Turndown Cond. #2 Weeping Factor for Objective Function (Sieve and Fixed Valve)
SAMPLE PROBLEM A grassroots depentanizer is to be designed
After running a PRO/II simulation for a depentanizer, it is determined that 16 theoretical stages are required to obtain the desired separation, with 7 stages above the feed (rectifying section) and 9 stages below the feed (stripping section). In addition, the stripping section (below the feed) limits the tower capacity. The highest loaded tray in the stripping section, theoretical stage 16, has the following design flowrates and conditions according to the PRO/II model:
Operating pressure = 66.4 psia Liquid temperature = 366.8 F
Liquid rate = 560.82 klb/hr Vapor rate = 322.58 klb/hr Liquid density = 217.2 lb/bbl Vapor density = 0.972 lb/ft3 Liquid viscosity = 0.1765 cP Vapor viscosity = 0.0096 cP Surface tension = 8.94 dyn/cm
Liquid molecular weight = 125.9 Vapor molecular weight = 123.3
A turndown of 50% is required. This means that all trays must be designed so that they can handle both the design loads and 50% of the design loads. For theoretical stage 16, a 50% turndown will be specified for design, but after designing the stripping section trays based on stage 16, it is necessary to check the minimum loaded tray in the section to ensure that it too can handle 50% of design rates without significant loss of efficiency.
1) It is assumed that sieve trays have already been determined to be the correct internals selection for this tower. The loadings, conditions, physical properties, and turndown requirements for theoretical stage 16 are entered into EMoTIP. In addition, the equilibrium slope from the McCabe-Thiele diagram has been determined from the composition profile in the PRO/II output report to be 0.700; this value is entered into EMoTIP for calculating efficiency. The following output report is obtained after running EMoTIP in design mode, using the default design algorithm limits:
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***** EXXONMOBIL USE ONLY Page No. 1
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***** EXXONMOBIL TOWER INTERNALS PROGRAM *****
***** VERSION 1.0 01-May-2003 *****
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Date: 02-May-03 Time: 10:48:16
User Name: sample user Tower Tag No: sample tag Project: Depentanizer sample Facility: sample facility Comments: Depentanizer stripping section design based on theo 16
Process: Powerformer Service: Depentanizer Tower Section: Bottom
Sieve Tray 2 Pass Design Case
Tower Diameter = 11.00 ft Tray Spacing = 18.0 in
---TRAY PERFORMANCE SUMMARY (EACH VALUE IS FROM ITS LIMITING PASS)
---PRIMARY DESIGN PARAMETERS CRITERIA
---Overall Percent Flood 77.4 % < 85.0 %
Percent Ultimate Capacity Tray 45.7 %
Percent Jet Flood 78.7 %
Percent Downcomer Flood 77.4 % Probability of Non-Flooding Design 99.2 % Turndown: % Thruput at 20 % Weep 40.4 % Design Overall Effic. Key Comp 1 88.0 %
---SECONDARY DESIGN PARAMETERS CRITERIA
---Liquid Entrainment (M-Method) 0.01 % < 10.0 %
Liquid Entrainment (E-Method) 0.14 % < 10.0 % DC Choking Due To Velocity 50.2 % < 100.0 % SQRT(DC Backup * DC Choke) 58.7 % < 70.0 % Liquid Rate, US gpm/in of weir 9.3 1.5 - 17.5 Froth/Spray Transition 55.0 % < 110.0 % Ultimate Capacity Universal 38.5 % < 85.0 %
Dry Tray Pressure Drop, in 1.9 1.25 - 5.50 Total Tray Pressure Drop, in 5.3
Velocity Under Downcomer, ft/s 1.04 < 1.30 Vapor Fraction Under DC 0.04
Downcomer Seal in 0.39 > -0.50
Liquid Weeping at Conditions 0.0 % < 20.0 %
---WARNING: See Page 6 for 5 warning messages.
