Steam System Basics.pdf

Steam System Basics Course No: D03-001

Credit: 3 PDH

Gilbert Gedeon, P.E.

Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 [email protected]

3Why Steam?There are three principal forms of energy used inindustrial processes: electricity, direct-fired heat,and steam. Electricity is used in many differentways, including mechanical drive, heating, andelectrochemical reactions. Direct-fired energydirectly transfers the heat of fuel combustion to a process. Steam provides process heating,pressure control, mechanical drive, and compo-nent separation, and, is a source of water formany process reactions.

Steam has many performance advantages thatmake it an indispensable means of deliveringenergy. These advantages include low toxicity,ease of transportability, high efficiency, high heatcapacity, and low cost with respect to the otheralternatives. Steam holds a significant amount ofenergy on a unit mass basis (between 1,000 and1,250 British thermal units per pound [Btu/lb])that can be extracted as mechanical workthrough a turbine or as heat for process use.Since most of the heat content of steam is storedas latent heat, large quantities of heat can betransferred efficiently at a constant temperature,which is a useful attribute in many process heat-ing applications.

Steam is also used in many direct contact appli-cations. For example, steam is used as a source ofhydrogen in steam methane reforming, which isan important process for many chemical andpetroleum refining applications. Steam is alsoused to control the pressures and temperatures of many chemical processes. Other significantapplications of steam are to strip contaminantsfrom a process fluid, to facilitate the fractionationof hydrocarbon components, and to dry all typesof paper products.

The many advantages that are available fromsteam are reflected in the significant amount of energy that industry uses to generate it. Forexample, in 1994, industry used about 5,676 trillion Btu of steam energy, which representsabout 34% of the total energy used in industrialapplications for product output.1

Steam use in the Industries of the Future2 is especially significant. For example, in 1994, the pulp and paper industry used approximately2,197 trillion Btu of energy to generate steam,accounting for about 83% of the total energyused by this industry. The chemicals industryused approximately 1,855 trillion Btu of energyto generate steam, which represents about 57% of the total energy used in this industry.The petroleum refining industry used about1,373 trillion Btu of energy to generate steam,which accounts for about 42% of this industrystotal energy use.3

Steam System OperationThis Sourcebook uses four categories to discusssteam system components and ways to enhancesteam system performance: generation, distribu-tion, end use, and recovery. These four areas follow the path of steam as it leaves the boilerand returns via the condensate return system.

GenerationSteam is generated in a boiler or a heat recoverysteam generator by transferring the heat of combustion gases to water. When water absorbsenough heat, it changes phase from liquid tosteam. In some boilers, a superheater furtherincreases the energy content of the steam. Underpressure, the steam then flows from the boiler or steam generator and into the distribution system.

A Sourcebook for Industry

Section 1: Steam System Basics

1 Arthur D. Little, Overview of Energy Flow for Industries in Standard Industrial Classifications 2039, December, 2000.2 DOEs Industries of the Future (IOF) include: aluminum, chemicals, forest products, glass, metal casting, mining,

petroleum refining, and steel.3 Resource Dynamics Corporation estimates.

Steam System Basics

4 DistributionThe distribution system carries steam from theboiler or generator to the points of end use.Many distribution systems have several take-offlines that operate at different pressures. Thesedistribution lines are separated by various typesof isolation valves, pressure-regulating valves,and, sometimes, backpressure turbines. A properlyperforming distribution system delivers sufficientquantities of high quality steam at the right pres-sures and temperatures to the end uses. Effectivedistribution system performance requires propersteam pressure balance, good condensate drain-age, adequate insulation, and effective pressureregulation.

End UseThere are many different end uses of steam.Examples of steams diverse uses include processheating, mechanical drive, moderation of chemi-cal reactions, and fractionation of hydrocarboncomponents. Common steam system end-useequipment includes heat exchangers, turbines,fractionating towers, strippers, and chemicalreaction vessels.

In a heat exchanger, the steam transfers its latentheat to a process fluid. The steam is held in theheat exchanger by a steam trap until it condenses,

at which point the trap passes the condensateinto the condensate return system. In a turbine,the steam transforms its energy to mechanicalwork to drive rotating machinery such as pumps,compressors, or electric generators. In fractionat-ing towers, steam facilitates the separation of various components of a process fluid. In strip-ping applications, the steam pulls contaminantsout of a process fluid. Steam is also used as asource of water for certain chemical reactions. Insteam methane reforming, steam is a source ofhydrogen.

RecoveryThe condensate return system sends the conden-sate back to the boiler. The condensate is returnedto a collection tank. Sometimes the makeupwater and chemicals are added here while othertimes this is done in the deaerator. From the col-lection tank the condensate is pumped to thedeaerator, which strips oxygen and non-condens-able gases. The boiler feed pumps increase thefeedwater pressure to above boiler pressure andinject it into the boiler to complete the cycle.

Figure 1 provides a general schematic descriptionof the four principal areas of a steam system. Thefollowing sections discuss the components inthese areas in greater detail.

Improving Steam System Performance

Steam System Basics

CombustionGases

CondensateReceiverTank

PressureReducing Valve

FeedPump

SteamTrap

SteamTrap

SteamTrap

Economizer

Combustion Air Condensate Pump

Process Heater

Process Heater

Isolation Valve

Boiler

Deaerator

Fuel

Combustion AirPreheater

Shell and TubeHeat Exchanger

Forced DraftFan

Distribution

Recovery

End Use

Figure 1. Steam System Schematic

Generation

5GenerationThe generation part of a steam system uses aboiler to add energy to a feedwater supply togenerate steam. The energy is released from thecombustion of fossil fuels or from process wasteheat. The boiler provides a heat transfer surface(generally a set of tubes) between the combustionproducts and the water. The most importantparts of the generating system include the boiler,the fuel supply, combustion air system, feedwatersystem, and exhaust gases venting system. Thesesystems are related, since problems or changes inone generally affect the performance of the others.

BoilersThere are two basic types of boilers: firetube andwatertube. The fundamental difference betweenthese boiler types is which side of the boiler tubescontains the combustion gases or the boilerwater/steam.

Firetube boilers. In firetube boilers, the combus-tion gases pass inside boiler tubes, and heat istransferred to water on the shell side. A represen-tative firetube boiler is shown in Figure 2. Scotchmarine boilers are the most common type ofindustrial firetube boiler. The Scotch marine boiler is an industry workhorse due to low initialcost, and advantages in efficiency and durability.Scotch marine boilers are typically cylindricalshells with horizontal tubes configured such thatthe exhaust gases pass through these tubes, trans-ferring energy to boiler water on the shell side.

Scotch marine boilers contain relatively largeamounts of water, which enables them torespond to load changes with relatively littlechange in pressure. However, since the boilertypically holds a large water mass, it requiresmore time to initiate steaming and more time to accommodate changes in steam pressure. Also, Scotch marine boilers generate steam on


Steam System Basics

Figure 2. Firetube Boiler4

4 Guideline for Gas and Oil Emission Factors for Industrial, Commercial, and Institutional (ICI) Boilers, American Boiler Manufacturers Association, Arlington, Virginia, 1997.

6the shell side, which has a large surface area, limiting the amount of pressure they can generate.In general, Scotch marine boilers are not usedwhere pressures above 300 psig are required.Today, the biggest firetube boilers are over 1,500 boiler horsepower (about 50,000 lbs/hr).5

Firetube boilers are often characterized by theirnumber of passes, referring to the number oftimes the combustion (or flue) gases flow thelength of the pressure vessel as they transfer heatto the water. Each pass sends the flue gasesthrough the tubes in the opposite direction. Tomake another pass, the gases turn 180 degreesand pass back through the shell. The turnaroundzones can be either dryback or water-back. Indryback designs, the turnaround area is refractory-lined. In water-back designs, this turnaroundzone is water-cooled, eliminating the need forthe refractory lining.

Watertube boilers. In watertube boilers, boilerwater passes through the tubes while the exhaustgases remain in the shell side, passing over thetube surfaces. A representative watertube boiler isshown in Figure 3. Since tubes can typicallywithstand higher internal pressure than the largechamber shell in a firetube, watertube boilers areused where high steam pressures (3,000 poundsper square inch [psi], sometimes higher) arerequired. Watertube boilers are also capable ofhigh efficiencies and can generate saturated orsuperheated steam. In fact, the ability of water-tube boilers to generate superheated steam makesthese boilers particularly attractive in applica-tions that require dry, high-pressure, high-energysteam, including steam turbine power generation.

The performance characteristics of watertube boilers make them highly favorable in processindustries, including chemical manufacturing,


Steam System Basics

5 1 boiler horsepower = 33,475 Btu/hr6 Guideline for Gas and Oil Emission Factors for Industrial, Commercial, and Institutional (ICI) Boilers, American Boiler

Manufacturers Association, Arlington, Virginia, 1997.

Figure 3. Watertube Boiler6

7pulp and paper manufacturing, and refining.Although firetube boilers account for the majorityof boiler sales in terms of units, water-tube boil-ers account for the majority of boiler capacity.7

Waste Heat Recovery Boiler (WHRB). These boilersmay be either firetube or watertube design anduse heat that would otherwise be discarded togenerate steam. Typical sources of heat forWHRBs include exhaust gases or high tempera-ture products from an external manufacturingprocess in refineries and chemical manufacturingfacilities, or combustion of a waste fuel in theboiler furnace.

