What is a boiler steam drum?
Watertube boilers use steam drums to ensure that the steam supplied to consumers is 'clean' (free of foreign bodies and moisture). Watertube boiler steam drums are located at the top of the boiler, above all other boiler parts. The steam drum is characterized by its long, cylindrical shape, and is usually made from thick steel plate to withstand the high pressures and temperatures at which it operates. A steam drum may be up to two meters in diameter and 30 meters in length.
Good to know - there are several ways of spelling 'watertube', 'water-tube', or 'water tube', but the meaning is the same. The same is true of 'firetube', 'fire-tube' and 'fire tube' e.g. firetube boiler. Water tubes contain water, whilst fire tubes contain exhaust gases (hence why they are also called 'smoke tube boilers').
Good to know - firetube boilers are low pressure boilers. Watertube boilers are used within power stations because they can supply a lot of steam, at high temperature and high pressure. Firetube boilers are limited in size due to their boiler shell design (see our firetube boiler article for further details).
Watertube Boiler Steam Drum Position
What are the main functions of a steam drum?
A steam drum has three main functions:
- It's primary purpose is to provide clean, dry steam, to downstream consumers, such as superheaters and steam turbines. The steam drum ensures that no moisture (water droplets) or wet steam reaches these downstream components, thus ensuring they are not damaged from problems such as carryover, water hammer, and excessive corrosion.
- Steam drums also act as a reservoir, holding a small amount of saturated water to address fluctuations in steam demand within the boiler system.
- Water treatment occurs within the steam drum. For example, the mixing of chemicals for internal boiler treatment occurs within the steam drum, as does blowdown (a process used to control water properties such as total suspended solids (TSS) and total dissolved solids (TDS)).
Watertube Boiler Schematic
What are the main parts of a watertube boiler steam drum?
A detailed description of a steam drum's main parts is given below. It is important to note the following definitions:
- Feedwater - water that has been treated, but has not yet entered the boiler.
- Boiler water - water that is within the boiler (including the steam drum).
- Condensate - steam that has condensed back into water. Condensate becomes feedwater again after it has been treated.
To learn about the water and steam flow paths around a watertube boiler, see our watertube boiler article. You can also learn more about different boiler types and all of their parts (steam header, furnace wall, etc.) in our Power Engineering Fundamentals Video Course.
Boiler Steam Drum Parts
- Feedwater Inlet: The feedwater pipe, which spans the entire length of the steam drum, evenly distributes feedwater within the drum. Feedwater is introduced into the drum through evenly spaced holes within the pipe, ensuring a uniform spread across the drum's length.
Good to know - 'feedwater' is also spelt 'feed water' or 'feed-water', but the meaning is the same.
- Downcomers and Risers: Downcomers distribute boiler water from the steam drum to the mud drums at the base of the furnace walls. As the water is heated, it rises within the furnace walls due to its density change (hotter fluids are less dense), transforming into a water-steam mixture as it does so. The water-steam mixture then returns to the steam drum via the steam headers.
Good to know - downcomers have a larger diameter than risers and are fewer in number. A riser pipe has a smaller diameter because this increases its heat transfer rate, whilst this is not a requirement for a downcomer pipe. Both risers and downcomers are usually manufactured from seamless carbon steel piping.
Good to know - risers are sometimes referred to as 'riser tubes', or 'riser pipes'. Similarly, downcomers are sometimes referred to as 'downcomer tubes' or 'downcomer pipes'.
- Saturated Steam Outlet: Once the steam is adequately cleaned and dried, it is discharged through steam discharge pipes located at the top of the steam drum. Discharged steam is heated by superheaters to increase its temperature before it is directed to its end consumer, such as a steam turbine. It is important to note that saturated steam is discharged from the steam drum, but it is converted to superheated steam once it has passed through the superheaters; this reduces the likelihood of water droplets forming in the steam. The steam output from a watertube boiler can be considerable, sometimes over 350 kg/s (771 lb/s), with the boiler system potentially operating at over 190 bar (2,755 psi), and over 500°C (932°F),
Good to know - superheaters add sensible heat to steam. Sensible heat increases the temperature of the steam, but does not change its state/phase i.e. no change of state from water to steam or vice versa. Latent heat is the energy added or removed during a change of state i.e. during the process of condensation or evaporation, but this energy change causes no temperature change.
Steam Drum Internals
- Chemical Dosing Line: The chemical dosing line introduces chemicals into the boiler. These chemicals, distributed evenly across the steam drum's length, reduce the likelihood of corrosion, scale build-up, and other potential issues within the boiler system. The turbulent environment inside the steam drum aids with the mixing of these chemicals.
