Watertube Boiler Parts Explained
A watertube boiler converts water into steam by heating it using the energy released from burning fuels; the exception to this is a heat recovery steam generator (HRSG) which converts water into steam using heat recovered from exhaust gases. Watertube boilers are used for many industrial applications, but this article is focused upon watertube boilers used within power stations.
Good to know – to aid understanding and familiarisation, this article uses the various forms of water-tube, watertube, and water tube, throughout; all expressions mean the same thing.
Good to know – please see our main Watertube Boilers Explained article if you require a general overview concerning how watertube boilers work, their designs, and operation.
Power Station Watertube Boiler
Water Tube Boiler Design
A large power plant boiler uses thousands of parts in order to produce steam efficiently and reliably. The main parts of a water tube boiler are given in this article, but there are many more! Use the below diagram as a reference as you work through this article.
Water Tube Boiler Parts and Locations
Water Tube Boiler Components and Locations
Steam Drum
A steam drum is a cylindrical vessel that holds water and distributes steam; it is located at the top of the boiler. The steam drum collects the steam generated by the riser tubes, which surround the furnace. Steam drums contain internal components like cyclone separators and scrubbers to ensure that the discharged steam is dry and free from water droplets, thus avoiding moisture carryover to the superheaters and steam turbine(s). Chemicals are dosed into the steam drum because the environment within the drum is turbulent, thus aiding mixing of the chemicals into the system.
Watertube Boiler Steam Drum
Mud Drum
Mud drums are positioned at the bottom of a watertube boiler; they collect sediment and impurities from the boiler water system. In some boiler designs, especially more modern or compact designs, a dedicated mud drum might not be present. Instead, water headers installed at the bottom of the boiler can perform the same function (these headers serve as collection points for impurities, effectively acting as mud drums).
Water Tube Boiler Steam and Mud Drum Locations
Water Tubes
Water tubes contain boiler water before it changes state/phase to steam. The name of a water tube changes depending upon its location within the boiler, for example:
- Downcomer tubes – carry cool water from the steam drum towards the lower part of the boiler.
- Riser tubes – carry heated water and steam from the lower part of the boiler to the steam drum.
Headers
Headers are large cylinders with multiple connections. Headers collect the working fluid for a group of tubes and are typically labelled as either upper headers or lower headers. If a header is installed in a high position within the boiler, it collects the water/steam mixture from its associated riser tubes, then channels it to the steam drum through multiple pipes; it is known as a steam header or upper header. If a header is installed in a low position in the boiler, it distributes water to its connecting pipes; it is known as a water header or lower header.
- Steam header – collects steam from the riser tubes and discharges it to the steam drum.
- Water header – distributes water to the base of the riser tubes.
Burner
The main inputs of a boiler are:
- Fuel – serves as the source of heat energy.
- Air – provides the oxygen necessary for combustion.
- Water – required to generate steam.
Coal-Fired Boiler Inputs
The burner is responsible for mixing fuel and air then igniting the mixture to create the flame that heats the water tubes; a burner ensures efficient combustion and a stable flame. There can be multiple burners in a single boiler and these can be arranged in various configurations:
- Front Wall – burners are arranged on the front wall of the boiler.
- Opposing Walls – burners are placed on opposing walls, facing each other.
- Corners – burners are positioned in the corners of the furnace.
Tangential burners and wall-fired burners are the two most common types of burners utilized in water-tube boilers.
Furnace
The core of a watertube boiler is its furnace, where fuel combustion occurs, generating heat. Surrounding the furnace are tubes full of water, which form the ‘water walls’ of the boiler. Boiler tubes absorb the heat generated by combustion, converting the water inside the tubes to steam; this steam can then be used to drive steam turbines or provide heat for various industrial processes.
Water Tube Boiler Internals
Furnaces are lined with refractory material to retain the heat generated and protect the boiler structure.
Good to know – the term ‘refractory’ is often confused with ‘insulation’, which is not strictly correct. Refractory is used for high-temperature applications, may be directly exposed to a flame, and is usually manufactured from ceramics or a brick-like material. Insulation is used for lower temperature applications, is not directly exposed to a flame, and is usually manufactured from fiberglass or mineral wool.
