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Questions and Answers

EASY What is the primary function of a low water fuel cut-off (LWCO)?

To monitor the boiler's pressure

To regulate the boiler's temperature

To optimize fuel efficiency

To shut off the boiler burner system if the water level drops to an unsafe level (correct)

Which type of boilers does not require a low water fuel cut-off?

High-pressure steam boilers

Cast iron sectional heating boilers

Natural gas-fired boilers

Electrode-type electric boilers (correct)

What is the lowest water level at which a boiler can be safely operated?

Maximum operating level

Lowest permissible water level (LPWL) (correct)

Minimum operating level

Minimum safe level

What is the purpose of the float chamber in the LWCO device?

To monitor the water level in the boiler Signup and view all the answers

What happens when the water level in the boiler drops below the minimum safe level?

The electrical switch opens Signup and view all the answers

What is the function of the diaphragm in the LWCO device?

To act as a seal between the water chamber and the switch Signup and view all the answers

Which type of boilers commonly uses the LWCO device shown in Figure 1?

Low capacity cast iron sectional heating boilers Signup and view all the answers

What is the purpose of the safety shut-off valve (SSOV)?

To shut off the fuel supply to the burner if the water level drops too low Signup and view all the answers

What is the highest normal operating level in the boiler?

Level A Signup and view all the answers

What happens when the water level drops to level C?

The burner circuit opens and the alarm circuit energizes Signup and view all the answers

What is the purpose of the magnetic low water fuel cut-off?

To shut off the fuel supply when the water level is low Signup and view all the answers

What type of low water fuel cut-off has no moving parts?

Electric probe type Signup and view all the answers

Why is it necessary to check and clean the electric probes?

To prevent scale or sediment build-up Signup and view all the answers

What type of boiler can experience low water conditions due to human error or mechanical failure?

Hot water heating boiler Signup and view all the answers

What is the purpose of the float in the magnetic low water fuel cut-off?

To lower the plunger Signup and view all the answers

What happens when the water level drops below the upper probe in the electric probe type low water fuel cut-off?

The power supply to the fuel valve is interrupted Signup and view all the answers

What is the advantage of the electric probe type low water fuel cut-off?

It contains no moving parts Signup and view all the answers

What is the purpose of the low water fuel cut-off device in hot water heating boilers?

To shut off the fuel supply when the water level is low Signup and view all the answers

What is the function of the bellows assembly in the low water fuel cut-off system?

To form a flexible steam and watertight seal between the float housing and the electrical junction box Signup and view all the answers

What happens to the electrical circuit when the boiler water level drops below the LPWL?

The circuit is opened Signup and view all the answers

What is the purpose of the mercury switch in the low water fuel cut-off system?

To open or close the electrical contacts inside the bulb Signup and view all the answers

What happens when the boiler water level drops and the float closes the two-wire switch?

The feedwater pump is started Signup and view all the answers

What is the function of the float in the float chamber?

To follow the fluctuations in water level Signup and view all the answers

What happens when the boiler water level continues to drop and the three-wire switch acts as a boiler emergency shutdown?

The electrical circuit to the SSOV is opened and the burner is shut off Signup and view all the answers

What is the purpose of the combination low water cutoff and feeder valve?

To safeguard against overheating of heat transfer surfaces Signup and view all the answers

What is the function of the float in the float controlled make-up water valve?

To move to a lower position when the boiler drum level drops Signup and view all the answers

What happens when the boiler water level drops to the point where heat transfer surfaces could overheat?

The combination low water cutoff and feeder valve offers the best protection Signup and view all the answers

What is the purpose of the float chamber in the low water fuel cut-off system?

To follow the fluctuations in water level Signup and view all the answers

What may cause a rapidly falling water level in the boiler?

A faulty feedwater level controller Signup and view all the answers

What is not a cause of a gradual reduction in boiler water level?

Normal operation of the boiler Signup and view all the answers

What happens when the water level drops to the low water cut-off point?

