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Total corrosion control for industrial gas turbines: High temperature coatings and air, fuel and water management
Surface and Coatings Technology, 32 (1987) 1
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TOTAL CORROSION CONTROL FOR INDUSTRIAL GAS TURBINES: HIGH TEMPERATURE COATINGS AND AIR, FUEL AND WATER MANAGEMENT* L. L. HSU Solar Turbines Inc., San Diego, CA (U.S.A.) (Received March 26, 1987)
Summary
Contaminants found in air, fuel or water can combine in the turbine hot section to produce corrosion, erosion and/or deposition. The fundamental role of high temperature coatings is to delay the onset of substrate deterioration and attendant effects on component structural integrity. To achieve this, the total concentration of each harmful contaminant in the turbine environment should be determined in order to select the appropriate material condition necessary, coated or uncoated, for the airfoil surface to survive the expected service time successfully. A method is presented whereby the total fuel equivalent concentration of any contaminant from all sources can be calculated. Utilization of this methodology is demonstrated by means of a hypothetical case of a turbine operating under specifically defined conditions. In this example, sodium (plus potassium) concentrations in air and fuel are assumed to be well within industry-accepted standards of 1 ppmw and it is shown that the use of untreated potable water for water injection and evaporative cooling can result in unacceptably high sodium concentrations. Subsequently, the extent and cost of water clean-up can be optimized by assessing the costs of water treatment equipment and the volume of water to be treated for emissions control and evaporative cooling. Field experiences with a turbine of 12000 horsepower further substantiate the diagnostic advantages of this approach. In three cited cases, the presence of excessive sodium in the turbine hot section resulted in Type I or high temperature hot corrosion degradation of stage 1 blades. The source of contamination was attributed to water-borne sodium, using the methodology described. Corrective action recommended was directed at reducing the sodium concentrations in both the evaporative cooling water and the injected water and with target levels specified to ensure compliance *paper presented at the 14th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., March 23 -27, 1987. Recipient of the Bunshah Award for the best paper in Symposium A: Coatings for Use at High Temperatures. 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
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with Solar’s specification limit of 1 ppmw (fuel equivalent concentration) of total sodium. A platinum-modified aluminide coating was also applied to first- and second-stage blading. It is demonstrated that, with this approach, total corrosion control can be realized by assessing the source(s) of contamination followed by application of appropriate corrective action to reduce the level of contamination economically.
1. Introduction Protective coatings have become integral components of turbine engines as a result of increasingly greater high temperature strength requirements for superalloys combined with more challenging applications using lower grade and non-fossil fuels. Whereas the aero-engine is limited by weight (thrust) considerations in conjunction with relatively tight restrictions on fuel, the land-based turbine engine, being free from weight and fuel limitations, must necessarily be more versatile and rugged as a broad-based power plant. Time between overhaul (TBO) of the order of 30 000 h is not uncommon, thus making life prediction of critical components a difficult task at best. As an original equipment manufacturer (OEM), Solar Turbines Inc., a subsidiary of Caterpillar Inc., has a worldwide fleet of industrial gas turbine engines operating in a range of environments and duty cycles, both on land and offshore. In some land-based installations, emissions control and power augmentation have necessitated the introduction of water as a third fluid component (as well as air and fuel) via water injection and evaporative cooling. In addition, it is not uncommon for fuels to vary in type and quality from pipeline natural gas to biomass fuels and non-hydrocarbon gases. Consequently, contaminants can enter the turbine engine in any of the three fluid media, air, fuel and water, as depicted schematically in Fig. 1. These contaminants can be in the form of gas, liquid or solid phases, depending on prevailing temperatures and pressures. In the compressor, the effect of air-borne contaminants on airfoils tends to be less severe while the engine is in operation because of low temperatures (very much less than 1000 °F)and the centrifuging action of the rotating blades. Physical or mechanical damage due to particulate erosion tends to be of more concern, particularly in the early stages of compression. Most of the low temperature corrosion damage observed in the compressor is the result of electrochemical or galvanic reactions that occur during shutdown or in aerodynamically stagnated recesses in the presence of liquid water. Surviving air-borne contaminants combine with fuel and (injected) water-borne contaminants in the combustor wherein the complex scenario of exothermic reactions produces highly energetic hot gases containing potentially corrosive and/or fouling chemical species.
3
Fuel
+
Water Contaminants
w
Water Soluble
+
I Hydrocarbons)
+
(Moisture)
+
Contaminants
Solid Contaminants (Colloids, 5~rbi~nat1es,
Fuel Water
Air
AF
Ambient Air +
Water—.-
EC
+
omp
Water Vapor I Ambient EvaporatedI
Solid (Particulates I
+
+
Comb
T
Exhaust
Liquid Water Droplets
Gas Phase Contaminants ISalts, Aerosols, etc. I
Fig. 1. Schematic diagram of fluids and entrained species entering industrial gas turbine engines.
In the face of this continuous onslaught of combustor effluence, turbine airfoils can be viewed as “consumables” in that surface degradation is expected to occur and does occur at a controlled rate with the expectancy that the airfoils remain functional (within specified performance ratings) over a defined lifetime, i.e. TBO. Therefore, the role of the hot-section coating is to protect, albeit temporarily, the substrate alloy from the environment in order to meet material performance expectations, i.e. delay the threshold of unacceptable alloy degradation, as shown in Fig. 2. A crucial aspect of materials engineering is the selection of appropriate coatings that can impart the additional life necessary to achieve designated TBOs under specific environmental conditions without any penalty in mechanical strength. The approach discussed in this paper was developed at Solar to establish a quantitative methodology for evaluating contaminant concentrations in air, fuel and water and to correlate their presence to component (coating or substrate) resistance. The objectives of such an approach are twofold: (1) a more accurate assessment of life or performance expectations of gas
Degradation. Baseline •i.I.I Coating Alloy
Uricoated Coated
U•U~
Surtace Degradation
Coated Component Life
Life
Fig. 2. Function of the hot-section coatings.
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path materials and (2) correlation of site conditions with coating requirements. Even though this is an ongoing effort at Solar, the fundamentals of this approach are presented in the first part of the paper followed by an ifiustrative example and discussion of field experiences in which the combined effects of air, fuel and water quality directly influenced the life of the hot-section components.
2. Contaminants Contaminants of concern to the land-based turbine operator and the mode of degradation by these contaminants are listed in Table 1 which also indicates the source(s) whereby these contaminants are known to enter the engine. The first five in the listing are of primary concern because of their critical role in hot-section degradation. Nevertheless, the net effect of any one of these contaminants on turbine hot-section components can be critical if the total allowable limit specified by the OEM is exceeded. In general, the severity of degradation is dependent on the properties of the existing chemical species, i.e. gas, liquid or solid state, thermodynamic activities, residence time in the combustor, reaction rates (kinetics), concentration, substrate (alloy or coating) composition, etc. Regardless of the mode of degradation, corrosion, erosion or deposition, the damage incurred is irreversible, even when the contaminant concentration level subsequently decreases to innocuous levels. From the OEM standpoint, specific limits on the maximum allowable concentration of critical species, particularly sulfur, sodium, potassium, vanadium and lead, in the total fluid intake of the engine must be clearly defined in order to control the rate and extent of damage by these contaminants. Solar’s standards for the five critical contaminants are given in Table 2.