Heat Recovery Steam Generators (HRSGs). HRSGstransfer energy from the exhaust of a gas turbineto an unfired or supplementary fired heat-recov-ery steam generator to produce steam. Exhaustgases leave the gas turbine at temperatures of1000F (538C) or higher and can represent morethan 75% of the total fuel energy input. Thisenergy can be recovered by passing the gasesthrough a heat exchanger (steam generator) toproduce hot water or steam for process needs. Ifthe amount of steam needed by the processexceeds the amount produced by simple heatrecovery, then supplementary fuel can be burnedin the ducting between the gas turbine and theHRSG.

Superheaters. Superheaters add energy to steam,resulting in a steam temperature that exceeds the saturation temperature at a specific pressure.Superheaters can be convective or radiant. Rad-iative superheaters rely on the energy transferreddirectly from the combustion flame to increasethe energy level of the steam, while convectivesuperheaters rely on the transfer of additionalenergy from the flue gases to the steam.

Economizers. In many boilers, the flue gases stillhave useful amounts of energy even after theyhave passed through the boiler. In many of these applications, economizers provide effectivemethods of increasing boiler efficiency by transferring the heat of the flue gases to incom-ing feedwater. There are two principal types ofeconomizers: noncondensing and condensing.Noncondensing economizers are usually air-to-

water heat exchangers. Since these economizersare not designed to handle flue gas condensation,noncondensing economizers must be operated attemperatures that are reasonably above the dewpoints of the flue gas components. The dewpoint of the flue gases depends largely on theamount of water in the gas, which, in turn, isrelated to the amount of hydrogen in the fuel.For example, to avoid condensation in theexhaust gases produced by burning natural gas,the exhaust gas temperature should typically bekept above 250F. Condensing economizers aredesigned to allow condensation of the exhaustgas components. Due to latent heat recovery,these economizers typically extract more energythan do noncondensing economizers. Often, special materials are required.

For more information on economizers, see theSteam Tip Sheet Number 3 titled Use FeedwaterEconomizers for Waste Heat Recovery in Appendix B.

Combustion air preheaters. Combustion air pre-heaters are similar to economizers in that theytransfer energy from the flue gases back into thesystem. In these devices, however, the energy istransferred to the incoming combustion air. Theefficiency benefit is roughly 1% for every 40Fincrease in the combustion air temperature.8

Boiler InsulationThe walls and combustion regions of boilers aretypically lined with insulating materials toreduce energy loss and to prevent leakage. Thereare several types of boiler insulating materials,including brick, refractory, insulation and lagging.The selection and design of boiler insulatingmaterials depends largely on the age and designof the boiler. Since the insulating lining isexposed to high temperatures and is subject todegradation, it should be periodically inspectedand repaired when necessary.

Boiler Control SystemBoiler control systems are designed to protect theboiler and to ensure proper boiler operation.These systems include the combustion controlsystem, flame safeguard, water level control, andfuel control.


Steam System Basics

7 GRI, Analysis of the Industrial Boiler Population, Final Report No.-96/0200, 1996.8 Boiler Efficiency Institute, Boiler Efficiency Improvement, 1991.

8Combustion control system. The combustion controlsystem regulates the fuel air mixture to achievesafe and efficient combustion and maintainssteam system pressure. Control systems havevarying levels of sophistication. Simple systemsuse a fixed linkage between the fuel-regulatingvalve and the combustion air damper. This iscalled single point positioning. A change insteam pressure makes a proportional change inthe combustion air and fuel. Advanced systemsrely on signals from transmitters to determineindependent fuel valve and air damper positions.This is called a full monitoring system.

For more information, see the Steam Tip SheetNumber 4 titled Improve Your Boilers CombustionEfficiency in Appendix B.

Burner flame safeguard system. A flame safeguardsystem is an arrangement of flame detection systems, interlocks, and relays which will sensethe presence of a proper flame in a furnace andcause fuel to be shut off if a hazardous conditiondevelops. Modern combustion systems are close-ly interlocked with flame safeguard systems andalso pressure-limit switches, low-water level cut-offs, and other safety controls that will stop theenergy input to a boiler when an unsafe condi-tion develops. The flame safeguard system sensesthe presence of a good flame or proper combus-tion and programs the operation of a burner system so that motors, blowers, ignition, and fuelvalves are energized only when they are neededand then in proper sequence.

Safety shutoff valve. Safety shutoff valves isolate thefuel supply to the boiler in response to certainconditions such as low or high gas pressure orsatisfied load demand. The type of safety shutoffvalves and the settings are often determined bycode or insurance requirements.

Water level control. The boiler water level controlsystem ensures a safe water level in the boiler.Typically, the control system provides a signal tothe feedwater control valve to regulate the feedrate. Simple water level control systems that onlysense water level are single element systems. Morecomplex systems incorporate additional datasuch as steam flow rate (dual element system)and feedwater flow (triple element system) andwill provide better water level control duringabrupt load changes.

Safety valve. The safety valve is the most importantvalve on the boiler and keeps the boiler fromexceeding its maximum allowable working pressure (MAWP).

Steam pressure control. Steam pressure controlsregulate the combustion equipment to maintaina constant pressure in the steam header. As thepressure rises above or falls below the pressuresetting, the control adjusts the burner firing rateto bring the pressure back to the setpoint.

Nonreturn valve. The nonreturn valve is a combi-nation shutoff and check valve that allows steamout of the boiler, but prevents backflow from thesteam header in the event the boiler pressuredrops below that of the header. The valve isopened only when the pressure inside the boilerrises slightly above the steam header pressure.

Steam flow meter. Steam flow meters are helpful in evaluating the performance of the system and can provide useful data in assessing boiler performance, calculating boiler efficiency, andtracking the amount of steam required by thesystem. In some systems, steam flow meters provide a measurement signal for the boiler control system. Additionally, steam flow meterscan be useful in benchmarking efforts.

There are three basic types of steam flowmeters:differential pressure (DP), vortex, and Coriolis.Differential pressure flowmeters rely on thechange in pressure as steam flows by an elementsuch as a nozzle, orifice, or venturi. This pressuredifference provides an indication of flow velocity,which, in turn, can be used to determine theflow rate. Vortex flowmeters rely on the principalthat flow past an element creates vortices thathave frequencies that correspond to the flowvelocity. Coriolis flowmeters rely on tubes placedin the steam flow path that twist according tothe velocity of the flow.

Boiler Feedwater SystemThe boiler feedwater system supplies water to theboiler. Sources of feedwater include returningcondensate and makeup water. Feedwater is typically stored in a collecting tank to ensurethat a steady supply of heated water is availableto the boiler.


Steam System Basics

9Feedwater flow control valve. A modulating feed-water flow control valve moves up or down inresponse to the water level transmitter(s). Onsmaller firetube boilers, it is not uncommon for the feedwater valve to operate in a closed or open position, depending on the water leveltransmitter signal.

Softener. Softeners remove hardness minerals,such as calcium, magnesium, and iron, from awater supply. The presence of hardness in boilerwater leads to many problems, including scalebuildup and foaming, which reduce boiler efficiency and can cause tube failure. Softenersreduce this problem through an ion exchangeprocess. As the hard water passes through achamber filled with resin, an exchange occursthat removes hardness minerals from the water.The sodium that replaces the hardness mineralshas a higher solubility in water and generally will not form scale.

Pretreatment equipment. Pretreatment equipmentimproves the quality of the incoming water sothat it may be used in the boiler without exces-sive scaling or foaming, which can reduce boilerefficiency and cause tube failure. Pretreatmentequipment includes, but is not limited to, clarifiers,filters, softeners, dealkalizers, decarbonators,reverse osmosis (RO) units, and demineralizers.

Deaerator, deaerating heater, and atmospheric deaera-tor. The presence of oxygen in the boiler systemcan be a significant problem due to its corrosivityat high temperatures. Deaerators and deaeratingheaters use heat, typically steam, to reduce theoxygen content in water. Deaerators and deaerat-ing heaters are typically pressurized tanks thatraise the water temperature to the point of satu-ration. They also break the incoming water intoeither fine droplets or thin sheets to facilitate theremoval of oxygen and other non-condensablegases. Depending on the design, the feedwateroxygen content can be reduced to levels rangingfrom 7 to 40 parts per billion (ppb).

Atmospheric deaerators are typically found insmaller, lower-pressure boiler systems. Theyoperate at atmospheric pressure, so the maximumoperating temperature is 212F. Most will operateat temperatures lower than this. Atmosphericdeaerators cannot achieve the same level of

oxygen removal as deaerators and deaeratingheaters, typically providing water with oxygen levels of 0.5 to 1 parts per million (ppm).

In applications that require lower oxygen levelsthan achievable with a deaerator, deaeratingheater, or open feedwater heater, a chemicalagent, known as an oxygen scavenger, can beused to remove more oxygen. In most systems,an oxygen scavenger is part of the systems watertreatment program.

For more information on these devices, see theSteam Tip Sheet Number 18 titled Deaerators inIndustrial Steam Systems, provided in Appendix B.

Feedwater pump. Feedwater pumps transfer waterfrom the deaerator to the boiler. Feedwaterpumps are driven by electric motors or by steamturbines. In a modulating feedwater system, thefeedwater pumps run constantly as opposed toan on/off operation in relatively small boilers.

Collecting/Storage tank. The return of condensateis often erratic due to changing steam require-ments by the end uses. The condensate is usually returned to a condensate receiver ordirectly to the deaerator if the system does nothave a receiver. Pretreated water may also be storedin a tank prior to use. This provides the boiler system with additional water capacity in case the pretreatment equipment malfunctions. Thecondensate and pretreated water, or makeup, are transferred from the storage tanks to thedeaerator prior to being sent to the boiler.

Boiler Combustion Air SystemThe combustion air system supplies the oxygennecessary for the combustion reaction. To pro-vide enough air for the amount of fuel used inindustrial boilers, fans are typically required.Dampers, inlet valves, or variable speed drivestypically control the amount of air allowed intothe boiler.

Forced draft fan. A forced draft fan is located atthe inlet of a boiler and pushes ambient air intothe burner region, ensuring that adequate air isdelivered to the combustion process. These fanseither pull air directly from the boiler room orconnect to a duct system that allows outside airto be drawn into the boiler.


Steam System Basics

10

Induced draft fan. Induced draft fans are locatedon the outlet gas side of the boiler and pull fluegases out. The induced draft fan creates a slightlynegative furnace pressure that is controlled byoutlet dampers on the boiler. In some systemswhere a bag house, mechanical collector, or precipitator is involved, special considerationsshould be given in sizing and selection of this fan.

Damper. Dampers control the amount of airallowed into and out of a combustion chamber.Dampers, in combination with fuel regulatingdevices, are positioned by the combustion controlsystem to achieve certain fuel-to-air ratios.Dampers on the boiler outlet are used to regulatethe negative furnace draft.

Boiler Fuel SystemThere are many different types of fuels used inboilers, requiring several different types of fuelhandling systems. Fossil fuels such as coal, oil,and gas are most commonly used. Waste fuels areused in many industries, particularly the forestproducts, petroleum refining, and chemical man-ufacturing industries where there is an availablesupply of waste products such as bark, woodchips, black liquor, and refinery gas.

Fuel regulating valve. In gaseous and liquid fuels,regulating valves control the fuel delivered to theboiler. In many systems, these valves can bequickly shut in response to an operating problem.

Fuel. The fuel types that are commonly used inboilers include natural gas, coal, propane, fueloils, and waste fuels (for example, black liquor,bark, and refinery gas). Fuel type significantlyaffects boiler operation, including efficiency, emissions, and operating cost. Natural gasaccounts for about 36% of the total U.S. industryboiler capacity. Coal accounts for about 14% ofthe boiler capacity. Fuel oils account for about21%. Other fuels, which include waste fuels,account for about 29% of the boiler capacity.9

Fuel flow meter. Fuel meters measure the amount offuel delivered to a boiler. Fuel meters provideessential data in determining boiler efficiency.Since fuel flow meters measure volume or mass offuel, it is important to know the energy contentof the fuel when determining boiler efficiency.

For more information, see the Steam Tip SheetNumber 15 titled Benchmark the Fuel Cost ofSteam Generation in Appendix B.

Burner. Burners combine the fuel and air to initiatecombustion. There are many different types ofburners due to the many different types of fuels.Additionally, burners have different performancecharacteristics and control requirements. Someburners are on/off while others allow precise setting of the fuel:air mixture over a range ofconditions. Some burners can fire different typesof fuel, allowing boiler operation to continuedespite the loss of one fuel supply.

Boiler Blowdown SystemThe boiler blowdown system includes the valvesand the controls for the continuous blowdownand bottom blowdown services. Continuousblowdown removes a specific amount of boilerwater (often measured in terms of percentage offeedwater flow) in order to maintain a desiredlevel of total dissolved solids in the boiler. Set-ting the flow for the continuous blowdown istypically done in conjunction with the watertreatment program. Some continuous blowdownsystems rely on the input of sensors that detectthe level of dissolved solids in the boiler water.

The bottom blowdown is performed to removeparticulates and sludge from the bottom of theboiler. Bottom blowdowns are periodic and aretypically performed a certain number of timesper shift or according to a set schedule. In somesystems, bottom blowdowns are controlled by an automatic timer. Bottom blowdown shouldnever be permitted unless it is recommended by the boiler manufacturer. This is because inhigher pressure boilers, especially those above700 pounds per square inch gauge (psig), bottom blowdown may cause water starvation in someportions of the boiler circuit.

Boiler blowdown heat exchangers and flash tank. The continuous blowdown water has the same temperature and pressure as the boiler water.Before this high energy water is discharged intothe environment, it is often sent to a heatexchanger and flash tank. Flash tanks permit therecovery of low-pressure flash steam, which canbe used in deaeration or process heating. Theyalso permit the use of a smaller heat exchanger


Steam System Basics

9 Derived from GRI, Analysis of the Industrial Boiler Population, Final Report No.-96/0200, 1996.

11

than would be required without the flash tank. Blowdown heat exchangers are most often usedto preheat boiler makeup water.

For more information on boiler blowdowns, seethe Steam Tip Sheets Numbers 9 and 10 titledMinimize Boiler Blowdown, and Recover Heat fromBoiler Blowdown in Appendix B.

DistributionThe distribution system transports steam fromthe boiler to the various end uses. Although distribution systems may appear to be passive, in reality, these systems regulate the delivery ofsteam and respond to changing temperature andpressure requirements. Consequently, proper performance of the distribution system requirescareful design practices and effective maintenance.The piping should be properly sized, supported,insulated, and configured with adequate flexibility.Pressure-regulating devices such as pressure-reducing valves and backpressure turbines shouldbe configured to provide proper steam balanceamong the different steam headers. Additionally,the distribution system should be configured toallow adequate condensate drainage, whichrequires adequate drip leg capacity and propersteam trap selection. Steam distribution systemscan be broken down into three different categories:buried pipe, above-ground, and building sections,and selection of distribution components (piping,insulation, etc.) can vary depending on the category.

PipingSteam piping transports steam from the boiler tothe end-use services. Important characteristics ofwell-designed steam system piping are that it isadequately sized, configured, and supported.Installation of larger pipe diameters may be moreexpensive, but can create less pressure drop for agiven flow rate. Additionally, larger pipe diametershelp to reduce the noise associated with steamflow. As such, consideration should be given tothe type of environment in which the steam piping will be located when selecting the pipediameter. Important configuration issues areflexibility and drainage. With respect to flexibility,piping (especially at equipment connections),needs to accommodate thermal reactions duringsystem start-ups and shutdowns. Additionally,piping should be equipped with a sufficient

number of appropriately sized drip legs to promoteeffective condensate drainage. Additionally, thepiping should be pitched properly to promotethe drainage of condensate to these drip lines.Typically, these drainage points experience twovery different operating conditions, normal operation and start-up; both load conditionsshould be considered in the initial design.

InsulationThermal insulation provides important safety, energy savings, and performance benefits. Interms of safety, insulation reduces the outer surface temperature of the steam piping, whichlessens the risk of burns. A well-insulated systemalso reduces heat loss to ambient workspaces,which can make the work environment morecomfortable. Consequently, the energy savingbenefits include reduced energy losses from the steam system and reduced burden on thecooling systems that remove heat from workspaces.In addition to its safety and energy benefits,insulation increases the amount of steam energyavailable for end uses by decreasing the amountof heat lost from the distribution system.

Important insulation properties include thermalconductivity, strength, abrasion resistance, work-ability, and resistance to water absorption. Thermalconductivity is the measure of heat transfer perunit thickness. Thermal conductivity of insulationvaries with temperature; consequently, it isimportant to know the right temperature rangewhen selecting insulation. Strength is the meas-ure of the insulations ability to maintain itsintegrity under mechanical loads. Abrasion resist-ance is the ability to withstand shearing forces.Workability is a measure of the ease with whichthe insulation is installed. Water absorptionrefers to the tendency of the insulation to holdmoisture. Insulation blankets (fiberglass and fabric) are commonly used on steam distributioncomponents (valves, expansion joints, turbines,etc.) to enable easy removal and replacement formaintenance tasks.

Some common insulating materials used in steamsystems include calcium silicate, mineral fiber,fiberglass, perlite, and cellular glass. The AmericanSociety for Testing and Materials (ASTM) providesstandards for the required properties of these and other insulation materials. Additionally, the North American Insulation ManufacturersAssociation (NAIMA) has developed a software


Steam System Basics

12

program titled 3E Plus that allows users to determine the energy losses associated with various types and thicknesses of insulation. The 3E Plus program facilitates the assessment ofvarious insulation systems to determine the mostcost-effective solution for a given installation.See Section 2, page 27 for more about 3E PlusInsulation software, which can help steam usersassess insulation opportunities.

