Document a4peE9Q1QqLX88B2mYQ3Qk38R

Here are the requirement* for succeuful condensatepump controlby hotwell water level. Alto given are the way pumps perform when their ,, discharge is regulated by this moans, as well as the ad vantages and disadvantages of control by submergence i Ii I By PIO NARDONt Methonleal Fngioeer fotter Wheeler Corp. CONDENSATE-PUMP INSTALLATION. Two 2-*topo centrifugol motor-driven rah densote pumps ore connected to the hotwell below the surface condenser serving o lorge steam turbine in o modem public-utility centro|.power generating pJgni 1 CHARACTERISTIC CURVES. Hood-capacity, system-heod ond broke-horsepower curves lor o contrllugol condensate ' < pump. When pump dliehcrpc Is decreased, its heod Increases Suction Submergence Controls Cohdbksate pumps remove liquid from vessels under vacuum, ouch os steam-turbine condenser hotwell*. Cen trifugal pumps ere generally used on these applications. Because of large lood changes on the turbine, discharge rate of these pumps must be adjusted to maintain hotwell liquid level within sofa operating limits. This can be done automatically by float or submergence control. The latter method ii discussed hi this article. Whot It Submergence? Submergence la generally considered as being the dif ference in elevation between the hot* weil level end the pump's center line (horizontal unit). This head, however, represents only approximately the en ergy available to deliver liquid to the Impeller eye, when the pump h near the hotwell and connected directly by oversize suction pipe. The true net posi tive suction bead (npih) at the pump's auction flange is the total available head (static plus velocity head) corrected to tho impeller's center line above the va por pleasure at this point. Even though the term submergence, because of its popularity, is used hereafter, it should be borne in mind that npth is the criterion for auccessful pump opera tion. Centrifugal pumps, as generally built, have a rising head-capacity charoeteriitie like Fig. 1- At constant speed, as the discharge decreases the total dynamic head developed by the pump increases. For example, in Fig. 1 at 37$ gpm the pump devdopa 275-ft head, point B, where at zero discharge the heed is 350 ft, point A. On the other hand, the total bead required to cause the liquid to flow through the system decreases as the volome decreases. Again, in Fig. 1 at point C, the system head at 400 gpm equals 2S8 ft. At 300 gpm, system head drops to 163 (t, point D. If flew from a centrifugal pump is not regulated there is only one condi tion where it will operate. This U the point where the syslem-head curve in tersects tho pump's head-copaeity curve, point C, Fig. 1. At this point, heod developed by the pump just over comes the system resistance to tho liquid delivered. At all lower capaci ties, head developed is greater than the system requires. With many installations, it is neeas sary at times to change the rite st' which liquid flow# through the system, which can be done by throttling the pomp's discharge. This artificially i>, creases the 'system held so (hst It' crosses the pump's head-capacity curve at a lower capacity, as is doae with system-bead curve 2, which crosses th* head-capacity curve at E. Capacity of the pump can alto bore duced by bypassing some of the liquid discharged by it back to the tueliasFor condensate lervice we may uis so other method, known as submergence control--that is, control by ft liquid level in tho holwelL Submergence Control Operotton. For successful operation of this nlr0* with a centrifugal pump a gW* mergence is required to cause a girt* quantity of liquid to enter the Impeller- At this submergence the pontP 6,0 liver no more than this quantity- At o given submergence and caps*7 the head developed by the puP . creases shorply because of partial oration of the liquid at the ilBP^ . eye. Point ot which this drop t* occuri is known as tho break. ^ head developed by the pump deoeaaf* 74 (680) POWER Nowmbef [ Condensate-Pump Discharge I* until It equals the system bead at this . espsrity. Fig. 2 shows the character- iilio curves (or a pump with only a few break points indicated. There are, how, ever, an infinite number of these points st which the pump can operate. For a pump to operate properly with mbmergenee control, it must be se; fccted so the base bead-capacity curve ^ intersects the system-head curve at some capacity, point C, beyond the maximum flow desired, point B. If submergence st a desired maximum flow were suffi cient, total head developed by the pump would be greater than the system head. 7111 unit must be selected so maximum nbuurgence available is greater than lbs submergence for operation at repdttd capacity. Operation will be as follows: . Rump Operation. 1( & maximum flow si 375 gpm, point 0, comes into the hot- from the condenser end the tubeUeoce is 4J2 ft, the pomp will op* *** st about 400 gpm, point C. Thus, 1 P.--utarp r.evmuiwovtecsj mmoorree wwaatieerr firroomm tuho "rirtll than the eondenecr adds to ItrtstweU level U lowered until the' subMgcnte i equal to 3.6'ft- At this ; lQWrgence the pump becomes stabil ized ond operates at point D. This is the point where tho hend-capielty curve (in the break region) crosses the sys tem-head curve. The pump then delivers just as much water as is added to the hotwell and the level remains constant. If the steam condensed decreases to where 300 gpm flows into the hotwell the pump tempo rarily continues to remove from It more water than Is added until sub mergence decreases to 3.1 ft, when the two corves cross ot E. The pump then becomes Itebilited once more and the hotwell level remains constant ot 3.1-ft submergence. The reverse applies if the steam flow increases. Thus, stabilization takes place rapidly end for any steam flow to the condenser up to point C. At all times, therefore, the total bead devel oped by the pump is equal to the sys tem heod and the pump delivers es much water as is added to the hotwell. Note thot a pump operating with sub mergence control always operates in the break regions or off tbe head-ca pacity curve. This is evidently so be cause the pump operates bn the base curve only at point C, which is greater than the maximum flow, or if the sub mergence were artificially maintained,*greater than the amount required for any particular Row. For the latter, some type of float control would be required and throttle or bypass control would be necessary. Main disadvantage of submergence control is that some vaporization of the liquid alwaya accompanies operation in the break regions. Because of the rise in pressure these vapor pockets or bubbles collapse os they flair through the impeller. This action Is accom panied by noise and in time by pitting and corrosion of tho impeller passages. Undue bearing wear may also result from varying shock loads Imposed when cavitation occurs (ptunp operating in the break region). But power for operating in tbe break regions will always be less than that required along the base head-capacity curve ABC since the total dynamic head that the pump develops Is leas tbon indicated on this curve. This decrease in power and the elimination of external controls are the chief merits of sub mergence control November 1949 (681) 75