TECHNICAL DESCRIPTION
3.10 Fan Testing
A good fan test requires test instrumentation and controlled conditions. Data collected from existing plant instrumentation can be used to identify possible problems but cannot be used to quantify the magnitude of a performance problem. Plant instrumentation is not accurate enough to conduct a good fan test.
The codes and standards that specify field performance testing of fans are ASME PTC-11, Fans;
AMCA 203, Field Performance Tests; and AMCA 803, Site Performance Test Standard. These codes provide guidance on the instruments and methods used to conduct high-quality field performance tests on fans.
The actual operating point can be determined from measurements of static pressure at the fan inlet and outlet and the calculated air or gas flow. The air or gas flow can be calculated from measurements of oxygen and boiler load. Although these calculations are only approximate, they are usually adequate for identifying fan performance problems.
Before the actual operating point is compared to the fan design performance, adjustments to actual operating conditions must be made. Fan performance is affected by inlet vane position (blade position for variable-pitch axial fans), fan speed, and fan inlet density.
3.11 Operation
A well-operated system improves efficiency and reduces auxiliary power consumption. To optimize draft fan system performance, operators must thoroughly understand the capabilities and limitations of their specific fan system.
The startup of any piece of major equipment requires following specific steps, which can be divided into two phases:
1. Prestart checks 2. Startup procedures 3.11.1 Prestart Checks
Prestart checks involve a detailed system walk through to verify that the fan will operate safely.
During prestart checks, the operator should perform the following tasks:
Verify that supportive auxiliary systems and monitoring equipment are operating correctly. For example, check for adequate lube oil and cooling water, proper temperature range of the lube oil and cooling water, and proper damper position.
Perform safety-related checks to verify that all DANGER, WARNING, CAUTION, and DO NOT OPERATE tags have been properly cleared; all safety-related interlocks, alarms, and similar operational interlocks have been cleared; and all personnel have been cleared from the fan and ductwork.
3.11.2 Startup Procedures
Startup procedures should consist of a well-defined sequence of steps that ensure fan safety and that prevent boiler explosions and implosions. In writing the startup procedures for a draft fan, the operating engineer should take into account the station’s practices and regulations, the level of automation, the number of personnel on the shift, and the manufacturer’s recommendations.
The startup procedures, in addition to the controls and interlocks, should follow the requirements of the current version of NFPA 85 [1].
Key Human Performance Point
The startup procedures, in addition to the controls and interlocks, should follow the requirements of the current version of NFPA 85.
3.11.2.2 Sequence of Steps for Startup
The procedure for starting up a draft fan should include the following:
• Sequence of all operator actions required for startup
• Alarm conditions to monitor
• Operating parameters to monitor
• Emergency actions
• Fan control requirements
• Electric motor restrictions
• Requirement for the control of inlet vanes
• Fan outlet dampers requirements
• Individual responsibilities
• Procedures to prevent a stall
3.11.3 Alarm Conditions to Monitor
Draft fan alarms are either executive or advisory. Executive alarms give operating personnel audible and visual warnings of a dangerous condition, such as excessive vibration, loss of lube oil, or a hot bearing. Through safety interlocks, an executive alarm condition may initiate a shutdown of a fan. The automatic boiler control system would then respond to maintain a safe condition in the furnace.
Advisory alarms are similar to executive alarms except that operating personnel must initiate the corrective action. A high differential pressure on the discharge filter of a circulating lube oil system is an example of an advisory alarm.
3.11.4 Operating Parameters to Monitor
The operating parameters that affect the safety of the equipment and personnel should be monitored.
Recommended parameters to monitor for operation include the following:
• Motor winding temperature
• Bearing oil temperature
• Bearing temperature
• Motor amperes
• Damper position
• Fan vibration levels
• Flue gas temperature (if applicable)
Additional parameters and analysis are discussed in the section on condition based monitoring.
3.11.5 Emergency Actions
The startup procedures should contain cautions and warnings to remind operators of problems that could develop. In draft fan emergencies where personnel or equipment safety are in danger, shutting down the fan should be a standard operating procedure.
Examples of such circumstances include the following:
• High vibration levels or excursions
• Temperature excursions
• Electrical fires and oil leaks
• Hot bearings and loss of fan control
• Fan stall
• Loss of damper or vane control
• Major flow unbalance on double inlet fans 3.11.6 Fan Control
Before placing control of the fan in the furnace automatic combustion control system, prestart checks verifying positive control of the fan should be complete. These checks include the following:
• Verifying that the control damper/inlet guide vane is operating correctly and that local position indicators agree with the remote indicators located in the control room
• Test operating the fluid drive control (if applicable)
3.11.6.1 Electric Motor Restrictions
During startup, it is important not to exceed the electric motor’s duty cycle, especially for an ID fan being started up with cold air. Frequent starts that exceed the duty for which the motor was designed form local hot spots or raise the operating temperature above the allowable temperature rating for the motor insulation. Such a condition could reduce the expected operating life of the motor or cause a premature failure of the motor’s insulation. Ensuring that the design duty cycle of a motor is incorporated into a written procedure for starting a draft fan will help in preventing damage to the motor.