For more information on insulation, refer to SteamTip Sheets Numbers 2 and 17 titled Insulate SteamDistribution and Condensate Return Lines and InstallRemovable Insulation on Uninsulated Valves and Fit-tings. Both can be found in Appendix B.

ValvesIn steam systems, the principal functions of valvesare to isolate equipment or system branches, toregulate steam flow, and to prevent overpressur-ization. The principal types of valves used in steamsystems include gate, globe, swing check, pressurereducing, and pressure relief valves. Gate, globe,and swing check valves typically isolate steamfrom a system branch or a component. Pressurereducing valves (PRV) typically maintain certaindownstream steam pressure conditions by con-trolling the amount of steam that is passed. Thesereducing valves are often controlled by transmit-ters that monitor downstream conditions. Pressurerelief valves release steam to prevent overpressur-ization of a system header or equipment.

Steam SeparatorsIn some steam systems, wet steam is generated.This wet steam contains water droplets that canreduce the effectiveness of the steam system.Water droplets erode turbine blades and passages,and pressure reducing valves, thus reducing effi-ciency and life. Furthermore, liquid water cansignificantly reduce heat transfer rates in heatexchange components, as well as result in waterhammer. Removing water droplets before theyreach end-use equipment is necessary.

Steam separators remove water droplets, generallyrelying on controlled centrifugal flow. This actionforces the entrained moisture to the outer wallwhere it is removed from the separator. Themeans of moisture removal could be a steam trap or a drain. Some manufacturers include the

trap as an integral part of the unit. Additionalaccessories include water gauge connections, thermometer connections, and vent connections.

Steam separators can be installed in either a hori-zontal or vertical line. They are capable of remov-ing 99% of particulate entrainment 10 micronsand larger over a wide range of flows. Separatorsare often designed in accordance with ASMECode, Section VIII, Division 1 with pressures to300 psig.

Steam AccumulatorsA steam accumulator is a large insulated pressurevessel, partially filled with hot water (saturatedliquid). When steam supply exceeds demand, theexcess high-pressure steam is charged into theaccumulator through special charging nozzles.The steam is condensed, giving up its latent heat, to raise the pressure, temperature, and heatcontent of the water body. When the steamdemand exceeds the supply, the pressure in theaccumulator drops and the additional requiredsteam flashes from the water, taking back the heatpreviously stored. A simple system of controlvalves and check valves regulates the chargingand discharging. The excess steam is charged quietly and smoothly, and when steam is needed,it is available with the speed of a control valveoperation. There is also an accumulator designthat stores hot water for use as boiler feedwater.

Steam TrapsSteam traps are essential for proper distributionsystem performance. During system start-ups,traps allow air and large quantities of condensateto escape. During system operation, the trapsallow collected condensate to pass into the condensate return system, while minimizing theaccompanying loss of steam. There are three primary types of traps: thermostatic, mechanical,and thermodynamic.10

Thermostatic TrapsThermostatic traps use temperature differential todistinguish between condensate and live steam.This differential is used to open or close a valve.Under normal operating conditions, the conden-sate must cool below the steam temperaturebefore the valve will open. Common types of ther-mostatic traps include bellows and bimetallic traps.


Steam System Basics

10The following discussion of steam traps is based extensively on C. B. Oland, Review of Orifice Plate Steam Traps, Oak Ridge National Laboratory, January 2001.

13

Bellows traps. Bellows traps include a valve element that expands and contracts in responseto temperature changes. Often a volatile chemi-cal such as alcohol or water is inside the element.Evaporation provides the necessary force to changethe position of the valve. At start-up, the bellowstrap is open due to the relative cold condition.This operating condition allows air to escape and provides maximum condensate removalwhen the load is the highest. Bellows traps canfail either open or closed. The configuration of abellows steam trap is shown in Figure 4.

Bimetallic traps. Bimetallic traps rely on the bend-ing of a composite strip of two dissimilar metalsto open and close a valve. Air and condensatepass freely through the valve until the tempera-ture of the bimetallic strip approaches the steamtemperature. After steam or relatively hot con-densate heats the bimetallic strip and causes it toclose the valve, the trap remains shut until thetemperature of the condensate cools sufficientlyto allow the bimetallic strip to return to its origi-nal shape and thereby open the valve. Bimetallictraps can fail in either the open or closed posi-tion. The configuration of a bimetallic steam trapis shown in Figure 5.

Mechanical TrapsMechanical traps use the difference in densitybetween condensate and live steam to produce achange in the position of a float or bucket. Thismovement causes a valve to open or close. Thereare a number of mechanical trap designs that arebased on this principle. They include ball float,float and lever, inverted bucket, open bucket,and float and thermostatic traps.

Ball float traps. Ball float traps rely on the move-ment of a spherical ball to open and close the

outlet opening in the trap body. When no con-densate is present, the ball covers the outletopening, thereby keeping air and steam fromescaping. As condensate accumulates inside thetrap, the ball floats and uncovers the outlet opening. This movement allows the condensateto flow continuously from the trap. Unless theyare equipped with a separate air vent, ball floattraps cannot vent air on start-up.

Float and lever traps. Float and lever traps are simi-lar in operation to ball float traps except the ballis connected to a lever. When the ball floatsupward due to accumulation of condensate insidethe trap body, the attached lever moves and causesa valve to open. This action allows condensate tocontinuously flow from the trap. If the condensateload decreases and steam reaches the trap, down-ward ball movement causes the valve to close,thereby keeping steam from escaping. Unless theyare equipped with a separate air vent, float andlever traps can not vent air on start-up. See the dis-cussion on float and thermostatic traps.

Inverted bucket traps. Inverted bucket traps aresomewhat more complicated than float and levertraps. At start-up, the inverted bucket inside thetrap is resting on the bottom of the trap body andthe valve to which the bucket is linked is wideopen. The trap is initially filled with condensate.As steam enters the trap and is captured insidethe bucket, it causes the bucket to move upward. This upward movement closes the valve and keepssteam from escaping. When the condensate collects and cools the steam, the bucket movesdownward. This movement causes the valve toopen, thereby allowing the condensate to escape.Unlike closed float traps, inverted bucket trapshave intermittent discharge. These traps can be


Steam System Basics

Steam &Condensate In

LiquidCondensate& Flash Out

Bellows Element

Valve

Seat

Steam and/orHot CondensateDepending on

Trap


LiquidCondensate& Flash Out

Hot orSubcooled Liquid

Condensate

Bimetallic Elements

Valve Seat

Figure 4. Thermostatic Steam Trap with a Bellows Element

Figure 5. Thermostatic Steam Trap with a Bimetallic Element

14

Steam System Basics


depleted of their condensate seal when appliedin superheated steam service. If this occurs, thetrap will continuously discharge live steam. Thistrap type is not recommended for superheatedsteam service, unless special installation condi-tions are met. The configuration of an invertedbucket steam trap is shown in Figure 6.

Open bucket traps. Open bucket traps consist of anupright bucket that is attached to a valve. Atstart-up, the bucket rests on the bottom of thetrap body. In this position, the valve is wideopen. As condensate accumulates in the trapbody on the outside of the bucket, the bucketfloats upward, causing the valve to close. Whensufficient condensate accumulates outside thebucket, it spills over the top and fills the insideof the bucket. At this time, the bucket sinks, causing the valve to open. This trap is also proneto failure when applied in superheated steamservice because of the loss of the condensate seal.Like inverted bucket traps, open bucket trapshave intermittent discharge.

Float and Thermostatic (F&T) traps. Float and ther-mostatic (F&T) traps are similar to float and levertraps except they include a thermostatic elementthat allows air to be discharged at start-up andduring operation. The thermostatic elementsused in these traps are the same as those used inthermostatic traps. The configuration of a floatand thermostatic steam trap is shown in Figure 7.

Thermodynamic TrapsThermodynamic traps use the difference in kinet-ic energy (velocity) between condensate and livesteam to operate a valve. The disc trap is themost common type of thermodynamic trap, butpiston or impulse traps are sometimes used.

Disc traps. Disc traps use the position of a flat discto control steam and condensate flow. When condensate flows through the trap, the disc israised, thereby causing the trap to open. Assteam and air pass through the trap, the discmoves downward. The force that causes the discto move downward is generated by the differencein pressure between the low-velocity steam abovethe disc and the high-velocity steam that flowsthrough the narrow gap beneath the disc. Disctraps commonly have an intermittent dischargeand, when they fail, they normally fail open. The configuration of a disc steam trap is shownin Figure 8. Generally, the air removal capabilityof this trap type is poor unless equipped withadditional components (like the float and thermostatic trap).

Piston traps. Piston or impulse traps utilize theheat energy in hot condensate, and the kineticenergy in steam, to open and close a valve. Likedisc traps, piston traps are phase detectors thatsense the difference between a liquid and gas orvapor. They continuously discharge any air andcondensate. Their primary failure mode is open.