TABLE 1 Listing of contaminants detrimental to turbine engine operation
Contaminant
Air
Sodium Potassium Vanadium Lead Calcium Sulfur Silicon Chlorine Fluorine
x x
Fuel
Water
Degradation mode
x
Corrosion Corrosion Corrosion Corrosion Erosion and deposition Corrosion Erosion and deposition Corrosion Corrosion
x X
x
x
x x
X
x
x x
x x x
x
x x
5 TABLE 2 Industry standards for maximum contamination levels
Sodium + potassium Vanadium Lead Calcium
Solar
Other OEMs
ASTM D2880
(ppmw FEC)
(ppmw)
(ppmw)
1.0 0.5 1.0 1.0
1 - 1.30 0.5 - 1.95 1.0 - 1.95 1.0-3.90
0.5 0.5 1.0 0.5
All values given are referenced to fuel.
2.1. Air
Ambient air quality can fluctuate from the pristine environments of rural hinterlands to the smog- and chemicals-laden air of highly industrialized areas. Unfortunately, data on many of the air-borne contaminants listed in Table 1 are meager and difficult to measure other than for those monitored by Air Pollution Control Districts [1] such as carbon monoxide, sulfur oxides, sulfates, nitrogen oxides, lead, ozone and aerosols (fine particulates). In addition, transient conditions such as dust storms, high wave conditions at sea, accidental or temporary plant or factory discharge, prevailing wind directions and seasonal weather-related parameters (temperature, relative humidity, precipitation etc.) can directly impact the concentration of airborne contaminants. For example, the National Gas Turbine Establishment has published data that correlate wind velocity with sodium chloride concentration in air for offshore installations (see Fig. 3). Depending on the particle size distribution of these salt water droplets (which in turn are affected by factors such as particle velocity, relative humidity and temperature), droplet removal by an appropriately specified air-cleaner system would remove most of the particles in the air stream at the rated equipment efficiency and particle cutoff size. However, experience has shown that some finite amount of airborne particles will elude the filter elements and enter the compressor. Therefore, it is possible that the concentration of sea salts ingested during a period of high wind velocity could be considerably higher than the norm and maximum allowable limits could be exceeded, thus enabling hot-section corrosion to occur temporarily. As a consequence, even if high wind velocity periods are infrequent, the degradation caused in the duration is non-reversible and self-propagating and tends to continue even after ingested concentrations drop to lower levels. Compressor intake is subjected to various thermal profiles depending on the engine size, efficiency and rating; typically, approximately 10% of compressor discharge is bled off for cooling and hence remains at temperatures less than 1200 °F; 15% 25% is used for combustion (primary zone) and the remaining 65% 75% is introduced downstream of stoichiometric combustion as secondary or tertiary cooling air [2,3]. Even though it is -
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recognized that not all air-borne contaminants are exposed to the elevated temperatures that are necessary for formation of corrosive species in the gas stream, it is assumed in this approach that all air-borne contaminants participate fully in turbine degradation. 2.2. Fuel The types of fuels typically associated with directly fired turbine engines are gases or liquids. (Current research and development in the area of directly fired coal-fired turbine engines will eventually allow the use of solid—liquid slurries as a fuel type as well.) A generic listing of fuel types is shown in Fig. 4 and it is evident from a scanning of the list that contaminant levels can vary from pipeline quality natural gas (nominal 0.5 wt.% sulfur for a San Diego utility) to sulfur- and vanadium-rich crude and high halogen fuels from landfills and waste. For several of the fuel types listed in Fig. 4, particulate contamination is a major concern because of the presence of ashforming constituents especially for fuels with high aromatic content, resulting in an increased potential for turbine deposition and fouling. It is customary at a gas turbine operating site to install a fuel processing skid to ensure that fuel properties satisfy specified requirements for reliable and safe operation. In addition to fuel compressors (or de-compressors) and heating devices to regulate pressure and temperature, a fuel filter and separator system is typically employed to remove entrained water, liquids and particulates. Water removal can reduce the level of sodium and potassium salts because, being water soluble, they are segregated to the water or moisture portion of the fuel. However, water-insoluble species such as vanadium, lead
Fuels
I
101
Total Salt
E ~
Gaseous
/ //
//
/ / Salt from / / Droplets / /
10~
4
<13 ~umDIA.
// 1/
Z
,7’// 10
Natural Gas
•
Crude
•
Coal
•
Natural Gas Liquids
•
Distillates
•
Wood
•
Residuals
•
Digester
.
— _____-~
g~
~rt~m<4
1nm DIA.
20 30 40 Wind Speed, Knots
•
Process
•
Coal Gas
•Nonhydrocarbons
• Coke Oven • Blast Furnace
Fig. 3. Sodium chloride concentration as a function of wind speed. Fig. 4. Generic classification of fuels.
Solids
.
Landfill • Gob Gas
/1’
Liquid
-~
7
and sulfur are organically bound to the hydrocarbon fuel and are not removable by on-site fuel treatment except as part of condensable liquid hydrocarbons.
2.3. Water Water chemistry and treatment, a technology in itself, plays a crucial, albeit often overlooked, role in gas turbine corrosion. Water can be intentionally introduced as injected water (or stream) for emissions (nitrogen oxides) control or it can also be a constituent of ingested air as water vapor via evaporative cooling for power augmentation or as liquid water carried over from evaporative cooler operation. There are three sources of naturally occurring waters: groundwater (springs and wells), surface water (rivers and lakes) and seawater. Water quality, on the basis of total dissolved solids (TDS), can range from “fresh” to “brine”, as defined in Table 3. For corrosion control, definition of water quality via TDS is inadequate as it expresses all dissolved ions in the water as though they were CaCO3. Additional data regarding specific concentrations 2 etc.) as determined by wet chemof of theare critical ions to (Na~,K~, icaleach methods required calculateS04 actual concentrations of critical contaminants. Where water injection and/or evaporative cooling is required, it is customary to install an ancillary water treatment package specifically designed to achieve target water quality levels. Water treatment processes can include any combination of softener, ion exchanger (cation, anion or mixed bed), demineralizer, reverse osmosis or distillation units. The price of treated water can differ by orders of, magnitude, depending on the extent of water clean-up. For example, soft San Diego water is estimated to cost $0.12 per gallon while demineralized San Diego water can be as high as $20 per gallon. Contaminants in injected water can be considered as being equivalent to contaminants in fuel because water flow rates are closely matched to fuel flow, i.e. typical water-to-fuel ratios range from 0.5 to 1.0 for Solar engines. In contrast, the rate of liquid water carryover from evaporative coolers is nominally much lower than fuel flow and this difference is reflected in the concentration ranges of the two water qualities as discussed in the latter part of this paper.
TABLE 3 Total dissolved solids content of water types
Type of water
TDS (ppmw CaCO 3)
Fresh water Brackish water Salty water Brine
0 1000 10000
1000 10000 - 100000 >100000 -
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From the ancillary equipment standpoint, it should be realized that, even though evaporative coolers are generally designed for zero water carryover, field experience has shown that liquid water may be and often is carried over into the air stream and can be a major source of contaminants in high temperature corrosion.
