Document O1yJmXazVwrzGnKEzgjeRjaw

Welding Saves Costly Pump Casing! : A. *: ; Shock from rapid load changes in these 1600-psi boiler feed pumps called for inner casings made of chromium steel, an alloy difficult to cast without flaws. Carl Barlow tells how care ful casting procedure and welding* licked the leak problem CAM N BAMOW Machenlcef tnglnocrlng Irtipcttar, Deitga A Construction Dir, Power Fyifem ot tho City of Los Angelai The Harbor Steam Plant ol the City of Lo Angeles is built directly ovor the dome of a large oil field. Be cause thb location makes it vulnerable to seismic shock, station design incor porates more safety foclora than con ventional plants of equal capacity. Original unit No. 1 and recently In stalled unit No. 2 are designed to pick up load quickly. While this is one of the world's most modern sleam-generatlng plants, five years' experience with unit No. 1 bears out the fact that improvements can be made in certoin operating parts of the plant Designed as a hot standby (or quick pickup if the other energy sources in tho city fail, No. 2 unit operates as a spinning reserve generating 5000 kw when idling. Capable of withstanding extreme shock, it picks up to 75,000 kw in a few minutes, delivers it for two hours, and then returns to 65,000 kw, its rated output Ail parts of this 900- A'VOstteft/M&if rapa/r. . ,No. t ntcriofi sto\ Inner coins tilde. Into Iho color borrol with due-fitting tolerance,. Arrow. Indicote tho cloture points lor testing. Suction passogo Is sealed at both ends per 915-K unit ere deiiill.1| , J .tend hisher load .hock Ur,0 J* t plonte operating ol lower p food Pump.. When "oulJe-1" in one or more transmission l the qulek pickup begins f one point of great load shock the boiler feed pumps. At lay they -force feedwater throagh iheater and into the boiler dmo. operates at 1050 ptl. When plan, j one of two pumps carries the lot maximum capacity, both pump, ^ parallel, with their output viiybi | tero to normal fulMoad csp,, against a variable discharge head, t lion pressure and temperature. I quick pickup they sometimes hi handle cold water. On standby, the. pumps normally t, water at 12 psi and 245 F from t <* ocreting-heater storage tank. Upon i "outage," pressure increases to l psi and temperature to 560 F. As iiei demand at the turbine throttle creases, subsequent flashing of watt the boiler drum surges the pumps I, zero pressure to full-load coediti This surging pula a terrific stress . the pumps because at thb time they each handling full capacity--575wfl lb an hr of water. Originally the p had inner-volute casings of steel, Fig. 1, and handling such enormous quantity of feedwater t these high-atreas oonditioni pi more than the metal could long m After 7000-hr service one ctain way. ft was repaired by fill-w. rcmachinlng to oversize dimes and filled with oversize wear rings, spore casing and element were tu while the manufacturer overhauled 1 carbon-steel casing--a costly ProcaJJ that might lead to the loss of a poapj during "outage" when it is most needoLff After 15,000 hr of service the casing*! were beyond repair, and it was evidartl they would hove to be mode of mereff enduring metol. Chromium Alloy. The steam-design, section of the Dept of Water and Po***J had conducted extensive research ffi an aUoy with higher wear-resbUflij qualities, which would olio resbt dization and be suitable for MgM*`j peralurc service. Many recent appl***"] Ows urt Uluarulm fw* P?Efta rej cm UlifMS Uattfi-rM-Fracrms Anr4 WkBOWltmUmaBWatW*rretwucMuladf liaMdfiddpW. fWabnWtwiSm'.'.* 3up view of seals on No. I suction and discharge parts it tn Fig. I m moke tests with cold water ot 1200 ptl After the Seal plates were wetded to the suction and dis charge ports they were drilled end tapped far rest piping |0( chromium-steel alloy for other ftp (adjected that it possessed all P&eslted requirements. Of those conj&d, UVb`13% chromium alloy fcjjTM No. A221-39, Class A) has the I?'ductility at high temperatures *ifcs least corresponding brittleness too temperatures. It bats about fire times os long as carbon HSuader extreme wear. |kuliirities of thb alloy make it "dolt to east AH parts of the mold `sections join mast be gated and flarge risers used. The Ll%-13% [am