Food Process Engineering Lab 3. BOILER OPERATION

 

Laboratory 1. Steam Boilers and Operation Learning Objectives 1.

To becom become e ffam amili iliar ar wit with h tthe he impor importan tantt c com ompon ponen ents ts of a boil boiler er and its functi function on..

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To star startt up an elect lectri rica call stea steam m boil boiler er safe safely ly..

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To sh shut ut do dow wn an an elec electr tric ical al stea steam m boi boile lerr safe safely ly..

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To be fam famili iliar ar with with tthe he ffact actory ory and and ma machi chiner nery y re regu gulat lation ion re regar gardin ding g press pressur ure e vesse vessell and and stea steam m boilers.

1.BOILER   Introduction Boilers are pressure vessels designed to heat water or produce steam, which can then be used to provide space heating and/or service water heating to a building and for process heating. In most commercial building heating he ating applications, the heating source in the boiler is a natural gas red burner burner.. Oil red burners and electric resistance heaters can be used as well. Steam is preferred over hot water in some applications, including absorption cooling, kitchens, laundries, sterilizers, and steam driven euipment. Boilers have several strengths that have made them a common feature of buildings. !he" have a long life, can c an achieve e#ciencies up to $%& or greater, provide an e'ective method of heating for f or processes reuiring heat, and in the case of steam s"stems, reuire little or no pumping energ". energ". (owever, fuel costs can be considerable, regular maintenance is reuired, and if maintenance is dela"ed, repair can be costl".

 Boilers are often one of the largest energ" users in a process. )or ever" "ear a boiler s"stem goes unattended, boiler costs can increase appro appro*imatel" *imatel" +& -+. Boiler operation and maintenance is therefore a good place to start when looking for wa"s to reduce energ" use and save mone".

How Boilers Work Both gas and oil red boilers use controlled combustion of the fuel to heat water. water.  !he ke" boiler components involved in this process process are the burner, burner, combustion chamber, heat e*changer, and controls.

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 !he burner mi*es mi*es the fuel and o*" o*"gen gen together and, with the assistance assistance of an ignition device, provides a platform for combustion. !his combustion takes place in the combustion chamber, and the heat that it generates is transferred to the water through the heat e*changer. e*changer. ontrols regulate the ignition, burner ring rate, fuel suppl", air ai r suppl", e*haust draft, water tempera temperature, ture, steam pressure, and boiler pressure. (ot water produced b" a boiler is pumped through pipes and delivered to euipment throughout the process s"stem, which can include hot water coils in air handling units, service hot water heating euipment, and terminal units. Steam boilers produce steam that 0ows through pipes from areas of high pressure to areas of low pressure, unaided b" an e*ternal energ" source such as a pump. Steam utilized for heating can be directl" utilized b" steam using euipment or can provide heat through a heat e*changer that supplies hot water to the euipment.  !he discussion of di'erent di'erent t"pes of boilers, below, pr provides ovides more detail on th the e designs of specic boiler s"stems

Types of Boilers In The Food Industry Boilers are classied into di'erent t"pes based on their working pressure and temperature, fuel t"pe, draft method, size and capacit", and whether the" condense the water vapor in the combustion gases. Boilers are also sometimes described b" their ke" components, such as heat e*changer materials or tube design. !hese other characteristics are discussed in the following section on 1e" omponents of Boilers.  !  !wo wo primar" t"pes of boilers include ) )iretube iretube and 2atertube 2atertube boilers. In a )iretube )iretube boiler, hot gases of combustion 0ow through a series of tubes surrounded b" water.. 3lternativel", in a 2atertube boiler,  water flows in the inside of the tubes and the hot water gases from combustion flow around the outside of the tubes. " drawing of a watertube boiler is shown in #igure 2.

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#iretube boilers are more commonly available for low pressure steam or hot water applications$ and are available in si%es ranging from &''$''' to (&$'''$''' BT) input *&+. ,atertube boilers are primarily used in higher pressure steam applications and are used e-tensively for comfort heating applications. They typically range in si%e from &''$''' to more than 2'$'''$''' BT) input *&+.

ast iron sectional boilers -gure 4 are another t"pe of boiler commonl" used in commercial space heating applications. !hese t"pes of boilers don5t use tubes. Instead, the"5re built up from cast iron sections that have water and combustion gas passages. !he iron castings are bolted together, similar to an old steam radiator.. !he sections are sealed together b" gaskets. !he"5re available for radiator producing steam or hot water, and are available in sizes ranging from 4%, to +6,, B!7 input -8. ast iron sectional boilers are advantageous because the" can be assembled on site, allowing them to be transported through doors and smaller openings. !heir main disadvantage is that because the sections are sealed together with gaskets, the" are prone to leakage as the gaskets age and are attacked b" boiler treatment chemicals.