The boiler will shut down automatically Signup and view all the answers

Why is it essential for the plant operator to conduct regular tests of the low water cut-off?

To ensure the boiler shuts down safely in case of an emergency Signup and view all the answers

MEDIUM What can cause an interruption of the water supply to the feedwater pump?

Interruption of the water supply to the feedwater pump Signup and view all the answers

What would cause a rapidly falling water level in the boiler?

Faulty feedwater level controller Signup and view all the answers

What could cause a gradual reduction in boiler water level?

Leaks in hot water supply or return piping Signup and view all the answers

What happens if the low water cut-off fails to shut down the boiler?

The water level may drop to a dangerous level Signup and view all the answers

Why is it essential for the plant operator to conduct regular tests of the low water cut-off?

To identify potential faults in the low water cut-off system Signup and view all the answers

What is a possible cause of an interruption of the water supply to the feedwater pump?

Ruptured tubes or open blowoff valves Signup and view all the answers

What is the primary purpose of the bellows assembly in the low water fuel cut-off system?

To provide a flexible steam and watertight seal between the float housing and the electrical junction box Signup and view all the answers

What happens when the boiler water level drops and the float closes the two-wire switch?

The feedwater pump circuit is opened Signup and view all the answers

What is the purpose of the combination low water cutoff and feeder valve?

To provide the best protection for low-pressure steam heating boilers Signup and view all the answers

What happens when the boiler water level continues to drop and the three-wire switch acts as a boiler emergency shutdown?

The electrical circuit to the SSOV is opened Signup and view all the answers

What is the function of the mercury switch in the low water fuel cut-off system?

To open or close the electrical circuit to the SSOV Signup and view all the answers

What happens when the water level in the boiler drops to the LPWL?

The mercury switch opens the electrical circuit to the SSOV Signup and view all the answers

What is the purpose of the float in the float chamber?

To follow the fluctuations in water level Signup and view all the answers

What happens when the boiler water level drops and the float moves to a lower position in the float controlled make-up water valve?

The make-up valve opens further to admit more water into the boiler Signup and view all the answers

What is the purpose of the cut-off switch at level C?

To shut off the burner circuit Signup and view all the answers

What happens when the water level continues to drop even after the switch has shut off the burner?

The pressure relief valve opens Signup and view all the answers

How does the electric probe type low water fuel cut-off work?

It relies on the flow of current between two probes to close the fuel valve Signup and view all the answers

What is the purpose of the float chamber in the low water fuel cut-off system?

To connect the steam and water space directly to the float chamber Signup and view all the answers

Why is it necessary to check and clean the electric probes?

To prevent sediment and scale buildup that can reduce current flow Signup and view all the answers

What is a feature of the magnetic low water fuel cut-off?

It uses a float to shut off the fuel supply Signup and view all the answers

What can occur in hot water heating boilers due to human error or mechanical failure?

Low water conditions Signup and view all the answers

At what level does the control switch close, and the feedwater pump starts?

Level B Signup and view all the answers

What happens when the water level drops below the upper probe in the electric probe type low water fuel cut-off?

The flow of current to the solenoid will stop Signup and view all the answers

What is an advantage of the electric probe type low water fuel cut-off?

It contains no moving parts Signup and view all the answers

What is the purpose of the low water fuel cut-off device in hot water heating boilers?

To shut off the fuel supply in case of low water level Signup and view all the answers

What happens when the water level drops to level A?

The feedwater supply switch is open Signup and view all the answers

What is the primary purpose of the low water fuel cut-off device in a boiler?

To prevent overheating and weakening of the pressure-retaining parts of the boiler Signup and view all the answers

Which of the following boilers does not require a low water fuel cut-off?

Electrode-type electric boilers Signup and view all the answers

What is the purpose of the flexible diaphragm in the low water fuel cut-off device?