Good to know – water walls are formed by an assembly of riser pipes that are aligned to form ‘walls’.
Boiler Water Wall Structure
Economizer
The economizer preheats boiler feedwater before it enters the steam drum. Due to its location within the boiler, an economizer recovers residual heat from the exhaust gases, thus improving boiler efficiency.
Superheater
Superheaters are designed to raise the steam temperature above its saturation temperature. Superheaters heat the steam discharged from the steam drum, thus increasing its temperature and consequently the amount of heat energy the steam contains. There are usually several superheaters within a single watertube boiler:
- Primary superheater – the first stage of superheating, which increases the steam’s temperature and reduces the liquid water content of the steam.
- Secondary superheater – further heats the steam to achieve the desired temperature.
Heat transfer within the boiler occurs through three primary methods:
- Radiation – transfer of heat via electromagnetic waves; this is a significant mode of heat transfer within the furnace because radiant energy from combustion heats the water tubes. Superheaters that absorb heat via radiation are known as radiant superheaters.
- Convection – occurs within a system via bulk movement of liquid or gas molecules. In boilers, natural circulation of water through a boiler is an example of convection. Evaporator tubes (where water evaporates to steam) are known as convective tube bundles. Superheaters that absorb heat via convection are known as convection superheaters.
- Conduction – heat transfer through a material or materials. Heat passing to a boiler’s metal tubes, then to the water inside the tubes, is an example of conduction.
Heat Transfer Within a Watertube Boiler
Superheaters can be classified based on how heat is transferred into them i.e. via radiation, convection, or conduction. The type of heat transfer depends upon the superheater’s location within the boiler. Radiated heat can only be transferred by line-of-sight, from the heat source to the heat absorber, thus radiant superheaters must have line-of-sight to the boiler flame, otherwise they cannot be radiant superheaters. Superheaters can be classified as:
- Radiant Superheaters – directly exposed to the furnace flame by line-of-sight; they absorb most of their heat through radiation.
- Convection Superheaters – located away from the furnace flame with no line-of-sight; they absorb heat via the hot combustion gases that flow around them.
Reheaters
Reheaters are used in multi–stage turbine systems to reheat steam after it has passed through a high–pressure turbine. Reheaters are similar in design and appearance to superheaters. Where there are multiple reheaters, they are installed in series. Steam entering the reheat system is called cold reheat steam and steam exiting the reheater is called hot reheat steam.
- Primary reheater – reheats steam for intermediate–pressure turbine stages.
- Secondary reheater – provides additional reheating for intermediate–pressure turbine stages.
Watertube Boiler Reheaters
Air Preheater
An air preheater is a heat exchanger that preheats primary and secondary air using the boiler’s exhaust gases; this enhances combustion efficiency and reduces fuel consumption.
Feedwater Pump
Feedwater pumps supply feedwater from the deaerator to the boiler. Feedwater pumps maintain the water level in the steam drum and ensure a continuous supply of water for steam generation. There will always be more than one feedwater pump due to their criticality (failure of the feedwater pumps can lead to severe damage to the boiler and surrounding area), and they will be installed at a lower elevation than the deaerator to reduce the risk of cavitation. Boiler feedwater pumps are usually driven electrically via an induction motor, or, using steam via a steam turbine. Water circulation within a boiler serves two primary purposes:
- Supplying Water – ensuring a constant supply of water that can be turned into steam.
- Heat Distribution – preventing overheating of the boiler tubes by distributing the heat evenly.
Feedwater pumps are usually rated as 50% or 100%, depending upon how much of the full boiler load they can cater for. For example, a 100% feedwater pump can cater for the entire water demand of the boiler when it is operating at maximum load. Water for attemperators (desuperheaters) is fed from the feedwater pumps. A steam turbine’s gland seals are also cooled using boiler feedwater.
