Impulse steam traps of the type shown in figure 13-35 are commonly used in steam drain collecting systems onboard ship. Steam and con-densate pass through a strainer before entering the trap. A circular baffle keeps the entering steam and condensate from striking on the cylinder or on the disk.

The impulse trap operates on the principle that hot water under pressure tends to flash into steam when the pressure is reduced. So that you under-stand how this principle is used, lets consider the arrangement of parts shown in figure 13-35 and see what happens to the flow of condensate under various conditions.

The only moving part in the steam trap is the disk, which is rather unusual in design. Near the top of the disk is a flange that acts as a piston. As you can see in the insert, the working surface above the flange is larger than the working sur-face below the flange. The importance of having this larger effective area above the flange is brought out later in this discussion.

A control orifice runs through the disk from top to bottom and is considerably smaller at the top than at the bottom. The bottom part of the disk extends through and beyond the orifice in the seat. The upper part of the disk (including the flange) is inside a cylinder. The cylinder tapers inward, so the amount of clearance be-tween the flange and the cylinder varies according 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 (put in service), 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. At the same time, a small amount of condensate (CONTROL FLOW) flows up past the flange and into chamber B. The control flow discharges through the con-trol orifice, into the outlet side of the trap.

Figure 13-35.-Impulse steam trap.

The pressure in chamber B remains lower than the pressure 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. This continues 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 be-tween 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.

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. The steam has a much larger volume than the water from which it is generated. Therefore, pressure is built up in the space above the flange (chamber B). The force exerted on the top of the flange pushes the disk downward and closes the valve.

With the valve closed, the only flow through the trap is past the flange and through the control orifice. When the temperature of the condensate entering the trap drops slightly, condensate enters chamber B without flashing into steam. Pressure in chamber B is thus reduced to the point that the valve opens and allows condensate to flow through the orifice in the valve seat. Thus the entire cycle is repeated.

With a normal condensate load, the valve opens and closes at frequent intervals. This discharges a small amount of condensate at each opening. With a heavy condensate load, the valve remains wide open and allows a heavy, continuous discharge of condensate. (You can tell if this valve is working properly by listening to it with an engineers stethoscope. A clicking sound indicates the valve disk is moving up and down on its seat.)