To separate the water chamber from the electrical switch Signup and view all the answers

What happens when the float switch opens in the low water fuel cut-off device?

The fuel supply to the burner is interrupted Signup and view all the answers

What determines the lowest permissible water level (LPWL) for a boiler?

The boiler manufacturer Signup and view all the answers

What is the purpose of the float chamber in the low water fuel cut-off device?

To mirror the boiler's water level Signup and view all the answers

Why is the low water fuel cut-off device an important boiler safety control?

It prevents the boiler from weakening due to low water levels Signup and view all the answers

What is the function of the safety shut-off valve (SSOV) in the low water fuel cut-off device?

To shut off the fuel supply to the burner when the water level drops Signup and view all the answers

HARD What is the most likely cause of a rapid fall in boiler water level if the make-up water or condensate return systems are functioning properly?

Leakage from the boiler due to ruptured tubes or open blowoff valves Signup and view all the answers

What is the primary consequence of a faulty low water cut-off system?

The water level may drop to a dangerous level Signup and view all the answers

Which of the following is NOT a cause of a gradual reduction in boiler water level?

Faulty feedwater level controller Signup and view all the answers

What is the purpose of regular testing of the low water cut-off system?

To prevent damage to the boiler and its components Signup and view all the answers

What is the most likely consequence of a faulty feedwater level controller?

A rapid fall in boiler water level Signup and view all the answers

What would happen if the feedwater supply switch fails to open when the water level drops to level B?

The water level would continue to drop to level C Signup and view all the answers

What is the purpose of the alarm switch in the low water fuel cut-off system?

To energize the alarm circuit Signup and view all the answers

How does the electric probe type low water fuel cut-off device work?

By using a small current flow between two probes to keep the fuel valve open Signup and view all the answers

What is the primary purpose of the low water fuel cut-off device in hot water heating boilers?

To prevent damage from low water conditions Signup and view all the answers

What happens when the water level drops below the upper probe in the electric probe type low water fuel cut-off?

The fuel valve closes Signup and view all the answers

Why is it necessary to check and clean the electric probes in the electric probe type low water fuel cut-off?

To prevent scale or sediment from reducing or stopping the current flow Signup and view all the answers

What happens when the water level in the boiler continues to drop even after the switch has shut off the burner?

The boiler cannot restart until the water level is restored to above level C Signup and view all the answers

What type of low water fuel cut-off has no moving parts?

Electric probe type low water fuel cut-off Signup and view all the answers

What is the purpose of the cut-off switch at level C?

To shut off the fuel supply Signup and view all the answers

What happens when the water level in the boiler drops to level C?

The burner circuit opens, shutting off the fuel supply Signup and view all the answers

What is the primary benefit of having a unique lowest permissible water level (LPWL) for each boiler design?

To reduce the risk of overheating and weakening of the pressure-retaining parts Signup and view all the answers

What is the significance of the float chamber being connected to the same boiler openings as the directly connected water gauge glass?

It enables the float to track changes in water level in the boiler Signup and view all the answers

What is the primary purpose of the flexible diaphragm in the LWCO device?

To act as a seal between the water chamber and the electric switch Signup and view all the answers

What would happen if the LWCO device were to fail to shut off the boiler burner system when the water level drops below the LPWL?

The boiler would be at risk of overheating and weakening of the pressure-retaining parts Signup and view all the answers

What is the primary reason why electrode-type electric boilers do not require a low water fuel cut-off?

They do not use water as a coolant Signup and view all the answers

What is the primary purpose of the safety shut-off valve (SSOV) in the LWCO device?

To shut off the fuel supply to the burner when the water level drops below the LPWL Signup and view all the answers

What is the significance of the boiler manufacturer identifying the LPWL for each boiler design?

It enables the LWCO device to be calibrated to the specific boiler design Signup and view all the answers

What is the primary benefit of using a LWCO device that directly detects the level of water in the boiler?

