Steam Basics Part 18: Selection of Medium and High Pressure Steam Flash Tank

By Chad Edmondson (JMP) and Norman Hall (RLD)

The last several weeks we've focused on steam and condensate handling. Today we'll talk about steam flash tanks.

Condensate flashes into steam in the return lines after the steam traps. When the steam pressure exceeds 15 PSIG and is defined as medium or high pressure, flash tanks are used to control where flashing occurs. The flash tank provides a “bubble” in the pipe with enough volume to allow the higher temperature condensate to flash into lower pressure steam. Flash tank outlets may be vented to atmosphere or may be isolated with a valve to capture the steam and reuse it in a lower pressure application.

In general, there are two types of steam flash tanks used in HVAC systems: horizontal and vertical.

Horizontal Condensate Flash Tanks

Horizontal flash tanks are the more traditional style. They are designed and sized to give a required disengaging area. This area is the length times diameter of a horizontal tank and is coupled with the tank volume above the water level to vent dry flash steam. Dry flash steam is steam that is not carrying excessive amounts of condensate with the flash steam. The more PPH of condensate entering, the larger the tank. The higher the steam pressure (and thus the condensate temperature), the larger the tank. The lower the flash pressure and corresponding temperature, the larger the tank.

Vertical Condensate Flash Tanks

Vertical flash tanks use a different principle. Vertical flash tanks use tangential openings and “spin” the condensate around the diameter of the tank. Flash steam is lighter than the water, so it moves to the inside and is vented up while the condensate moves down. This is the same operating principle of air separators that use tangential openings and centrifugal force to actively remove air from the water. Vertical tanks that are more efficient and can typically be sized smaller than horizontal tanks.

Cemline Corporation, a manufacturer of ASME flash tanks, has an online sizing program in which users input the key values and the program generates the correct tank size and opening sizes and provides CAD ready drawings. Here's a screenshot of what that program looks like: