Document O1yNjp7KKdB4rL6JQxeqG9kYX

--J ORIGINAL PLANT LAYOUT feotured two 5000*kw outomoile-extroetlon rmritiq. i bp turbines which took 400-psig 700-F tteom and supplied 20*pslg itcom to mQ aaioMIZID PLANT LAYOUT USES new 7500-kw bockpres* matte-extractIon turbine*. Preferred operation posses all steom ^turbine old units as combined mixed-pressure ond outo- Ihrou0h new unit first, old unit's low-pressure end os needed TRIP-THROTTLE VALVE, obovo. reptecei the oll-tripped nonreturn vclve, shown be* low, in the turbine-oxtroetion lino to ollow the units to oporoie on mixed pressure 120,000 lb per hr continuously el 400 psig end 700 F, burning pulverised coal. The obvious addition teemed to be a new 7500-lew condensing automatic*** traction turbine alongside the existing unite. But a study of steam ahd elec* trical loada showed that while the mill runs the steam demand ia never leas then 100,000 lb per hr. This suggested a backpressure turbine. Further study showed that a heat balance could not be achieved at all limes without gener* ating some condensing kw. Since the condensing power is olways small, con* sideration was given to rebuilding the existing turbines lo mixed-pressure op* eration. This could be done by simply replacing the positive-closing nonreturn valve with a trip throttle valve in the extraction line of each unit, sketch above. This allows either admission or extraction of low-pressure steam as de mands require, top of next page. Over the years, steam pressure In the driers had been raised from 20 to 40 psig; it may be relied lo 50 psig later. The left-hand performance curve, at bottom of p 73, shows maximum How to exhaust of 40,000 lb per hr with an extraction pressure of 20 psig <35 psia). Flow through a turbine section varies directly with the absolute steam pressure ahead of that section. Hence, with 40 psig (55 pais) at the extrac tion opening, (low through the turbine exhaust section would be 63,000 lb per hr. Adapting Condensers. Each condenser has 6000 sq ft of surface and handles 40,000 lb per hr of steam. With increased steam flow through turbine exhaust, condenser capacity had to bo increased. An idea to replace the exist ing units with new, larger ones was discarded in favor of the following scheme. It was found that by raising circulating-pump speed from 690 to 870 rptn enough water could be furnished to each condenser to hoodie 60,000 lb per hr of steam. With 80-F cooling water an absolute pressure of 2 in. Hg could bo maintained. This requires a SO-hp motor to replaco the old 30-hp one. The higher cooling-water velocity not only increases heat transfer but helps keep condenser tubes free of silt. Spaed Governing. To benefit most from the high efficiency of the 7500-kw back-pressure turbine, as much steam os possible should be passed through it. To do this, frequency will be held by the new unit. The old 5000-kw turbine, running parallel with the 7500-kw unit, will hold the mill steam pressure (40 psig). Speed governor of the 500Wtw unit will be set to keep its high-pres sure valves closed at frequencies above 59 cycles. Thus, these valves will ordi narily be closed. First high-pressure valve to open end the grid-extraction valve will he drilled to pass a steam leakage of 5000 lb p*L hr, to ventilate or cool the turbine. Tho^ backpressure turbine will furnish iteta j 4* governor would give about 6% JLi change lor this load range. Therefore, . this turbine Is being to the mill up to either Its capacity ; the demand. If steam demand Is greater' equipped with a hydraulic speed corffOor to keep speed changes within than will pass through this turbine t; sitMestory limits (*0.1% for gradual the prevailing kw load, the reducing!, valve will furnish the balance end floi^Vcondensing turbine will merely load or flow change). Pressure governor on the 5000-kw nised-pressure turbine must govern for passing 5000 lb per hr dear through tf| far ranges far beyond its original cap the condenser. This cooling steam : abilities. This control (1928 design) la not quite carry the no-load losses my ; betag replaced by a modern design that there is no danger of runaway. f ' will bold pressure within *54 psi when If, as will normally prevoll, the k'J * Sow changes are gradual. With these load exceeds the power generated bf i ' alterations it Is expected that both mill steam, the control on the gridj ^ speed and pressure governing will be valve will open It to allow enough stein t uecessfuL to post through the backpressure toM l Ptrismeace Comporfson. Haring de bine into the mixed-pressure turbine ts [ ( termined that the turbines and conden the condenser to generate the needed j. * sers could be adapted easily, a study kw. This arrangement will produce toy*1 * was undertaken of the economics of the kw demand from 0 to 12,300 kw aadji ' alternate schemes. The turbine mnnu- any mill demand from 0 to 210,000 ttj ; fseturer furnished data on (1) the old per hr within the limits of the right- f , turbines after rebuilding (2) a new hand chart, bottom of page 73. \ Speed governor on the backpreoun} ; turbine must govern over a kw ranges!. 1 0 to 12,300 kw. This Is 12300/7500, of / | 164% of Its rating. An ordinary j ' 7 7500-kw backpressure turbine (3) a new 7500-kw condensing automatic-ex traction turbine. Combined perform ance of a new 7500-kw backpressure turbine, operating in conjunction with one rebuilt 5000-kw condensing mixedpressure turbine, was plotted in solid lines as shown in lower right-hand graph. Performance of a 7500-kw con densing extraction turbine, operating alone under the same conditions, is shown by the triangular points; 5000kw unit performance is shown by the circular points. At all flows to throttle above 94,000 lb per hr the combination is best If flows less than this prevailed, the kw load would be low enough so one 5000kw turbine would be run alone anyway. At 5000-kw 60,000-lb-per-hr extrac tion, performances of all these arrange ments are identical. At high flows the combination arrangement shows as much as 20,000 lb per hr less flow for a given kw and steam demand. This U because the efficiency of a straight non* condensing turbine is so ranch greater than the head end of on extraction turbine. Fuel saving with this operation has been estimated at 335,000 per year. Cost comparison of a 7500-kw con densing extraction turbine and the plan chosen Is shown in the table at the bot* tom of p 72. The backpressure turbine is less expensive; also, no new conden ser and piping are needed. A new con denser would have meant installing a new 200-ft 24-in. water line from the eanel. The investment saving as listed Is only partial. This figure of 158,000 does not take into account the lower foun* dation cost for a noncondenslng tur bine, lower-cost piping layout and lower turbine-installation cost In addition to these odvantages there is sufficient ca` parity to supply a new large paper ma chine when conditions warrant its in stallation. n <741 POWER February >**9