---FOR CONTRACTOR AND VENDOR USE Page No. 2
---TRAY GEOMETRY SUMMARY * INDICATES INPUTTED GEOMETRY
---* Tray Type = Sieve Avg. Adi = 10.054 ft2 10.6 %
No. of Pass = 2 Avg. Ado = 5.385 ft2 5.7 %
Diameter = 11.00 ft Tot. BA = 62.692 ft2 66.0 % Tray Spacing = 18.0 in Tower Area = 95.033 ft2
PASS A PASS B
FLOW DIRECTION CNTR-SIDE SIDE-CNTR
* DC Type Sloped Sloped
DC Top Area ft2 10.071 10.036
DC Top Width in 21.500 11.000
DC Btm Area ft2 5.392 5.378
DC Btm Width in 14.000 5.875
Wasted DC Area ft2 0.000 0.000
Bubble Area ft2 32.067 30.625
Bubble Waste Area ft2 0.000 1.464
Free Area ft2 39.809 39.785
Volum. Waste Area ft2 0.000 0.000
Flow Path Length in 38.63 38.50
DC Top Chord Len in 97.48 130.15
DC Btm Chord Len in 81.29 131.48
DC Clearance in 2.500 2.125
Outlet Weir Ht in 2.375 3.500
Tray Thickness in 0.074 0.074
Hole Diameter in 0.5000 0.5000
Number of Holes 2057 1965
Hole Area ft2 2.806 2.680
Hole/Bubble Area % 8.75 8.75
NON-STANDARD TOWER INTERNALS
DC Lip Radius in 0.00 0.00
Inlet Weir Ht in 2.50 0.00
Inlet Weir Distance in 2.50 0.00
Recessed Inlet Box No No
Picket Fence Weir No Yes
Swept Back Weir No
Eff Weir Length % 100.00 74.90
Eff Weir Length in 97.48 97.48
Eff DC Btm Length % 100.00 100.00
Eff DC Btm Length in 81.29 131.48
Eff Flow Path Len % 100.00 100.00
Anti-Jump Baffle Req'd
NOTE: Values for DC areas, widths, and waste area for Pass B are 50% of the total for the associated center DC.
NOTE: The specified pass for an inlet weir is the pass upstream of the downcomer it seals.
---LOADING AND PHYSICAL PROPERTIES
---Service: Depentanizer
Pressure = 66.40 psia Temperature = 366.80 degF
Liquid Flow = 1807.43 US gpm Vapor Flow = 92.22 ft3/s
= 560.8 klb/h = 322.6 klb/h
Liquid T/D Fact = 50.00 % Vapor T/D Fact = 50.00 % Liquid Density = 217.200 lb/bbl Vapor Density = 0.972 lb/ft3 Liquid Viscosity = 0.176 cP Vapor Viscosity = 0.010 cP Surface Tension = 8.941 dyn/cm
Liquid Mol Wt = 125.903
Foaming Factor = 1.00 Fouling Factor = 1 (Clean) Defaults for this tower and section: foam fac = 1.00 foul fac = 1
---EXXONMOBIL USE ONLY Page No. 3
---TRAY HYDRAULICS
---PASS A PASS B
FLOW DIRECTION CNTR-SIDE SIDE-CNTR
PRIMARY PERFORMANCE INDICATORS
Overall Flood % 77.45 76.56
Ult Capacity Tray % 45.69 45.72
Jet Flood % 76.84 78.66
DC Flood % 77.38 66.35
Prob. of Non-Flood % 99.16 99.35
T/D to 20% Weep % 40.04 40.39
SECONDARY PERFORMANCE INDICATORS
Liquid Ent M-Method % 0.01 0.01
Liquid Ent E-Method % 0.09 0.14
DC Choke % 50.01 50.18
SQRT(DCBackup*Choke)% 58.75 49.39
Froth/Spray Trans % 52.48 54.96
Ult Capacity Univ % 38.52 38.52
Dry Tray Press Drop in 1.91 2.00
Tot Tray Press Drop psi 0.106 0.119
Velocity Under DC ft/s 1.43 1.04
Downcomer Seal in 6.27 0.39
Vapor Frac Under DC 0.04 0.04
Liquid Weep % 0.00 0.00
TURNDOWN PERFORMANCE INDICATORS
Downcomer Seal in 4.29 -0.11
Liquid Weep % 4.01 3.44
MISCELLANEOUS CALCULATIONS ---JET FLOOD PARAMETERS
Liquid Load US gpm/in 9.270 9.270
C-Fact Based on BA ft/s 0.231 0.242
DOWNCOMER BACK-UP (IN OF HOT LIQUID)
FRI Tray Cl Liq Ht in 2.83 3.34
Tot Tray Press Drop in 4.74 5.34
DC Fric Head Loss in 0.73 0.85
Head Loss Under DC in 1.49 0.66
Tray Inlet Head in 4.78 1.86
Hofhius Tray CLH in 1.86 1.91
Tray Froth Density 0.271 0.257
Tray Froth Height in 6.85 7.44
DC Clear Liq Height in 11.92 8.87
DC Froth Density 0.848 0.849
DC Froth Height in 14.06 10.45
DC Backup % 69.01 48.61
Note: Downcomer calculations use Hofhius clear liquid height equation.
