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Questions and Answers
What is the main purpose of the feed forward signal in a boiler level control system?
What is the main purpose of the feed forward signal in a boiler level control system?
In a cascade control system, which controller acts as the primary controller?
In a cascade control system, which controller acts as the primary controller?
Which of the following is a component of a 3-element control strategy in a boiler system?
Which of the following is a component of a 3-element control strategy in a boiler system?
What does the purge interlock prevent in a boiler system?
What does the purge interlock prevent in a boiler system?
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What action does the loss of flame interlock trigger in a boiler system?
What action does the loss of flame interlock trigger in a boiler system?
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What is the primary function of a feedwater pump in a water tube boiler?
What is the primary function of a feedwater pump in a water tube boiler?
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Which component of a water tube boiler is responsible for increasing the temperature of steam beyond its saturation point?
Which component of a water tube boiler is responsible for increasing the temperature of steam beyond its saturation point?
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What role does an economizer play in boiler operation?
What role does an economizer play in boiler operation?
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Which structure in a water tube boiler serves as a collection vessel for both steam and water?
Which structure in a water tube boiler serves as a collection vessel for both steam and water?
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How does a water tube boiler primarily operate to prevent overheating?
How does a water tube boiler primarily operate to prevent overheating?
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What function does the air heater serve in a water tube boiler system?
What function does the air heater serve in a water tube boiler system?
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In terms of pressure control, how does a water tube boiler manage high pressure levels?
In terms of pressure control, how does a water tube boiler manage high pressure levels?
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What is the primary purpose of a mud drum in a water tube boiler system?
What is the primary purpose of a mud drum in a water tube boiler system?
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What role does the feedforward trim play in the firing rate control system?
What role does the feedforward trim play in the firing rate control system?
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What does the 3-element control strategy specifically measure?
What does the 3-element control strategy specifically measure?
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In the pressure control system described, how is the firing rate demand signal generated?
In the pressure control system described, how is the firing rate demand signal generated?
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What issue can arise from a low boiler drum liquid level alongside falling supply pressure?
What issue can arise from a low boiler drum liquid level alongside falling supply pressure?
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Which variable is NOT typically measured in a 3-element control strategy?
Which variable is NOT typically measured in a 3-element control strategy?
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What is the primary function of cascade control in this context?
What is the primary function of cascade control in this context?
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How does the pressure controller PIC-101 respond to changes in steam pressure?
How does the pressure controller PIC-101 respond to changes in steam pressure?
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What would likely happen if the flow controller fails to sense variations in supply conditions?
What would likely happen if the flow controller fails to sense variations in supply conditions?
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Study Notes
Boiler Level Control Systems
- Boiler level control systems often use a cascade control strategy with a level controller as the primary controller and a flow controller as the secondary controller.
- The level controller adjusts the set point of the flow controller to maintain the desired boiler drum liquid level.
- Feedforward control is frequently incorporated to compensate for changes in steam demand.
- The steam flow can be used as a feedforward signal to adjust the set point of the flow controller, or directly manipulate the boiler feedwater control valve.
- Most boiler level control systems add the feedforward signal to the level controller output, affecting the set point of the secondary (feedwater flow) controller.
Boiler Safety Interlocks
- Purge interlock: Prevents fuel from being admitted to an unfired furnace until it's thoroughly purged.
- Low air flow interlock: Shuts off fuel upon loss of air flow or combustion air fan operation.
- Low fuel supply interlock: Shuts off fuel supply upon loss of fuel to prevent unstable flame conditions.
- Loss of flame interlock: Shuts off all fuel upon loss of flame.
- Fan interlock: Stops the forced draft fan upon loss of the induced draft fan.
Boiler Steam Temperature Control
- Steam temperature is measured and compared to a set point.
- A temperature control system uses a temperature control relay (TRC) to generate an actuating signal for the desuperheater water flow valve to adjust steam temperature.
Boiler Pressure Control
- Boiler pressure control systems use a feedforward model to estimate the true load on a boiler.
- The pressure controller adjusts the ratio of firing rate to estimated load.
- A feedforward trim based on steam flow is added to speed up the response to load changes.
- Steam flow anticipates changes in steam demand, allowing for faster adjustments to firing rate without waiting for pressure fluctuations.
3-Element Control in Boiler Water Feed
- 3-element control uses three process variables (PVs) to control the boiler feedwater control valve:
- Liquid level in the boiler drum
- Flow of feedwater to the boiler drum
- Flow of steam leaving the boiler drum
- The level controller determines the desired feedwater flow, which it transmits as a set point to the flow controller.
- The flow controller adjusts the feedwater valve to meet the set point, compensating for supply pressure variations.
Boiler Types
- There are two main categories of boilers:
- Fire-tube boilers: Hot gas inside tubes heats the surrounding water.
- Water-tube boilers: Water inside tubes is heated by the surrounding hot gas.
Boiler Applications
- Boilers are widely used in:
- Thermal power plants
- Industrial processes
- Heating
- Sanitation
- Sterilizing equipment
Water-Tube Boiler Design
- Water-tube boilers consist of two drums:
- Steam drum: Collects steam and water, separating them.
- Water/mud drum: Collects water from the downcomers.
- Downcomers: Carry cold water from the steam drum to the mud drum.
- Risers: Carry steam from the mud drum to the steam drum.
- Superheaters: Increase steam temperature beyond its saturation point.
- Economizers: Recover heat from flue gas to preheat feedwater.
- Air heaters: Preheat combustion air using waste heat from flue gas.
Boiler Feedwater
- Feedwater is supplied by a feedwater pump and valve.
- Water is sourced from a demineralization (dm) plant to prevent scaling.
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Description
This quiz covers boiler level control systems and safety interlocks, focusing on cascade control strategies using level and flow controllers. It also explores feedforward control mechanisms and essential safety features like purge and air flow interlocks in boiler operations.
