STEAM DRUM INTERNAL PARTS

STEAM DRUM

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

1. 1. Steam  is  purified  by  the  steam  drum  internals  which  may  have  the  baffle  separators,  cyclone 

1. separators, Chevron separators, demisters / Screen driers.  

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

1.  surfaces. 

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

PARTS OF STEAM DRUM

1. Feed water distributor pipe

2. Safety valve

3. Chemical dosing pipe

4. Continuous Blow down pipe

5. Drum level gauge

6. Cyclone box Chamber

7.  Steam Purifier/ demister/ steam Driers

8. Saturated Steam Outlet Pipes

9.Down comer

10 intermittent Blow down Pipe

Feed water distributor pipe

Should distribute the fresh water throughout entire drum length , so that the fresh water mixes well with boiler water. The feed distributor should not disturb the drum water level. It should not obstruct the water entry into the down comer. Otherwise it may lead to circulation failure of evaporator section. The  pipe  should  not  direct  the  cold  water  to  drum  surface.  It  may  lead  to  differential  metal 

temperature  and  may  strain  the  metal. The  feed  distributor  pipe  shall  be  flanged  for  easy  removal  and  flanges  shall  have  proper machining done on the mating faces. Gaskets are not to be used as they fail over a period.

Safety Valve

The safety valve is to release the excess pressure created in the drum than the set point of the valve. It should be so located that the steam when vented out should not disturb the steam drawl by the   saturated steam links to super heater. It should not lead to starvation of Super heater. Hence it is 

generally located outside the steam drier unit.

Chemical Dosing Pipe

In order to maintain desirable boiler water chemistry, suitable chemical is added directly into the drum 

water. The chemical is dosed through the chemical distributing pipe.

Concentrated chemicals are being dosed in the boiler water.  Hence the piping shall be of non 

corroding material. The piping shall be of stainless steel construction. The chemical should be well distributed throughout the drum length. Or else it can be connected to feed water pipe itself so that it helps for better mixing. The chemical distribution piping should not get choked due to chemical. The hole diameter shall be 6 mm. Holes shall be pitched at 300 mm.  

The chemical should not get drained off through the blow down pipe. It should be so located that  the chemical gets mixed well with the fresh water before entering in to the down comers.

Continuous blow down Pipe (CBD)

The boiler water is evaporated continuously and the relative pure steam leaves the boiler drum. This mechanism is leads to concentration of boiler water with more dissolved solids. Since the solubility of various  dissolved  solids  is  limited  based  on  the  operating  pressure,  it  is  necessary  to  limit  the concentration of dissolved solids in the boiler water. There is always a carryover of dissolved solids of boiler water in to the steam. This is called vaporous carry over. The percentage of carryover of each constituent is a  function  of  operating  pressure  of  the  boiler.  Hence it  is  necessary  to  limit  the concentration of boiler water with dissolved solids. Silica is a bad water constituent that leads to hard deposits on turbine blades. The CBD pipe should be able to draw the boiler water from the entire length steam drum. Any localized sampling will lead to a wrong sample of the boiler water. It should blow down the high TDS boiler water only. In should not lead to blow down of fresh water.

Drum Level Gauge

The exact level of the Steam Drum water level is very important factor in the Power plant. Because if the drum level is high, then the quantity of the steam production is less and it may affect the turbine blades due to low quality of steam.​ if drum level is low then there is a chance of starvation of the boiler tube and it may change the metallurgy of the tubes and there by the tube will be damaged, prematurely.

Hence it is necessary to get the correct feedback from the field regarding drum level.

For a Boiler operating up to a pressure of 28 Kg/Cm2, use of only one gauge glass is recomended.but for the power Boiler which is operating above 28 Kg/Cm2,an combination of the following is used.

•  Two gauge glass in continue service and visible by the operator.
• Two independent remote transmitters with one single gauge glass.
• One independent remote transmitter and one single gauge glass(continuously in service) both of the instrument must be continuously visible/Monitored by the operator from control room.

Types of Level Gauge

There are mainly four types of water level gauges 

1.     Tubular Type

2.     Reflex Type

3.     Flat Glass Type

4.     Ported Type

Tubular type water level Gauge glass limited to 17Kg/Cm2 operating pressure.

Reflex type can be used for the pressure up to 24 Kg/Cm2.

Flat glass type can be used for pressure up to 140 Kg/Cm2.

Ported Type gauge glass can be used up to 210 Kg/Cm2.

Cyclone Box Chamber

When  the  boiler  is  fitted  with  cyclone  separators,  all  the  water  steam  mixture  is  collected  in  a chamber. The mixture is made to pass through multiple cyclones. The cyclones are mounted through 

flange system to the cyclone box chamber.  The cyclones separate the water and return to the water 

space of the drum. The steam leaves through the top chevron unit. The total system should meet the 

following requirements.

Steam Purifier / Demister

Demister can be two stage Chevron driers / demister / screen driers.  Its purpose is to trap the droplets and lead them back to drum water

Downcomer.

The water from the entire length of the drum is allowed to pass through the down comer pipe and circulates through the water wall panel through natural or forced circulation.

Intermittent Blow Down Pipe

It is apt to name it as emergency drain valve through which the water level is brought down in case of 

high level.  In  low  pressure  boilers,  the  intermittent  blow  off  valve  was  strategically  located  for removal of settled sledges generated by phosphates reaction with calcium and magnesium salts