Boiler feedwater pumps usually have a variable frequency drive (VFD) which allows their electrical frequency to be regulated, and consequently their speed and discharge volume. A minimum flow rate from a boiler feedwater pump is catered for by using an auto recirculation valve (ARV).
Boiler Feedwater Pump (Multistage Centrifugal Pump)
Safety Relief Valves (SRVs)
Safety relief valves (SRVs) prevent overpressure in a boiler. Safety relief valves are designed to automatically release steam when a boiler’s steam pressure exceeds a preset limit, thus ensuring the boiler is not over-pressurised. A boiler’s safety relief valves are classified as ‘non-assisted’ or ‘assisted’. A non-assisted valve will open once a certain system pressure is reached; it is strictly a mechanically operated valve usually relying upon one or multiple springs. An assisted valve will only open if a certain system pressure is reached and an additional external force is applied to ‘assist’ the valve to open. For example:
- Non-assisted – system pressure acting against the safety valve disc causes the valve’s spring to be compressed and the valve to open.
- Assisted – system pressure acting against the safety valve disc causes the valve’s spring to be compressed, but this does not cause the valve to open until an external force is applied. The external force applied is usually via a hydraulic or pneumatic system, often using a ram, cylinder, or diaphragm.
SRVs are typically installed on the steam drum and superheater headers. There are often two SRVs installed in each position, thus giving 100% redundancy should one valve fail to operate as intended.
Good to know – excessive valve vibration is caused when a valve opens and closes rapidly; this phenomenon is known as chattering. Assisted type valves avoid this problem by moving the valve directly from the fully closed to fully open position.
Spring Loaded Safety Valve (non-assisted)
Water Level Indicators
A water level indicator shows the water level inside a boiler steam drum. Water level indicators may be installed locally and viewed locally, such as with a boiler gauge glass, or, they may be installed locally and viewed remotely, such as with pressure level transmitters that measure the water level locally and relay it in the form of a 4-20 mA electrical signal to a remote location e.g. a control room. The water level within a boiler can be measured directly or indirectly, depending upon which instrumentation is used. Sometimes a light will be installed behind the boiler sight glass and a surveillance camera aimed at the sight glass; this solution allows operators to remotely monitor the gauge level making it a cost-effective alternative to electronic instrumentation.
IMPORTANT: Maintaining the correct water level within a boiler is the most critical task when operating any boiler!
Mounted Boiler Level Gauge Glass (Sight Glass)
Blowdown Valve
Regular blowdown helps maintain water quality and prevent scale buildup. A bottom blowdown valve is used to remove sediment and impurities from the mud drum; this type of blowdown is intermittent (not constant).
Firetube Boiler Bottom Blowdown Valve
A surface blowdown valve is used to remove impurities from the surface of the water within the steam drum; this type of blowdown is continuous.
Firetube Boiler Surface Blowdown Valve
Control Systems
Modern water tube boilers are equipped with advanced control systems that simultaneously regulate the fuel supply, air flow, water level, and steam pressure. Large watertube boilers utilise three element control for drum level control, but smaller boilers do not require such advanced systems (e.g. two element control or similar is adequate). A boiler control system performs the following tasks:
- Monitors and controls all critical parameters, including drum level, steam temperatures, air/fuel ratio, based on process values received from field-mounted instruments.
- Sounds alarms for process deviations.
- Initiates a trip in case of a critical process deviation, such as low/high drum water level, high steam temperature, or high steam pressure.
- Monitors boiler auxiliaries for problems and performs auto changeovers, when necessary. For example, bringing a standby pump online in case of a main feedwater pump trip.
Boiler Drum Three Element Control
Good to know – three element control includes a fourth element, but it is often not shown or considered when referring to three element control. The fourth element is the pressure measurement of the steam drum. It is essential to know the steam drum pressure because it is responsible for phenomena such as swelling and shrinking, which causes the recorded level in the steam drum to fluctuate considerably; this makes the recorded level unreliable unless the steam drum pressure is accounted for.