Total tray pressure drop uses FRI clear liquid height equation.
Percent Thruput at
85% Overall Flood (Const L/V) 108.703 109.070
Liq Vel into DC Top ft/s 0.200 0.201
---EXXONMOBIL USE ONLY Page No. 5
---TRAY EFFICIENCY CALCULATIONS
---KEY COMPONENT NUMBER 1
DESIGN OVERALL EFFICIENCY: 87.97 % EFFICIENCY MODEL USED: M-Method
TYPICAL DESIGN OVERALL EFF: 75 % FOR: Depentanizer
PHYSICAL PROPERTIES USED ONLY IN EFFICIENCY CALCULATIONS Slope of Eqm Line = 0.700
Liquid Molecular Weight = 125.90 Vapor Molecular Weight = 123.28
Vapor Diffusivity = 0.8244E-05 ft2/s (Calculated) Liquid Diffusivity = 0.8627E-07 ft2/s (Calculated)
PASS A PASS B
FLOW DIRECTION CNTR-SIDE SIDE-CNTR
MASS TRANSFER PARAMETERS
Lambda 0.41 0.41
NOG 1.75 1.82
Liquid Phase Control
(from E-Method) % 14.07 13.52
Clear Liquid Height in 2.31 2.69
Froth Density 0.22 0.22
EFFICIENCY CALCULATIONS
Vert Mixing Pools, j 100.00 100.00
Point Efficiency % 82.31 83.54
Horiz Mixing Pools, N 7.88 5.77
Uncorrected Murphree
Tray Efficiency, EMV % 101.91 102.39
Lng FPL/Sml DBA Corr. 1.00 1.00
Weepage Correction 1.00 1.00
Entrainment Corr. 1.00 1.00
Corrected Tray Effic. % 101.90 102.37 Pass Averaged Tray Eff. % 102.14
Effective Tray Effic. % 102.14
Overall Lambda 0.41 Expected Overall Effic. % 103.50 Design Overall Effic. % 87.97
Note: 0.85 design safety factor included in Design Overall Efficiency
E-Method DESIGN OVERALL EFFICIENCY: 76.61 %
Note: The E-Method efficiency model is not suggested for this service.
---EXXONMOBIL USE ONLY Page No. 6
---COMPLETE LIST OF WARNING MESSAGES
---Please see your local Fractionation Specialist regarding assistance with your tray design and advice regarding warning messages and intrepreting the output. Technology ownership of this program is the Fractionation Technology Group in the CDFS section located at the Central Engineering Office in Fairfax, VA.
WARNING: PAGE 4 OF THE OUTPUT REPORT (HYDRAULICS RATIOS) ONLY RELEVANT FOR 3-PASS AND 4-PASS TRAYS. PAGE 4 NOT PRINTED FOR THIS CASE.
WARNING: OUTLET WEIR HEIGHT IS GREATER THAN 1/6 OF THE TRAY SPACING.
WARNING: ANTI-JUMP BAFFLES ARE REQUIRED ON CENTER DOWNCOMERS WHEN LIQUID LOAD > 4.20 US gpm/in of weir .
WARNING: USER SHOULD FINE-TUNE DESIGNS TO MINIMIZE THE NUMBER OF WARNING MESSAGES BY MAKING GEOMETRY ADJUSTMENTS IN RATING MODE FOR NON-RATING OPTIONS.
WARNING: PICKET FENCE WEIRS APPLIED DUE TO LOW LIQUID RATE, OR TO BALANCE THE PASSES OF 2-PASS OR 4-PASS TRAYS. CONTACT A FRACTIONATION SPECIALIST BEFORE APPLICATION OF PICKET FENCE WEIRS.
