium without sig-nificant carry-over of these metals into the gasturbine. However, very careful attention todetail is necessary in order to realize this level ofperformance. This includes proper orientationof the media packs, correct flows of air andwater, uniform distribution of water over themedia surface, and proper drainage back to thetank. Any deficiencies in these areas may make itpossible for water to become entrained in theair, wi th potent ia l ly ser ious resu l t s .Consequently, installation and maintenance ofevaporative cooling equipment is very impor-tant. In areas where the water exceeds 133 ppmsodium and potassium it is good practice to per-i- :odicahy check the rate at which these elementsenter the gas turbine by means of a mass-bal-ante calculation. Any discrepancy between therate at which sodium and potassium enter in the

20

feedwater and the rate at which they leave in theblowdown can be attributed to carry-over. It willbe recalled that the concentration of these ele-ments in the inlet air should typically be held to0.005 ppm or less; this is equivalent to an inges-tion rate of 0.01 lb./h. for an MS7001 gas tur-bine, 0.005 lb./h. for an MS5000, and 0.0002lb./h. for an MS3002.

Operational ExperienceWhen medium-type coolers were first placed

in service, some units exhibited unacceptablecarry-over. It was found that this problem hadthree possible causes: damaged or improperlyinstalled media, entrainment of water from thedistribution manifold, or local areas of exces-sive velocity through the media. The first causewas removed by new procedures for shippingand installing the media blocks. Carry-overfrom the manifold was eliminated by installa-tion of blanking plates downstream of the sprayelements. The third problem, high flow veloci-ty through portions of the media, was the mostdifficult to solve. After considerable effort, twosolutions were developed. The first incorporat-ed features in the design to force more uni-form flow, so that velocities everywhere werewithin the acceptable range. The second solu-tion was more radical. It involved a new designwhich accepted some carry-over from themedia, but which eliminated carry-over intothe gas turbine by use of eliminator blades(similar to the vanes of a moisture separator)immediately downstream of the evaporativemedia. Both approaches have proven successfulin the field and both approaches are now takentogether to ensure no water carry-over. Therehas been no problem in meeting goals for cool-er effectiveness in any of the more than 75media-type coolers now in the field.

Inlet Cooling Coils

AS with the evaporative cooler, the actualtemperature reduction from a cooling coil is afunction of equipment design and ambientconditions. Unlike the evaporative cooler, how-ever, cooling coils are able to lower the inletdry bulb temperature below the ambient wetbulb temperature. The actual temperaturereduction is limited only by the capacity of thechilling device, the effectivness of the coils, andthe acceptable temperature/humidity limits ofthe compressor.

Figure 36 shows a typical cooling cycle basedon an ambient dry bulb temperature of ZOOOFand 20 percent relative humidity. Initial coolingfollows a line of constant humidity ratio. As theair approaches saturation, moisture begins tocondense out of the air. If the air is cooled fur-ther, more moisture condenses. Once the tern-perature reaches this regime, more and more ofthe heat removed from the air is used to con-dense the water. This leaves less capacity for tem-perature reduction. Because of the potential forwater condensation, General Electric recom-mends that drift eliminators be installed down-stream of the coils to prevent excessive wateringestion by the gas turbine. The exact point atwhich further cooling is no longer feasabledepends upon the desired gas turbine outputand the capacity of the chilling system.

It is readily apparent in Fig. 36 that the aircan easily be cooled below the ambient wetbulb temperature. Therein lies one of themajor benefits of the cooling coil system. Itmust be pointed out, though, that the lowerlimit of cooler operation is a compressor inlettemperature of 45OF with a relative humidity of95 percent. At temperatures below 45’F withsuch high relativepressor is probable.

humidity, icing of the com-

INLET CHILLING PROCESS

DRY su.B TEMPERATURE r GT21141

EFFECT OF EVAPORATIVE COOLER ON AVAILABLE OUTPUT85% EFFECTIVE

;:TE/\SE IN OUTPUT - X,,o~ RH-

12 - 2Qx

30x

40x

50%

60%

TEMP - ‘F

Figure 36 Figure 3721

Power IncreaseThe exact increase in power available from a

particular gas turbine as a result of inlet air cool-ing depends upon the machine model and sitealtitude as well as ambient temperature andhumidity. However, Fig. 37 can be used to makean estimate of this benefit for evaporative cool-ers. As would be anticipated, the improvement isgreatest in hot, dry weather. The power increasefrom a cooling coil is also dependent upon thechiller capacity so it is difficult to make a gener-al estimation. The addition of an inlet cooler iseconomically viable when the value of theincreased output exceeds the initial and operat-ing costs, and appropriate climatic conditionspermit effective utilization of the equipment.

SUMMARYIt has been shown that there are many envi-

ronments which are naturally hostile to gas tur-bine operation, but that General Electric hasdeveloped a wide range of inlet air treatmentequipment which permits its machines to adaptto these conditions and operate successfully.With the information given in this paper, it ishoped that gas turbine users will be able to iden-tify potential needs for air treatment, and toknowledgeably consider equipment options.General Electric applications engineers havemany years of experience in this field and areready to assist in selection of suitably equippedinlet compartments to enhance gas turbine per-formance, reliability, and maintainability.

REFERENCES1. Beltran, A.M., and Shores, D.A., “Hot

Corrosion,” Chapter 11 from Superalloys,C.T. Sims and W.C. Hagel, eds., John Wileyand Sons, 1972.

2. Fairman, L., “Mechanism of AcceleratedOxidation by Vanadium-Containing FuelAsh,” Corrosion Science, Vol. 2 1962, pp.293-296.

3. Bradury, B.J. Hancock, P., and Lewis H.,“‘The Corrosion of Nickel-Base Material inGas Turbine and Boiler Atmospheres,”Metallurgia, January 1963. pp. 3-14.

