Boiler Drum is a very crucial part in case of Sub-critical boilers. It is cylindrical in shape and situated at the top of the boiler. In Super-critical units, water is directly converted into steam so drum is not required, but in case of subcritical boilers, the whole water quantity is not directly converted into steam. So proper steam and water separation is required which is done by the Drum internal mechanism. So we will explain you the need of Boiler Drum and how it works one by one.
Why Boiler Drum?
- It separate steam from steam-water mixture
- It stores the water quantity comes from economizer and mixing with water which is separated from steam
- Acts as a point for Chemical treatment of water in boiler
- Acts as point for blow down of water in order to reduce the impurity in boiler.
Steam/Water Flow in Drum
The feed water comes from the BFP (Boiler Feed Pump) through FRS (Feed Regulating Station) and Economizer to the Boiler Drum and from there the feed-water goes to the down-comers and then to Bottom Ring Header. This Bottom Ring Header is connected to the evaporating tubes which are exposed to the fire ball in the furnace and through these evaporating tubes the feed water again goes to drum through the Top Ring Header. The whole cycle can be seen from the diagram below.
This circulation takes place due to the density difference of water of down-comer and water-steam mixture in the evaporating tubes. In evaporating tubes water gains the Sensible Heat as well as Latent Heat and this water is partially converted into the steam.In some units, where the drum pressure is very high, there we use forced circulation as the density difference is not that much higher to assist the natural flow. So we used assisted circulation pumps known as BCP (Boiler Circulating Pumps) to assist in circulation of water through evaporating tubes.
The main motive of providing the Boiler Drum is to separate steam from the water-steam mixture which comes from the evaporating tubes through the riser tubes in the drum. Because we are not supposed to pass steam and water mixture into the Turbine as Dry/Super-heated steam drives the Turbine. So this separation takes place in 3 stages in the drum –
When the steam-water mixture comes in the boiler drum through riser tube, there is a zig-zag path or we call it Baffle Plates. In these baffle plates the steam-water mixture change its path continuously and Heavy water content fall down due to gravity and steam still follows this path and move forward.
After baffle plates steam-water mixture comes to the Cyclone Separator or we call it Turbo- separators. These turbo separators are placed along the length of the drum. They are placed in a single row or in a double row format in the drum. The steam water mixture enters the turbo-separators tangentially and in turbo separators, there are spinning blades or vanes present. When steam-water mixture passed through this blades heavy water particle due to centrifugal and gravity force falls down and steam moves upside.
After the mixture passes through Turbo Separators, a bulk part of the steam separates with water but there is a chance that some of moisture is still present in the steam and to separate this we use Dryers. This Dryer is in the form of mesh or corrugated plates or screen. Mostly we use screen type dryers. These dryers are present at the top of the drum, at the outlet of the steam path. This dryer separate moisture from the steam and insure saturated steam exits without any water particles in it.
Apart from this Separation Mechanism there are many connections attached to the boiler drum. The Main Connections to the boiler drum are:
- Feed water line – Through this line the feed water comes in the drum. There is a feed water header placed along the length of the drum at the bottom part in the drum. There is feed nozzles in the feed line through which water comes directly to the feed header.
- Down comers – These are big pipes connected at the bottom part of the drum and through this down-comer water comes down and circulates in the evaporating tubes. This down comer is outside of heating zone. The no. of down-comer varies from unit to unit according to their capacity. At the inlet of down-comer there is an Anti- vortex spyder. This anti-vortex spider restricts the formation of the vortex which causes the hammering in the down-comer.
- Steam Outlet Line – Through this line saturated steam exit from the drum and goes to the superheater tubes.
- Riser Tubes – After evaporating tubes water comes to the top ring header which is near to the drum. Riser tubes are connected between the drum and top ring header. Through this riser tubes steam-water mixture enter in the drum.
Apart from these main connections there are some Auxiliary Connections to the drum-
- Blow Down Lines – In Re-circulation type of boilers due to continuous circulation of water, the salt concentration increase in boiler drum and to reduce this salt concentration and remove impurity we give blow downs. In the blow down process, we drain some amount of water from the bottom part of the drum and there are 2 types of blow down
CBD (Continious Blow Down)
EBD (Emergency Blow Down)
- CBD (Continuous Blow Down) – If the silica concentration increases in the boiler water then we give the continuous blow down. If this silica concentration is more in the boiler water then at high pressure there may be a chance of silica carryover from the boiler drum to the turbine blades and it is difficult to remove silica deposition in the turbine blade. To restrict this we give CBD. This blow down line is directly connected to the CBD tank. Where the flashing of hot water takes place at low pressure. The steam generate after flashing goes to deaerator for heating purpose. After this CBD tank the remaining portion of water goes to the IBD (Intermittent blow down) In this tank again flashing of water takes place. The steam from IBD tank directly goes to the atmosphere and the water portion is also drain out or again use for equipment cooling purpose.
- EBD (Emergency Blow Down) – During operation of the boiler if the level of drum goes higher then there is also provision to drain the higher level of water through the EBD (emergency blow down) line. This EBD line is connected to the higher level water side of drum. If the level goes high then it drains the water till its not reaching normal level. This is done to restrict the flow of water into the Turbine.
- Chemical Dozing Line – In order to maintain the boiler water chemistry we dose chemicals in the drum water. Generally we dose Tri- Sodium Phosphate. This chemical dozed maintains the pH, Silica carryover and also restricts the scale formation in the drum, But the chemical quantity dozing in the drum should be optimum. If we doze excess in quantity then there is chance of casting embrittlement in the drum and if we doze less in the quantity then the dozing will not be proper and there is chance of corrosion in the drum and also at low pH there is more chance of silica carryover to the Turbine. In drum, this tri sodium phosphate reacts with the scale and forms the sludge, which is non-reacting in nature and does not affect the water quality or drum material.
3Cacl2 + 2 Na3po4 —— Ca3(PO4)2 + 6 Nacl
**Also this tri sodium phosphate increases the pH value of the water and at higher pH there is less chance of silica carryover.
Na3PO4 +H2o ——- Naoh + Na2HPO4
- Safety valve – the drum is operating at a very high pressure. Because of some operating reason there may be chance of increase of the drum pressure with time. So to save drum from explosion there is provision of safety valve. When the drum pressure is reach at the set pressure of safety valve then this safety valve operate automatically and release the excess pressure to the atmosphere. The set pressure of this safety valve may vary unit to unit but it is usually 10% of drum operating pressure.
- Air vent – air vent is mounted at the top of the drum. At the time of starting of unit we fill the drum with water and the air is evacuated through this air vent.
- Boiler Filling Line and Nitrogen Filling Line – to initially fill the boiler with water we use this boiler filling line. Also there are 2 types of preservation of boiler drum at the time of shut down which is dry and wet preservation. In wet preservation we totally fill the drum with water and pressurize this by boiler initial filling line. In some plant we use dry preservation, for this there is provision of nitrogen filling line to the drum. Nitrogen is inert in nature and not reacts with the drum material. By this 2 method we pressurize the boiler at the time of shut down.
- Other Instruments – Apart from this main and aux. connection there are some equipment placed on the drum which are –
- Hydrastep – it shows the drum level and operates on the principle by measuring the conductivity of water and steam.
- Gauge glass / local drum level indicator– To check the drum level locally.
- Pressure and Temperature Gauge – To check the temperature and pressure of the drum.