Control Valves
Control valves regulate the flow of boiler feedwater to the steam drum. For large water tube boilers, there will often be redundant feedwater valves. During routine operation, feedwater control valves actuate automatically to keep the steam drum water level near the desired setpoint.
Instrumentation
Gauges, indicators, transmitters, and sensors are instruments used for monitoring of a boiler’s different process variables (pressure, temperature, flow, etc.). Power station water-tube boilers use many different types of instruments for local and remote viewing purposes. For example:
- A pressure gauge is used for local pressure indication.
- A temperature gauge is used for local temperature indication.
- A flow transmitter can be used for local and remote flow indication.
- A glass level gauge is used for local level indication.
- A level transmitter relays a measured level in real-time to the control system for remote monitoring.
Steam Drum Differential Pressure (DP) Level Transmitter
Good to know – if a sensor is deemed critical to the safe operation of a boiler, it will be installed multiple times even though it is measuring the same variable! For example, the water level within the steam drum of a boiler is very important and must be constantly monitored, thus it is common practice to install three independent level transmitters on each steam drum, and these all measure the same water level. If one sensor fails, it is assumed that the other two are still reliable, and the boiler can remain in service whilst the failed sensor is repaired; this setup is called a two-out-of-three (2oo3) voting logic and is employed throughout the engineering world for safety critical systems. For example, to initiate a boiler trip on a high drum water level condition, two out of three level transmitters must be on or above the trip setpoint for a predefined period of time (usually a few seconds).
Good to know – redundancy is one method of ensuring that all safety critical systems are effectively monitored, but there is also the ‘variety’ method. The variety method relies upon the variety of sensors used for a given measurement, rather than their quantity. For example, a boiler’s steam drum level may be measured using conductivity sensors, floats, or differential pressure transducers; installing all three gives results from a variety of sensors, which is considered more reliable than simply three measurements from the same sensor type.
Exhaust Gas Stack
The exhaust gas stack expels the boiler’s exhaust gases to atmosphere. For some boilers, exhaust gases can be expelled without being treated, but this is not true for fossil-fired boilers, such as those burning oil, coal, or bio-fuel; waste-fired boilers also require exhaust gas cleaning. Standard pollution control equipment includes electrostatic precipitators, flue gas desulphurisers, and/or bag filters.
Electrostatic Precipitator
Soot Blower
Soot blowers use compressed air or steam to clean soot from the boiler tubes, radiant furnace surfaces, economizers, and air heaters. Cleaning the boiler internals of soot ensures that heat transfer efficiency is maintained (soot on the tubes acts as a thermal insulator and prevents heat transfer, thus leading to a reduction in boiler efficiency). Effective heat transfer may be inhibited by several issues, the most common being:
- Scale Formation – accumulation of solidified minerals on the interior surfaces of the boiler, which insulates them and reduces heat transfer efficiency.
- Soot Buildup – occurs in fossil-fired boilers, where incomplete combustion leads to soot accumulation on the boiler’s heat transfer surfaces, thus leading to a reduction in heat transfer.
Attemperator
Attemperators control steam temperature by injecting high-pressure boiler feedwater into the steam flow, consequently preventing overheating that could damage downstream components. Attemperators can be installed:
- At the boiler’s main steam discharge line (after the final superheater).
- Between superheaters, for example, between the second and third superheater.
- In a primary and secondary configuration, where the primary is installed between superheaters and the secondary is installed at the main steam discharge line.
Usually, attemperators operate automatically to maintain steam temperatures at or near a given setpoint. Attemperators are critical to the safe operation of a boiler and its steam consumers. A malfunctioning attemperator can result in flooded steam lines, leading to water hammer in steam piping and damage to any downstream steam turbines due to moisture carryover. To prevent such problems occurring, the steam’s temperature is carefully monitored directly after the attemperators.
Good to know – attemperators are also known as ‘desuperheaters’.
Steam Piping
Steam piping transports steam from the boiler to the point of use, such as turbines or process equipment. Steam piping will be insulated to reduce heat losses and may be supported by piping hangers or piping supports (sliding feet supports etc.).
Variable Piping Hanger