Vent Hole

Seat Steam Spaces

Condensate LevelSteam Bubbles

Inverted BucketValve

Lever

Liquid Condensate & Flash Out

Seat

Air Vent

Valve

Float Lever

SteamSpace

CondensateLevel

Liquid Condensate& Flash Out


Liquid Condensate & Flash Out

Valve Disc

Outlet Port

Seating Surface Inlet Port

Bonnet Chamber

Flash Vapor ClosesValve Disc

Steam & Condensate In

Steam & Condensate In

Figure 6. Inverted Bucket Steam Trap

Figure 7. Float and Thermostatic Steam Trap

Figure 8. Thermodynamic Disc Steam Trap

15

Lever traps. Lever traps are a variation of the thermodynamic piston trap. They operate on thesame principal as a piston trap but with a leveraction to pass large amounts of condensate andair on a continuous basis. Their primary failuremode is open.

Orifice traps. Orifice traps are of two basic types:orifice plate and short tube. Both trap types oper-ate under the exact same principles. A simple orifice plate steam trap consists of a thin metalplate with a small-diameter hole (orifice) drilledthrough the plate. When installed, condensatethat accumulates is continuously removed as thesteam pressure forces the condensate through theorifice. During conditions when no condensateis present, a limited amount of steam flowsthrough the orifice. The report Review of OrificePlate Steam Traps on page 49 of the Where toFind Help section provides information for making informed decisions about when orificeplate steam traps should be considered for use in new or existing steam systems.

Additional information regarding steam traps isavailable in the Steam Tip Sheet Number 1 titledInspect and Repair Steam Traps, found in Appendix B.

Steam MetersThe use of flowmeters within the distributionsystem can provide important data for monitor-ing the efficiency of a process or an end use.Tracking the amount of steam required can beparticularly useful in benchmarking efforts. Thetypes of steam flowmeters are discussed in theGeneration Section.

End UseSteam system end-use equipment transfers steamenergy into other forms of useful energy. Commonend-use equipment includes heat exchange devicesto transfer thermal energy and turbines to recovermechanical energy. In manufacturing industries,steam end uses often directly support production,making their performance and reliability essentialto plant productivity. Improvements in end-useefficiency and effectiveness also tend to result inbetter performance and increased reliability. Thereis a wide range of end-use equipment, largely due to the advantages of steam that are discussed

in the Introduction. Some of the major end-usecomponents are discussed in this section.

For the purposes of this discussion, steam end-useequipment is grouped into three basic categories:

Industries of the Future11 (IOF) key end-use equipment

Conditioning and control equipment

Additional equipment.

The key IOF equipment category includes thelargest uses of steam in those industries. AlthoughIOF facilities use steam for other services as well,the key end uses account for the largest amountof steam use. The conditioning equipment category includes equipment that facilitates theeffective use of steam. The additional equipmentcategory includes equipment that is used in otherindustries and, though significant, does notaccount for most of the steam use in IOF industries.

Industries of the Future Key End-Use EquipmentIn the three IOF industries of forest products, petroleum refining, and chemicals, steam accountsfor the largest amount of end-use energy. Inanother IOF industry, steel production, steamrepresents a significant amount of end-use energyand is used to generate most of that industryson-site electric power. Table 1 provides a list ofkey steam-supplied end-use equipment for IOFindustries.

CondensersIn steam applications, condensers are associatedwith condensing steam turbines and with multi-ple stage ejector systems. In steam turbine appli-cations, condensers typically operate under avacuum. They remove energy from the exhauststeam, allowing it to be recovered as condensate.In steam ejector applications, condensers increasethe effectiveness of the ejectors by condensingboth the motive steam and condensables pulledfrom the process, reducing the amount of motivesteam required.

Condensers can be surface type or barometric.Surface condensers are supplied with coolingwater that circulates through condenser tubes,providing a cool surface area that causes steamcondensation. The condensate is typically


Steam System Basics

11 Industries of the Future (IOF) include: aluminum, chemicals, forest products, glass, metal casting, mining, petroleum refining, and steel.

16

collected in a condensate well, and pumped intothe condensate return system. Barometric con-densers rely on direct contact between the cool-ing water and the steam. In petroleum refiningand chemical manufacturing applications, con-densers are also used to condense componentsfrom gaseous mixtures. In these applications, the condensers use a cooling medium to extractenergy from the gases and collect the condensedcomponents.

Distillation TowersThe petroleum refining and chemical manufac-turing industries use large amounts of steam tofacilitate the separation of crude oil or chemical

feedstocks into various components. This separa-tion process relies on differences in the boilingpoints of these hydrocarbon components. Fraction-ating towers use a furnace to heat crude oil above700F. As the volatile components boil off andrise up the tower, they cool and condense on trays.Steam is injected into the bottom of these towersto reduce the partial pressures of the hydrocarbons,which facilitates their separation, and to reducecoke formation on tray and tower surfaces.

DryersDryers reduce the water content of a solid. Dry-ers account for the largest end use of steam inthe pulp and paper industry.12 The chemical


Steam System Basics

Equipment Process Application IndustryCondenser Steam turbine operation Aluminum, Chemical Manufacturing, Forest

Products, Glass, Metal Casting, PetroleumRefining, and Steel

Distillation tower Distillation, fractionation Chemical Manufacturing, Petroleum RefiningDryer Drying Forest ProductsEvaporator Evaporation/concentration Chemical Manufacturing, Forest Products

Petroleum RefiningProcess heat Alkylation, Process air heating, Process water Aluminum, Chemical Manufacturing, Forestexchanger heating, Gas recovery/Light ends distillation, Products, Glass, Metal Casting, Petroleum

Isomerization, Storage tank heating Refining, and SteelVisbreaking/Coking

Reboiler Fractionation Petroleum RefiningReformer Hydrogen generation Chemical Manufacturing, Petroleum RefiningSeparator Component separation Chemical Manufacturing, Forest Products,

Petroleum RefiningSteam ejector Condenser operation, Vacuum distillation Aluminum, Chemical Manufacturing, Forest

Products, Glass, Metal Casting, PetroleumRefining, and Steel

Steam injector Agitation/blending, Heating Chemical Manufacturing, Forest Products,Petroleum Refining

Steam turbine Power generation, Compressor mechanical Aluminum, Chemical Manufacturing, Forestdrive, Hydrocracking, Naphtha reforming, Products, Glass, Metal Casting, PetroleumPump mechanical drive, Feed pump Refining, and Steelmechanical drive

Stripper Distillation (crude and vacuum units), Chemical Manufacturing, Petroleum RefiningCatalytic cracking, Asphalt processing,Catalytic reforming, Component removal,Component separation, Fractionation,Hydrogen treatment, Lube oil processing

Thermocompressor Drying, Steam pressure amplification Forest Products

Table 1. Key IOF Steam End-Use Equipment

12 Giese & Associates, Scoping Study of the Pulp and Paper Industry, EPRI, 1988.

17

manufacturing, textiles, and food processingindustries also use large amounts of steam fordrying. Dryers can be indirect or direct. Indirectdryers remove moisture thermally as energy istransferred from condensing steam, flue gases, or high temperature process fluid to the productbeing dried. Common indirect dryer types arecoil and rotating drum. Direct dryers use hotgases that have been heated with steam or fluegases to directly contact and dry a product.

Dryers, like evaporators, can be arranged in multi-ple-stage configurations. Multiple-stage steamdryers use a cascading set of steam pressures, allow-ing steam released from an upstream stage to sup-ply steam to the next stage. In many multiple-stagedryers, thermocompressors are used to increasethe steam pressure of downstream-effect stages.

EvaporatorsEvaporators reduce the water content of a liquid,generally by heating it with steam in order to concentrate the product. Evaporators are usedextensively in industries such as food processing,chemical manufacturing, steel, forest products,and textiles.

In most cases, evaporators are shell and tube heatexchangers with the steam on the shell side andthe product being concentrated in the tubes.Evaporators can be single effect or multiple effect.A single-effect evaporator uses steam at one set ofpressure and temperature conditions to boil off thevapor from a product. Multiple-effect evaporatorstake the vapor produced from one evaporator anduse it to heat the product in a lower-pressureevaporator. Multiple-effect evaporators are gener-ally more efficient at concentrating a fluid thansingle-effect evaporators.

Heat ExchangersHeat exchangers transfer thermal energy from onefluid to another. In manufacturing facilities, steamis a common source of heat for many reasons,some of which are discussed in the Introduction.There is a wide range of heat exchanger designsthat use steam, largely due to the wide range ofproducts that are heated with steam. Many processand product considerations must be incorporatedinto the selection of a heat exchanger. Some

basic heat exchanger types are discussed below,including:

Tubular

Plate and frame

Jacketed

Coil.

Tubular heat exchanger. Tubular heat exchangersare tube bundles that are surrounded by theheated or heating medium. This type of heatexchanger includes finned tube and shell andtube designs as shown in Figure 9. Finned tubeheat exchangers are often used to heat air fordrying and space heating applications. Shell andtube heat exchangers are often used for liquidheating and evaporation. Since the tube side ofshell and tube heat exchangers can be designedto withstand high pressures, sometimes exceed-ing 1,500 psig, heat exchangers of this type areoften used in high temperature and high-pressureapplications.

