Steam traps are installed in steam lines to drain condensate from the lines without allowing the escape of steam. There are many different designs of steam traps; some are suitable for high-pressure use and others for low-pressure use.
TYPES OF STEAM TRAPS
Some types of steam traps that are used in the Navy are the mechanical steam traps, bimetallic steam traps, and orifice-type steam traps.
Mechanical Steam Traps
Mechanical steam traps in common use include bucket-type traps and ball-float traps.
The operation of the bucket-type steam trap, shown in figure 9-34 is controlled by the condensate level in the trap body. The bucket valve is connected to the bucket in such a way that the valve closes as the bucket rises. As condensate continues to flow into the trap body, the valve remains closed until the bucket is full. When the bucket is full, it sinks and thus opens the valve. The valve remains open until enough condensate has blown out to allow the bucket to float, thus closing the valve.
Figure 9-35.-Ball-float steam trap.
Figure 9-35 shows a ball-float steam trap. This trap works much in the same way as the bucket trap. Condensate and steam enter the body of the trap, and the condensate collects at the bottom. As the condensate level rises, the ball float rises until it is raised enough to open the outlet valve of the trap. When the outlet valve opens, the condensate flows out of the trap into the drain system, and the float level drops, shutting off the valve until the condensate level rises again.
Bimetallic Steam Traps
Bimetallic steam traps of the type shown in figure 9-36 are used in many ships to drain
Figure 9-36.-Bimetallic steam trap.
condensate from main steam lines, auxiliary steam lines, and other steam components. The main working parts of this steam trap are a segmented bimetallic element and a ball-type check valve.
The bimetallic element has several bimetallic strips fastened together in a segmented fashion, as shown in figure 9-36. One end of the bimetallic element is fastened rigidly to a part of the valve 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 keeps the valve open. When steam enters the trap body, the bimetallic element expands unequally because of the different response to the 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, an unbalance of line pressure across the valve is created; since the line pressure is greater on the upper side of the ball of the check valve, the valve now opens wide and allows a full capacity flow of condensate.
Orifice Steam Traps
Aboard ship, continuous-flow steam traps of the orifice type are used in systems or services in which condensate forms at a fairly steady rate.
Figure 9-37 shows one orifice-type steam trap. Several variations of the orifice-type steam trap exist, but all have one thing in commonthey have no moving parts. One or more restricted passageways or orifices allow condensate to trickle
Figure 9-37.-Constant-flow drain orifice.
through but do not allow steam to flow through. Besides orifices, some orifice-type steam traps have baffles.
A strainer is installed just ahead of each steam trap. The strainer must be kept clean and in good condition to keep scale and other foreign matter from getting into the trap. Scale and sediment can clog the working parts of a steam trap and seriously interfere with the working of the trap.
Steam traps that are not operating properly can cause problems in systems and machinery. One way to check on the operation of a steam trap is to listen to it. If the trap is leaking, you will probably be able to hear it blowing through. Another way to check the operation of steam traps is to check the pressure in the drain system. A leaking steam trap causes an unusual increase in pressure in the drain system. When observing this condition, you can locate the defective trap by cutting out (isolating from the system) traps, one at a time, until the pressure in the drain system returns to normal.
You should disassemble, clean, and inspect defective steam traps. After determining the cause of the trouble, repair or replace parts as required. In some steam traps, you can replace the main working parts as a unit; in others, you may have to grind in a seating surface, replace a disk, or perform other repairs. You should reseat defective trap discharge valves. Always install new gaskets when reassembling steam traps.