Separation Process in Steam Drum

Steam drum is a collector and separator equipment for water and steam from water wall tubes. According to separation concept of water and steam in steam drum, separation process can be classified in three types such as natural gravity driven separation, baffle assisted primary separation and mechanical primary separation.

-          Natural Gravity Driven Separation

In the process of natural separation, mixture of water and steam is separated due to differences in density. Steam has less density than water so steam will go to the top of drum and water will fall to bottom of drum. This separation process depends on the location out of steam and water, the speed and position of the incoming steam, the quality of steam and so on. But the natural separation process has some disadvantages such as Figure 1 below:

Figure 1: Natural Gravity Driven Separation in Steam Drum

In the figure shown, if the steam entered through the bottom of drum, the steam will mix with boiler water so reduce temperature of steam and increase temperature of water which will be distributed to downcomer pipe or convection wall tubes. If velocity of steam is low, steam will not be able to pass water and its quality will be reduced while if the velocity of steam is too high, some steams will go to downcomer pipe so the water level in steam drum will rise, thus disturbing the accuracy of water level gauges on drum.

If steam is entered to the center of drum, the water level in steam drum will be uneven and some steams will enter to downcomer pipe. If the steam is entered from the top of drum, steam will affect to water level in steam drum.

-          Baffle Assisted Separation

In this separation process, a mixture of steam and water coming out of the water wall tubes will be separated by baffle with redirecting the flow into baffle, so water separate from steam and the flow of steam will be directed so it does not mix with water in the steam drum as shown in Figure 2 below.

Figure 2: Baffle Assisted Separation in Steam Drum

-          Mechanical Primary Separation

The working principal of mechanical primary separation is use separation process due to centrifugal force and radial force by passing the mixture of steam and water in cyclone shaped equipment. Steam will be separated from water because centrifugal force and radial force occur when the mixture flow this cyclone. The figure below is sample of cyclone equipment such as conical cyclone, curved arm cyclone and horizontal cyclone. 

Figure 3: Type of Cyclone in Steam Drum