Document 9JLxvw9M9MzmOwakpbBZZkxk7

"rCCf& T^re&dwie Op*fie tor fer9tt$ ~~fe btowoff m Timty retult-Wtt fttam Bfmw%'trhtrged ogsinst the intermit- Sf&Womi ,h"d- Thei' (*> KftjuVr to maltdsin uniform solids PSvstio". ketchc* P m (2) ,he ^n'-errot-operelor's failure to repSffir to blow down (3) difficulty in IKSGtifftltiiouupous blowtdeocwonv,crTt-he second Rtfia'tesi method of removing concengftjfiolids, answers these major dis"Aniiutei Bui >l* **lu* begins to tell CofiOtt\tgtt rotvrot f5^Ji!r8WmEAL 11 J Bicoft Con'i/ivovt blpwoff igtttin boilers si hotter than 4000- to l&phfapnr-b*' steeming capacity, oper100 jsj and above. As the j5m6Vug|*JA total solids leave the ^i^r,'continuously through a pipe that .few the water surface st the point grSghest concentration. KtfQver and above the continuous pipe is 1' ?* B&)ncilon, a takeoff pipe goes' in at Here are the two bask chokes you can make in... HOMs'/boller low points. Then an occadSnSl manual blowdown in the convenRIms! manner removes sny sludge ae- tnotoff Kumulstion In the mud drum. ICctfntiflUoui blowdown gives these Bringing Blowdown Under Control sui| drainages: uniform solids con centration, entirely automatic operaI uo^(excep( for relatively minor adjust* ! menii if rate of solids concentration Wafer treatment can change the nature of all your boiler scale, bur you still have to remove a!/ the soHcfs that make scale from the boiler cycle. Here Is how and why The job of Slowdown seems straightforward enough. It's meant to control the build-up of too many solids in the boiler drum. Left to themselves, these solids bring on serious scaling prob lems and add still more to operating problems by laying the groundwork for possible foaming, priming, carryover. The way these solids build up or con centrate shows up readily with calcium sulfate, a moderately soluble salt. You'll find it a fairly common member of most boiler makeup waters- Suppose we consider a feedwater, holding 32 ppm of this calcium sulfate and slated for a boiler operating at 300 pel. The corresponding saturation temperature, 422 F, is a point where calcium sulfate's solubility is 640 ppm, 20 times what we start out with in our feedwater sam ple. This means we can expose our . feedwater to 20 cycles of evaporation or concentration. Or stated another way, we can evaporate 20 gallons of feedwater down to not less than one gallon without going beyond the cal cium sulfotc solubility. Out any more than this, and the excess drops out of solution: Blowdown Needs. To keep on mak ing steam we have to bring In fresh water as replacement. This incoming water brings with it 32 ppm of dissolved calcium sulfate that drops right out of solution and creates a seating problem. If, however, we pull out one gallon of boiler water, we can add 20 gallons of fresh water before we're up against this same problem. In onr case this would mean only 1/20th of the hoiler feed, or S%--a fairly reasonable blowdown rote. But the exact amount ol blowdown isn't determined by calcium sulfate alone- Instead, what decides the safe blowdown rate Is all the indivlduol compounds the feedwater brings to the boiler drum, the water-treatment chem icals you add, plua their effect on one another to form possibly still different compounds of different solubility lim its, plus the boiler operoltng pressure and temperature and their effect on solubility. When you come to pick blowdown rate for your boiler you'd better'call in a .reliable water-treatment consultant. He'll set you itroight. Table, left on p 137, Is only a rough guide. Over and above this problem of hold ing total dissolved solids in the boiler water within safe solubility limits is the Dtwlaord fr*i wiutlil iDixulnt In UM N* I. IIM Itpx at Ow ArmlUMf Trip jjtful**. AmUrm* MhWiw Wait*. Tfcm nt*r. IlIiA. 1 one of keeping a curb on suspended1, solids--that Is, those that are no longer bound in solution but are,free to drop' out. We have it on ptetty good au-^ thorlty that most of these suspended;!1 solids do not settle to the lowest point] in the boiler at you might guess- Some' small percentage do, and you have lo*j put in special takeoffs at these low.j' points to pull them out during blow-'] down. '? But most of these suspended solids, are held just below the water'Surface , in ihe boiler drum by the buoyant force of boiling and rising steam bubbles.''M Concentration can get so high at thit^r point that the water becomes soupy,.ft) its surface tension considerably in* creased. Then normal steam release is retarded and foaming or priming esn Ji develop. So much for the need for { blowdown. How is ft done^ Blowdown Methods. There are.two primary methods of boiler blowdown-- Iniermiitcnt and continuous. The in- lermiuent or manuol system is the gronddaddy of blowdown methods- And it is still doing e job on the emailei boilers. The takeoff pipes ere usually in the low point, or mud drum, and when you blow down, sludge as well ss dissolved end suspended solids go out. But because the takeoff pipes do not connect up to the point of highest solids concentration, the blowdown amount m runs higher than absolutely necessory to bring* the water down to the allow able solubility limits. In addition, three major disadvan- 136 PUNT OPERATION AND MAINTENANCE SECTION POWIB {(ianges because of, ssy, makeup water), 'easy best recovery. jj&KoQt Recovery, The steps you take Htfibest recovery: Fig.), 2, may or may jiot include flash tanka. If you can use 'Mat) steam, you pass the blowdown Swiuer Into a flash tank or tanks first, find (eke off the steam there. Then the Jjanflashed water is passed through heat tHchangers. If you have no use for |ftuh steam all the blowdown water -tees through the heat exchangers only. li* ehher arrangement, heat lost is Represented by the difference in tern-' :po*lure between the liquid being .jfested and the blowdown water as each ;dj**ei'Ute last heat eichanger. ^The costs that intermittent blowdown ^represents, in table at right, give you me Idea of the importance ol your r-bbwdown water. For a large boiler, 'dn* savings from coniinuous blowdown Jpay Islrly fast for the heai-recovery pjuipment and Justify itj purchase. Allowable Boiler Solltfs fAmsvice" Bailer Mlrs Assn) w* t wit*i hmmrSflM *i> TM `zppmr1 1500 1000 1500 1000 1500 1250 1000 jftoCTOllR 1933 wotra r'r'0' Average Costs ol Blowdown I <i. tft / INTERMITTENT or monuol blowdown Vets o certain amount of steam, depending an pressure and valve sixes used, escape. The cost to you shows In the obove table ,OH OPEHATION AND MAINTENANCE SECTION 137 1: