Enhancing Energy Efficiency of Induced Draft Fans in Thermal Power Plants

Enhancing Energy Efficiency of Induced Draft Fans in Thermal Power Plants

R.P. Mandi and U.R. Yaragatti (India)Keywords

Energy efficiency, Auxiliary power, fan efficiency, SEC, VFD, excess air, furnace air ingress

Abstract

This paper describes the various methods for enhancing the energy efficiency of induced draft fans in coal fired thermal power plants. The auxiliary power used by induced draft fans forms about 1.4 to 1.8 % of the total gross energy generation. ID fans are the second largest (in rating) motor in a thermal power plant after Boiler Feed Pump (BFP). The detailed energy audit of auxiliary equipment in various thermal power stations, operational optimization and appropriate control system had shown ample scope for improving the energy efficiency of induced draft fans. The implementation of energy conservation measures reduce the average auxiliary power used by ID fans for 210 MW plant from 1.63 % to 0.97 % of gross energy generation and the energy conservation schemes are economically attractive with a payback period of 1 to 5 years.

This is written to describe the type of machinery that can be found in a typical fossil fuel (gas, oil or coal-fired) electric generating plant to better understand what type of instrumentation is being used for vibration monitoring.

Typical Power PlantA typical power plant will usually have 5 or 6 power generators of several sizes. It might have 2 larger capacity units, 2 or 3 medium capacity units, and 1 or 2 "peaking units" that have a small generating capacity each. Peaking units are used during periods of high demand for electricity, like hot summer nights, thus the name. Otherwise they sit idle in a stand-by mode. Peaking units use gas turbines or diesel engines to drive the electric generators. Large steam turbines are the most common type of machine used to drive medium and large electric generators.

Power plants are rated by the combined generating capacity of all of the units at the site, in megawatts. Following is a generalized size breakdown based on the generating capacity of each unit.

SizeMegawatts

SmallLess than 50

Medium50 to 150

Large150 to 300

Very LargeMore than 300

Monitoring PracticesAll power generating units consist of primary or major machinery and secondary or BOP machinery. With little exception, all major machinery is monitored with a permanently installed continuous monitoring system. Most plants monitor their BOP machinery on a periodic basis using a portable data collector and database program. The continuous monitoring system for a medium or large steam turbine generator is called a Turbine Supervisory Instrumentation System (TSI System). A TSI System includes the typical radial displacement vibration and axial position measurements associated with high speed rotating machinery but also includes measurements that are unique to the long and heavy rotors and machine cases required for steam turbines. Among these are; case expansion, differential expansion, runout (shaft bow), zero speed, valve position, and absolute shaft vibration.

Periodic monitoring provides a great solution for power plants for BOP machinery. This is because of the large number of points to be monitored each month, usually 700 or 800. When you consider taking vertical and horizontal readings at each bearing and one axial reading on each machine case, the numbers add up quickly. Too many to have continuous monitoring on all of them!

Still, some plants have moved some of their measurement points (on periodic monitoring) to continuous monitoring (using vibration transmitters, signal conditioners, scanning monitors and even API 670 type monitors) for various reasons. The reasons include; machines with higher than normal maintenance, personnel safety (must climb to reach measurement points), better operating condition visibility, hard to get to machines (like cooling towers), and increased workloads for personnel.

Following is a list of the typical machines found in a single unit in a power plant.

NameQtyTypeFunction

Steam turbine generator1CriticalMain generator

Boiler feed pump2Critical or BOPdepending on sizeCirculate water to boiler

Condensate pumps3-6BOPCollect condensed steam and feed water to boiler feed pumps

Feed water booster pumps3-6BOPFeed water to boiler feed pumps

Circulating water pumps3-6BOPMaintain cooling water flow to turbine condensers

Induced draft fans or Forced draft fans2BOPCirculate air through the boiler and stack optimizing use of heat

Gas recirculation fans2BOPRecirculate partially burned gas back through the boiler

Cooling towers fans8-10BOPCool water from the turbine condenser

Other pumps, fans and compressorsmanyBOPAir supply, fuel supply, lubrication, coal-fired plants have coal handling and processing machinery

To get a perspective of how much machinery a power plant has, multiply the number of steam turbine generators times the numbers in this table for each group of machines!

New Monitoring OpportunitiesWhen a plant decides to move monitored points to continuous monitoring, Hardy has several good solutions! These are; HI 5701VT Vibration Transmitter, DI 5500 Series Signal Conditioners, and the HI 2601 Monitoring System.

Choosing the best measurement solution will depend on the measurement objective. For example, if the DCS system is going to monitor the vibration channels (and do the alarming) then the options are to use Vibration Transmitters or Signal Conditioners. Both of these will connect to the DCS using analog input channels (4-20 mA). Form and function will help choose which is appropriate for the application. The following guidelines will help.