Plate and frame heat exchanger. In plate and frameheat exchangers, the two heat exchange fluidsare separated by plates. The plates are corrugated,or ridged, as shown in Figure 10, to increase thesurface area available for heat transfer. Plate andframe heat exchangers are often used in low-viscosity applications, where the risk of cloggingis less severe. The plate ends are typically sealedby gasketed covers that can be removed to allow disassembly and cleaning. This heat exchangertype is used when temperatures and pressures aremoderately low, typically below 300F and 370 psi.Plate and frame heat exchangers also have acommon design variation that has the plateswelded or brazed together. This allows highertemperature service but eliminates the possibilityof mechanical cleaning.

Jacketed heat exchangers. Jacketed heat exchang-ers use an enclosure to surround the vessel that contains the heated product. A common exampleof a jacketed heat exchanger is the jacketed kettle. A representation of a jacketed heatexchanger is shown in Figure 11. Jacketed heatexchangers are practical for batch processes andfor product types that tend to foul or clog tubebundles or coils.


Steam System Basics

Coil heat exchangers. Coil heat exchangers charac-teristically use a set of coils immersed in themedium that is being heated. Coil heat exchang-ers are generally compact, offering a large heattransfer area for the size of the heat exchanger.

ReboilersReboilers are typically used in distilling processesto increase component separation. Reboilers useheat, often provided by steam, to evaporate thevolatile components of a product that has beendrawn from a fractionating tower. These volatilecomponents are sent downstream for further processing. The residual components are sentback into the fractionating tower or sent on to avacuum distillation process. There are several typesof reboilers, including jacketed kettle, kettle, inter-nal reboiler, and thermosyphon reboiler. Thesedesigns differ from one another in the way theproduct is heated with steam.

ReformersSteam reformers are used togenerate hydrogen, typicallyfrom a hydrocarbon feedstocksuch as methane (the largestcomponent of natural gas). Inturn, hydrogen is used in manypetroleum refining and chemi-cal manufacturing processes.Reformers use steam for bothenergy and as a source ofhydrogen. Steam is injectedwith the hydrocarbon feed-stock to initiate the followingreaction:

Reformers often have secondary stages that areused to convert the carbon monoxide to carbondioxide and additional hydrogen. Although largeamounts of steam are used throughout thereforming processes, steam is also generated bythe reformers and is sometimes exported forother uses.

Steam EjectorsSteam ejectors use steam flow through a nozzleto create a vacuum (similar in operation to ther-mocompressors). They are used in several differenttypes of system applications and process equip-ment. Low-pressure conditions promote theevaporation of liquids at reduced temperatures.

18

Steam System Basics


Tube Side Fluid

Tube BundleTubesheet

Shell Side Fluid

Baffles

Plates

Compression Fasteners

Frame

Kettle

Steam

Condensate

SteamJacket

Figure 9. Shell and Tube Heat Exchanger

Figure 10. Components of a Plate and Frame Heat Exchanger

Figure 10. Configuration of a Jacketed KettleHeat Exchanger

CH4 + H2O CO + 3H2Methane Steam Carbon Hydrogen

monoxide

19

Consequently, many chemical manufacturingprocesses use steam ejectors to increase the concentration of a product. In petroleum refining, steam ejectors are commonly used in the vacuum distillation of heavy hydro-carbon products. Steam ejectors are also used to initiate and maintain vacuum conditionin the condensers of condensing turbines.

Steam InjectorsSteam injectors are used to inject steam directlyinto a tank or a pipe containing a process fluid,generally for heating purposes. Many injectortypes use a nozzle and a diffuser to pull processfluid into the steam before the mixture is injectedinto the process fluid, to promote an even distri-bution of heat. Important performance character-istics of injectors include accurate control of theamount of steam injected and effective mixing of the steam and process.

Steam TurbinesSteam turbines are used to drive electric generatorsor other rotating machinery such as compressors,pumps, and fans. Steam turbines are used inmany different system designs, depending on the relative requirements for steam, electricity, orother mechanical loads. Steam turbines providean effective means of stepping down steam pressure while extracting mechanical work.

Additional information regarding steam turbinesis available in Steam Tip Sheets Numbers 15 and21 titled Benchmark the Fuel Costs of Steam Gener-ation and Consider Steam Turbine Drives for Rotat-ing Equipment, found in Appendix B.

Some turbines have interstage take-offs thatallow steam to be extracted at various pressuresbefore reaching the turbine exhaust. Theseextractions provide flexibility in meeting com-peting requirements of both the steam systemand the mechanical load. For example, if the turbine is connected to an electric generator,adjusting the amount of extracted steam canallow more or less electric power to be generated,while making respectively less or more steamavailable to the plant.

Backpressure turbines. Backpressure turbinesexhaust steam at pressures that are higher thanatmospheric, and the exhaust steam is then used

for other services. By extracting mechanical workfrom steam, backpressure turbines can provide an efficient means of supplying lower-pressuresteam from a high-pressure header.

Condensing turbines. Condensing turbines exhauststeam to vacuum (sub-atmospheric) conditions.This steam is condensed in a heat exchanger,referred to as a condenser, and transferred to thecondensate return system. Condensing turbinestypically require a source of cooling water to condense the steam.

StrippersSteam strippers are used to remove contaminantsfrom a solution. Strippers are commonly foundin petroleum refining and chemical manufactur-ing applications, where process solutions containcomponents that have different boiling pointsand removal of one or more of the componentsis necessary. Injecting steam into the processsolution lowers the partial pressure of volatilecomponents, allowing some of them to vaporizeand get transported away with the steam. Steamcan also raise the temperature of the mixture,lowering the solubility of the objectionable mate-rial and causing it to strip off with the steam.Often, the steam and the contaminants are con-densed and separated, allowing recovery of thecondensate and disposal or further processing ofthe contaminant.

ThermocompressorsThermocompressors combine high-pressure andlow-pressure steam to form an intermediate-pressure steam supply (see Figure 12). Often thelow-pressure steam does not have enough energyto be feasibly used; however, discharging it to thecondensate return system can be an unnecessaryenergy loss. Thermocompressors use a high-pres-sure steam source to recover the energy from thislow-pressure source, providing an intermediatesteam supply that can be feasibly used.

Conditioning and Control EquipmentConditioning equipment is generally used toimprove the performance of, or to protect theend-use equipment. For example, desuperheatersare often used to control the energy of a steamsupply to end-use equipment to reduce the riskof damage to the equipment or to effectivelyimprove temperature control of the process.


Steam System Basics

20

Desuperheaters. The purpose of a desuperheater is to remove the superheat from steam. Themajority of heating and process equipment performs more efficiently using saturated ratherthan super-heated steam. Desuper-heaters injecta very fine mist of high-purity water, such ascondensate, into the steam flow. The superheatedvapor gives up heat to the water mist, and bydoing so, reduces its temperature.

Vacuum breakers. Vacuum conditions can developin a steam system when steam flow into a com-ponent or a branch is throttled or shut off. If therate of downstream steam use exceeds the steamsupply, the pressure decreases and vacuum condi-tions can form. Vacuum conditions also resultwhen the load on the heat exchanger is signifi-cantly less than the heat exchanger capacity. Ifthe pressure in the heat exchanger drops too far,the condensate will not drain from the trap dueto a higher pressure on the traps downstreamside. If uncorrected, the condensate level will rise in the heat exchanger, reducing the availableheat transfer area and increasing the risk of corrosion by condensate. Vacuum breakers arepressure-controlled devices that essentially vent a heat exchanger or system branch in which avacuum has formed. By allowing in air whenthey open, vacuum breakers restore pressure andallow the condensate to drain.

Air vents. Before start-up, the steam system con-tains air that must be removed. The presence of air in a steam system reduces heat transfereffectiveness and promotes condensate corrosion.Air vents remove this air. Air vents are often thermostatic devices, similar to thermostaticsteam traps that rely on the temperature differ-

ence between air and steam.When exposed to the lowertemperature air in the systemside, the vent opens. As thehigher temperature steam reach-es the vent, it closes, prevent-ing the escape of steam.

Traps. Steam traps are importantto the performance of end-useequipment. Traps provide forcondensate removal with littleor no steam loss. If the traps donot function properly, excesssteam will flow through theend-use device or the conden-sate will back up into it. Excess

steam loss will lead to costly operation whilecondensate backup will promote poor perform-ance and may lead to water hammer. Traps canalso remove non-condensable gases that reduceheat exchanger effectiveness. There are severaldifferent types of steam traps, which are dis-cussed in the Distribution section of this Source-book.

InsulationEnd-use equipment, such as heat exchangers and turbines, should generally be insulated dueto the significant heat loss that the surface areasof this equipment can provide. The various typesof insulation are discussed in the Distributionsection of this Sourcebook. Where end-use equip-ment requires frequent inspection or maintenance,removable insulation should be considered.

Additional EquipmentThe additional equipment category refers to enduses throughout industry, which, though still significant users of steam, generally account forless steam energy than the key IOF end uses.

