PRINCIPLES OF NAVAL ENGINEERING 



tapers inward, so the amount of clearance be- 

 tween the flange and the cylinder varies accord- 

 ing to the position of the valve. When the valve 

 is open, the clearance is greater than when the 

 valve is closed. 



When the trap is first cut in, pressure from 

 the inlet (chamber A) acts against the underside 

 of the flange and lifts the disk off the valve seat. 

 Condensate is thus allowed to pass out through 

 the orifice in the seat; and, at the same time, a 

 small amount of condensate (called control flow) 

 flows up past the flange and into chamber B, The 

 control flow discharges through the control ori- 

 fice, into the outlet side of the trap, and the pres- 

 sure in chamber B remains lower than the pres- 

 sure in chamber A. 



As the line warms up, the temperature of the 

 condensate flowing through the trap increases. 

 The reverse taper of the cylinder varies the 

 amount of flow around the flange until a balanced 

 position is reached in which the total force 

 exerted above the flange is equal to the total 

 force exerted below the flange. It is important 

 to note that there is still a pressure difference 

 between chamber A and chamber B. The force 

 is equalized because the effective area above 

 the flange is larger than the effective area below 

 the flange. The difference in working area is 

 such that the valve maintains an open, balanced 

 position when the pressure in chamber B is 86 

 percent of the pressure in chamber A. 



As the temperature of the condensate ap- 

 proaches its boiling point, some of the control 

 flow going to chamber B flashes into steam as 

 it enters the low pressure area. Since the steam 

 has a much greater volume than the water from 

 which it is generated, pressure builds up in the 

 space above the flange (chamber B). When the 

 pressure in this space is 86 percent of the inlet 

 pressure (chamber A), the force exerted on the 

 top of the flange pushes the entire disk down- 

 ward and so closes the valve. 



With the valve closed, the only flow through 

 the trap is past the flange and through the con- 

 trol orifice. When the temperature of the con- 

 densate entering the trap drops slightly, con- 

 densate enters chamber B without flashing into 

 steam. Pressure in chamber B is thus reduced 

 to the point where the valve opens and allows 

 condensate to flow through the orifice in the 

 valve seat. Thus the entire cycle is repeated 

 continuously. 



With a normal condensate load, the valve 

 opens and closes at frequent intervals, dis- 

 charging a small amount of condensate at each 



opening. With a heavy condensate load, the valve 

 remains wide open and allows a continuous 

 discharge of condensate. 



ORIFICE -TYPE STEAM TRAPS.-Aboard 

 ship, continuous-flow steam traps of the orifice 

 type are used in some constant service steam 

 systems, oil heating steam systems, ventilation 

 preheaters, and other systems or services in 

 which condensate forms at a fairly constant 

 rate. Orifice-type steam traps are not suitable 

 for services in which the condensate formation 

 is not continuous. 



There are several variations of the orifice- 

 type steam trap, but all types have one thing 

 in common— they contain no moving parts. One 

 or more restricted passageways or orifices 

 allow condensate to trickle through but do not 

 allow steam to flow through. Some orifice-type 

 steam traps have baffles as well as orifices. 



BIMETALLIC STEAM TRAPS. -Bimetallic 

 steam traps of the type shown in figure 14-8 

 are used on many ships to drain condensate 

 from main steam lines, auxiliary steam lines, 

 and other steam lines. The main working parts 

 of this steam trap are a segmented bimetallic 

 element and a ball-type check valve. 



The bimetallic element consists of several 

 bimetallic strips2 fastened together in a seg- 

 mented fashion, as shown in figure 14-8. One 

 end of the bimetallic element is fastened rigidly 

 to a part of the trap body; the other end, which 

 is free to move, is fastened to the top of the 

 stem of the ball-type check valve. 



Line pressure acting on the check valve 

 tends to keep the valve open. When steam enters 

 the trap body, the bimetallic element expands 

 unequally because of the differential response 

 to temperature of the two metals; the bimetallic 

 element deflects upward at its free end, thus 

 moving the valve stem upward and closing the 

 valve. As the steam cools and condenses, the 

 bimetallic element moves downward, toward 

 the horizontal position, thus opening the valve 

 and allowing some condensate to flow out through 

 the valve. As the flow of condensate begins, a 

 greater area of the ball is exposed to the higher 

 pressure above the seat. The valve now opens 

 wide and allows a full capacity flow of condensate. 



2The principle of bimetallic expansion is discussed 

 in chapter 7 of this text. 



370 