TypeMeasurements and Functions

HI 5701VT Vibration Transmitter Larger size requires more mounting room

1 inch NPT flex conduit attachment

Velocity range; 0-1 ips (HI 5701VT-3 is 0-3 ips)

Frequency range; 360-6,000 cpm

Has "raw" data output for data collector

DI 5500 Series Signal Conditioners Separate industrial accelerometer is smaller

2-pin MS connector used at sensor

Signal Conditioner clips on "DIN" rail

Measure Acceleration, Velocity or Displacement

Full scale range choices, some field adjustment

Frequency range; 60-600,000 cpm

Filtering for selective measurement ranges

Sensor and wiring "OK" indication

Buffered "raw" data output for data collector

Optional set point and relay modules

DI 5504 and DI 5505 used with eddy probes

Most of the BOP machinery will not be shut down automatically, based on vibration. The one exception to this is the cooling tower fans. These can do a lot of damage if they throw a blade because they become severely out of balance. A lot of damage would result if they were not stopped. Additionally, one cell in a cooling tower can usually be taken out of service without stopping the generation of power. Set point Modules and Relay Modules can be added to the signal conditioner "building blocks" to provide alarm and shutdown capabilities.

The HI 2601 Monitoring System would be a good choice if the user wants a local display. It has a 16 channel capability so it can measure 16 points. This monitoring system can be configured with individual relays. That means alarms or trips can be configured per machine or alarms can be configured per groups of machines, as may be required.

Summary of Power Plant Machinery

Steam is generated in the Boiler that is sent to the High Pressure section of the Steam Turbine. The waste steam is passed through a regenerator which heats it prior to entering the Intermediate Pressure and Low Pressure sections of the Steam Turbine, turning the Generator.

Condensate is collected from the Condenser (as well as from the Steam Turbine directly) and is pumped by the Condensate Pumps to the Feed Water Booster Pumps where the pressure is raised. Water is then pumped to the Boiler Feed Pump which feeds the Boiler. In large plants, the Boiler Feed Pump might be driven by a small steam turbine. This sketch shows a large electric motor driven Boiler Feed Pump.

Induced Draft Fans (ID Fans) and Forced Draft Fans (FD Fans) are used to control air flow through the stack, maximizing the efficiency of the Boiler. Gas Recirculation Fans collect unburned gas and send it back to be burned again, reducing the particulates that are emitted to the air.

Circulating Water Pumps keep water moving through various functions in the system. The sketch shows water from the Cooling Tower being circulated through the Condenser. Circulating Water Pumps are also used to feed fresh water to the plant from a pond as well as return excess water back to its' source.

Boiler Feed Pumps - Large horizontal pumps that are driven by large electric motors. (In some cases, a small steam turbine is used as the driver). The motor is coupled to the pump through a hydraulic coupling which acts, in a sense, like an automatic transmission.

Induced Draft and Forced Draft Fans - Large fans driven by large electric motors developing 2,000 to 6,000 hp. The motor is coupled to the fan through a hydraulic coupling (as described above).

Gas Recirculation Fans - Smaller than IF or FD Fans, driven by electric motors developing 2,500 hp.

Condensate Pumps - A medium size vertical pump driven by an electric motor. The motor is direct coupled to the pump which may be 10 to 15 feet below the surface. Suction is at the bottom and output is at deck level, just below the motor.

Circulating Water Pumps - Much like the Condensate Pumps, medium size vertical pump driven by an electric motor. The motor is direct coupled to the pump which may be 10 to 15 feet below the surface.

Feed Water Booster pumps - A medium size horizontal pump driven by an electric motor. The motor is usually direct coupled to the pump.

Cooling Towers - Fans are slow turning using long blades (much like a large 3-bladed propeller) that are made out of aluminum or fiberglass. Medium size motors drive gear assemblies through long drive shafts. Motor speeds are 1,200 to 1,800 rpm while the fans speeds are 250 to 300 rpm. The fans keep air moving to cool the water as it flows down from the top of the cells. A cooling tower is usually made up of 4 to 8 cells. Each cell has a motor, gear assembly, and fan.

Plants located along the coasts may use seawater for cooling and therefore will not have cooling towers. In this case, there are additional pumping requirements

Coordinated control for power plant forced and induced draft fans during startup and fan speed changes Document Type and Number:

United States Patent 4181099

Abstract:

In a power plant, a solid state sequence is provided for starting an FD or ID fan, or for changing the fan speed in response to the position of the fan inlet vanes. The sequential process is self-calibrated since process events control the stepping operation of the sequencer. At the operating fan speed, boiler air flow and pressure are controlled by fan inlet vane position in accordance with plant load demand. When the fan speed is changed, a multiplier circuit causes the fan inlet vanes to reposition so as to minimize air flow changes as the fan speed changes. When one of multiple FD fans or multiple ID fans has its speed changed at a time a fan balancer control, which normally equalizes air flow, is biased to permit different fan flows through the different speed FD or ID fans.