The criteria used to establish the minimum number of starts for a large motor (that is, 500 hp and greater) are provided in NEMA Standard MG-l, section 20.43:
• Two starts in succession, coasting to rest between starts, with the motor initially at ambient temperature.
• One start with the motor initially at a temperature not exceeding its rated load operating temperature.
Specific manufacturer’s requirements for starting operations after a motor has undergone a cycle of two cold starts and one hot start can be found in the manufacturer’s operating instruction manual or on the motor’s starting plate. Requirements that should be incorporated include the following:
• Minimum time the motor must run before it is shut down
• Length of time the motor must be at a standstill before additional starts are attempted
• Maximum operating temperature of the motor windings
• Number of total starts per day that should not be exceeded
Another consideration involves operating the draft fan at or above the motor’s maximum rated current level. A loss in fan efficiency could cause the motor to be operated above its rating.
According to NEMA Standard MG-l, motors are to be designed to run at a maximum horsepower and full-load current without exceeding a specified temperature rise. It is normal design practice for a utility power plant motor to have a design margin of 15% for horsepower for the following reasons:
• To allow margin for a demand increase from the driven equipment under unusual or infrequent operating conditions
• To prolong the operating life of the motor in the event of unusual or infrequent operating conditions
The life of a motor is originally determined by the effective life of its insulation. The numbers of starts and operating temperature of the motor directly influence a motor’s operational life. By operating the motor under normal conditions at less than rated horsepower (and therefore less than rated maximum current), the motor operating temperature is kept below its maximum operating temperature.
3.11.7 Control of Vanes and Dampers
The startup procedures should specify the position of the inlet and outlet dampers and vanes during startup. The degree of automation will determine what the operator should do and how the control system performs. For systems that require the operator to initiate all actions, startup procedures should address the sequence of opening and closing the dampers and vanes, the time limits between each step, the requirements for a visual verification of the damper position, and indications that a vane or damper has failed to open.
3.11.8 Fan Outlet Dampers
Most fan manufacturers recommend starting constant-speed fans with the outlet dampers closed.
A closed outlet damper reduces the starting time and load on the motor. The torque required at any given speed can be three to four times higher with an open versus closed outlet damper.
Some plants have found that the required torque and startup time for a draft fan system
configured with both inlet vanes and dampers and outlet dampers may not differ significantly if the outlet dampers are kept open and the inlet control devices are closed. Outlet dampers are typically high maintenance items—especially on ID fans—and may be required only for startup and fan isolation during maintenance. Some stations have found outlet dampers unnecessary, but this depends on the fan design, the motor capabilities, and the design and condition of the inlet vanes or inlet dampers.
The best practice is to follow the fan manufacturer’s recommendation. If a change in damper operation is desired, data on startup times and motor current should be collected and discussed with the fan and motor suppliers.
Key O&M Cost Point
The best practice is to follow the fan manufacturer’s recommendation. If a change in damper operation is desired, data on startup times and motor current should be collected and discussed with the suppliers of the fan and motor.
3.11.9 Stall Prevention for Axial Fans
The aerodynamic term stall is often used to describe a phenomenon that can occur in an axial flow fan. Under certain pressure and flow combinations, the gas flow cannot accommodate the guiding surface of the fan blade, and flow separation occurs. Pronounced separation results in circulator flow within the fan and a significant reduction in flow through the fan as well as pressure rise across the fan. An abrupt change in flow and pressure will have an adverse effect on boiler operation and can cause a unit trip. In addition, stall can damage fan blades.
To prevent a fan stall, operators must understand the fan curves of the fans they are operating.
The startup procedures should describe how to prevent fan stalls. Operators must understand how they can prevent fan stalls by monitoring the operating values of static pressure, airflow, and blade position on axial fans. The procedures should show the fan curves and identify points on the curve that, if exceeded, will stall the fan. The procedures should also describe the following indications of a fan stall:
• Abnormal flow volume (pulsations) and power consumption (motor amps displaying abnormal fluctuations)
• High vibration levels
• Failure of the variable-pitch blades to move on command
• Loud, abnormal noise levels
The stall line is usually identified on the performance curve (head versus flow) for axial fans.