3. Calculation of total contaminant(s) concentration Regardless of the media through which contaminants are introduced into the engine, the net effect in the turbine section can be expressed as a sum of the contaminants contributed by air, fuel and water. At Solar, the accepted convention is to express total contaminant concentration as a fuel equivalent concentration (FEC), i.e. parts per million with reference to a standard fuel with a lower heating value (LHV) of 18 380 Btu lb1: =
F, + (AFR)A 1 + (WFR)W1 + (CFR)G1
(1)
where T1 is the total concentration of the ith contaminant entering the engine, F. is the concentration of the ith contaminant in the fuel supply, AFR is the air-to-fuel ratio, A1 is the concentration of the ith contaminant in ambient air, WFR is the injected water-to-fuel ratio (or stream), W~is the concentration of the ith contaminant in the injected water, CFR is the carryover water (from evaporative cooler)-to-fuel ratio and C7 is the concentration of the ith contaminant in the evaporative cooling water. The above expression can be converted to a more useful form by incorporating factors that influence the effective concentration in the turbine section. Examples of such factors are efficiency of air clean-up (filter), extent of on-line fuel processing (filters and separators), evaporative cooler design (recirculating or once-through), water carryover rate, adjustment of fuel flow rate (normalized to the LHV of the standard fuel) and mist eliminators efficiency (if used in conjunction with evaporative cooling). The final expression, with appropriate definition of units, has the following form: 18380 LHV F7(1 —K) + (AFR)(1 _N)A~5mb {4.99 x iO~R(LHV)(1 —E)}C.
=
+
+
(WFR)W1 (2)
where T7 (ppmw FEC) is the total concentration of the ith contaminant, F7 (ppmw) is the concentration of the ith contaminant in fuel, K is the efficiency rating for the fuel clean-up system, AFR is the air-to-fuel ratio, N is the efficiency rating for the air clean-up system, A7amb (ppmw) is the concentration of the ith contaminant in ambient air, WFR is the water-to-fuel ratio, W- (ppmw) is the concentration of the ith contaminant1)inisthe water theinjected rate of water at the point of entry into the combustor, R (gal min’
9 carryover from the evaporative cooler, LHV (BtU lb’) is the lower heating value of actual fuel, E is the efficiency rating for the mist eliminator system, C, (ppmw) is the concentration of the ith contaminant in the evaporative cooler feedwater and f (MBtu h’) is the actual fuel flow rate. Obtaining reliable data from field sites to use in the above expression can be difficult and is often not feasible in practice. Nevertheless, estimates can be derived by extrapolating available data. Correlation of T, values for each critical contaminant, given a set of ambient conditions, can be used as a means of assessing the economics of coating costs and air, fuel and water treatment with respect to the economics of maintenance (overhaul and maintenance, parts repair and recoating).
4. Application Solar’s largest turbine engine is the Mars, of 12000 horsepower, with 15 stages of axial compression, an annular combustor design and two turbine stages. Typical air and fuel flow rates are of the order of 80 lb s’ and 1.5 lb s’ respectively. A cutaway view is shown in Fig. 5.
EXHAUST COLLEcI0R
TURBiNE EXHAUST DIFFUSER
FUEL MANIFOLi
-
FUEL iNJECTO’
COMPRESSOR ROTOR
-
-
ACCESSORY
-
-i..
-
- ..
/
.
~
I
-..--~,r —
.-
-
PRODUCER INE ROTORS -
COMPRESSOR CASE
,ARiABLE STATOR VANES
Fig. 5. Cutaway view of the Mars engine.
-. ..~--TURBiNE - TOnS
-.
/
-,
-
-
-
“A.
OR
10
To illustrate this concept of total corrosion analysis, a hypothetical example is taken of a Mars engine operating with water injection and evapo-
rative cooling. Operating conditions, based on nominal values for this engine, are cited in Table 4, with sodium selected as the contaminant of interest. For illustrative purposes, an initial calculation of total sodium is performed using city water (similar to the quality of San Diego water) without further treatment for evaporative cooling and control of nitrogen oxides. Calculations of total sodium, expressed as parts per million FEC, are based on three levels of water carryover, i.e. 5%, 10% and 15% of feedwater to the evaporative cooler, which is estimated to span the range (time averaged) that can be expected in practice with such devices. Other conditions and assumptions are as follows: an industrial environment of 0.010 ppmw sodium in air; a nominal air-to-fuel ratio of 56; the efficiency of the standard air filter is 0.95%; the average sodium content of the local utility gas supply is estimated at a very low level of 0.01 ppmw; the LHV of the gas fuel is 20 100 TABLE 4 Sample calculation of hypothetical Mars operationa Carryover
(%)
5
10
15
Concentration of air (ppmw) Air-to-fuel ratio Air clean-up factor FEC in air (ppmw)b
0.010 56 0.95 0.026
0.010 56 0.95 0.026
0.010 56 0.95 0.026
Concentration of fuel (ppmw) Fuel LHV (Btu lb’) Fuel clean-up factor 1) Fuel flow rate (MBtu h FEC in fuel (ppmw)”
0.0100 20100 0.00 104.00 0.0091
0.0100 20100 0.00 104.00 0.0091
0.0100 20100 0.00 104.00 0.0091
Concentration of injected water (ppmw) Water-to-fuel ratio FEC in injected water (ppmw)b
100.00 0.80 73.1
100.00 0.80 73.1
100.00 0.80 73.1
Concentration of evaporative cooling water (ppmw) Evaporative cooler carryover rate (gal min1) Mist eliminator efficiency FEC in evaporative cooling water (ppmw)b
100.00 0.85 0.00 7.5
100.00 1.70 0.00 15.0
100.00 2.55 0.00 22.5
Total FEC (ppmw)
80.7
88.2
95.7
Maximum allowable limit of sodium per Solar specification is 1.0 ppmw FEC. aNatural gas; city water (500 ppmw TDS CaCO 3, 20% Na); water injection (water-to-fuel ratio 0.8); evaporative cooling (no mist eliminator). For the evaporative cooling application, the water carryover is estimated at 5%, 10% and 15% of feedwater, T(dry bulb) air in is assumed to be 90 °F,T(dry bulb) air out is assumed to be 75 °F, the relative humidity is 1.assumed to be 30% and the evaporative cooling feedwater is assumed to be 17 gal bFEC min is the fuel equivalent concentration referenced to standard fuel with an LHV of 18 380 Btu lb1.
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Btu lb~ the nominal fuel flow is 104 MBtu h’; a 0.8 water-to-fuel ratio is assumed to be capable of meeting the emissions regulations; the typical feedwater rate to the Mars-size evaporative cooler is 17 gal min1. With the above conditions, total sodium levels of 80.7 ppmw FEC, 88.2 ppmw FEC and 95.7 ppmw FEC for 5%, 10% and 15% carryover respectively are attained and clearly underscore the point that, in most geographic locations, potable water is not acceptable for water injection and evaporative cooling. In order to assess the extent of water clean-up economically necessary to comply with Solar’s sodium limit of 1 ppmw FEC, eqn. (2) is used with T 1 = 1 ppmw FEC. The relationship between the injected water quality and the evaporative cooling water quality can be determined iteratively and
calculated values are plotted in Fig. 6 for three levels of water carryover. Consequently, Fig. 6 can be used as a guide for minimizing equipment costs. For example, if a significant cost break from the equipment supplier
can be taken advantage of for treated water containing 0.7 ppmw sodium, a corresponding maximum limit for the sodium content of the evaporative cooling water must be set at 6 ppmw, if a 5% carryover rate is assumed.
~II ~
12
5% Water
Mars
ETQiTe
Sodium CoTcentration in lTjected Water.
pprnw
Fig. 6. Correlation between the sodium concentration of the injected and the evaporative cooling water for a hypothetical example.
5. Field experiences First-stage turbine blades in the Mars engine are fabricated from MarM421, a l6Cr—4.5A1 nickel base superalloy. From a hot corrosion standpoint, this alloy is rated for service under conditions of up to 1 ppmw FEC
of sodium plus potassium. For non-standard fuels or corrosive applications, a platinum aluminide coating is applied to external surfaces for added protection. Three field experiences are cited here involving uncoated Mar-M421 Mars blades that were unintentionally subjected to high sodium environments during service. In each instance, initial indications of unusually severe conditions were manifested in the presence of white deposits on both
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pressure and suction sides of the air-cooled blade, paralleling metal temperature profiles, as shown in Fig. 7. Subsequent analysis identified the deposits as being sodium sulfate. Further deterioration of the base metal resulted in performance losses and eventual shutdown. During engine disassembly, it was readily evident that all three units had experienced severe hot corrosion degradation, resulting in various stages of airfoil deterioration, ranging from that shown in Fig. 7 to localized areas where penetration through the component wall resulted in breakaway losses such as is shown in Fig. 8. Metallurgical evaluation of corroded hardware from all three units confirmed Type I high temperature hot corrosion. Characteristic morphologies are shown in Fig. 9, consisting of oxide scale formation over a depleted alloy zone containing discrete chromium sulfide particles as well as intergranular attack in the affected zone immediately adjacent to the oxide or alloy interface. On the basis of on-site investigations in each case, high sodium and potassium (salt) concentrations in the water injection and/or the evaporative cooler water carryover were suspected of being responsible for hot-section corrosion. A common occurrence in all three cases was excessive feedwater flow to the evaporative cooler resulting in oversaturation of the cooling media and high liquid water carryover into the air inlet. This problem was corrected by reducing and adjusting the water flow such that the evaporative cooler media was sufficiently saturated and a surface film of water can be maintained at
Suction Side
Prussuru Sick,
Fig. 7. Firste stage Mar-M421 turbine blade removed from service in an ultrahigh sodium environment.
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-
Suction Side
Pressure Side
Fig. 8. Worst-case example of first stage Mars blade removed from service in an ultrahigh sodium environment.
all times and water breakaway minimized. As further insurance against possible recurrence, a mist eliminator device was installed downstream of the evaporative cooler just prior to the compressor air inlet. Another common factor in the three cases was the unreliability of the water monitoring device that triggers the purge cycle or dump valve when resistivities (corresponding to TDS) levels exceed the set-point. Replacement with a properly adjusted monitor resolved this issue. Yet another chronic problem discovered was water leakage via the air inlet and this was suspected of being the means whereby excessive sodium entered the engine. This suspicion was confirmed on inspection of the air intake which revealed the presence of white salt and rust deposits on internal surfaces of duct~(see Fig. 10) as well as excessive encrusted salts on the sidewalls of the evaporative cooling media as indicated in Fig. 11. Subsequent action taken secured water lines and eliminated the source of water leakage. Total site analyses were conducted for sodium, based on the best available data from the operating sites. The results are given in Table 5. Columns A, B and C represent site conditions that are believed to have prevailed during service, resulting in the hardware conditions shown in Fig. 7 9. It is clearly evident, in all three cases, that the sodium concentrations in the air and fuel are well within Solar’s specification limit of 1 ppmw FEC and that water-borne sodium is the key contributor responsible for the unusually severe corrosion deterioration. Column D in Table 5 reflects the total sodium concentration as a result of the various changes implemented. Operation at this reduced sodium level -
Pull. .001
“
Section Showing Typical Depth of Attack Mag: 266X Mount No. 1866
~, -. -
~
-
I
~-~.0O1” }—I Section Showing Chromium Sulfide Particles (Arrows) in Depleted Zone Below Oxide Scale. Mag: 532X Mount No. 1865 Fig. 9. Section through a Mars first-stage turbine blade showing typical high temperature hot corrosion (type I) morphology.
of 0.24 ppmw FEC thus far has been satisfactory with further reports of excessive corrosion degradation. 6. Concluding remarks The concept of this approach is summed up in Fig. 12 which compares capitalized cost (costs of air, fuel and water treatment and coatings) with the
cost of maintenance (overhaul, parts refurbish and recoating). For a given set
______
-
____
15
Fig. 10. Downstream view of an air-silencer device showing excessive rusting and mineral (salt) deposits due to water carryover from the evaporative cooler.
-Si
-,
.
~
~
I
White Salt Deposits
-
~
V
~
_~_~
‘—.
Fig. 11. Side-walls of the evaporative cooling media encrusted with white salt deposits.
of environmental conditions, the cost of corrosion is the sum of these two cost factors which is reflected in the TBO that can be expected in service. For example, minimal treatment of air, fuel and water with the absence of protective coating(s) could result in substantially increased maintenance
16 TABLE 5 Calculated total sodium concentrations of Mars applications
Case history A
B
C
D
Concentration of air (ppmw) Air-to-fuel ratio Air clean-up factor FEC in air (ppmw)
0.010 59.300 0.990 0.007 8
0.010 59.300 0.990 0.0078
0.010 59.300 0.990 0.0078
0.010 59.000 0.999 0.0008
Concentration of fuel (ppmw) Fuel LHV (Btu/h’) Fuel clean-up factor Fuel flow rate (Btu/h’) FEC in fuel (ppmw)
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
Concentration of injected water (ppmw) Water-to-fuel ratio FEC in injected water (ppmw)
1.16
0.37
4.80
0.10
0.80 1.2
0.80 0.4
0.80 5.0
0.80 0.1
Concentration of evaporative cooling water (ppmw) Evaporative cooler carryover 1) rate eliminator (gal min~ efficiency Mist
11.00
22.00
22.00
22.00
2.00
2.00
2.00
1.00
0.00 1.8
0.00 3.7
0.00 3.7
0.99 0.02
3.2
4.2
8.9
0.2
FEC in evaporative cooling water (ppmw) Total FEC (ppmw)
Maximum allowable limit of sodium per solar specification is 1.0 ppmw FEC.
Total Cost of Corrosion
Cost
~rnUmcostofAlrFeI
TBO
Fig. 12. Total corrosion management.
costs in terms of component replacement. Conversely, installation of pro-
hibitively high cost treatment packages may not necessarily be economically justifiable by reduced maintenance costs. Therefore, it is desirable to achieve a minimum balance of startup costs us. maintenance costs, as indicated in Fig. 12, for optimized corrosion management.
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References 1 1984 —Air Quality in San Diego, Air Pollution Control District, County of San Diego, CA, 1984. 2 V. M. Sood, Solar Turbines Inc., personal communication, 1986. 3 P. Carter, The correlation of HP turbine blade corrosion attack in dissimilar marine gas turbines, Proc. 4th Con f. on Gas Turbine Materials in a Marine Environment, Annapolis, MD, 1 979, Naval Sea Systems Command, Washington, DC.