Working Pressure and Temperature Boilers are classied as either low pressure or high pressure and are constructed to meet 3S9: Boiler and ;ressure ;ressure pressure =ow>pressure boilers are limited to a ma*imum working pressure of +% psig -pound>force per suare inch gauge for steam and +? psig for hot water -8. 9ost boilers used in (pressure boilers. (igh>pressure boilers are constructed to operate above the limits l imits set for low>pressure boilers, and are t"picall" used for

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power generation. Operating water temperatures for hot water boilers are limited to 8%o ) -8.

Fuel Type In commercial buildings, natural gas is the most common boiler fuel, because it is usuall" readil" available, burns cleanl", and is t"picall" less e*pensive than oil or electricit". electricit". Some boilers boiler s are designed to burn more than one fuel -t"picall" natural gas and fuel oil. @ual fuel boilers provide the operator with fuel redundanc" in the event of a fuel suppl" interruption. !he" also allow the customer to utilize the fuel oil during Apeak time rates for natural gas. In times when the rates for natural gas are greater than the alternate fuel, this can reduce fuel costs b" using the cheaper alternate al ternate fuel and limiting natural gas use to occur onl" during Ao' peak times. :lectric boilers are used in facilities with reuirements for a small amount of steam or where natural gas is not available. :lectric boilers are known for being clean, uiet, and eas" to install, and compact. !he lack of combustion results in reduced comple*it" in design and operation and less maintenance. (eating elements are easil" replaced if the" fail. fail . !hese t"pes of boilers can be used to produce low or high pressure steam or water, and ma" be good alternatives for customers who are restricted b" emissions regulations. Sizes range from 4, to ++,, B!7 input with overall e#cienc" generall" in the range of C8& to C?& -8.

Draft Methods  !he pressure di'erence di'erence between the boiler combustion cha chamber mber and the 0ue -also called the e*haust stack produces a draft which carries the combustion products through the boiler and up the 0ue. Datural draft boilers rel" on the natural buo"anc" of hot gasses to e*haust combustion products up the boiler 0ue and draw fresh air into i nto the combustion chamber. chamber. 9echanical draft boilers includeE )orced @raft, where air is forced into the combustion chamber b" a fan or blower to maintain a positive pressureF and Induced @raft, where air is drawn through the combustion chamber b" a fan or blower to maintain a negative pressure. Size and Capacity 9odular Boilers are small in size and capacit" and are often intended to replace a large single boiler with several small boilers. !hese modular boilers can easil" t through a standard doorwa", and be transported in elevators and stairwa"s. !he units can be arranged in a variet" of congurations to utilize limited space or to accommodate new euipment. 9odular boilers can be staged to e#cientl" meet the demand of the heating load. Condensing Method  !  !raditional raditional hot water boilers operate without condensing condensing out water vapor from the 0ue gas. !his is critical to prevent corrosion of the boiler components.

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ondensing Boilers operate at a lower return water temperature than traditional boilers, which causes water vapor to condense out of the e*haust gasses. !his allows the condensing boiler to e*tract additional heat from the phase change from water vapor to liuid and increases boiler e#cienc". e#cienc". Some carbon dio*ide dissolves in the condensate and forms carbonic acid. 2hile some condensing boilers are made to handle the corrosive condensation, others reuire some means of neutralizing the condensate. !raditional !raditional non>condensing boilers t"picall" operate in the range of $%& G H?& combustion e#cienc", while condensing boilers generall" operate in the range of HH& to C%& combustion e#cienc" -8.

Key Components of Boilers  !he ke" elements of a boiler include the burner, combustion combustion chamber, heat e*changer, e*haust e*haust stack, and controls. Boiler accessories including the 0ue gas economizer are also commonl" used as an e'ective method to recover heat from a boiler and will be discussed brie0" in the section Best ;ractices for :#cient Operation. Datural gas boilers emplo" one of two t"pes of burners, atmospheric burners, also called natural draft burners and forced fo rced draft burners, also called power burners. @ue to more stringent federal and state air ualit" regulations, low DO* burners and pre>mi* burners are becoming more commonl" used and even reuired in some areas. B" ensuring e#cient mi*ing of air ai r and fuel as it i t enters the burner, these t"pes of burners can ensure that DO* emissions are reduced.

 !he combustion chamber, usuall" made of cast iron or steel, hous houses es the burners and combustion process. !emperatures inside the combustion chamber can reach several hundred degrees ver" uickl". (eat e*changers ma" be made from cast iron, steel tube bundles, or, in the case of some smaller boilers, boile rs, copper or copper>clad steel. !he e*haust stack or 0ue is the piping that conve"s the hot combustion gasses awa" from f rom the boiler to the outside. !"picall" !"picall" this piping is made of steel, but in the case of condensing boilers it needs to be constructed of stainless steel to handle the corrosive condensate. 3nother consideration is whether the e*haust stack will be under a positive or negative pressure. !his can determine how the oints of the e*haust stack must be sealed.

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Boiler controls help produce hot water or steam in a regulated, e#cient, and safe manner.. ombustion and operating controls regulate the rate of fuel use to meet manner the demand. !he main operating control monitors hot water temperature or steam pressure and sends a signal to control the ring rate, the rate at which fuel and air enters the burner. burner. ommon burner ring seuences include on/o', high/low/o' and modulating. Boiler safet" controls include high pressure and temperature, high and low gas/oil pressure, and high and low water level and 0ame safeguard controls.  !hese controls are are considered safeties or limits that that break the electrical circuit to prevent ring of the boiler. boiler. )or e*ample, in the event pressure in the boiler e*ceeds the pressure pressure limit setting, the fuel valve is closed to prevent an unsafe, high pressure condition. !he safet" circuit of a 0ame safeguard control s"stem t"picall" includes switch contacts for low water cuto', high limits, air proving switches, redundant safet" and operating controls, and 0ame detectors. )lame detectors often consist of 0ame rods, and ultraviolet or infrared scanners to monitor the 0ame condition and deactivate the burner in the event of a non ignition or other unsafe condition. )lame )l ame safeguard controls are programmed to operate the burner and c"cle it through the stages of operation.

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2. Boiler Operation  " boiler boiler operates operates using the the feedwater feedwater system$ the ste steam am system$ the fuel system and the draft  draft  system. The feedwater system supplies water to the boiler. The steam system controls and directs the steam produced in the boiler. The fuel system supplies fuel and controls combustion to produce heat. The draft system regulates the movement of air for combustion and evacuates gases of combustion. ,ater$ steam fittings and accessories are re/uired to supply and control water and steam in the boiler. Boiler fittings or trim are components such as valves directly attached to the boiler. "ccessories are pieces of e/uipment not necessarily attached to the boiler$ but re/uired for the operation of the boiler.

Safety Valves are the most important fittings on the boiler. They should open to release pressure when pressure inside the boiler e-ceeds the ma-imum allowable wor0ing pressure or ",. Safety valves are installed at the highest part of the steam side of the boiler. 3o other valve shall be installed between the boiler and the safety valve. Safety valve capacity is measured in the amount of the steam that can be discharged per hour. The safety valve will remain open until sufficient steam is released

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and there is a specific amount of drop in pressure. This drop in pressure is the blowdown of the safety valve. Safety valve capacity and blowdown is listed on the data plate on the safety valve. Spring loaded safety valves are the most common safety valves. " spring e-erts pressure on the valve against the valve seat to 0eep the valve closed. ,hen pressure inside the boiler e-ceeds the set popping pressure$ the pressure forces the valve open to release. The "S4 5ode specifies the design$ materials and construction of safety valves. The number of safety valves re/uired and the fre/uency and procedures for testing safety valves is also specified by the "S4 5ode. "djustment or  repairs to safety valves must be performed by the manufacturer or an assembler authori%ed by the manufacturer. Typical e-amples of safety valves used on steam systems are shown below.

,ater fittings and accessories control the amount$ pressure and temperature of water supplied to and from the boiler. ,ater in the boiler must be maintained at the normal operating water level or 3O,L. Low water conditions can damage the boiler and could cause a boiler e-plosion. 6igh water conditions can cause carryover. 5arryover occurs when small water droplets are carried in steam

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lines. 5arryover can result in water hammer. ,ater hammer is a banging condition caused by hydraulic pressure that can damage e/uipment. Feedwater Valves control the flow of feedwater from the feedwater pump to the boiler. #eedwater stop valves are globe valves located on the feedwater line. They isolate the boiler from feedwater accessories. The feedwater stop valve is positioned closest to the boiler to stop the flow of water out of the boiler for maintenance$ or if the chec0 valve malfunctions. The feedwater chec0 valve is located ne-t to the feedwater stop valve and prevents feedwater from flowing from the boiler bac0 to the feedwater pump. The feedwater chec0 valve opens and closes automatically with a swinging disc. ,hen water is fed to the boiler it opens. 8f water flows bac0 from the boiler the valve closes.

