POWER PLANT BOILER STEAM DRUM INTERNALS

The boiler steam drum is an essential part of any power plant.it is also called the “heart of boiler “Because of its importance in steam generation. The boiler steam drum acts as a reservoir, providing the necessary head for water circulation through the evaporation zone. The boiler steam drum has its feed water storage capacity, which changes according to the design of the plant’s requirements.

Steam drum supplied the water to the boiler after that unit suddenly stopped or tripped off the plant. Steam drum steam is purified with the help of steam drum internal like baffle separators, cyclone separators, chevron separators, and demister pad/screen driers. Feed water distribution and chemical dosing arrangements are made inside the steam drum. This article discusses how power plant steam drums work internally and their installation’s importance. Drum internals include steam separation and purification equipment and other secondary equipment such as the following:-

1. DRUM LEVEL GAUGE

2. CONTINUOUS BLOW DOWN PIPE

3. CHEMICAL DOSING PIPE

4. SAFETY VALVE

5. SATURATED STEAM OUTLET PIPES

6. PURIFIER / STEAM DRIERS / DEMISTER

7. CYCLONE BOX CHAMBER

8. INTERMITTENT BLOW DOWN PIPE

9. DOWN COMER

STEAM DRUM WORKING PRINCIPLE :

A steam drum is a crucial component in steam boilers. It’s a large, cylindrical vessel that separates steam from water in the boiler’s steam-water mixture.  

Here’s how it works:

Water-Steam Mixture: The heated water in the boiler turns into a mixture of steam and water, which enters the steam drum through riser tubes.  

Separation: Inside the drum, special internal components called demisters separate the steam from the water. These demisters can be of various types, such as centrifugal, gravity, or cyclone separators.  

Dry Steam: The separated dry steam is collected at the top of the drum and then piped to its intended use, such as driving a turbine or providing process heat.  

Water Return: The remaining water, saturated with dissolved solids, is recirculated back to the boiler through downcomer pipes.  

Critical Functions of a Steam Drum:

Phase Separation: The primary function is to separate steam from water.  

Storage: It acts as a reservoir for steam, ensuring a steady supply.  

Water Conditioning: It can add chemicals to treat boiler water.  

Blowdown: It facilitates the removal of impurities from the boiler water.  

The steam drum is like a steam spa, separating the steam from the water to provide a clean, dry energy source.

1. DRUM LEVEL GAUGE

This is a glass tube, or a pair of flat glass, fitted to the boiler drum to visually indicate the water level. Through this device, the boiler operator can determine the water level and the general condition of the boiler. This glass is manufactured from highly chemical—and corrosion-resistant low-expansion borosilicate glass, well-known for its clarity and mechanical strength. Gauge glass should be regularly checked for any signs of clouding, scratching, erosion, or corrosion.

In power plant boilers, four types of gauge glass are used to show the exact water level in the steam drum. These gauge glasses are the following:-

a. Flat Glass Type

b. Ported Type

c. Tubular Type

d. Reflex Type

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

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

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

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

2. CONTINUOUS BLOW DOWN PIPE

The pipe header is uniformly distributed throughout the steam drum length in the boiler steam drum CBD (CONTINUOUS BLOW DOWN). A continuous blowdown header is placed just below the working water level of the drum. This header is connected to the drum blowdown pipe. Blowdown is given to maintain the concentration level of boiler water. When the concentration of TDS (total dissolved solids ) of boiler water is increased, there is a high chance of deposits of solids on the turbine blades, so blow down is given to maintain the concentration of these solids and protect the turbine blades from further deposition.

3. CHEMICAL DOSING PIPE

In the steam drum, the chemical dosing pump is connected to the dosing pump header inside the steam drum, which is evenly distributed throughout the steam drum length. This dosing pump pressure is always more significant than the steam drums. The TSP ( Tri sodium phosphate ) chemical maintains the feed water chemistry inside the boiler steam drum. The chemical dosing header distributed the chemical evenly throughout the boiler steam drum. Chemical dosing piping should be made of stainless steel because it can be choked due to chemical reactions. The hole diameter shall be 6 mm. Holes shall be pitched at 300 mm. The chemical added to the boiler drum should not get drained out through the blowdown pipe. It should be so located that the chemical mixes well with the freshwater before entering the downcomers. The chemical could be dosed in a feed water pipe before the drum. This will ensure a good mixing of the water & chemicals. It would also provide a perfect distribution.

4. SAFETY VALVE:-

The safety valve is also an essential part of the steam drum. When the steam pressure exceeds a safe working limit, the safety valve opens to realise the exceeding pressure and closes at the predetermined pressure limit. This should not lead to starvation of the Superheater. Hence, it is generally located outside the steam drier unit.

5. STEAM PURIFIER / STEAM DRIES / DEMISTERS

Steam is allowed to pass through closely spaced screens. Dry steam can pass easily through these screens, and the suspended water particles carried by steam are collected and fall back by gravity. Many boilers have several screens, called primary screens and secondary screens. This screen is called a demister’s pad.

6. SATURATED STEAM OUTLET PIPE

After the heaviest particles separate from the steam with the help of demister pads, I come out for the dryer. The steam then goes to the saturated steam outlet pipe. A baffle plate protects the steam before it enters the steam outlet pipeline.

7. CYCLONE BOX CHAMBER

It’s an important part of the steam drum and is vital in purifying it. In this chamber, moisture and steam are separated by centrifugal force. The steam and water mixture is passed through a cyclone separator chamber, which creates a spinning action on the steam. Due to centrifugal action, the heavier water particles pass away, collide with the cyclone wall, and fall.

8. INTERMITTENT BLOW DOWN PIPE

IBD (Intermittent blow down) its name indicates that it is an emergency drain valve through which the water level is brought down in case of high levels. The intermittent blow-off valve was strategically located in low-pressure boilers to remove settled sludge from phosphate reaction with calcium and magnesium salts. It also maintained the feed water chemistry to get the highest steam purity.

9. DOWN-COMER

The comer’s tube is connected to the bottom of the steam drum. These tubes are usually seen from the outside of the field. These tubes take the water from the boiler to the downcomer ring header of the boiler. These tubes take the water equally throughout the boiler length and do not cause fluctuations in the boiler drum level. They also maintain the boiler’s circulation ratio.

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joginder chauhan

Hi, all power engineers. I am Joginder Chauhan. I am the founder of the website www.askpowerplant.com. I am also a power plant engineer with over ten years of experience in the power sector. During this period, I learned about different power technologies like AFBC, CFBC, TRAVELLING GRATE, PULSATING GREAT, PF BOILER, WHRS, and many more. My motive for this site is to share my power sector knowledge with every person who belongs to this field; on this website, I share information related to boiler operation, boiler maintenance, turbine operation & maintenance, boiler & turbine question answers, D.M plant, and all topic related to power plant sector please join this website for latest updation regarding power plant ………..thanks ( JOGINDER CHAUHAN )

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