It reduces the risk of human error in monitoring the water level Signup and view all the answers

What is the primary function of the bellows assembly in the low water fuel cut-off system?

To form a flexible steam and watertight seal between the float housing and the electrical junction box Signup and view all the answers

What happens when the boiler water level drops to the level where the mercury switch tilts to the opposite side?

The electrical circuit to the SSOV opens, and the burner shuts down Signup and view all the answers

What is the advantage of using a combination low water cutoff and feeder valve in low-pressure steam heating boilers?

It offers better protection against heat transfer surfaces overheating Signup and view all the answers

What is the purpose of the vacuum inside the mercury switch?

To prevent oxidation of the mercury and corrosion of the electrical contacts Signup and view all the answers

What happens when the float in the float controlled make-up water valve moves to a lower position?

The make-up valve opens further to admit more water into the boiler Signup and view all the answers

What is the purpose of the lever system in the low water fuel cut-off system?

To transmit the movement of the float to the mercury switch Signup and view all the answers

What happens when the boiler water level continues to drop even after the three-wire switch has shut off the burner?

The boiler temperature or pressure rises until the pressure relief valve opens Signup and view all the answers

What is the purpose of the float in the float chamber?

To follow the fluctuations in water level and transmit the movement to the mercury switch Signup and view all the answers

What is the function of the electrical contacts inside the mercury switch?

To open or close the electrical circuit to the SSOV Signup and view all the answers

What happens when the two-wire switch closes due to a drop in water level?

The electrical circuit to the feedwater pump closes, and the pump starts Signup and view all the answers

Study Notes

Low Water Fuel Cut-Off (LWCO)

A float or probe-operated switch that detects the water level in a boiler and shuts off the boiler burner system if the level drops to a predetermined unsafe level.
Prevents overheating and weakening of the pressure-retaining parts of the boiler.

Importance of LWCO

Each boiler design has a unique lowest permissible water level (LPWL) identified by the boiler manufacturer.
LWCO ensures the boiler is shut down before the water level drops to the LPWL.

Types of LWCO

Float-operated LWCO:
- Uses a float chamber connected to the boiler openings, with a float that follows the changes in water level.
- Electric switch is actuated by the float, which shuts off the fuel supply to the burner when the water level drops below the LPWL.
Mercury switch LWCO:
- Uses a mercury switch that opens or closes the electrical circuit when tilted.
- Tilted by the movement of the float and rod, which is connected to the bellows assembly.
Magnetic LWCO:
- Uses a permanent magnet attached to a pivoted mercury switch.
- When the water level drops, the plunger drops out of the magnetic field, and the switch tilts to open the circuit.
Electric probe type LWCO:
- Uses two electric probes immersed in the water.
- When the water level drops below the upper probe, the flow to the solenoid stops, and the switch opens.

Applications of LWCO

Used in steam boilers, except for electrode-type electric boilers, coil tube and fin tube boilers, and those under continuous operator attendance.
Used in hot water heating boilers, with modifications to accommodate higher pressure ratings.

Causes of Low Water Level

Rapidly falling water level:
- Faulty feedwater level controller.
- Feedwater or condensate pump failure.
- Interruption of the water supply to the feedwater pump.
- Leakage from the boiler.
Gradual reduction in boiler water level:
- Leaks in hot water supply or return piping.
- Damaged condensate return piping.
- Defective condensate return pumps.
- Excessive blowoff or blowdown.
- Relief valve discharge caused by full expansion tanks.

Importance of Regular Testing

Regular testing of the LWCO is essential to ensure the boiler shuts down automatically in case of low water level.
Failure to test the LWCO can lead to a dangerous drop in water level.

Low Water Fuel Cut-Off (LWCO)

A float or probe-operated switch that detects the water level in a boiler and shuts off the boiler burner system if the water level drops to a predetermined unsafe level.

Importance of LWCO

Prevents overheating and weakening of the pressure-retaining parts of the boiler.
Essential for boiler safety control.

