Boiler Drum in Sub-critical Boiler

Boilers operating below the critical pressure are generally fitted with a steam drum, which is known as the heart of the boiler. In the steam drum, the main objective is to separate the steam from the steam-water mixture, which is coming through the riser tubes. Basically In the boiler, the feed-water comes through the feed-water distribution Pipe to the boiler drum and then the feed water flows through the down-comers (All the down-comers and the evaporating tubes are connected to the bottom-ring header), then through the evaporating tubes the steam-water mixture moves up after absorbing the heat from the furnace and the steam water mixture enters into the boiler drum. It’s a complete cycle and the working principle of a sub-critical boiler has been explained earlier.

(Boiler Drum)

The boiler drum separates steam from the mixture by the following principles:

1. Gravity Method: This method is employed in case of boilers having low rate of steam generation. Here the steam-water mixture comes to the drum through the riser tubes and due to low rate of steam generation, the steam being lighter goes up through the screen-dryer to the super heater and the water being heavier settles down in the drum. Due to the lower rate of steam generation the steam-water mixture gets sufficient time for the separation by gravity. This method has become obsolete as now a day we are going for boilers having high rate of steam generation.
2. Gravity Method assisted by Baffle Plates: this method is same as that of the gravity method, but here the baffle plates are provided in steam space to provide obstruction for the water drops which are getting carried away by the steam and due to impact with the baffle plates the water drop comes down by gravity. This method is only used in case of boilers having low rate of steam generation.
3. Turbo Separators (Gravity Method assisted by Centrifugal force): This method is widely used in case of modern sub-critical boilers.in this method Turbo Separators are provided with baffle plates. The turbo separators separate steam by the help of centrifugal force.it consists of two sections.

Primary Separator: It consists of two concentric cylinders. The inner cylinder is of rotating type and the outer cylinder is of stationary type. The steam-water mixture enters the inner cylinder which is rotating and due to the centrifugal force the mixture moves up and the steam enters the secondary separator (corrugated sheet) being lighter in mass and the water being heavy moves to the inner portion of the outer cylinder due to centrifugal force, then water comes down from the outer cylinder and mixes with the feed-water present in the drum.

Secondary Separator: Secondary separator is nothing but a set of corrugated sheets which traps the water drops which are getting carried away by the steam. It’s basically a filter.In modern sub-critical boilers, turbo separators are widely used due to their effectiveness in separating steam from steam-water mixture.

(Drum Schematic Diagram)

Objectives of boiler drum:

1. The main purpose of the boiler drum is to provide necessary space for separating steam from steam-water mixture at a certain pressure.
2. It provides a point for chemical dosing in order to maintain the concentration of the impurities and the ph of the water
3. It provides a point for blow down to maintain the concentration of the impurities as well as in case of emergencies like high drum level; it helps to drain out the excess water.
4. It provides necessary space for housing the drum internals.

Drum Internals:

1. Turbo Separators
2. Baffle Plates: Baffle plates are provided to change the flow pattern to assist the gravity separation in the boiler drum and these are located in steam space.
3. Chemical dosing Pipe: It is a pipe present in the boiler drum throughout the length, which evenly distributes the chemical in the drum for maintaining the pH as well as the concentration of the impurities in DM water.
4. Feed-water distribution pipe: This pipe is present along the full length of the boiler and it evenly distributes the feed-water throughout the length. The reason we are evenly distributing the hot feed-water is to avoid stresses in the drum metal.
5. Screen Dryer: It is provided at the steam outlet in the drum as there is a chance that the steam leaving the boiler drum may carry some residual moisture, which will damage the HP turbine blades.
6. Anti-vortex Spyder: This is provided at the suction of the down-comer to avoid formation of vortex, which may lead to hammering of water and formation of steam.

Drum Connections:

1. Tapings for Riser Tubes
2. Feed-water intake Line
3. Tapings for safety valve
4. Taping for Steam outlet
5. Taping for drum level gauge

(Specification of the boiler-drum in KMPCL,Chattisgarh)