Document 3e661qg6DoNBdxJDbJ2d1Qv0a

crates at relatively low speeds, geared turbines should be considered, as they usually offer tremendous improvements in economy. Listed below are speeds generally considered as the lowest min* imora acceptable in average applica* tlons where efficiency is not highly evaluated: Approximate minimum "direcKonnectcd" Up rating speed, rpm 50 1000 400 1500 1000 2000 2000 2500 Table II: Special Features Modify Basic Standard Design* J fMlVN PrtMort wtobtor Tempaatm conhoOn To Dump prnr* camteifl, at bold comm *etn pt*MM bv rvfoltllitf twtolM *o*d ft*rab Urad let noct pan* cl!<*lio** ewfloiUriv boil*, <td porno*. O* b* **d to k**o b&m on *(* mite flwdbv wwteo j la hold const**! tmeentura bv ttfuUv Died for rdrlyvitar convenor Abe cor*o< 1st boblna tSMtd L 1 To hold oncon* le eihewsl (to* toad**! C*n be eied on dorian mflcadoM ahere tpetd by etnnabf ste**i flew b held bv odo V To provide rtecntMlbiy on Mneendmw loo enlb when opentln* candmiles Sb*t*-*toyi eete ra boldly owncond--lr>sr In condemhit openllo* Ihb fetan nd bo added. Cormooly ewd oo mNi for Standby nnlee eb *onm*rtlM eondtmbe operelloo - > f i Te provide ofl end** qmmow lei beerWf* fnowdlr wvee eddt*S *1 ** I* Atom* ib> { Naturally, these limits will be higher whenever economy la more important. Partial*Load Economy. Efficiency at part load may be of considerable im* portance. This item should be studied efter filing the desired level of effi* ciency. On most small turbines a single valve controls the steam flow to the total area of first-stage nozzles. At light loads, the futl throttling causes considerable energy loss. By sectionallzlng the first* stags nozzles and controlling each tec* tion with a separate valve, either by hand or automatically, the throttling fltnote ipeed emVol P*it p**d cento! Trlppbif devfca* Solenoid told vcfve Anakawar vtlva To entd ewter-Atvn peed dmte Cww**U*L Uawnm To provld* ewnntMv cenUa pedibel* Und lor wntilni tpeed end hall rp*d an o ton central lor evtemetic w**d tattlaf necWne Atvas to *a*e Una To provld* eutoewU* or renoto contol AddMon*l otefowd* tor weienw MwAUora ei Ctotdown dorlce Te provld* for letomllc oulA fljrUni Fa* aanrsenev floodby mlb To provide Mfh flaan flow lor * hlfb . Ui*d oo drive* la* aaibsmifl wMi Myh feartU . flarltns tarena loss reduces. The importance of de* creasing this throttling loss depends on the application. In noncondensing units where the demand for exhaust steam exceeds the amount furnished by the turbine, increased efficiency has practically no value. Where throttling causes heat loss to the overall cycle, however, os in'condensing applications, the added valving has considerable value. While hand-valve arrangements are inexpensive, they have doubtful value with rapid or frequent load swings. Operators must change the settings with every sizable load change in order to realize a gain. Automatic sectional valves are comparatively expensive for small units, not being used normally for ratings less then 500 to 600 hp. Above these ratings automatic seetlonal valves become increasingly important Cett of Steam. In making any study to determine efficiency, load factor and steam cost are of greatest importance. Load factor can usually be established with fair accuracy. Computing the cost of steam may not be as easy. Cost at the boilers is usually, known. When the heat divides between power genera* tion and process requirements, however, computing the cost becomes difficult. This is particularly true when the di vision of heat varies on irregular swings. In the simplest case where all the Row for process always passes through turbines, an approximste divisioo of Table III: Accessories Available For Mechaelcal-Orlve Turblses Ascasaorr flamarbs ......................... Oaes.lhanBOr*a*rs,Udiornaton,baaoialar. *U ......................... Oban henMndby umdedarerof Aiven *a*lo****t ......................... ForbebAhra FUtlbl* covplinvt.................... Low oll-prcmr* tip................. Retld velvet......................... Hlfh badtprassura tip.............. (Srdwra Aroolh Urbina asmafactuni aey pove coovanlanl Added ptotectloo ***lnd let* of oil prenure . Primary protection ayalafl owproMOO I* cctlnf Added protection wind overprenatn In c*iln| OS flit***, perilIvt. *nd tp*c!l coclwt................................... Keep oil d**o **d cool ndr tdvwt* coedUtora AeiltUnr ell pomp............. For standby and startini orvlca InseUUoo or bfiln................. Save* hi Ce<tpll*| nwdi........................ Sdty 0 lb% power and 8595 process can be applied to the cost of steam at the boilers. In such cases, steam cost chargeable to the turbine may be a small percentage of the total steam cost. A careful study of this situation is recommended. Other Factors. Faclots other than turbine cost may influence an installa tion decision. For example, if a unit is to be installed in a location need ing a long run of pipe, either inlet or exhaust, cost of the larger pipe re quired to handlo the increased flow of a less efficient unit may outweigh the lower coat of such a unit. Or, the' need for low exhaust temperature may dictate the selection of a more effi cient unit. Governing. Speed-governing system* used on mechanical-drive turbines. range from simple direct-acting flybsB. governors, through hydraulic govern-. ing systems, to the.highly accurate,, long-range electrical type. .. Most governor designs are arranged , so that pressure regulators can be ap-. plied to control speed In response to j remote signals, such as pump discharge pressure, differential pressure across feed valves, combustion requirements, temperature, etc. Through the use of, regulators, turbine control is very flex* '; (Continued on page 140) IM (438) ROWER July lW4j Pointers on Pumps WHAT HS NPSH? NPSH (net positive suction heed) is an absolute term covering hydraulic conditions nt the auction side of a cen trifugal pump. It can be used instead of the relative terms "suction lift" and "suction head" to assure the proper ap plication of centrifugal pumps. These relative'terms are useful in calculating the total head against which the pump niust operate; but they should not be used when considering the auction con dition as It affects proper centrifugal pump selection. Actually, every centrifugal pump has Its own NPSH characteristic, usually referred to as "required NPSH" The PUmp imoeUer GDenino mint tw a* Sp. Or* Where ffi -- Static heed In feet P a Pressure on surface of Dqutd Pv = Vapor pressure ot pumping tomperature Hf a Friction tosses In feet H = Entrance toss from took lo pipe NOTIi for boFRftfl tlqiridi, P imiofty oquah emitted from itie problem. rbb Item It Owroforo velocity needed to put this quantity into the Impeller opening must come from an external head. The value of this ex ternal head is the "required NPSH". Obviously, if the liquid pumped is not to vaporize in the entrance to the Im peller, the required NPSH must be measured over and above the vapor pressure of the liquid at the pumping temperature. The required NPSH will vary roughly os the square of the pump capacity through the useful range of opplication of that pump. For YOUR Pumping Needs Worthington has the most.complete line of centrifugal .pumps -- all pres sures, all capacities.-- and' engineers ready to give you expert advice on any pumping problem. For further details proving there's more worth in Worthington, contact our nearest Dis trict Office, or write to Worth- turn ington Pump-and Machinery .Corporation, CentrifugalPump Division, Harrison, N. J WORTHINGTON