The design algorithm has determined that a 11.0 ft diameter, 2-pass tray with 18 inch tray spacing is the best design for the stripping section. This determination is based on a pseudo-cost objective function in the EMoTIP design algorithm. However, the balance between diameter, number of passes, and tray spacing cannot be accurately captured in a simplified cost objective function. Many factors, such as design of other sections of the tower, maximum total tower height, area available for the tower footprint, and maintenance considerations can play a very important role in determining the optimum tower design.
For this reason, a design debug (*.DBG) file is created when a design case is run, that allows the user to compare alternate designs. The present design case gives the following design comparison table at the end of the debug file:
Optimum Diameters, ft
Num Pass 4 2 1
Tray Spacing, in 36.0 10.0 9.5 10.0 Tray Spacing, in 33.0 10.0 9.5 10.0 Tray Spacing, in 30.0 10.0 9.5 10.5 Tray Spacing, in 27.0 10.0 10.0 10.5 Tray Spacing, in 24.0 10.0 10.0 11.0 Tray Spacing, in 21.0 10.5 10.5 13.5 Tray Spacing, in 18.0 10.5 11.0 *****
For new designs, 18 inch tray spacing may be insufficient to allow for easy maintenance. If the designer wanted to increase tray spacing from 18 inches, the table above shows the minimum tower diameter needed to meet all of the design limits as a function of tray spacing. Perhaps a 10.0 ft diameter, 24-inch tray spacing tower is the best design for this application. Or, the designer could decide to use 1-pass trays, in which case an 11.0 ft diameter, 24 inch tray spacing may be the best choice.
Another option available to the designer is the ability to specify design limits. Default limits are built into the program, and are recommended for typical designs for which no additional information is known.
Geometry design limits may be set if certain geometric constraints are known (for example, if the tower cannot be greater than 12 ft in diameter).
Performance design limits may also be set by the user, if the user wishes to make the tower more or less conservative than the default. This option is recommended for advanced users only.
A design case with the default design limits can take several minutes to run, because the algorithm searches the entire design space for the best design (the sample depentanizer design looked at over 2.6 million tray geometries). Limiting the design window can save time by reducing the number of tray geometries considered in the design algorithm.
2) Assuming that the 2-pass, 11.0 ft diameter, 18 inch tray spacing design is chosen, the design efficiency for the stripping section design is 87.97% as given on page 5 of the output report. (9 theoretical stages) / (0.8797) = 10.2 actual trays. This is rounded up to 11 actual trays for the depentanizer stripping section.
3) For design of the depentanizer rectifying section, a similar process is performed as above. However, it must be determined whether a swedged tower is appropriate. This is not a large tower, so economics would likely justify keeping the same diameter for the stripping and rectifying section.
4) As mentioned in step 1, after designing each section, the least loaded tray must be rated at turndown conditions to ensure that the tray will not exhibit excessive weeping, downcomer unseal, or very low dry tray pressure drop at turndown rates.
5) For comparison, the design results of sieve tray program 1133 in Pegasys 5.2 (the older tower internals program) are displayed below for the stripping section.
WARNINGS AND ERROR MESSAGES
---HOLE DIAMETER NOT SPECIFIED, DEFAULT TO 0.5 IN (12.7 MM) DESIGN LIQUID RATE DENSITY ASSUMED TO BE #/FT3
***** CAUTION ***** DESIGN CASES UNDER THE FOLLOWING CONDITIONS MAY NOT BE OPTIMUM
AN ANTI-JUMP BAFFLE MUST BE PROVIDED ON THE INBOARD DOWNCOMER IF THE LIQUID RATE EXCEEDS 4.2 GPM/IN (10 DM3/S/METER) OF DIAMETER/PASS
SIEVE TRAY DESIGN PROGRAM NUMBER 1133 VERS. 7.5
TOWER DIAMETER FT 11.00
TRAY SPACING INCHES 18.000
NO. OF LIQUID PASSES 2.
HOLE AREA PER TRAY FT2 4.8325
FRACTIONAL WEEPAGE (MAX= 0.20) 0.032
DC FILLING, % (MAX= 50.0) 50./ 47.
* JET FLOOD, % (MAX= 90.0) 85./ 76.