Exemptions from LWCO Requirement

Electrode-type electric boilers.
Coil tube and fin tube boilers that require forced circulation to prevent overheating.
Boilers under continuous operator attendance.

Lowest Permissible Water Level (LPWL)

The lowest water level at which a boiler can be safely operated, identified by the boiler manufacturer.
The LWCO turns off the burner system before the water level drops to the LPWL.

Types of LWCO Designs

Figure 1: Float chamber connected to the same boiler openings as the directly connected water gauge glass.
Figure 2: Float chamber connected directly to the steam and water space, independent of the gauge glass.
Figure 3: Mercury switch used in the low water fuel cut-off, which opens or closes the electrical contacts inside the bulb.
Figure 4: Float arrangement that combines a feedwater pump control switch with a low water fuel cut-off and alarm.
Figure 5: Float-controlled make-up water valve combined with a low water fuel cut-off.
Figure 6: Float chamber of the low water fuel cut-off combined with the water column.
Figure 7: Magnetic low water fuel cut-off actuated by a float and magnet.
Figure 8: Electric probe type low water fuel cut-off with two probes immersed in the water.
Figure 9: Electric probe type low water fuel cut-off with a single probe, using the boiler shell or probe housing as a second probe.
Figure 10: Float-operated low water fuel cut-off device installed on a hot water heating boiler.

Possible Causes of Low Water Level

Faulty feedwater level controller.
Feedwater or condensate pump failure.
Interruption of the water supply to the feedwater pump.
Leakage from the boiler, due to ruptured tubes or open blowoff valves.
Leaks in hot water supply or return piping.
Damaged condensate return piping.
Defective condensate return pumps.
Excessive blowoff or blowdown.
Relief valve discharge caused by full expansion tanks.

Low Water Fuel Cut-Off (LWCO)

A float or probe-operated switch that detects the water level in a boiler and shuts off the boiler burner system if the water level drops to a predetermined unsafe level.

Importance of LWCO

Prevents overheating and weakening of the pressure-retaining parts of the boiler.
Essential for boiler safety control.

Exemptions from LWCO Requirement

Electrode-type electric boilers.
Coil tube and fin tube boilers that require forced circulation to prevent overheating.
Boilers under continuous operator attendance.

Lowest Permissible Water Level (LPWL)

The lowest water level at which a boiler can be safely operated, identified by the boiler manufacturer.
The LWCO turns off the burner system before the water level drops to the LPWL.

Types of LWCO Designs

Figure 1: Float chamber connected to the same boiler openings as the directly connected water gauge glass.
Figure 2: Float chamber connected directly to the steam and water space, independent of the gauge glass.
Figure 3: Mercury switch used in the low water fuel cut-off, which opens or closes the electrical contacts inside the bulb.
Figure 4: Float arrangement that combines a feedwater pump control switch with a low water fuel cut-off and alarm.
Figure 5: Float-controlled make-up water valve combined with a low water fuel cut-off.
Figure 6: Float chamber of the low water fuel cut-off combined with the water column.
Figure 7: Magnetic low water fuel cut-off actuated by a float and magnet.
Figure 8: Electric probe type low water fuel cut-off with two probes immersed in the water.
Figure 9: Electric probe type low water fuel cut-off with a single probe, using the boiler shell or probe housing as a second probe.
Figure 10: Float-operated low water fuel cut-off device installed on a hot water heating boiler.

Possible Causes of Low Water Level

Faulty feedwater level controller.
Feedwater or condensate pump failure.
Interruption of the water supply to the feedwater pump.
Leakage from the boiler, due to ruptured tubes or open blowoff valves.
Leaks in hot water supply or return piping.
Damaged condensate return piping.
Defective condensate return pumps.
Excessive blowoff or blowdown.
Relief valve discharge caused by full expansion tanks.

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Description

Learn about the function and importance of low water fuel cut-off in boilers, and exemptions for certain types of boilers.