Bucket traps are either of the upright or inverted design and are used in both low- and high-pressure steam heating systems. Both types of bucket traps are designed to respond to the difference in density between steam and condensation. The construction of a bucket trap is such that it has good resistance to water hammer. On the other hand, most bucket traps, unless modified, have limited air-venting capabilities. Bucket traps also have a tendency to lose their waterseal and blow steam continuously during sudden pressure changes.
In an upright bucket trap (see Figure 10-29), the condensation enters the trap and fills the space between the bucket and the walls of the trap. This causes the bucket to float and forces the valve against its seat, the valve and its stem usually being fastened to the bucket. When the water rises above the edges of the bucket, it floats into it and causes it to sink, thereby withdrawing the valve from its seat. This permits the steam pressure acting on the surface of the water in the bucket to force the water to a discharge opening. When the bucket is emptied, it rises and closes the valve and another cycle begins. The discharge from this type of trap is intermittent.
In the inverted bucket trap (see Figure 10-30), steam floats the inverted submerged bucket and closes the valve. Water entering the trap fills the bucket, which sinks, and through compound leverage opens the valve, and the trap discharges.
An inverted bucket trap with its seat open to vent air or drain condensate is shown in Figure 10-31. During start-up, air is vented into the return line through a bleed hole located at the top of the bucket. The condensate enters the trap, moves around the bucket, and drains from the open trap seat.
As steam flows into the trap, it collects at the top of the bucket. When enough steam has collected there, its buoyancy causes the bucket to rise and close the trap seat (see Figure 10-32). The closed trap seat blocks the exit of the steam until more condensate enters the bucket trap. The cycle repeats itself as long as the boiler is producing heat. Some inverted bucket traps have a thermal vent at the top of the bucket, which produces faster venting during start-up.
Typical installations for bucket traps are shown in Figure 10-33. Note the following installation recommendations:
• Provide enough room around the trap to allow easy access for service and maintenance.
• Locate the trap as close as possible to and below the equipment being drained.
• Install the trap in a straight run of horizontal pipe that is slightly pitched to allow condensate to flow down into the trap inlet.
