Steam Partial Pressure #3: Air Removal (Part 2)

From Where on the Equipment is Air Removed?

The air filling the steam space is pushed along by the flow of steam, and since heating begins when steam enters the equipment, it is critical that all the air be completely discharged. Particular care must be taken for equipment with complex steam space configurations or equipment that uses a siphon pipe, as air has a tendency to become locked in these types of equipment. Even if steam traps with a high-performance air venting function are used and condensate and air are discharged through the condensate outlet, the air in any locked steam will not be discharged. In such cases, the use of an air vent for steam is effective. Air vents for steam are a type of automatic valve with a configuration similar to that of thermostatic steam traps—the valve opens and closes in response to the difference between the temperatures of the steam and the air. Air can be effectively removed by installing this type of air vent on the equipment at locations where the equipment configuration would lead one to expect air locking.

How Hot Air is Removed

The above discussion dealt with the initial air found in the equipment at startup, but let's consider the hot air in the equipment during operation. In some instances, insufficient treatment of the boiler feed water causes carbon dioxide or other non-condensable gases to be fed to the equipment during operation, and this air inside the equipment is heated by the steam to high temperatures. If this air or non-condensable gas is not completely removed, operation can be impaired. If the hot air is removed by means of an air vent, a noticeable difference will result.

The valve opening temperature on bimetal type air vents is more or less fixed, so this type of air vent is not well-suited for the removal of hot air. The valve on balanced pressure type air vents, however, opens at a temperature that is a certain number of degrees lower than the saturation temperature, so hot air is discharged at all times regardless of fluctuations in steam pressure.

Air Vents for Steam

Operation of Air Vent for Steam (LA21)

At equipment start-up, the x-element valve is in the open position and large quantities of air are quickly discharged, thereby greatly reducing equipment start-up time. When the inflow of steam begins, the x-element valve immediately closes. The valve remains closed as long as the temperature around it is near saturation temperature. As the inflow of air causes the temperature to drop, the valve once again opens and the air are rapidly discharged.

Balanced Pressure Type (X-element) Air Vents

Balanced Pressure Type (X-element) Air Vents Graph

With the X-element of balanced pressure types, the opening / closing temperatures automatically adjust in response to the steam pressure. Accordingly, there is no need to adjust them and yet they are able to discharge very hot air. In addition, the response time is fast - virtually eliminating steam blowing.

As seen from the preceding explanation, when removing air from steam spaces it is important to understand both the correct placement of the air vent and the particular type of the air vent.