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Table I: Array lor Combining Outage Probabilities Two Groups of Unitai Three 15 Mw and Three 83 Mw, Outage >03 Proba- Proba- Proba- Proba- Unto Mw bilily Unto Mw bUUy Unto Mw fcility Unto Afio fciitfy Table II: Summarr Probability ol Outages Ter Groups of Three 1 S-Mw and Three 22-Mw Uoits . .i--i-- - . ,, .Three 15-Mw Unit Group Separately......... 0 0 . 912673 1 15 .084681 2 30 .002619 3 4S .000027 --------- -Three 22-Mw Unit Group Separately ' -- 0 0 .912673 1 22 .084681 2 44 . 002619 3 * 66 .000027 0 0 .832972 1 22 .077286 2 44 .002390 3 66 .000025 l 15 .077286 2 37 .007171 3 59 .000222 4 81 .000002 2 so .002390 3 52 .000222 4 74 .000007 5 96 -- "3 45 .000025 4 67 .000002 5 .89 6 111 or point of connection of ties? By ating one dsy every seven peak seasons. This a common steam header for a group of compared with an expectancy of one units? By interconnecting two systems? day every peak season for the entire By changes in load factor? system. Fig. 9 shows variation of reserve Fig. 12 shows the combined effects of needs with the number of units of equal system growth and change in daily capacity end for three values of load* maximum load ratio on the load-loss loss probabilities, and an outage rate probability of the Consolidated Edison of q 0.01 Fig. 10 shows the effect system. The objective reserve for 1938 of interconnecting two stations with a tie. Curve a gives the variation of load* lots probability of each station with the was 320 mw, corresponding to a loadloaa probability of 0.0Q042 of 7d% daily maximum load ratio. For 1968, tie capacity, assuming local reserve at ' with an expected doily maximum load each station remaining constant at 80% ratio of 80% the reserve would be 455 of maximum load. Curve 6, on the mw for the same level of reliability os other hand, gives the local reserve in 1938. Should the load ratio remain needed to maintain load-loss probabil at 70% the reserve would drop to 430 ity at 0.00946. Both curves were figured mw. These figures represent contin for a load-duration curve shape oi gency reserve requirements opart from Fig. 6. considerations of overhauling. Fig. 11 shows the effect of variation in daily peak-load variations for a group of 60 units equivalent to those of the Consolidated Edison system in 1938. At that time the dally maximum load ratio = (sum of daily peak loads) 100/(Annual peak load X 365) was of the order of 70%. For an objective re serve of, say, 320 mw the load-lots probability was 0.00042. The curve ebows, for instance, that with the same reserve if the ratio had been 80% the load-loss probability would rise to Overhauling Units. The time avail able for periodic maintenance over hauls of generating units is determined by the amount of capacity represented by the difference in annuel and daily peak loads, os shown by Fig. 7. 'Strictly speaking, the contingency reserve should be calculated, excluding the units on overhaul. For all practical purposes, considering the nature of the problem, it may usually be assumed that the contingency reserves remain constant through the year. 0.000658, an increase of about 60%. Kegordlcss of the method used in For constant conditions of load fac calculating the reserve of a system, the tor, outage rate and unit aize, the de gree of service reliability should be lower with Increasing number oi units. final value must be checked to insure that it Is sufficiently large to allow per forming all extended maintenance A given outage in excess of reserve be work. Take into consideration the over comes lees severe as the system grows.' haul cycle and duration of both turbineConsistent with this criterion, the ob generators and boilers. jective reserves of (be major stations In 1936 the valley In the curve of of the Consolidated Edison system as of doily maximum loads. Fig. 7, provided 1938 were set to correspond to on ex ample room for performing mainte pected outage in excess of reserve of nance work. In addition, os a result of Mw 0 15 22 30 37' 44 45 62 59 66 67 74 81 89 111 Probability qf Outage; Exadly equaling indicated Exceeding indicated Mw Mw .832972 .077286 .077286 .002390 .007171 .167028 .089742 .0L24S6 .010066 .002895 .002390 .000025 .000222 .000222 .000025 .000504 .000480 .000258 .000036 .000012 .000002 .000007 .000002 -- -- .000009 .000002 -- --- -- the depression, the actual reserve of the system exceeded the calculated ; value. During the war, and under the , impact of the emergency, reserve stand- ards were lowered and our actual re- . serves fell short of the value considered f adequate. We are trying hard now to / reestablish those standards, but be- . cause of tho current shortage of mate- .< rials and consequent delayed deliveries, we do not expect to do so for some time. Valley in (be curve, Fig. 7, Is gradually filling up so It Is now anticipated that j the long-range contingency reserve ' must be increased to fulfill overhauling J requirements. Conelusion. Though the method ot \ probability has grown into a powerful tool for aolring reserve problems, the applications msdo up to the present J time have not exhausted its potsibiU- lies. The method coo7be further 1-^, proved by using indexes of service^* reliability based on load-duration curves V to includo the effect of load variation. J This purpose can be accomplished with little extra work by using straight-lino >j curves and, usually, by replacing step j outage-probability curves with approxi mate continuous ones, as in Fig. 2. The very nature of the problem entirely-j justifies these approximations without | affecting the engineering occuracy of the results. 104 (426) POWER July 1741 Consider These Factors in Specifying Mechanical-Drive Steam Turbines Second article of a ierie, by F S Kohl, turbine divUion, Gen eral Electric Co, review, condition, that should be studied when considering installing mechanical-drive steam turbines Psorss ArruCATlON ot mechanicaldrive turbines requires evaluation of various turbine characteristics and the conditions under which they operate. These factors ore (1) ratings (2) speeds (3) steam conditions (4) efficiency (5) governing (6) special features and (?) accessories. The principles stated here provide a good background of in formation for proper turbine drives. Sines standard designs offer great advantages to the user In initial cost, shipment, stocking of spare parts, etc, they should be applied whenever pos sible. Where standard designs are not adequate, however, modified standard units or special designs covering a great variety ol requirements ore available. Rotlagi ond Speed*. Range of horse power and speed ratings arc almost limitless. Capacities range Iron less than 5 to more than 25,000 hp and speeds from less than 1000 to over 12,000 rpm. Speed-reduction gears ex tend the lower limit to below 200 rpm, while speed-increasing gears raise the upper limit to exceed 40,000 rpm. AU ratings arc not normally avail able at all speeds as applications da not call for all possible combinations. For exsmplc, It would be very unlikely to find s piece of driven equipment wed even 10,000 bp operating at speeds ever 4000 rpm. Flexibility of turblpe sod gear design, however, can meet any application requirement. The great majority of applications need less than 800 bp with speeds be- w 4500 rpm. Most of these can be t with standard toilt* and almost "off* e-shelf' design. Semi-standard da8 *n* uaiog standardized components caver the bulk of the remainder. loom Conditions. Mecbanlcal-drive urbines are built for a wide range of steam conditions. Tnl'*"1 preanre, leu than SO to na 2000 p,|g. Title I: Golds to Mott Likely Turbine For Glveo Application Coodltlonx Rtrite tea Cm* SOO bp tOO-SOO Ibe 300-1000 bp tooe-ecoo be On MOO he COO DESIGNATION Ai &i<i ebpe. wl> Nwol <i Mdlbugt, lew WUdmcv B. SbwW-M***, hr** wbttl* Di KWtbUt*. wufWw Wlkimcv Ei MrfUdtp*, Mfh tSlduKT U*S Fact*4 Cw4iilt| Cm) Stttu, Kelt Uw NtMtrioilti Cpct il hum Kfeb Uw Low Mtdhti KOfb Lew Mute HWt 9A a * C0 0c ED A e B B C Owl A A A A A B tw Mtdl-- HJib CC Dc E0 cB o cC U> MiAb Klfb Cc DD Ee c8 06 4C Lev Mtdhw hfljb DD 6E 1E 0c 8D 11 Owtw elate oowdy fftrt bight, effldicy. Atv 1 rtaarfbp untwi njlun--tbet $0% blgb-bwi totdtd *lMlv Ml food habit* opnctOon, Initial quality or temperature, 3% moisture to 1050 F. Backpressure, 1 In. Hg to over 400 pslg. Standard designs ora suitable for initial conditions up to 600 pslg, 750 F, and with backpressures up to 75 prig. Scm(standard designs range up to initial conditions of about 2200 pslg, 950 F and backpressures up to 250 psigEfflcieecy. One of the most important points considered will be efficiency de sired. For given horsepower, speed end steam conditions, any one of several designs' can be used. From * an efficiency standpoint, the decision on design should be based upon a cost study. But'experience proridu some approximate rales of riinmh that can be used In the majority of coses. Table I summarizes the most usual turbine selection after cost studies for given operating conditions. Another- consideration In efficiency Is the question of geared vs direet-eoonacted drive. Speed of the driven equipment is tbc most Important factor in this selection, although rating and team conditions have considerable effect. Turbines ere-inherently highspeed machines, low .turbine speed sacrifices economy. If the driven equipment op* 'OWE* Jhr 1444 (427) 164