AIR VENTING

The removal of this air from steam or condensate system is termed as ‘air venting’. Air enters the steam system through the leakages in the steam piping and along with the steam itself from the boiler.

The feedwater if not de-aerated will be a carrier of air and other non-condensable gases which are eventually liberated at elevated temperature in the boiler drum. The air in the steam system occupies space in the piping dead ends, elevated risers, and is often carried to the heat exchangers and process equipment.

Air thus has to be vented in the boiler feedwater circuit, steam distribution lines, process equipment, and condensate lines.

A. Air removal from boiler feedwater

Air in the boiler feedwater can be removed by thermal de-aeration, chemical de-aeration, or a combination of both. The details of the de-aeration are discussed in Unit-2, chapter-3.

B. Air removal from the steam distribution system

Air from steam mains and distribution lines needs to be removed on an ongoing basis but specifically at start-up. Air removal can be done manually through a hand-operated vent or automatically by an air vent.

1. Manual hand-operated vent

The manual hand-operated vent cock is fitted on the steam space. The operator needs to open this before the steam is turned on and close when steam starts escaping through the vent. It is difficult to differentiate between steam and steam/air mixture and hence the operation is sole to the experience and skills of the operator.

The manual air venting system can be employed only at start-up, for continuous air venting, it is not feasible to operate the vent frequently. This is an inefficient method of removal of air removal. In most cases, it is observed that there is huge steam wastage build-up of air in the system towards the end of batch or day. The laundry calendars are fitted with air vent cocks. These cocks need to be opened in the morning during the start-up to vent out the air. The opening and closing of these vents are carried by prediction. It is common to find that at the end of the day the ability of the calendar to dry the work decreases. The reason for this is the gradual building of a high air/steam ratio.

2. Automatic air vent

The principle of a thermostatic air vent is the same as that of the thermostatic trap. The factor that differentiates air/steam mixture and pure steam is the temperature. This parameter is used in the operation of an automatic air vent.

The difference of temperature between the saturation temperature of water at any pressure and the boiling point of the liquid filling of the thermostatic element at the same pressure should be as small as possible. This difference in the boiling point of the liquid in the thermostatic element and steam saturation temperature is the sub-cooling of the trap. The degree of sub-cooling determines at what temperature the thermostatic element will close and open. If for a low degree of sub-cooling there will be minimum residual air in the steam system.

At the startup, the thermostatic element is in the open position and allows the discharge of air. When steam begins to flow the element immediately expands, which ultimately closes the valve, the valve remains in a closed position till the temperature is above sub-cooling temperature. As the temperature drops, the valve opens and the cycle continues.

Fig Valve Open

Fig: Valve Close

 

It is generally not usual to employ these on the superheated steam. Problems like water hammers occur due to superheated steam, bimetallic air vents are used in this case. Bimetallic air vents are like bimetallic steam traps.

They employ a stack of bimetallic strips instead of a thermostatic element. Bimetallic air vents operate at a higher sub-cooling as compared to thermostatic air vents.

Position of air vent

The factors that should be kept in mind while deciding the position of the air vent are:
The air should be quickly removed before it mixes with the incoming steam. If the air and steam are mixed the air is carried to the process equipment. The incoming steam tries to push the air in front of it till it reaches the remote point where it is collected. If not removed or vented it forms a pocket and slowly diffuses in steam. The temperature where the air pocket exists has low temperature than that in contact with steam.

Though the air is heavier than steam at the same pressure and temperature. However, this doesn’t result in air occupying the bottom space of steam lines. The space occupied by air in steam systems is a function of the steam line flow (entry path) and geometry of the steam equipment or pipe layout.
In most cases in the supply and distribution of steam, the flow is directed in the upward direction. For process heating, the steam flow is directed in the downward direction. Hence air venting position is different based on application.

In steam distribution lines when the line is charged air is collected in the remote places and dead-ends at the top position and hence air vents should be fitted on the top. For distribution lines, air vents should be preferably fitted with air bottles for effective air venting.