Page 203 - Root Cause Failure Analysis

Steam Traps 191

brated, the disk closes tightly against a seat when steam is present and opens when
condensate, air, and other gases are present.

Two key advantages of bimetallic traps are their compact size relative to their conden-
sate load-handling capabilities and immunity to hydraulic-hammer damage.

Their biggest disadvantage is the need for constant adjustment or calibration, which
usually is done at the factory for the intended steam operating pressure. If the trap is
used at a lower pressure, it may discharge live steam. If used at a higher pressure, con-
densate may back up into the steam system.

Thermostatic or Thermal Element

Thermostatic, or thermal-element, traps are thermally actuated using an assembly
constructed of high-strength, corrosion-resistant stainless steel plates seam-welded
together. Figure 15-5 shows this type of trap.

On startup, the thermal element is positioned to open the valve and purge condensate,
air, and other gases. As the system warms up, heat generates pressure in the thermal
element, causing it to expand and throttle the flow of hot condensate through the dis-
charge valve. The steam that follows the hot condensate into the trap expands the ther-
mal element with great force, which causes the trap to close. Condensate that enters
the trap during system operation cools the element. As the thermal element cools, it
lifts the valve off the seat and allows the condensate to discharge quickly.

Figure 15-5 Thermostatic trap.