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TOTAL CORROSION CONTROL FOR INDUSTRIAL GAS TURBINES: HIGH TEMPERATURE COATINGS AND AIR, FUEL AND WATER MANAGEMENT* L. L. HSU Solar Turbines Inc., San Diego, CA (U.S.A.) (Received March 26, 1987)
Summary
Contaminants found in air, fuel or water can combine in the turbine hot section to produce corrosion, erosion and/or deposition. The fundamental role of high temperature coatings is to delay the onset of substrate deterioration and attendant effects on component structural integrity. To achieve this, the total concentration of each harmful contaminant in the turbine environment should be determined in order to select the appropriate material condition necessary, coated or uncoated, for the airfoil surface to survive the expected service time successfully. A method is presented whereby the total fuel equivalent concentration of any contaminant from all sources can be calculated. Utilization of this methodology is demonstrated by means of a hypothetical case of a turbine operating under specifically defined conditions. In this example, sodium (plus potassium) concentrations in air and fuel are assumed to be well within industry-accepted standards of 1 ppmw and it is shown that the use of untreated potable water for water injection and evaporative cooling can result in unacceptably high sodium concentrations. Subsequently, the extent and cost of water clean-up can be optimized by assessing the costs of water treatment equipment and the volume of water to be treated for emissions control and evaporative cooling. Field experiences with a turbine of 12000 horsepower further substantiate the diagnostic advantages of this approach. In three cited cases, the presence of excessive sodium in the turbine hot section resulted in Type I or high temperature hot corrosion degradation of stage 1 blades. The source of contamination was attributed to water-borne sodium, using the methodology described. Corrective action recommended was directed at reducing the sodium concentrations in both the evaporative cooling water and the injected water and with target levels specified to ensure compliance *paper presented at the 14th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., March 23 -27, 1987. Recipient of the Bunshah Award for the best paper in Symposium A: Coatings for Use at High Temperatures. 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
2
with Solar’s specification limit of 1 ppmw (fuel equivalent concentration) of total sodium. A platinum-modified aluminide coating was also applied to first- and second-stage blading. It is demonstrated that, with this approach, total corrosion control can be realized by assessing the source(s) of contamination followed by application of appropriate corrective action to reduce the level of contamination economically.
1. Introduction Protective coatings have become integral components of turbine engines as a result of increasingly greater high temperature strength requirements for superalloys combined with more challenging applications using lower grade and non-fossil fuels. Whereas the aero-engine is limited by weight (thrust) considerations in conjunction with relatively tight restrictions on fuel, the land-based turbine engine, being free from weight and fuel limitations, must necessarily be more versatile and rugged as a broad-based power plant. Time between overhaul (TBO) of the order of 30 000 h is not uncommon, thus making life prediction of critical components a difficult task at best. As an original equipment manufacturer (OEM), Solar Turbines Inc., a subsidiary of Caterpillar Inc., has a worldwide fleet of industrial gas turbine engines operating in a range of environments and duty cycles, both on land and offshore. In some land-based installations, emissions control and power augmentation have necessitated the introduction of water as a third fluid component (as well as air and fuel) via water injection and evaporative cooling. In addition, it is not uncommon for fuels to vary in type and quality from pipeline natural gas to biomass fuels and non-hydrocarbon gases. Consequently, contaminants can enter the turbine engine in any of the three fluid media, air, fuel and water, as depicted schematically in Fig. 1. These contaminants can be in the form of gas, liquid or solid phases, depending on prevailing temperatures and pressures. In the compressor, the effect of air-borne contaminants on airfoils tends to be less severe while the engine is in operation because of low temperatures (very much less than 1000 °F)and the centrifuging action of the rotating blades. Physical or mechanical damage due to particulate erosion tends to be of more concern, particularly in the early stages of compression. Most of the low temperature corrosion damage observed in the compressor is the result of electrochemical or galvanic reactions that occur during shutdown or in aerodynamically stagnated recesses in the presence of liquid water. Surviving air-borne contaminants combine with fuel and (injected) water-borne contaminants in the combustor wherein the complex scenario of exothermic reactions produces highly energetic hot gases containing potentially corrosive and/or fouling chemical species.
3
Fuel
+
Water Contaminants
w
Water Soluble
+
I Hydrocarbons)
+
(Moisture)
+
Contaminants
Solid Contaminants (Colloids, 5~rbi~nat1es,
Fuel Water
Air
AF
Ambient Air +
Water—.-
EC
+
omp
Water Vapor I Ambient EvaporatedI
Solid (Particulates I
+
+
Comb
T
Exhaust
Liquid Water Droplets
Gas Phase Contaminants ISalts, Aerosols, etc. I
Fig. 1. Schematic diagram of fluids and entrained species entering industrial gas turbine engines.
In the face of this continuous onslaught of combustor effluence, turbine airfoils can be viewed as “consumables” in that surface degradation is expected to occur and does occur at a controlled rate with the expectancy that the airfoils remain functional (within specified performance ratings) over a defined lifetime, i.e. TBO. Therefore, the role of the hot-section coating is to protect, albeit temporarily, the substrate alloy from the environment in order to meet material performance expectations, i.e. delay the threshold of unacceptable alloy degradation, as shown in Fig. 2. A crucial aspect of materials engineering is the selection of appropriate coatings that can impart the additional life necessary to achieve designated TBOs under specific environmental conditions without any penalty in mechanical strength. The approach discussed in this paper was developed at Solar to establish a quantitative methodology for evaluating contaminant concentrations in air, fuel and water and to correlate their presence to component (coating or substrate) resistance. The objectives of such an approach are twofold: (1) a more accurate assessment of life or performance expectations of gas
Degradation. Baseline •i.I.I Coating Alloy
Uricoated Coated
U•U~
Surtace Degradation
Coated Component Life
Life
Fig. 2. Function of the hot-section coatings.
4
path materials and (2) correlation of site conditions with coating requirements. Even though this is an ongoing effort at Solar, the fundamentals of this approach are presented in the first part of the paper followed by an ifiustrative example and discussion of field experiences in which the combined effects of air, fuel and water quality directly influenced the life of the hot-section components.
2. Contaminants Contaminants of concern to the land-based turbine operator and the mode of degradation by these contaminants are listed in Table 1 which also indicates the source(s) whereby these contaminants are known to enter the engine. The first five in the listing are of primary concern because of their critical role in hot-section degradation. Nevertheless, the net effect of any one of these contaminants on turbine hot-section components can be critical if the total allowable limit specified by the OEM is exceeded. In general, the severity of degradation is dependent on the properties of the existing chemical species, i.e. gas, liquid or solid state, thermodynamic activities, residence time in the combustor, reaction rates (kinetics), concentration, substrate (alloy or coating) composition, etc. Regardless of the mode of degradation, corrosion, erosion or deposition, the damage incurred is irreversible, even when the contaminant concentration level subsequently decreases to innocuous levels. From the OEM standpoint, specific limits on the maximum allowable concentration of critical species, particularly sulfur, sodium, potassium, vanadium and lead, in the total fluid intake of the engine must be clearly defined in order to control the rate and extent of damage by these contaminants. Solar’s standards for the five critical contaminants are given in Table 2.
