BOILER FUNDAMENTALS

Basic Boilers

A boiler is comprised of two basic systems. One system is the steam water system also called the waterside of the boiler.In the waterside,water is introduced and heated by transference through the water tubes,converted to steam,and leaves the system as steam. Boilers must maintain a chemical balance.The manner in which this is done can interact with the feedwater control system.The amount of blowdown must be considered in the feedwater control scheme,especially if the blowdown is continuous.Often,the blowdown flow is divided by the concentration ratio times the feedwater flow.Continuous blowdown is the common method for controlling the chemical concentration.On large boilers this may be done auto-matically by measuring the boiler water conductivity to control the blowdown rate.The blowdown rate may also be achieved by combining the conductivity with ratio control of blowdown,ratioing blowdown to feedwater flow.In utility plants,conductivity is usually meas-ured and blowdown is achieved manually.This is required on a periodic basis or when the conductivity gets too high.

Conductivity is measured in micro mhos which is equal to the reciprocal of 1 mechanical ohm (resistance).The other boiler system is the fuel air-flue gas system,also referred to as the  fireside of the boiler.This system provides the heat that is transferred to the water.The inputs to this system are the fuel and air required to burn the fuel.The fuel and air chamber is also referred to as the windbox.The outputs are the flue gas and ash.

Boiler Components

Note that in the boiler diagram,the steam goes to a header.This is common in industrial power plants;however,in utility plants the boiler is directly connected to the turbine.The firing demand in industrial plants is based on header pressure or drum pressure.Utility plants control the firing rate on megawatt demand,or throttle pressure.The firing rate demand depends on
the particular system.

Furnace

The combustion chamber/furnace releases the heat and becomes the heat transfer system.There are three T’s required for combustion to take place in the furnace:time,temperature,and turbu-lence.The control of the furnace draft is required to maintain a negative pressure in the furnace in a balanced draft boiler.This pressure is defined by the boiler manufacturer.Negative 0.5 inches is a common control point.The control set point may be raised during inspection rounds from
0.5 inches to 1.0 inches to minimize the possibility of flame coming out of inspection doors.Draft pressure control setting is defined by the boiler manufacturer and environmental equipment. Under certain conditions,the furnace pressure may be controlled positive.

Fans

The boiler consists of an ID (Induced Draft) fan and an FD (Forced Draft) fan.Large utility boilers may have two ID fans and two FD fans.The ID fan pulls air through the boiler producing a negative pressure in the furnace,thus creating draft control.The FD fan pushes air for combustion through the boiler. On utility boilers,FD fans normally supply secondary and overfire (tertiary air) with primary air (P.A.) flow being supplied by the P.A.fans.Industrial boilers often have separate fans for the tertiary air as well.

Due to the 1990 Clean Air Act amendments,there is often emission control equipment such as precipitators,bag houses,and sulfur dioxide scrubbers on the discharge of boilers.  If environmental equipment is added,booster ID fans may be required.The draft pressure control is defined by both the boiler manufacturer and environmental equipment. Windbox The windbox distributes secondary air to the burners.The windbox may have damper adjust-ments to create turbulence to improve combustion.

Flue Gas Heat Exchangers

To reduce heat loss in the boiler flue gases and to improve boiler efficiency,heat exchangers are added to the boiler to recover heat and to cool the flue gases. 

Combustion Air Preheater

The combustion air preheater is one type of heat exchanger.(See Figure 1-4.) As the flue gas leaves the boiler,it passes through the combustion air preheater.The combustion air passes through the air preheater heat exchanger before being mixed with the fuel.

Since the flue gas temperature is higher than the air temperature,heat is transferred from the flue gas to the combustion air via the convection heat transfer surface of the combustion air preheater.This transfer of heat cools the flue gas and thus reduces its heat loss and reduces the temperature of the air to the stack.The added heat in the combustion air entering the furnace enhances the combustion process.This reduces the fuel requirement in an amount equal in heat value to the amount of heat that has been transferred in the combustion air preheater,thus improving efficiency. By the use of an air preheater,approximately one percent of fuel is saved for each 40°F rise in the combustion air temperature.

Economizer

Another flue gas heat recovery method is through the use of an economizer.The economizer heats the feedwater to improve boiler efficiency and reduce heat loss to the stack.The increased heat in the feedwater reduces the boiler’s requirement for fuel and combustion air.In the econ-omizer arrangement ,the flue gas leaves the boiler and enters the economizer where it makes contact with the heat transfer surface,in the form of water tubes, through which the boiler feedwater flows.Since the flue gas is at a higher temperature than the water,the flue gas is cooled and the water temperature is increased.Cooling the flue gas reduces its heat loss in an amount equal to the increased heat in the feedwater to the boiler. Both types of heat exchangers are often used in large boilers.

When both an air preheater and an economizer are used,the normal practice consists of passing the flue gases first through the economizer and then through the combustion air preheater. Utility boilers normally have economizers and air heaters. While economizers are used to recover heat from the flue gas,the use of Selective Catalytic Reduction (SCR) requires flue gas temperatures above a specified minimum temperature to operate.To extend the range of operation of the SCR,the economizer surface may be bypassed to raise flue gas temperatures at lower loads.

Superheater

The superheater provides additional heat to the steam to remove any moisture from the steam, thereby improving the quality of the steam.The dryness of the steam (in percent) is the deter-mining factor of its quality.When there is no moisture in the steam,the quality is 100 percent.

Boiler Drums

High pressure utility boilers used for power generation have only an upper drum. Boilers may consist of an upper drum,or steam or water drum,and a lower drum,or mud drum.The mud drum terminology comes from the function of the lower drum.Although the water is treated to eliminate dissolved solids,some solids always remain in the water.These solids collect in the lower drum,and a drum blowdown is required to remove the solids that collect in the lower drum.The blowdown may be manual or automatic.

Piping and Instrument Diagrams (P&IDs)

The purpose of P&IDs is to provide an initial design basis for the boiler.The P&ID provides the engineering requirements to identify the measurements and functions that are to be con-trolled.It may be used to define the number of inputs and outputs and may also consist of a design basis check list (DBCL).The DBCL lists all the instruments and functions.Utility plants generally define the control systems on Scientific Apparatus Makers Association (SAMA) draw-ings and do not have a P&ID.

By identifying the required measurements and control functions,an I/O count can be defined. By defining the control functions,the memory requirements can be established.Memory and I/O capabilities can be important in selecting a basic process control system and/or logic system.Utility plants commonly use the term process control system (PCS).

The controls system may consist of panel mounted instruments,a distributive control system (DCS),logic system,or a combination.This also includes the amount of redundancy of both measurement and final control elements.

The numbering system will be based on the standard for the plant.The ISA and SAMA letter identification is used by most companies.ISA and SAMA identification letters are the same. (See ISA and SAMA identification tables in the Reference section for more information.) The letter configuration identifies the letter definition.For example,when T is the first letter,it rep-resents temperature.T as a succeeding letter is a transmitter.The numbering will consist of the identification letters,such as a PT,and the tag number.The tag number can consist of a system or loop number.An example would be:PT 115 1001.The PT represents the pressure trans-mitter,the 115 the system number,and 1001 the instrument number.

In the examples on the design basis check list,the PT is a pressure transmitter.Note P is used for both pressure and vacuum.PV is the pressure valve.V is used for valves,vanes,or dampers. PIC is the abbreviation used for a pressure indicating controller.