ULTIMATE CAPACITY, % (MAX= 90.0) 44./ 46.
SPRAY TRANSITION, % (MAX=100.0) 49./ 53.
ENTRAINMENT, % (MAX= 20.0) 0./ 0.
++ DC INLET VEL FT/SEC (MAX= 0.516) 0.307 / 0.502 DC OUTLET VEL FT/SEC (MAX= 0.600) 0.307 / 0.502 DC INLET CHOKING (MAX= 1.0) 0.287 / 0.765 DOWNCOMER SEAL INCHES (MIN=-0.25) 0.310 / 0.572
FINAL TRAY DESIGN
---TOWER DIAMETER FT 11.00
TRAY SPACING INCHES 18.00
NO. OF LIQUID PASSES 2.00
HOLE SIZE INCHES 0.500
HOLE AREA PER TRAY FT2 4.8325
NO. OF HOLES 3544.
TRAY DECK THICKNESS INCHES 0.074
OUTBOARD INBOARD
DC INLET RISE INCHES 16.000 8.750
DC INLET AREA FT2 6.555 8.015
CHORD LGTH AT TOP OF DC INCHES 86.171 131.710
DC OUTLET RISE INCHES 16.000 8.750
DC OUTLET AREA FT2 6.555 8.015
CHORD LGTH AT BTM OF DC INCHES 86.171 131.710
DC CLEARANCE INCHES 1.500 1.500
RECESSED BOX NO NO
SHAPED DC LIP YES YES
DOWNCOMER TYPE CHORDL
OUTLET WEIR HEIGHT INCHES 1.500 1.500
INLET WEIR HEIGHT INCHES 0.000 0.000
CROSS SECTIONAL AREA FT2 95.033
FREE AREA FT2 87.018 81.923
WASTE AREA FT2 0.000 0.000
BUBBLE AREA FT2 73.908 73.908
HOLE/BUBBLE AREA PCT 6.5 6.5
BUBBLE/CROSS SECT AREA PCT 77.8 77.8
FLOW PATH LENGTH FT 3.802 3.802
VAPOR - LIQUID RATES AND PROPERTIES AT CONDITIONS
---KILOLBS/HR OF VAPOR (DESIGN/MIN) 322.580/ 161.290 LB/FT3 OF VAPOR AT COND (DES/MIN) 0.9716/ 0.9716
VAPOR VISCOSITY AT COND CP 0.0096
FT3/SEC OF VAPOR AT COND 92.2247
VAPOR LOAD AT COND FT3/SEC 14.8028
TRAY LIQUID TEMPERATURE DEGF 366.8000
OPERATING PRESSURE PSIA 66.4000
KILOLBS/HR OF LIQUID (DESIGN/MIN) 560.8200 / 280.4100 LB/FT3 OF LIQUID AT COND (DES/MIN) 38.6850 / 38.6850 LIQUID RATE (DESIGN/MIN) US GAL/MIN 1807.3064 / 903.6532 SURFACE TENSION AT COND DYNES/CM 8.941
LIQUID VISCOSITY AT COND CP 0.176
SYSTEM TYPE NON-FOAMING HYDROCARBON
$ DOWNCOMER FILLING CALCULATIONS (INCHES ARE OF LIQUID AT CONDITIONS)
---OUTBOARD/INBOARD DRY TRAY PRESSURE DROP (HED) INCHES 3.30/ 3.30 CLEAR LIQUID HEIGHT (HC) INCHES 1.88/ 1.66 ##
TOTAL TRAY PRESSURE DROP (HT) INCHES 5.18/ 4.96 TOTAL TRAY PRESSURE DROP (HT) PSI 0.12/ 0.11
INLET HEAD (HI) INCHES 1.66/ 1.88
DC HEAD LOSS (HUD) INCHES 0.98/ 0.42
DC FILLING(DENSITY CORR) (HD) INCHES 8.98/ 8.40 DC FILLING, % (50.00 MAXIMUM) 49.90/ 46.68
DOWNCOMER VELOCITY CALCULATIONS
---++ INLET VELOCITY, FT/SEC (0.516 MAXIMUM) 0.307/ 0.502 OUTLET VELOCITY, FT/SEC (0.600 MAXIMUM) 0.307/ 0.502 DC INLET CHOKING (1.00 MAXIMUM) 0.287/ 0.765
TRAY CAPACITY CALCULATIONS
---* JET FLOOD, % ( 90.0 MAXIMUM) 85./ 76.