TABLE 1 Listing of contaminants detrimental to turbine engine operation
Contaminant
Air
Sodium Potassium Vanadium Lead Calcium Sulfur Silicon Chlorine Fluorine
x x
Fuel
Water
Degradation mode
x
Corrosion Corrosion Corrosion Corrosion Erosion and deposition Corrosion Erosion and deposition Corrosion Corrosion
x X
x
x
x x
X
x
x x
x x x
x
x x
5 TABLE 2 Industry standards for maximum contamination levels
Sodium + potassium Vanadium Lead Calcium
Solar
Other OEMs
ASTM D2880
(ppmw FEC)
(ppmw)
(ppmw)
1.0 0.5 1.0 1.0
1 - 1.30 0.5 - 1.95 1.0 - 1.95 1.0-3.90
0.5 0.5 1.0 0.5
All values given are referenced to fuel.
2.1. Air
Ambient air quality can fluctuate from the pristine environments of rural hinterlands to the smog- and chemicals-laden air of highly industrialized areas. Unfortunately, data on many of the air-borne contaminants listed in Table 1 are meager and difficult to measure other than for those monitored by Air Pollution Control Districts [1] such as carbon monoxide, sulfur oxides, sulfates, nitrogen oxides, lead, ozone and aerosols (fine particulates). In addition, transient conditions such as dust storms, high wave conditions at sea, accidental or temporary plant or factory discharge, prevailing wind directions and seasonal weather-related parameters (temperature, relative humidity, precipitation etc.) can directly impact the concentration of airborne contaminants. For example, the National Gas Turbine Establishment has published data that correlate wind velocity with sodium chloride concentration in air for offshore installations (see Fig. 3). Depending on the particle size distribution of these salt water droplets (which in turn are affected by factors such as particle velocity, relative humidity and temperature), droplet removal by an appropriately specified air-cleaner system would remove most of the particles in the air stream at the rated equipment efficiency and particle cutoff size. However, experience has shown that some finite amount of airborne particles will elude the filter elements and enter the compressor. Therefore, it is possible that the concentration of sea salts ingested during a period of high wind velocity could be considerably higher than the norm and maximum allowable limits could be exceeded, thus enabling hot-section corrosion to occur temporarily. As a consequence, even if high wind velocity periods are infrequent, the degradation caused in the duration is non-reversible and self-propagating and tends to continue even after ingested concentrations drop to lower levels. Compressor intake is subjected to various thermal profiles depending on the engine size, efficiency and rating; typically, approximately 10% of compressor discharge is bled off for cooling and hence remains at temperatures less than 1200 °F; 15% 25% is used for combustion (primary zone) and the remaining 65% 75% is introduced downstream of stoichiometric combustion as secondary or tertiary cooling air [2,3]. Even though it is -
-
6
recognized that not all air-borne contaminants are exposed to the elevated temperatures that are necessary for formation of corrosive species in the gas stream, it is assumed in this approach that all air-borne contaminants participate fully in turbine degradation. 2.2. Fuel The types of fuels typically associated with directly fired turbine engines are gases or liquids. (Current research and development in the area of directly fired coal-fired turbine engines will eventually allow the use of solid—liquid slurries as a fuel type as well.) A generic listing of fuel types is shown in Fig. 4 and it is evident from a scanning of the list that contaminant levels can vary from pipeline quality natural gas (nominal 0.5 wt.% sulfur for a San Diego utility) to sulfur- and vanadium-rich crude and high halogen fuels from landfills and waste. For several of the fuel types listed in Fig. 4, particulate contamination is a major concern because of the presence of ashforming constituents especially for fuels with high aromatic content, resulting in an increased potential for turbine deposition and fouling. It is customary at a gas turbine operating site to install a fuel processing skid to ensure that fuel properties satisfy specified requirements for reliable and safe operation. In addition to fuel compressors (or de-compressors) and heating devices to regulate pressure and temperature, a fuel filter and separator system is typically employed to remove entrained water, liquids and particulates. Water removal can reduce the level of sodium and potassium salts because, being water soluble, they are segregated to the water or moisture portion of the fuel. However, water-insoluble species such as vanadium, lead
Fuels
I
101
Total Salt
E ~
Gaseous
/ //
//
/ / Salt from / / Droplets / /
10~
4
<13 ~umDIA.
// 1/
Z
,7’// 10
Natural Gas
•
Crude
•
Coal
•
Natural Gas Liquids
•
Distillates
•
Wood
•
Residuals
•
Digester
.
— _____-~
g~
~rt~m<4
1nm DIA.
20 30 40 Wind Speed, Knots
•
Process
•
Coal Gas
•Nonhydrocarbons
• Coke Oven • Blast Furnace
Fig. 3. Sodium chloride concentration as a function of wind speed. Fig. 4. Generic classification of fuels.
Solids
.
Landfill • Gob Gas
/1’
Liquid
-~
7
and sulfur are organically bound to the hydrocarbon fuel and are not removable by on-site fuel treatment except as part of condensable liquid hydrocarbons.
2.3. Water Water chemistry and treatment, a technology in itself, plays a crucial, albeit often overlooked, role in gas turbine corrosion. Water can be intentionally introduced as injected water (or stream) for emissions (nitrogen oxides) control or it can also be a constituent of ingested air as water vapor via evaporative cooling for power augmentation or as liquid water carried over from evaporative cooler operation. There are three sources of naturally occurring waters: groundwater (springs and wells), surface water (rivers and lakes) and seawater. Water quality, on the basis of total dissolved solids (TDS), can range from “fresh” to “brine”, as defined in Table 3. For corrosion control, definition of water quality via TDS is inadequate as it expresses all dissolved ions in the water as though they were CaCO3. Additional data regarding specific concentrations 2 etc.) as determined by wet chemof of theare critical ions to (Na~,K~, icaleach methods required calculateS04 actual concentrations of critical contaminants. Where water injection and/or evaporative cooling is required, it is customary to install an ancillary water treatment package specifically designed to achieve target water quality levels. Water treatment processes can include any combination of softener, ion exchanger (cation, anion or mixed bed), demineralizer, reverse osmosis or distillation units. The price of treated water can differ by orders of, magnitude, depending on the extent of water clean-up. For example, soft San Diego water is estimated to cost $0.12 per gallon while demineralized San Diego water can be as high as $20 per gallon. Contaminants in injected water can be considered as being equivalent to contaminants in fuel because water flow rates are closely matched to fuel flow, i.e. typical water-to-fuel ratios range from 0.5 to 1.0 for Solar engines. In contrast, the rate of liquid water carryover from evaporative coolers is nominally much lower than fuel flow and this difference is reflected in the concentration ranges of the two water qualities as discussed in the latter part of this paper.
TABLE 3 Total dissolved solids content of water types
Type of water
TDS (ppmw CaCO 3)
Fresh water Brackish water Salty water Brine
0 1000 10000
1000 10000 - 100000 >100000 -
8
From the ancillary equipment standpoint, it should be realized that, even though evaporative coolers are generally designed for zero water carryover, field experience has shown that liquid water may be and often is carried over into the air stream and can be a major source of contaminants in high temperature corrosion.
