Document NeRRN9doj3q6E8B0yK9Jzk7qw

TECHNICAL BRIEFS Latest engineering developments for busy power men 9 Digests lor jo 0Ir PUMPS FUELS & FIRING STATION DESIGN ELECTRICITY c55 I 50 t 49 8 40 ?39 J 30 25 o 80 2.5 Ze 10 o9 50 0 400 600 1200 1600 2000 P.sdwottr flow-gpm 2400 ALLOWABLE RATE o( decay starts a. tero at no Hew, maximizes a. approximately 700 ppm par pump and (oils bock to toro at 11SO ppm Transient Conditions in Boiler Feed Pumps Attract Greater Attention Centrifugal Bailor Food Pumps Under Transient Operating Conditions, By Igor J Karauik, Worthington Pump Corp, G H Boaworth, Bechtel Corp, and IP D Elaion, Pacific Caj and Electric Corp. The study of transient conditions in a leant power plant and of their effect on the centrifugal boiler feed pumps as sumes the characteristics of a moppingup operation. For years we have been developing power plants to operate at ever increasing steam pressures and temperatures to improve overall thermal efficiencies. It follows, therefore, that the problems introduced by transient conditions have grown in complexity and severity with time. Yet it has been only recently that much concern--or even interest---has been given to these problems. The authors' present paper supple ments the theories presented earlier with experimental data, develops a simpli fied method of evaluating the relative safety of e given Installation and en larges the scope of the field covered by including an anaylsis oi problems which may arise in closed feedwater eyeles. In addition, problems akin to those pre sented by transient conditions and aris ing from tho use of common spare pumps in adjoining units--in either open or closed cycles--are included in the scope of this poper. These problems for open cycles sub divide into two groups, namely, prob lems arising from an Interruption or reduction in turbino load and problems caused by sudden increases in this load. Here is how the authors approach theso problems. The effect of a sudden drop in elec trical load is for the turbine governor to reduce the inlet steam flow so as to maintain the proper relation to the gen erated power. The consequence of this reduction is a proportionate pressure reduction at all of the successive stages of the turbino, including tha bleed stage supplying steam to the direct contact heater. It is possible during this period that the NPSH (net positive suction head) available for the boiler feed pump may be reduced seriously because of the changing relationship between the de caying beater pressure, above, aod the feedwater temperature at the suction end of the pump. The magnitude and duration of this reduction will ' vary according to the load drop that is ex perienced and will depend on a number of factors peculiar to each Installation The allowable rate of decay starts at wto at no (low (because the residence time is infinite), rises to a marimam somewhere in the vicinity of ?00 per pump for the example eese the au thors pick and (alls back to xero at 1250 gpm, where the available NPSH exactly balances the required NPSH We must remember, however, that open otion of boiler feed pumpa at abnormal )y low flows is prevented by automatic controls and that 1250 gpm per pump is greatly In excess' of the maximnm feedwater flow demand for the unit the authors have under consideration. Thus, If we assume that the minimum permlsBible flow without recirculation U 125 gpm, the allowable pressure decay rata at low flowa will be 10.5 feet per min ute, or opproximalety 4.4 pal per min ute. It should he considered that under low load operation a single pump may he running. Under these conditions, tbs residence time for any flow is effectively doubled and therefore the allowable rate of pressure decay shown oa graph is approximately halved. Since such a situation may Increase the danger of operation, it follows that, for a rigorous analysis of an installation where more than one pump ii installed (or normal operation, the allowable rate of pres sure decay must be determined for sin gle pump operation as wed as for all pumps on the line. The assumptions concerning tha ae- tion of tho controls involved are subject to wide interpretation. For instance, the action of tho feedwater regulator makes It difficult to predict how soao after the load drop the feedwater Bo* to the boiler will become stabilised with relation to the turbine throttle fla*. While the majority of large steam power plant installations are provided with three-element feedwater control, there are variations as to tho manner la which those controls are adjusted. In general, the feedwater input is regulated to par* allel closely steam output, hut tha con trol is sometimes adjusted to vary tho boiler water level with the load as a precaution against the effects of "swell and "shrinkage." Thus, while the water flow will go down with steam flo*rate of drop may not be quits as fait as that of the steam flow. In additiontho transient condition problems '* just as likely to oecur in Installed0* (Continued on page 156) 134 READER SERVICE SECTION K>*tt Reduce your fuel costs and build better furnace linings with JM-3000 INSULATING FIRE BRICK {RETSTHBONLY insulating firebrick bit withstandsa full 3000F. It's highly fficiem both ns an exposed refractory Loing or as back-up insulation. 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