alloy has amazing thermal less and does not readily flow restricted areas. After pouring, tho metal is no longer plastic and ilidg between 700 to 400 F, it has a sad suppressed transformation. creates internal stresses that result :gErsekjng. (Tbe alloy must cool completely in imold as It will air-quench and set *uMed stresses at temperatures as Fas 400 F. Since this sluggishness ^inherent quality of metals of this it Is natural to suppose they would ^ show s tendency to crack from S^ses caused by high, heat during IlSjjdtag, or immediately thereafter. But Sj** the proper procedures for prepglln4, welding and stress relieving followed they can ba welded withdamage. ^o# the Alloy. The Byron JackCo, Vernon, Colif, had the origi- watract to supply the pumps for l.No. 1 with- eaibcn-ateel volute ler easing*. The company was also tjjiTM Job of replacing these cas** because it was the successful bid- wt pumps for unit No. 2. They or castings for both contracts from the Utility Electric Steel Foundry of ditional units under construction, the Vernon. Ilia castings were poured dur plant will have a total of 18 pumps. ing a damp foggy night On arrival ot A conference on the new acting pro the foundry next day It was learned the cedure resulted In revising the foundry night crew, with heavy production and patterns to strengthen certain sections. the need for molds and apace, had re Six replacement costings were ponred moved the castings before thoy were on a bright, warm day and left in the completely cold. The air-quenched alloy at once be gan to show its peculiarities. Internal mold till cold to prevent surface airquenching. No surface cracks or other exterior defects were apparent upon stresses were manifested by a myriad removal from the molds. of liny cracks on the surface and some Hydrostatic Test. Because the con deeper pits. The foundry elected to tractor did not wish to chance a repeti chip certain sections and fill with weld tion of former trouble he gave the metal before sending to the contractor. castings a hydrostatic tat before ma After receiving the 'castings the con chining them. Steel plates were fitted to tractor proceeded with the rough ma the suction and discharge porta, welded chining. As it progressed, smoll cracks in place, and drilled ood tapped for localized into larger ones that extended pipe connections. Tata with cold water through the castings. at 1500 psi were made. No exterior After several machinings the castings leaks were found, so the castings were were subjected to e hydrostatic test and delivered for machining. Fig. 2 and 3 many looks found. An attempt to make show method of sealing porta. repairs by fill-welding was not success Bough machining was completed ful. The contractor now realized that after the exterior test. Next came part the procedure would have to be more of the semifinished machining, only the precise for alloy eastings than for the mating tacea and semlfinlsh bore being earbon steel previously used. done without removing the seals. The Thu costly experience showed the casing was then hydroststially tested need of a different procedure. Time again. In the second tat the inner cat* requirements of the original delivery ing was assembled in the outer barrel date had expired and the production of and tested with cold water at the dis now, completed casings would require charge nozzle of the outer barrel, thus 120 days. The Dept of Water and Power * giving a crushing prasure similar to extended the delivery dote, 1)008000 of shutoff head in actual operation. the serious problems involved in this This tat similata the action daring new experimental type of casing. Ob "outage" when the pump works at taining finished inner-volute casings of maximum discharge pressure and the this alloy was deemed extremely im boiler drum reaches required liquid portant because they would save many level. At that time the automatic valves replacements in the Harbor Steam close'and the entire prasure is sud Plant. With one unit operating, o sec denly thrown into the area between in ond scheduled for operation early in ner casing and outer barrel, Fig. 1. 1948 (now in operation) and three ad Factory test is conducted at 3800 psi. 74 {740| POWER Jo"' E>W| . June 1948 (341) 7&