3. Calculation of total contaminant(s) concentration Regardless of the media through which contaminants are introduced into the engine, the net effect in the turbine section can be expressed as a sum of the contaminants contributed by air, fuel and water. At Solar, the accepted convention is to express total contaminant concentration as a fuel equivalent concentration (FEC), i.e. parts per million with reference to a standard fuel with a lower heating value (LHV) of 18 380 Btu lb1: =
F, + (AFR)A 1 + (WFR)W1 + (CFR)G1
(1)
where T1 is the total concentration of the ith contaminant entering the engine, F. is the concentration of the ith contaminant in the fuel supply, AFR is the air-to-fuel ratio, A1 is the concentration of the ith contaminant in ambient air, WFR is the injected water-to-fuel ratio (or stream), W~is the concentration of the ith contaminant in the injected water, CFR is the carryover water (from evaporative cooler)-to-fuel ratio and C7 is the concentration of the ith contaminant in the evaporative cooling water. The above expression can be converted to a more useful form by incorporating factors that influence the effective concentration in the turbine section. Examples of such factors are efficiency of air clean-up (filter), extent of on-line fuel processing (filters and separators), evaporative cooler design (recirculating or once-through), water carryover rate, adjustment of fuel flow rate (normalized to the LHV of the standard fuel) and mist eliminators efficiency (if used in conjunction with evaporative cooling). The final expression, with appropriate definition of units, has the following form: 18380 LHV F7(1 —K) + (AFR)(1 _N)A~5mb {4.99 x iO~R(LHV)(1 —E)}C.
=
+
+
(WFR)W1 (2)
where T7 (ppmw FEC) is the total concentration of the ith contaminant, F7 (ppmw) is the concentration of the ith contaminant in fuel, K is the efficiency rating for the fuel clean-up system, AFR is the air-to-fuel ratio, N is the efficiency rating for the air clean-up system, A7amb (ppmw) is the concentration of the ith contaminant in ambient air, WFR is the water-to-fuel ratio, W- (ppmw) is the concentration of the ith contaminant1)inisthe water theinjected rate of water at the point of entry into the combustor, R (gal min’
9 carryover from the evaporative cooler, LHV (BtU lb’) is the lower heating value of actual fuel, E is the efficiency rating for the mist eliminator system, C, (ppmw) is the concentration of the ith contaminant in the evaporative cooler feedwater and f (MBtu h’) is the actual fuel flow rate. Obtaining reliable data from field sites to use in the above expression can be difficult and is often not feasible in practice. Nevertheless, estimates can be derived by extrapolating available data. Correlation of T, values for each critical contaminant, given a set of ambient conditions, can be used as a means of assessing the economics of coating costs and air, fuel and water treatment with respect to the economics of maintenance (overhaul and maintenance, parts repair and recoating).
4. Application Solar’s largest turbine engine is the Mars, of 12000 horsepower, with 15 stages of axial compression, an annular combustor design and two turbine stages. Typical air and fuel flow rates are of the order of 80 lb s’ and 1.5 lb s’ respectively. A cutaway view is shown in Fig. 5.
EXHAUST COLLEcI0R
TURBiNE EXHAUST DIFFUSER
FUEL MANIFOLi
-
FUEL iNJECTO’
COMPRESSOR ROTOR
-
-
ACCESSORY
-
-i..
-
- ..
/
.
~
I
-..--~,r —
.-
-
PRODUCER INE ROTORS -
COMPRESSOR CASE
,ARiABLE STATOR VANES
Fig. 5. Cutaway view of the Mars engine.
-. ..~--TURBiNE - TOnS
-.
/
-,
-
-
-
“A.
OR
10
To illustrate this concept of total corrosion analysis, a hypothetical example is taken of a Mars engine operating with water injection and evapo-
rative cooling. Operating conditions, based on nominal values for this engine, are cited in Table 4, with sodium selected as the contaminant of interest. For illustrative purposes, an initial calculation of total sodium is performed using city water (similar to the quality of San Diego water) without further treatment for evaporative cooling and control of nitrogen oxides. Calculations of total sodium, expressed as parts per million FEC, are based on three levels of water carryover, i.e. 5%, 10% and 15% of feedwater to the evaporative cooler, which is estimated to span the range (time averaged) that can be expected in practice with such devices. Other conditions and assumptions are as follows: an industrial environment of 0.010 ppmw sodium in air; a nominal air-to-fuel ratio of 56; the efficiency of the standard air filter is 0.95%; the average sodium content of the local utility gas supply is estimated at a very low level of 0.01 ppmw; the LHV of the gas fuel is 20 100 TABLE 4 Sample calculation of hypothetical Mars operationa Carryover
(%)
5
10
15
Concentration of air (ppmw) Air-to-fuel ratio Air clean-up factor FEC in air (ppmw)b
0.010 56 0.95 0.026
0.010 56 0.95 0.026
0.010 56 0.95 0.026
Concentration of fuel (ppmw) Fuel LHV (Btu lb’) Fuel clean-up factor 1) Fuel flow rate (MBtu h FEC in fuel (ppmw)”
0.0100 20100 0.00 104.00 0.0091
0.0100 20100 0.00 104.00 0.0091
0.0100 20100 0.00 104.00 0.0091
Concentration of injected water (ppmw) Water-to-fuel ratio FEC in injected water (ppmw)b
100.00 0.80 73.1
100.00 0.80 73.1
100.00 0.80 73.1
Concentration of evaporative cooling water (ppmw) Evaporative cooler carryover rate (gal min1) Mist eliminator efficiency FEC in evaporative cooling water (ppmw)b
100.00 0.85 0.00 7.5
100.00 1.70 0.00 15.0
100.00 2.55 0.00 22.5
Total FEC (ppmw)
80.7
88.2
95.7
Maximum allowable limit of sodium per Solar specification is 1.0 ppmw FEC. aNatural gas; city water (500 ppmw TDS CaCO 3, 20% Na); water injection (water-to-fuel ratio 0.8); evaporative cooling (no mist eliminator). For the evaporative cooling application, the water carryover is estimated at 5%, 10% and 15% of feedwater, T(dry bulb) air in is assumed to be 90 °F,T(dry bulb) air out is assumed to be 75 °F, the relative humidity is 1.assumed to be 30% and the evaporative cooling feedwater is assumed to be 17 gal bFEC min is the fuel equivalent concentration referenced to standard fuel with an LHV of 18 380 Btu lb1.
11
Btu lb~ the nominal fuel flow is 104 MBtu h’; a 0.8 water-to-fuel ratio is assumed to be capable of meeting the emissions regulations; the typical feedwater rate to the Mars-size evaporative cooler is 17 gal min1. With the above conditions, total sodium levels of 80.7 ppmw FEC, 88.2 ppmw FEC and 95.7 ppmw FEC for 5%, 10% and 15% carryover respectively are attained and clearly underscore the point that, in most geographic locations, potable water is not acceptable for water injection and evaporative cooling. In order to assess the extent of water clean-up economically necessary to comply with Solar’s sodium limit of 1 ppmw FEC, eqn. (2) is used with T 1 = 1 ppmw FEC. The relationship between the injected water quality and the evaporative cooling water quality can be determined iteratively and
calculated values are plotted in Fig. 6 for three levels of water carryover. Consequently, Fig. 6 can be used as a guide for minimizing equipment costs. For example, if a significant cost break from the equipment supplier
can be taken advantage of for treated water containing 0.7 ppmw sodium, a corresponding maximum limit for the sodium content of the evaporative cooling water must be set at 6 ppmw, if a 5% carryover rate is assumed.