ULTIMATE CAPACITY, % ( 90.0 MAXIMUM) 44./ 46.
SPRAY TRANSITION, % (100./100. MAX) 49./ 53.
ENTRAINMENT, % ( 20.0 MAXIMUM) 0./ 0.
TRAY FLEXIBILITY CALCULATIONS (AT MINIMUM RATES)
---FRACTIONAL WEEPAGE ( 0.20 MAXIMUM) 0.032
$ DOWNCOMER SEAL, INCHES (-0.25 MINIMUM) 0.310/ 0.572
MISCELLANEOUS CALCULATIONS
---DESIGN LIQUID RATE (L) GPM/INCH OF WEIR/PASS 10.487/ 6.861 VAPOR LOAD/FREE AREA FT/SEC 0.170/ 0.181 JET FLOOD (VL/AF) ALLOW FT/SEC 0.199/ 0.238 SURFACE TENSION - VISCOSITY PARAMETER 0.933
MAXIMUM RECYCLED VAPOR, % 0./ 0.
TRAY FROTH DENSITY (OUT/INBOARD) 0.252/ 0.243 (FRACT FROTH VOL OCCUPIED BY LIQ)
EST. LIQUID HOLDUP (DECK+DC), FT3 21.415/ 15.855 EST. DOWNCOMER LIQ. HOLDUP, FT3 9.813/ 5.613
* DENOTES INPUTTED HARDWARE INFORMATION
++ LOW/MODERATE PRESSURE CORRELATION USED FOR MAX INLET VELOCITY
$ WEEP CORRECTED CLEAR LIQUID HEIGHT NOT USED IN CALCS
## INBOARD PASS CHORD LENGTH USED FOR INBOARD PASS CLEAR LIQUID HEIGHT CALC AS OF NOV. 1998
SIEVE TRAY EFFICIENCY CALCULATIONS - OUTBOARD PASS VERS. 7.5
COPY NUMBER 1
ALL CALCULATIONS ON THIS PAGE ARE MADE AT DESIGN RATES AND INCLUDE THE EFFECTS OF WEEPING EXCEPT WHERE OTHERWISE NOTED EQUILIBRIUM PARAMETERS
---COMPONENT EQUILIBRIUM LAMBDA SLOPE
KEY COMP NO.1 0.700 0.411
MASS TRANSFER PARAMETERS
---VAPOR MASS TRANSFER COEFFICIENT, KG CM/SEC 0.887 LIQUID MASS TRANSFER COEFFICIENT, KL CM/SEC 0.096
NG 1.763
NL 4.368
COMPONENT NOG PERCENT LIQUID PHASE CONTROL
KEY COMP NO.1 1.512 14.2
PHYSICAL PROPERTIES AND LOADINGS
---LIQUID RATE LB-MOLES/HR 4454.382
LIQUID MOLECULAR WEIGHT LB/MOLE 125.903
LIQUID MOLECULAR DIFF (FRI) CM2/SEC 0.745E-04
VAPOR RATE LB-MOLES/HR 2616.581
VAPOR MOLECULAR WEIGHT LB/MOLE 123.283
RESIDENCE TIME CALCULATIONS
---FRACTION WEEPING 0.002
CLEAR LIQUID HEIGHT INCHES 1.883
FROTH DENSITY 0.252
LIQUID RESIDENCE TIME SECONDS 2.880
VAPOR RESIDENCE TIME SECONDS 0.499
MISCELLANEOUS CALCULATIONS
---EFFECTIVE FLOW PATH LENGTH FT 3.802
NUMBER OF MIXING POOLS 32.180
INTERFACIAL AREA CM2/CM3 3.982
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* DESIGN NO WEEP MIN. RATE *
* POINT TRAY OVERALL OVERALL OVERALL *
* EFFIC. EFFIC. EFFIC. EFFIC. EFFIC. *
* *
* KEY COMP NO.1 70.2 80.4 72.1 72.1 68.9 *
* *
* * * * * ALL EFFICIENCIES DEBITTED 10% ON POINT EFFICIENCY * * * * * *
Normal Program Completion