Boiler Steam Drum is the most important component of a boiler, this horizontally placed cylindrical shaped feature act as a phase separator for steam water mixture.
In ancient time Boilers are designed up to four drums placing them at the top as well as bottom and are interconnected with a network of tubes. Now a days mostly single and bi drums are used.
What is a Steam Drum in a Boiler?
Steam Drum or upper drum is an essential component of a water tube boiler which is mounted at the top portion of a boiler in the shape of a horizontal cylinder.
The function of Boiler Steam Drum is to collect the steam produced and also provide a space where steam can separate from boiler water. Steam is generated in the water tubes of a boiler and at the saturation point steam vapors starts to form bubbles and separates from the liquid.
Due to the difference in mass, steam vapor moves up through the tubes which are connected at the upper portion of the steam drum.
There are basically two stages of separation in a steam drum. In the primary stage only water is removed from the steam which is achieved due to the adequate space above the normal steam drum for the steam liquid detachment. In the second stage water droplets are removed from the steam
Purpose of Steam Drum in Boiler
Steam Drum is an essential part of a water tube boiler.
The main purposes of using a Boiler Steam Drum are:
Provide a storage space for steam water mixture.
Act as a place where steam produced in the boiler is separated from boiler water.
At the top portion of the Steam Drum a layer of vapor is accumulated and water layer remains below due to the difference in mass. Pipes are connected at the top portion of the drum and through these pipes steam is circulated for different applications like heating, power generation, sanitizing some area etc.
How Does a Boiler Steam Drum Work?
A Boiler Steam Drum is an essential part of a water tube boiler.
The steam generated inside the water tubes of the boiler due to the combustion of fuel are collected in a horizontal cylindrical shaped drum placed at the top position through riser tubes. The steam produced has lower density than the water and therefore will accumulate at the highest level in the vessel.
Boiler Steam Drum Material
Steam Drum is the pressure chamber at the extreme upper position of boiler circulatory system.
Boiler Steam Drums are manufactured with exclusive quality material and using modern technology to provide qualities like robust construction, high durability, excellent functioning, reliable etc. This upper drum is made of HCS( High Carbon Steel) to withstand temperature of 390 degree and pressure above 350 psi.
The thickness of carbon steel plates for Steam Drum are selected depending on the design pressure of the boiler. On the other hand volume of a drum is decided by the necessary volume required for a clean separation of water from steam to get dry saturated steam. The Drum also a store a sufficient portion of steam.
Boiler Steam Drum Parts
Boiler Steam Drum is the reservoir for steam and water mixture and provide necessary head for water circulation through evaporation zone.
The internal parts of a Boiler Steam Drum are:
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
10 intermittent Blow down Pipe
Boiler Steam Drum thickness calculation
The formula to calculate Boiler Drum thickness are mentioned within ASME Boiler and Pressure Vessel Code PG-27 for cylindrical components under internal pressure, page 10.
The formula used to calculate the minimum thickness as per mentioned in ASME, para PG-27.2.2, page no.-10 is written below:
The formula used to determine maximum allowable working pressure is mentioned below:
t= minimum design wall thickness in inch.
P=design pressure in psi
D=tube outside diameter in inch
e=thickness factor(0.04 for expanded tubes, 0 for strength welded tubes)
S= maximum allowable stress as per ASME section II
Boiler Steam Drum Level Measurement
Boiler Steam Drum acts as a important part of a power plant industry due its steam generation ability.
Drum water level is steady when operating conditions are steady and flows are balanced. Differential pressure transmitter is the instrument used to measure the drum level. Optimizing the steam drum and using them as safely and efficiently as possible requires a reliable water level measurement.
The steam drum creates a steady water level under steady operating conditions and for the balanced flows. Water level of a steam drum is a parameter which should be monitored and controlled properly.
Boiler Steam Drum Vent
While start up, saturated steam accumulate in the steam drum which raises the pressure, through the air vents air and dissolved gases are released. During boiler shut down these vent valves allow fresh air to enter the boiler drum to avoid its collapse under pressure.
Boiler Steam Drum Vents remove the vacuum during shut down due to condensing of steam in boiler drum.
Waste Heat Boiler Steam Drum
Heat Boilers mainly recycle thermal energy, heat is recovered from exhaust gases and used to generate steam as an output which reduces the overall operating costs
A waste heat boiler is attached with an overhead steam drum for water and steam circulation with a higher recirculation rate for effective cooling. Steam drums are equipped with two stage phase separation technique to get high purity steam. Internal cyclone separators and demister pads are used for steam water separation.
The steam drums are fabricated as per ASME or other codes so that they can satisfy the waste heat boilers requirements. Suitable instruments are provided with the drum to ensure safe and smooth operation of waste heat boiler.
Water Tube Boiler Steam Drum
Due to the difference in densities of steam and water, make it possible to accumulate saturated steam and water inside the steam drum. Steam Drum hold water( for 10 to 60 seconds) to feed the boiler preventing the loss of liquid in down comer and riser tubes.
The size of the Steam drum required for a water tube boiler is fixed by the required volume for the separation of steam and water to produce dry saturated steam and steam drum should also provide adequate space for steam storage.
Boiler Without Steam Drum
Even tough a Benson Boiler is a water tube boiler, it has a unique characteristic of the absence of a steam separating drum, in simple words it is a drum less boiler.
At supercritical pressure both steam and water have same densities which prevent bubble formation. Therefore, no need of a steam drum to separate air bubbles from water in Benson Boiler.
Due to the absence of steam drum weight of these type of boilers are at least 20% less than the other types of boilers.
Steam Drum of a water tube boiler is an essential component which provide a space for separation of steam and water. we should have a proper knowledge of different facts related to a steam drum to ensure efficient running of a water tube boiler.