~II ~
12
5% Water
Mars
ETQiTe
Sodium CoTcentration in lTjected Water.
pprnw
Fig. 6. Correlation between the sodium concentration of the injected and the evaporative cooling water for a hypothetical example.
5. Field experiences First-stage turbine blades in the Mars engine are fabricated from MarM421, a l6Cr—4.5A1 nickel base superalloy. From a hot corrosion standpoint, this alloy is rated for service under conditions of up to 1 ppmw FEC
of sodium plus potassium. For non-standard fuels or corrosive applications, a platinum aluminide coating is applied to external surfaces for added protection. Three field experiences are cited here involving uncoated Mar-M421 Mars blades that were unintentionally subjected to high sodium environments during service. In each instance, initial indications of unusually severe conditions were manifested in the presence of white deposits on both
12
pressure and suction sides of the air-cooled blade, paralleling metal temperature profiles, as shown in Fig. 7. Subsequent analysis identified the deposits as being sodium sulfate. Further deterioration of the base metal resulted in performance losses and eventual shutdown. During engine disassembly, it was readily evident that all three units had experienced severe hot corrosion degradation, resulting in various stages of airfoil deterioration, ranging from that shown in Fig. 7 to localized areas where penetration through the component wall resulted in breakaway losses such as is shown in Fig. 8. Metallurgical evaluation of corroded hardware from all three units confirmed Type I high temperature hot corrosion. Characteristic morphologies are shown in Fig. 9, consisting of oxide scale formation over a depleted alloy zone containing discrete chromium sulfide particles as well as intergranular attack in the affected zone immediately adjacent to the oxide or alloy interface. On the basis of on-site investigations in each case, high sodium and potassium (salt) concentrations in the water injection and/or the evaporative cooler water carryover were suspected of being responsible for hot-section corrosion. A common occurrence in all three cases was excessive feedwater flow to the evaporative cooler resulting in oversaturation of the cooling media and high liquid water carryover into the air inlet. This problem was corrected by reducing and adjusting the water flow such that the evaporative cooler media was sufficiently saturated and a surface film of water can be maintained at
Suction Side
Prussuru Sick,
Fig. 7. Firste stage Mar-M421 turbine blade removed from service in an ultrahigh sodium environment.
13
-
Suction Side
Pressure Side
Fig. 8. Worst-case example of first stage Mars blade removed from service in an ultrahigh sodium environment.
all times and water breakaway minimized. As further insurance against possible recurrence, a mist eliminator device was installed downstream of the evaporative cooler just prior to the compressor air inlet. Another common factor in the three cases was the unreliability of the water monitoring device that triggers the purge cycle or dump valve when resistivities (corresponding to TDS) levels exceed the set-point. Replacement with a properly adjusted monitor resolved this issue. Yet another chronic problem discovered was water leakage via the air inlet and this was suspected of being the means whereby excessive sodium entered the engine. This suspicion was confirmed on inspection of the air intake which revealed the presence of white salt and rust deposits on internal surfaces of duct~(see Fig. 10) as well as excessive encrusted salts on the sidewalls of the evaporative cooling media as indicated in Fig. 11. Subsequent action taken secured water lines and eliminated the source of water leakage. Total site analyses were conducted for sodium, based on the best available data from the operating sites. The results are given in Table 5. Columns A, B and C represent site conditions that are believed to have prevailed during service, resulting in the hardware conditions shown in Fig. 7 9. It is clearly evident, in all three cases, that the sodium concentrations in the air and fuel are well within Solar’s specification limit of 1 ppmw FEC and that water-borne sodium is the key contributor responsible for the unusually severe corrosion deterioration. Column D in Table 5 reflects the total sodium concentration as a result of the various changes implemented. Operation at this reduced sodium level -
Pull. .001
“
Section Showing Typical Depth of Attack Mag: 266X Mount No. 1866
~, -. -
~
-
I
~-~.0O1” }—I Section Showing Chromium Sulfide Particles (Arrows) in Depleted Zone Below Oxide Scale. Mag: 532X Mount No. 1865 Fig. 9. Section through a Mars first-stage turbine blade showing typical high temperature hot corrosion (type I) morphology.
of 0.24 ppmw FEC thus far has been satisfactory with further reports of excessive corrosion degradation. 6. Concluding remarks The concept of this approach is summed up in Fig. 12 which compares capitalized cost (costs of air, fuel and water treatment and coatings) with the
cost of maintenance (overhaul, parts refurbish and recoating). For a given set
______
-
____
15
Fig. 10. Downstream view of an air-silencer device showing excessive rusting and mineral (salt) deposits due to water carryover from the evaporative cooler.
-Si
-,
.
~
~
I
White Salt Deposits
-
~
V
~
_~_~
‘—.
Fig. 11. Side-walls of the evaporative cooling media encrusted with white salt deposits.
of environmental conditions, the cost of corrosion is the sum of these two cost factors which is reflected in the TBO that can be expected in service. For example, minimal treatment of air, fuel and water with the absence of protective coating(s) could result in substantially increased maintenance
16 TABLE 5 Calculated total sodium concentrations of Mars applications
Case history A
B
C
D
Concentration of air (ppmw) Air-to-fuel ratio Air clean-up factor FEC in air (ppmw)
0.010 59.300 0.990 0.007 8
0.010 59.300 0.990 0.0078
0.010 59.300 0.990 0.0078
0.010 59.000 0.999 0.0008
Concentration of fuel (ppmw) Fuel LHV (Btu/h’) Fuel clean-up factor Fuel flow rate (Btu/h’) FEC in fuel (ppmw)
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
1.80 14000 0.95 109.30 0.12
Concentration of injected water (ppmw) Water-to-fuel ratio FEC in injected water (ppmw)
1.16
0.37
4.80
0.10
0.80 1.2
0.80 0.4
0.80 5.0
0.80 0.1
Concentration of evaporative cooling water (ppmw) Evaporative cooler carryover 1) rate eliminator (gal min~ efficiency Mist
11.00
22.00
22.00
22.00
2.00
2.00
2.00
1.00
0.00 1.8
0.00 3.7
0.00 3.7
0.99 0.02
3.2
4.2
8.9
0.2
FEC in evaporative cooling water (ppmw) Total FEC (ppmw)
Maximum allowable limit of sodium per solar specification is 1.0 ppmw FEC.
Total Cost of Corrosion
Cost
~rnUmcostofAlrFeI
TBO
Fig. 12. Total corrosion management.
costs in terms of component replacement. Conversely, installation of pro-
hibitively high cost treatment packages may not necessarily be economically justifiable by reduced maintenance costs. Therefore, it is desirable to achieve a minimum balance of startup costs us. maintenance costs, as indicated in Fig. 12, for optimized corrosion management.
17
References 1 1984 —Air Quality in San Diego, Air Pollution Control District, County of San Diego, CA, 1984. 2 V. M. Sood, Solar Turbines Inc., personal communication, 1986. 3 P. Carter, The correlation of HP turbine blade corrosion attack in dissimilar marine gas turbines, Proc. 4th Con f. on Gas Turbine Materials in a Marine Environment, Annapolis, MD, 1 979, Naval Sea Systems Command, Washington, DC.