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Steam Drum



                          Steam Drum                             

          Steam drum is most important part of boiler. Boiler Drum is a pressure vessel it is used to separate steam and water mixture.

Steam Drum

Principles of working:

                    A steam drum is a standard feature of the boiler water pipes. It is  a reservoir of water/steam at the upper end of the water wall. Drum keep the steam produced in the water tubes and act as a phase separator for mixed steam/water. The density difference between hot and cold water helps in the accumulation of water / saturated steam into the steam drum.

A steam drum is a standard feature of a water-tube boiler:

             The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture.

           The difference in densities between hot and cold water helps in the accumulation of the "hotter"-water/and saturated-steam into the steam-drum. The mixture of steam and water entering the steam drum through riser tubes, internal drum consisting of demister separates water droplets from steam to produce dry steam.
Saturated water vapor at the bottom of the drum flows down through the down comer pipe, usually heated, for the header and water drums.


Theory of steam separation :

          There is a considerable difference between the density of water and steam at low pressure but as the pressure increases towards critical point the density difference decreases.

Steam Separation



Different method of steam separation:

1.    Primary stage
2.    Secondary stage.

Primary steam-water separation:

           Primary steam-water separation removes nearly all the steam from the water so that very little steam is recirculated from the drum bottom towards the heated tubes through the outlet connection (downcomer).

Primary separation equipment generally takes one of the following three forms:

- gravity-driven separation: it is generally considered uneconomical and its use nowadays is very limited,
- baffle-assisted separation: simple screens and baffle arrangements are used for improving the steam-water separation process. Baffles provide among others changes in the direction, more even distribution of the steam-water mixture as well as additional flow resistance. Their use is mostly limited to smaller, low capacity boilers.
- Mechanical primary separators: they make use of centrifugal force or radial acceletation. They are nowadays in use almost worldwide for state of the art steam-water separators. Conical cyclone, horizontal cyclone, vertical cyclone separators are some of the technologies used.

                     Steam leaving the primary separators typically still contains too much liquid in the form of droplets containing contaminants. Therefore, the secondary separation stage is also needed.

Secondary steam-water separation:

                At secondary separation stage, steam is passed through a secondary stage of separators or scrubber elements (usually closely spaced corrugated parallel plates) for final water droplet removal. Steam is then exhausted through various ports.

Drum Internals

Steam drum Internal parts:

 A steam drum of the boiler is meant for performing the following:

1.    Steam is purified by the steam drum internals which may have the baffle separators, cyclone separators, chevron separators, demisters / screen driers.

2.    Acts as reservoir and provides the necessary head for the circulation of water through evaporative surfaces.

3.    Supplies water for a brief period in the event of stoppage of water supply.

Internals

Parts Of Steam Drum:
  1. Feed Water line
  2. Safety line
  3. Chemical Dosing line
  4. Continuous blow down lin (CBD)
  5. Drum level gauge
  6. Cyclone box chamber
  7. Steam purifier
  8. saturated steam outlet pipes
  9. Down comer
  10. intermittent blowdown line



Safety Valve:

Safety valve

Chemical dosing line:

                    To reduce chances of scale forming salt by converting it to sludge and removal through low point drain. Tri Sodium Phosphate is used to react with scale forming salts like Calcium Chloride, Calcium Sulphate Etc. and convert them into sludge.

Continuous blow down line (CBD:

                    To keep concentration of impurities within specified limit it is necessary to drain a portion of water from the drum continuously and compensate the same with fresh make up water having lower impurities so that no scaling occurs in side tubes and to prevent silica carry over. Normally % of CBD will be maximum 1% of the steam generation


                    
     CBD % =      _ TDS in make up wter in PPM ____      = % of make up  
                           TDS allowed in boiler water in PPM


Blowdown


Drum level gauge:

          This is an important fitting, which indicates the water level inside the boiler steam drum to an observer. It is a safety device, upon which the correct working of the boiler depends. This fitting may be seen in front of the boiler. It is mostly employed in the steam boiler. It has a strong glass tube fitted to two hollow gun metal castings with the help of stuffing box. The lower end of this indicator communicates with water and the upper end with steam in the boiler.

 Boiler steam drum water level is one of the most important power plant parameters to both measure and control. Control of the proper water level in the boiler is critical for safe operation of the boiler. If the level is too low, boiler tubes will be damaged by overheating. If the level is too high, steam separators will not function properly, temperature control will be difficult, and the super heater tubes and turbine could be damaged by moisture or water treatment chemical carryover. In addition, poor level control will also adversely affect the drum pressure control. The sliding operating pressure of modern 3 drum Heat Recovery Steam Generators, along with frequent startup and shutdown, has added to the challenge of selecting the proper mix of instruments and maintaining correct water levels under all conditions.

Level Gauge


Saturated steam outlet pipes:

                     Saturated steam is steam in equilibrium with water (e.g., steam that holds all of the moisture it can hold and still remain a vapor). Saturated steam can be used to purge process equip-ment or perform other functions, or it can be superheated.As long as the steam and water are in contact with each other, the steam is in a saturated condition. Saturated steam cannot absorb additional water vapor, but the boiler can continue to add heat energy to it. Steam that continues to take on heat energy or get hotter is known as superheated steam.

Down comer:

                      Down comers are tubes that transfer water from the steam drum to the mud drum. As cooler water descends from the steam drum and flows through the down comers, it picks up heat from the firebox and replenishes the water supply to the mud drum.










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Steam Drum Steam Drum Reviewed by Thermal Power Tech on September 11, 2019 Rating: 5

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