4. “Air Quality Data-1978 Annual Statistics,”EPA-450/4-79-037, U.S. EnvironmentalProtection Agency, Research Triangle Park,North Carolina.

5. Artifical Stimulation of Rain, GeophysicsResearch Directorate, Air Force CambridgeResearch Center and Geophysics Branch,Office of Naval Research, Pergamon Press,NY, 1957, p. 163.

6. Anderson, A.W. and Neaman, R.G., “FieldExperience with Pulse-Jet Self-Cleaning AirFiltration on Gas Turbines in a DesertEnvironment,” ASME Paper 82-GT-283.

7. Tatge, R.B., Gordon, C.R., and Conkey, R-S.,“Gas Turbine Inlet Air Filtration in MarineEnvironments-Part I: Marine Aerosols andEquipment for Their Control,” ASME Paper8O-GT-174.

8. Krulls, G.E. and Lastella, J., “Gas Turbine SaltIngestion Analysis.* ASME Paper 77GT20.

9. Toba, Y, “On the Giant SeaSalt Particles inthe Atmosphere,” Tellus, Vol. 17, NO. 1,1965, pp. 131-145.

10. Randles, R.H. and Ansari, F., “Evaluation ofthe Peerless Mark 1 Spray Eliminator forProtection of Marine Gas Turbine AirIntakes,” Naval Marine Wing Note No.30/71, National Gas Turbine Establishment,Pyestock, Hams, England.

11. Jungle, C.E., ed., “Aerosols,” Air Chemistry;I’ 2ytioactivity, Academic Press, 1963, pp.

12. Tatge, RB., Gordon, C.R., and Conkey, R.S.,“Gas Turbine Inlet Air Filtration in MarineEnvironments-Part II: Commercial Experi-ence and Recommended Practice,” ASMEPaper 8@GT-175.

13. Chappell, M.S. Stationary Gas Turbine IcingProblems: The Icing Environment, NationalResearch Council, Ottawa, Canada, January1973.

14. Appelman, H., ‘“Ihe Cause and Forecastingof Ice Fogs,” Bull. Am. Meteorological SOc.,Vol. 34, No. 9, November 1953.

15. Grabe, W. and Chappell, M.S. StationaryGas Turbine Icing Some Design Guidelinesfor Blowing Snow Environments, NationalResearch Council , Ottawa, Canada,September 1974.

Q 1991 GE Company

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>

CONVERSION FACTORS

The following is a list Of COWerSiOn factors mostcommonly used for gas turbine performance.

To Convert To Multiply By To Convert To Multlply By

acres hectares 4.047 x 10” hp (U.S.) hp (metric) 1.014 .atm kg/cm* 1.0333 in. cm 2.540 .atm lb/in.* 1.47 x 10’ in. mm 2.54 x 10’bars atm 9.869 X 10-l in.2 mm* 6.452 x lo*bars lb/in.* 1.45 x 10’ in. of mercury kg/cm* 3.453 x lo-*Btu J (joules) 1.055 x lo3

in. of waterBtu kcal 2.52 x 10” (at 4°C) kg/cm* 2.54 x 1O-3Btu/h kcallh 2.520 x 10-l

Btulh kJlh 1.0548 in. of water(at 4X)

W (watts) 2.931 x 10”lb/in.* 3.613 x lo-*

Btu/hBtulhp-h kcal/kWh 3.379 x 10” J . Btu 9.486 x 1O-4

Btulhp-h kJlkWh 1.4148 kg lb 2.2046BtulkWh kcal/kWh 2.5198 x 10-l kg/cm* lb/in.* 1.422 x 10’BtulkWh kJlkWh 1.0548 kg-m ft-lb 7.233Btullb kcallkg 5.555 x 10” kg/m3 lb/f@ 6.243 x lo-*Btullb kJlkg 2.3256 km miles (statute) 6.214 x 10”“C “F (“C x 915) + 32 kW hp 1.341“C K “C + 273.18 I fts 3.531 x 10-2cm3 ft3 3.531 x 10’~ lb kg 4.536 x 10”

cm3 in.3 6.102 x lo-* lb/In.* kg/cm* 7.03 x lo-*

‘F “C (‘F-32) x 519 lb/in.* Pa 6.8948 x lo3

ft m 3.048 x 10” lb-ft* kg-m* 4.214 x 10-l

ft* m* 9.29 x 10-2 llmin ftsls 5.886 x 1O-4

fta I (liters) 2.832 x 10’ llmin galls 4.403 x 1o-3

fts m3 2.832 x lo-* m ft 3.281

ft-lb Btu 1.286 x 1O-3 m* ft* 1.076 x 10’

ft-lb kg-m 1.383 x 10-l m3 fts 3.531 x 10’

ftlmin km/h 1.8288 x lo-* mile (statute) km 1.6093

ft3/min I/s 4.720 x 10-l tons (metric) kg 1.0 x lo3

ft3!min m3/min 2.832 x lo-* tons (metric). lb 2.205 x lo3

gal m3 3.785 x 1O-3 W Btulh 3.4129

gallmin I/s 6.308 x lo-* W Btulmin 5.688 x lo-*

hectares acres 2.471 W ft-I b/s 7.378 x lo-’

hp (U.S.) kW 7.457 x 10” W hp 1.341 x,10-s

For further information, contact your GE FieldRepres&tative or write to GE CommunicatiDn Programs

G E I n d u s t r i a l &G E I n d u s t r i a l &Power,SystemsPower,Systems\\

General Electric CompanyGeneral Electric CompanyBuilding 2, Room 1158Building 2, Room 1158one River RoadOne River RoadSchenectady, NY 12345Schenectady, NY 12345

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