- Continuous Blowdown Line (CBL): To regulate the concentration of chemicals within the boiler, the continuous blowdown line constantly removes a portion of the boiler water. This process is vital since water evaporation can lead to an accumulation of non-evaporative chemicals, thus increasing the number of total dissolved solids (TDS).
- Water Level Monitoring: The blue line indicated within the steam drum represents the normal operating waterline (NOWL or NWL), sometimes also called the 'normal water line'. Instrumentation holes in the steam drum accommodate water level indicators such as level sensors and gauge glasses, as well as pressure sensors, and temperature sensors. These instruments help ensure that the drum water level remains within defined limits. See our drum level control article for more information concerning how the water level within a watertube boiler is maintained.
Steam separators - water and steam are separated within the steam drum using the following methods:
- Centrifugal Separation: The water-steam mixture, upon entering the drum, passes through a centrifugal separator. This device uses the principle of centrifugal force to separate some of the water from the steam.
- Gravity and Density Separation: The denser water molecules naturally settle at the base of the steam drum due to gravitational forces.
Good to know - a density difference within a fluid is always created whenever there is a temperature difference, this is the basis for natural convection.
- Torturous Flow Path: The semi-clean steam then passes through a scrubber located at the top of the steam drum; this provides a torturous flow path. This path causes water molecules to collide with the scrubber's surfaces, where they coalesce into larger droplets and drop back into the drum. The scrubber, usually made of metal mesh screens pressed together, is sometimes referred to as a chevron drier or demister, and it ensures that over 99.5% of the water is separated from the steam.
Swelling and Shrinkage in Steam Drums
Sudden pressure fluctuations play a role in the behaviour of water and steam within a steam drum. There are two main problems that pressure fluctuations cause: swelling, and shrinkage.
- Swelling - there is sudden high steam demand, causing a pressure decrease within the steam drum, and this causes the water level to rise. The increase in water level occurs because steam vapour bubbles suspended in the water expand (increase in volume) as the drum pressure decreases, which causes the water level to rise. In a simple single-element control boiler, the automatic feedwater inlet valve closes in response to the increased water level, which effectively means that water fed into the boiler has decreased even though steam consumption has increased (a dangerous situation if not corrected).
- Shrinkage - conversely, when there is a sudden reduction in steam demand, the steam drum pressure increases, causing the suspended steam vapour bubbles to decrease in volume and the water level to decrease. The automatic feedwater inlet valve opens in response to the decreased water level, causing an even larger water level reduction. This occurs because the relatively cold water entering the steam drum cools the water and steam vapour bubbles, causing the vapour bubbles to collapse.
Fluctuations in pressure, and the subsequent expansion or contraction it causes, can affect the water level of a steam drum considerably. If not properly managed, these changes can lead to water carryover to the superheaters and steam turbines, risking damage and inefficiencies. If the water level decreases due to thermal contraction, the boiler heat transfer surfaces (boiler tubes etc.) may not receive sufficient water and might overheat, posing a risk to the boiler's integrity.
Steam Drum Three-Element Control
There are many factors that can affect the water level within a steam drum. The water level must be maintained within defined limits at all times to ensure the boiler's safe operation, efficient operation, and the protection of downstream components. To ensure accurate drum level readings, the boiler system employs a method known as three-element control. Three-element control measures:
- Steam flow output.
- Feedwater flow input.
- Steam drum pressure.
- Actual water level within the steam drum.
Three-Element Control
These factors help determine the mass balance of the boiler (what has been put into the boiler compared to what has come out). By considering all of these factors, the system can accurately determine the water level, accounting for potential discrepancies caused by pressure changes. You can learn more about single-element control, two-element control, and three-element control in our Power Engineering Fundamentals Video Course.
What is the difference between a steam drum and mud drum?
Steam drums are located at the top of the boiler, whereas mud drums (or mud drum) are located at the lowest point of the boiler. The name ‘mud drum’ derives from the dirty material (‘mud’) that accumulates within the drums.
Additional Resources
https://boilersinfo.com/boiler-steam-drum-internals-function/
https://www.rasmech.com/blog/steam-drum-101-boiler-basics/
https://www.processindustryinformer.com/optimising-boiler-and-steam-drum-level-control/
https://www.vega.com/en/company/blog/2020/steam-boiler-drum-level-measurement-technology-comparison