Water Column minimi%es the water turbulence in the gage glass to provide accurate water level reading. ,ater columns are located at the 3O,L$ with the lowest part of the water column positioned at least 9 above the heating system. ,ater columns for high pressure boilers consist of the main column and three tricoc0s. 6igh and low water alarms or whistles may be attached to the top and bottom tricoc0s. The Gage Glass is used to visually monitor the water level in the boiler. 8solation valves located at the top and bottom permit the changing of gage glasses.  " Blowdown Valve at the bottom of the gage glass is used to remove sludge and sediment. Tubular gage glasses are used for  pressure up to !'' psig. "ll boilers must have two methods of determining the boiler water level. The gage glass serves as the primary method of determining boiler water level. 8f the water cannot be seen in the gage glass$ the tricoc0s are used as a secondary method of determining boiler water level. The middle tricoc0 is located at the 3O,L. 8f water comes out of the middle tricoc0$ the gage glass is not functioning properly. 8f water comes out of the top tricoc0$ there is a high water condition in the boiler. 8f water comes out of the bottom tricoc0$ water may be safely added to the boiler. 8f steam comes out of the bottom tricoc0$ water must not be added to the boiler. Secure the fuel immed imm ediat iatel el . " "ddi ddin n water water cou could ld caus cause e a boil boiler er ee- losio losion. n. .

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Makeup Water replaces boiler water lost from lea0s or from the lac0 of condensate returned in the boiler. a0eup water is fed manually or automatically. Boilers can have both manual and automatic systems. 8f the boiler has both$ the manual always bypasses the automatic automatic system. Boiler operators must 0now how to supply ma0eup water /uic0ly to the boiler in the event of a low water condition. anual systems feed city water with a hand operated valve. "utomatic systems feed city water with a float control valve mounted slightly below the 3O,L. 8f the float drops from a low water level$ the valve in the city cit y water line is open. "s the water level rises$ the float rises to close the valve. The Low Water Fuel Cut Off shuts off fuel to the burner in the event of a low water condition in the boiler. The low water fuel cut off is located 29 to 9 below the 3O,L. Low water fuel cut offs are available with or without an integral water column. Low water fuel cut offs must be tested monthly or more often depending on plant procedures and re/uirements. Low water fuel cut offs operate using an electric probe or a float sensor. The float senses a drop in water level. Switches in the low water fuel cut off are wired to the burner control to shut off fuel to the burner when the water level drops in the chamber. The Feedwater Regulator maintains the 3O,L in the boiler by controlling the amount of condensate return pumped to the boiler from the condensate return tan0. The correct water level is maintained with a feedwater regulator$ but boiler water level must still be chec0ed periodically by the boiler operator. Feedwater Pumps are used with feedwater regulators to pump feedwater to the boiler. ressure must be sufficient to overcome boiler water pressure to maintain the 3O,L in the boiler. #or ma-imum safety$ plants having one steam driven feedwater pump must have a bac0 up feedwater pump driven by electricity. #eedwater pumps may be reciprocating$ centrifugal or turbine. Re!proat!ng feedwater pumps are steam driven and use a piston to discharge water to the feedwater line. They are limited in capacity and are used on small boilers. Centr!fugal feedwater pumps are electric motor or steam driven. They are the most common feedwater pump. 5entrifugal force moves water to the outside edge of the rotatingimpeller. The casing directs water from the impeller to the discharge piping. ;ischarge pressure is dependent on impeller speed.

"ur#!ne feedwater pumps are steam driven and operate similarly to centrifugal feedwater pumps. Feedwater $eaters heat water before it enters the boiler drum to remove o-ygen and other gases which may cause corrosion. #eedwater heaters are either open or closed. Open feedwater heaters allow steam and water to mi- as they enter an enclosed steel chamber. They are located above the feedwater pump to produce a positive pressure on the suction side of the pump. 5losed feedwater heaters have a large number of tubes inside an enclosed steel vessel. Steam and water do not come in contact$ but feedwater goes through the tubes and steam is allowed in the vessel to preheat the feedwater. They are located on the discharge side of the feedwater pump.

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Bottom Blowdown Valves release water from the boiler to reduce water level$ remove sludge and sediment$ reduce chemical concentrations or drain the boiler. Two valves are commonly used$ a /uic0 opening and screw valve. ;uring blowdown the /uic0 opening valve is opened first$ the screw valve is opened ne-t and ta0es the wear and tear from blowdown. ,ater is discharged to the blowdown tan0. " blowdown tan0 collects water to protect the sewer from the hot boiler water. "fter blowdown$ the screw valve is closed first and the /uic0 opening valve is closed last Steam F!tt!ngs % &essor!es remove air$ control steam flow$ and maintain the re/uired steam pressure in the boiler. Steam fittings are also used to direct steam to various locations for heating and process. Steam Pressure Gages and vacuum gages monitor pressure inside the boiler. The range of these gages should be 1