The fan control system should monitor the head and flow and give the operator a stall warning so that the operator can take action before a stall occurs.
Key Human Performance Point
The fan control system should monitor the head and flow and give the operator a stall warning so that the operator can take action before a stall occurs.
3.11.10 Draft Fan Shutdown
The draft fan shutdown procedure should be developed around the safety of personnel and equipment. There are two types of draft fan shutdown, referred to here as a controlled and uncontrolled shutdown.
3.11.10.1 Controlled Shutdown
A controlled shutdown of a fan occurs in a logical and orderly manner. During shutdown, operators will perform the following:
• Reduce firing rate demands
• Stop the associated fan(s)
• Perform fan post-shutdown-related checks
Written procedures for a controlled shutdown consist of the following:
• Sequence of steps required to safely shut down the draft fan
• Post-shutdown checks
3.11.10.2 Uncontrolled Shutdown
An uncontrolled shutdown is characterized by a loss of one or more draft fans. An operator error or a problem that went undetected by the monitoring system could result in an uncontrolled shutdown. The following conditions could cause an uncontrolled shutdown:
• Control system upsets and/or failures
• High vibration level
• Uncontrolled hot bearing
• Loss of lube oil
• Electrical fire at the motor controller or fan motor
• Fires in ductwork and/or fan housing (GR or PA fan)
• Loss of electrical power
Direct consequences of an unexpected loss of a draft fan include the following:
• Reduced unit load capability
• Loss of a draft fan cascading into a boiler explosion or implosion (this occurs if boiler controls do not function properly)
• Loss of a unit in case its pair of ID or FD fans is shut down unexpectedly
A written procedure for an uncontrolled shutdown of a draft fan should incorporate adequate measures to ensure boiler safety.
Boiler safety is addressed through a system of mandatory safety interlocks that protect against an unstable operating condition. As with any mechanical or electrical system, this safety interlock system can provide only a limited level of assurance in preventing conditions that might lead to a boiler explosion or implosion. Two critical factors must be recognized about the reliability of automatic safety systems:
• The effectiveness of the system depends on how well it is maintained.
• No safety system is 100% reliable.
Written procedures and training on the system requirements and operator responsibilities in the event of a fan loss provide an additional margin of safety. NFPA 85 [1] discusses conditions resulting from the loss of a draft fan that may lead to a boiler explosion or implosion. The following summarizes the required safety system interlock responses from NFPA 85 [1] in the event of an uncontrolled shutdown of draft fan(s):
• Response to the loss of one draft fan (either one ID or one FD)
– Close dampers of the affected fan unless it is the last FD or ID fan in service.
– If the unit’s automatic safety interlock system is designed to start, stop, or trip ID and FD fans in pairs, trip the corresponding paired fan; close dampers of these fans unless they are the last ones in service.
• Response to the loss of all FD or all ID fans:
– Trip the corresponding ID or FD fans.
– Close dampers of all fans (to avoid any pressure excursion during fans coasting down).
– Open all dampers after the required time delay (to provide appropriate natural ventilation of the boiler).
3.11.11 Parallel Fan Operation
When two or more fans are brought into parallel operation, the operator must ensure that the airflow from the fans is balanced. An improper fan paralleling operation can lead to serious consequences, such as the following:
• High vibration levels, stalled fan(s), airflow pulsations in the ductwork, pressure excursions to the boiler, or damage to the electric motor.
• Potential for losing both fans. This is particularly serious if the unit loses all of its FD or ID fans because this may lead to furnace damage and will trip the master fuel valve, resulting in the loss of the unit.
When paralleling two fans, the operator should observe the following:
• Verify that the second fan to be brought on-line is mechanically ready.
• In the case of axial fans, bring the on-line fan to a pressure below the fan’s stall point; this
• Bring the second fan up to speed.
• Adjust flow control devices of the second fan to match those of the on-line fan.
• Confirm that the load of the on-line fan decreases accordingly as the second fan is brought on-line.
• Place the second fan in automatic and confirm that the system balances both fans.
An on-line monitoring program should be designed to do the following:
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Recognize a change in the equipment’s normal operating conditions or parameters.
Provide a method to warn operators when changes in the operating conditions and parameters have exceeded safe limits.
The following are the primary objectives of an on-line monitoring program:
• Reliably assesses the fan’s mechanical condition and displays the results in a format that is user-friendly to O&M personnel.
• Quickly and accurately recognizes any changes in the fan’s operating parameters and the significance of the changes.
• Assesses and displays the severity of a problem in a user-friendly format that requires no additional interpretation.
• Provides a means to initiate a shutdown of the fan in the event that the problem is determined to be severe.
• Maintains data files for long-term trending and machinery history.