Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGtapers inward, so the amount of clearance betweenthe flange and the cylinder varies accordingto the position of the valve. When the valveis open, the clearance is greater than when thevalve is closed.When the trap is first cut in, pressure fromthe inlet (chamber A) acts against the undersideof the flange and lifts the disk off the valve seat.Condensate is thus allowed to pass out throughthe orifice in the seat; and, at the same time, asmall amount of condensate (called control flow)flows up past the flange and into chamber B, Thecontrol flow discharges through the control orifice,into the outlet side of the trap, and the pressurein chamber B remains lower than the pressurein chamber A.As the line warms up, the temperature of thecondensate flowing through the trap increases.The reverse taper of the cylinder varies theamount of flow around the flange until a balancedposition is reached in which the total forceexerted above the flange is equal to the totalforce exerted below the flange. It is importantto note that there is still a pressure differencebetween chamber A and chamber B. The forceis equalized because the effective area abovethe flange is larger than the effective area belowthe flange. The difference in working area issuch that the valve maintains an open, balancedposition when the pressure in chamber B is 86percent of the pressure in chamber A.As the temperature of the condensate approachesits boiling point, some of the controlflow going to chamber B flashes into steam asit enters the low pressure area. Since the steamhas a much greater volume than the water fromwhich it is generated, pressure builds up in thespace above the flange (chamber B). When thepressure in this space is 86 percent of the inletpressure (chamber A), the force exerted on thetop of the flange pushes the entire disk downwardand so closes the valve.With the valve closed, the only flow throughthe trap is past the flange and through the controlorifice. When the temperature of the condensateentering the trap drops slightly, condensateenters chamber B without flashing intosteam. Pressure in chamber B is thus reducedto the point where the valve opens and allowscondensate to flow through the orifice in thevalve seat. Thus the entire cycle is repeatedcontinuously.With a normal condensate load, the valveopens and closes at frequent intervals, discharginga small amount of condensate at eachopening. With a heavy condensate load, the valveremains wide open and allows a continuousdischarge of condensate.ORIFICE -TYPESTEAM TRAPS.-Aboardship, continuous-flow steam traps of the orificetype are used in some constant service steamsystems, oil heating steam systems, ventilationpreheaters, and other systems or services inwhich condensate forms at a fairly constantrate. Orifice-type steam traps are not suitablefor services in which the condensate formationis not continuous.There are several variations of the orificetypesteam trap, but all types have one thingin common—they contain no moving parts. Oneor more restricted passageways or orificesallow condensate to trickle through but do notallow steam to flow through. Some orifice-typesteam traps have baffles as well as orifices.BIMETALLIC STEAM TRAPS. -Bimetallicsteam traps of the type shown in figure 14-8are used on many ships to drain condensatefrom main steam lines, auxiliary steam lines,and other steam lines. The main working partsof this steam trap are a segmented bimetallicelement and a ball-type check valve.The bimetallic element consists of severalbimetallic strips2 fastened together in a segmentedfashion, as shown in figure 14-8. Oneend of the bimetallic element is fastened rigidlyto a part of the trap body; the other end, whichis free to move, is fastened to the top of thestem of the ball-type check valve.Line pressure acting on the check valvetends to keep the valve open. When steam entersthe trap body, the bimetallic element expandsunequally because of the differential responseto temperature of the two metals; the bimetallicelement deflects upward at its free end, thusmoving the valve stem upward and closing thevalve. As the steam cools and condenses, thebimetallic element moves downward, towardthe horizontal position, thus opening the valveand allowing some condensate to flow out throughthe valve. As the flow of condensate begins, agreater area of the ball is exposed to the higherpressure above the seat. The valve now openswide and allows a full capacity flow of condensate.2The principle of bimetallic expansion is discussedin chapter 7 of this text.370