Absorption chillers. Absorption chillers providecooling using an interesting variation of thevapor compression cycle. Instead of a compres-sor, which is generally used in chillers, absorp-tion chillers exploit the ability of one substanceto absorb a refrigerant at one temperature andthen release it at another. In ammonia-based systems, water is the absorbent and ammonia is the refrigerant. In lithium bromide-based systems, lithium bromide is the absorbent, whilewater is the refrigerant. An absorption chilleruses a pump instead of a compressor to increase


Steam System Basics

Discharge(intermediate

pressure)

Motive Steam(high pressure)

Suction (low pressure)

Figure 12. Thermocompressor Operation

21

refrigerant pressure. Once it is at the higher pressure, the absorbent/ refrigerant solution isheated, often with steam, which releases the refrig-erant. Although absorption chillers generally havelower coefficients of performance (COP) (indicatinglower thermodynamic efficiency) than traditionalchillers, they use less electric power per ton of cool-ing and are well suited for use with steam systems.

Humidifiers. Humidifiers inject steam into an airor other gas source to increase its water vaporcontent. In humidification, steam is used as asource of both water and energy. Humidificationapplications are found in the chemical man-ufacturing industry where control of ambienttemperature and moisture content are critical for product quality.

Preheat/Reheat air handling coils. Steam is oftenused in space heating applications to preheat and reheat air. In many HVAC systems, the conditioned air must have both its temperatureand humidity adjusted. In preheat applications,steam is used to heat an air supply, which is typically a mixture of return air and outside air.The air is then conditioned to achieve a certainhumidity and temperature. In reheat applica-tions, the air is cooled to a particular dew pointto remove water and achieve a desired humidity.As a result, before the air is delivered back to theworkspaces, steam coils must reheat the processair stream up to the proper temperature. In bothreheat and preheat applications, finned tube heatexchangers are generally used.

Tracing. In tracing applications, steam is used tomaintain the temperature of a fluid in a pipe. A common application of tracing lines is to prevent the freezing of a process fluid in pipingthat runs outside of a temperature controlledarea. Since tracing lines are exposed to freezingconditions, proper insulation, steam flow, andcondensate drainage are essential to preventfreezing of the tracing lines as well as the process piping.

Meters. Steam meters are used to measure steamflow, and are important for tracking the steamuse of a particular part of a steam system or aparticular end use. Discussion of different metertypes is provided in the Steam Generation section of this Sourcebook.

RecoveryThe recovery components of a steam system collect and return condensate back to the genera-tion part of the system. Condensate recoveryprovides thermal and water treatment benefits.Condensate that is not returned must be com-pensated for by the addition of makeup water,which is generally much cooler than condensate.Condensate temperature often exceeds 200Fwhile makeup water temperature is typicallybetween 50F and 80F. As a result, the enthalpydifference between condensate and makeupwater is generally over 120 Btu/lb, an amount of energy that is often more than 10% of theenergy in the boiler generated steam.

Additionally, makeup water is generally treatedwith chemicals that remove minerals and estab-lish certain pH levels in the boiler water and inthe system. Reducing the amount of makeupwater added to the system reduces chemical use.Additionally, some of the treatment chemicalsthat are contained in condensate are problematicto a plants wastewater treatment facility. Indus-trial steam plants often extend across large areas.Recovering condensate from steam systemsrequires piping, collecting tanks, pumping equip-ment, and, in many cases, flash steam separators,meters, and filtration/cleanup equipment. How-ever, the cost savings available from avoiding thepurchase, treatment, and heating of makeupwater often make investments in condensaterecovery systems highly feasible.

For more information on condensate recovery,see the Steam Tip Sheet Number 8 titled ReturnCondensate to the Boiler, provided in Appendix B.

Condensate Return PipingCondensate return piping transports condensateas it drains from distribution and end-use equip-ment piping back to the boiler. Condensate piping should be adequately sized and insulated.Although the installation of larger pipe diametersis more expensive, larger pipes create less pressuredrop for a given flow rate; this reduces the loadon the condensate pumps. Larger pipe diametersalso reduce the noise associated with condensateflow and are more suitable for carrying flashsteam. Insulating the condensate piping helps toretain the thermal energy that provides much ofthe benefits of a condensate recovery system.


Steam System Basics

22

InsulationInsulation provides energy savings and safetybenefits. In terms of energy savings, insulationreduces heat loss from the condensate piping andrecovery equipment surfaces, which can makethe surrounding work environment more com-fortable. Reducing this heat loss can also reducethe burden on the cooling systems that supportsurrounding workspaces. In terms of safety, insulation reduces the outer surface temperatureof the piping, which lessens the risk of burns.Important insulation properties and characteris-tics of piping insulation are discussed in the Distribution section of this Sourcebook.

Condensate Receiver TanksCondensate receiver tanks collect and store condensate. These tanks are usually locatedremotely around the condensate system and areconfigured in conjunction with condensatepumps, as shown in Figure 13. Condensate flowscan be highly variable due to changes in steamdemand, especially during system start-ups.Receiver tanks minimize the effects of this flowvariability on condensate pumps by providingstorage, which maintains a minimum water levelthat prevents downstream condensate pumpsfrom running dry. Since many condensate pumpsare centrifugal types, it is important to keep a certain suction pressure to prevent cavitationdamage. By maintaining a minimum condensatelevel, receiver tanks provide enough static pressure to avoid cavitation.

Most systems also contain a large condensatereceiver tank that collects all the condensatereturned from the system. This tank may also be used to store pretreated water.

Condensate PumpsCondensate pumps move condensate from receiv-er tanks back to the boiler room. Condensatepumps can be driven by electric motors, steam, orcompressed air, depending on the availability ofthese sources. Motor-driven condensate pumps areusually centrifugal type pumps. In many cases,receiver tanks and motor driven pumps are pack-aged together and equipped with a control systemthat de-energizes the pump under low water levelconditions. Steam or compressed air powered condensate pumps are used where electricalpumps would not be suitable, and are generallypressure powered pumps.

Condensate pumps also can be important to theperformance of end-use equipment. Effective useof condensate pumps can eliminate condensateback up into end-use equipment, improvingprocess control and reducing potential equipmentproblems from condensate acidification andwater hammer.

Flash Steam VesselsFlash steam vessels allow the recovery of steamfrom condensate lines, as illustrated in Figure 14.By removing steam from the condensate system,flash steam vessels provide an efficient source ofsteam to low-pressure end uses. For example,250F condensate has a saturation pressure ofabout 15 psig. Consequently, steam that is separated by flash steam vessels can be used inlow-pressure steam applications such as spaceheating and preheating.

For more information on flash steam vessels, seethe Steam Tip Sheet Number 12 titled Flash High-Pressure Condensate to Regenerate Low-PressureSteam provided in Appendix B.

Condensate MetersCondensate meters measure the flow rate of condensate in the return system. Knowing the condensate flow rate can be helpful in monitor-ing the condensate system and the condition ofsteam traps. Condensate meters are often inlinerotary types, relying on turbine or scroll rotationto measure flow rate.

Filtration/Cleanup EquipmentIn many systems, the flow of steam and conden-sate picks up rust, scale, and trace contaminants


Steam System Basics

CondensatePump

CondensateReceiver

Tank

Outlet

Inlet

Figure 13. Condensate Receiver Tank and PumpCombination

23

that are either carried over from the boiler orthat form in carbon steel piping and on copperalloy heat exchange surfaces. Although strainersand filters are used to catch the particulate matter, some contaminants are dissolved in thecondensate and can cause problems if returnedto the boiler. In systems that require a high levelof cleanliness, condensate polishers are used.Condensate polishers use ion exchange to removethese contaminants, preventing the redepositionof these contaminants on boiler surfaces.


Steam System Basics

Saturated Vapor Supply

Steam Trap

High-PressureCondensate

CondensateDischarge

LevelController

Low-PressureFlash Vessel

SaturatedVapor

SaturatedLiquid

Figure 14. Flash Steam Recovery Vessel

Appendix A: Glossary of TermsThis appendix contains a glossary of terms usedin steam systems.

Appendix B: Tip SheetsThis appendix contains a series of steam systemtip sheets. Developed by the U.S. Department of Energy, these tip sheets discuss commonopportunities that industrial facilities can use to improve performance and reduce fuel use.

Appendix C: Guidelines for CommentThis appendix contains a form that provides avehicle for submitting comments for improvingthe Sourcebook.

Appendices

55A Sourcebook for Industry

Appendices

The following appendices have been included in the Sourcebook:

A Sourcebook for Industry 57

Absorption chillingThis is a water chillingprocess in which cooling of a solution is accom-plished by the evaporation of a fluid (usuallywater), which is then absorbed by a different solution (usually lithium bromide), then evapo-rated under heat and pressure. The fluid is then condensed with the heat of condensation reject-ed through a cooling tower.

Air ventA device that allows the release ofnon-condensable gases from a steam system.

AlkalinityA measure of the concentration of carbonate, bicarbonate, and hydroxyl ion inwater, usually expressed in equivalent ppm (partsper million) of calcium carbonate.

Backpressure turbineA turbine that exhaustssteam above atmospheric pressure. The exhauststeam is usually sent to other services.

BiomassOrganic matter which is available on arenewable basis, including agricultural crops andagricultural wastes and residues, wood and woodwastes and residues, animal wastes, municipalwastes, and aquatic plants.

Blowdown ratioFor boilers, the ratio of waterremoved by blowdown to the amount of feedwa-ter provided to the boiler in equivalent units forthe same time period (both usually reported inpounds per hour).

BoilerA vessel or tank in which heat producedfrom the combustion of fuels such as natural gas, fuel oil, wood, or coal is used to generate hot water or steam for applications ranging from building space heating to electric powerproduction or industrial process heat.

Boiler blowdownThe periodic or continuousremoval of water from a boiler to remove concentrations of dissolved solids and/or sludgeaccumulating in the boiler.

Boiler horsepowerA unit of rate of water evaporation equal to the evaporation per hour of34.5 pounds of water at a temperature of 212Finto steam at 212F. One boiler horsepowerequals 33,475 Btu per hour.

British thermal unit (Btu)The amount of heatrequired to raise the temperature of one poundof water one degree Fahrenheit; equal to 252calories. It is roughly equal to the heat of onekitchen match.

ChloridesChemical compounds found in boil-er water consisting of metallic ions with chlorineatoms, part of a group of compounds called salts.The most prevalent are magnesium chloride andsodium chloride.

CogenerationThe simultaneous production ofelectrical or mechanical work and thermal energyfrom a process, thus reducing the amount of heator energy lost from the process. Also known ascombined heat and power (CHP).

Combined Heat and Power (CHP)The simul-taneous production of electrical or mechanicalwork and thermal energy from a process, thusreducing any waste heat or energy lost from theprocess. Also known as cogeneration.

Combustion efficiencyThis measure repre-sents the amount of fuel energy extracted fromflue gases. It is a steady state measure and doesnot include boiler shell losses or blowdown loss-es. The losses identified in this efficiency calcula-tion are the stack losses. Stack loss is anindication of the amount of energy remaining inthe flue gases as they exit the boiler.

Combustion turbineA turbine that generateselectric power from the combustion of a fuel.

CondensateCondensed steam.

Condensate pumpA pump that pressurizes condensate allowing it to flow back to a collec-tion tank, or boiler plant.

Appendix A: Glossary of Terms


Improving Steam System Performance58

CondenserA device that condenses steam. Sur-face condensers use a heat exchanger to removeenergy from the steam, and typically operateunder vacuum conditions.

Condensing turbineA turbine that exhaustssteam to typically sub-atmospheric conditionswhere the steam is condensed. These turbines areusually used in power generation applications.

Continuous blowdownThe process of removingwater, on a continuous basis, from a boiler toremove high concentrations of dissolved solids,chlorides, and other products. Water is replaced bytreated make-up water added to the condensatereturn. The withdrawn blowdown water is usuallydischarged to a sanitary drain.

DeaeratorA device that uses steam to stripfeedwater of oxygen and carbon dioxide.

DesuperheaterA device used in steam systemsto control the energy level and/or temperature ofsteam supplied to an end use, typically by inject-ing a fine mist of condensate into the steam flow.

Dew pointThe temperature to which a vapormust be cooled, in order for saturation or condensation to occur.

Dissolved solidsThe minerals and impuritiesin boiler make-up water. These increase in concentration as water is boiled into steam.

Distillation columnA device that separates components of a compound relying on differ-ences in boiling points.

DryerA device that removes moisture from a solid.

Duty cycleThe duration and periodicity of theoperation of a device.

Excess airExcess air in a combustion process isthat amount of air present in excess of the theoretically required air for complete combustion.Excess air absorbs some of the heat of combus-tion and is carried away as waste heat with theexhaust combusted gases.

EvaporatorA device that removes moisturefrom a liquid, resulting in increased concentra-tion of the remaining compounds.

FeedwaterWater sent into a boiler or a steamgenerator. Feedwater typically meets cleanlinesscriteria, contains treatment chemicals, and hasbeen stripped of oxygen.

Feedwater pumpA pump that sufficiently pres-surizes feedwater to supply a boiler or generator.

Full load efficiencyThe stated efficiency (in percent) of a process in terms of energy orwork output divided by energy input at ratedproduction (full load).

Greenhouse gas emissionsThose gases, such aswater vapor, carbon dioxide, tropospheric ozone,methane, and low level ozone that are transpar-ent to solar radiation, but opaque to long waveradiation, and which are believed to contributeto climate change.

Heat exchangerA device used to transfer heatfrom one medium by either direct or indirect contact.

Heating, Ventilating and Air Conditioning (HVAC)All the components of the appliance orsystem used to condition the interior air of afacility.

Heat Recovery Steam Generator (HRSG)A device that captures the thermal energy in anexhaust stream or a process stream and transfersit to water to generate steam.

Kilowatt hour (kWh)A unit of measure of electricity supply or consumption of 1,000 Wattsover the period of one hour; equivalent to 3,412 Btu.

Latent heatThe change in heat content thatoccurs with a change in phase and withoutchange in temperature. Changes in heat contentthat affect a change in temperature are calledsensible heat changes.

Life Cycle Cost (LCC)The sum of all the costsboth recurring and nonrecurring, related to a product, structure, system, or service duringits life span or specified time period.



Make-up waterWater brought into a boiler system from outside to replace condensate notreturned to the boiler plant, water used in blow-down, steam lost through leaks, or water lostthrough evaporation or mist.

MMBtuA unit of one million British thermalunits (Btu).

PaybackThe amount of time required for posi-tive cash flows to equal the total investmentcosts. This is often used to describe how long itwill take for energy savings resulting from usingmore energy-efficient equipment to equal thepremium paid to purchase the more energy-effi-cient equipment.

Potable waterWater that is suitable for drink-ing, as defined by local health officials.

Pounds per square inch gauge (psig)A unit of pressure in pounds force in comparisonto local atmospheric pressure (as measured by agauge).

Pressure Reducing Valve (PRV)A valve thatregulates the amount of steam allowed from ahigh-pressure service to a low-pressure service.

Radiant energyEnergy (transfer of heatthrough matter or space by means of electromag-netic waves) that transmits away from its sourcein all directions.

ReboilerA heat exchanger used in petroleumrefining and chemical manufacturing applica-tions that adds heat to a process stream.

ReformerA device that produces hydrogenfrom a hydrocarbon compound such as methane,typically by using steam.

Relative humidityA measure of the percent ofmoisture actually in the air compared with whatwould be in it if it were fully saturated at thattemperature. When the air is fully saturated, itsrelative humidity is 100 percent.

R-ValueA measure of the capacity of a material(or assembly of materials) to resist heat transfer,sometimes called thermal resistance. The R-Valueis the reciprocal of the heat transfer coefficient(U-Value). The larger the R-Value, the greater itsinsulating properties.

SilicaThe concentration of silica dioxide foundin water measured in parts per million. Silicaforms a tenacious scale with low thermal con-ductance. Silica is soluble in steam and can carryover into steam where it can condense on tur-bine blades or erode surfaces.

SludgeA general term applied to a relativelyloose accumulation of material that has settled tothe bottom of a boiler.

Stack heat lossSensible and latent heat con-tained in combustion gases and vapor emitted tothe atmosphere.

Stand-aloneA device that operates independ-ent of or is not connected to other systems.

Standby lossesIn boiler systems, standby lossesare energy losses that are associated with naturaldraft through a boiler in a standby operatingmode.

Steam ejectorA device that uses a relativelyhigh-pressure motive steam flow through a noz-zle to create a low-pressure or suction effect.

Steam injectorA device that injects steam intoa process, often used in direct steam heating applications.

Steam trapAn automatic control valve thatallows for the release of condensate, air, CO2,and other non-condensable gases, yet keeps livesteam in the system.

StripperA device that separates componentsfrom a compound.

Surface blowdownBoiler blowdown takenfrom the upper part of the boiler just below thewater surface to reduce the amount of dissolvedsolids.

Thermal conductivityThis is a positive con-stant, k, that is a property of a substance and isused in the calculation of heat transfer rates formaterials. It is the amount of heat that flowsthrough a specified area and thickness of a mate-rial over a specified period of time when there isa temperature difference of one degree betweenthe surfaces of the material.


Improving Steam System Performance60

Thermal load factorA term used to describethe relative stability of use of heating in a facilityor a process. Often used in cogeneration where ahigh factor denotes a continuous use of steadyheating.

ThermocompressorA device that uses high pressure steam to increase the pressure of a low-pressure steam supply to generate a more usefulsteam service.

ThrottlingRegulating flow rate by closing avalve in the system.

TurbineA device that converts the enthalpy ofsteam into mechanical work. See condensing turbine and backpressure turbine.

Vacuum breakerA device that responds to theformation of vacuum conditions in equipmentsuch as a heat exchanger by allowing in airwhich promotes condensate drainage.

Variable Frequency Drive (VFD)A type of variable speed motor drive in which the motor is supplied with electrical power at frequenciesother than standard 60 Hertz through a converter.

Variable Air Volume (VAV)Variable air volumeis a term for changing the quantity of air flow tomeet heating, cooling, or exhaust loads ratherthan changing air temperatures.

Waste heatHeat that is discharged from amechanical process, wastewater, or ventilationexhaust system that could be reclaimed for usefulpurposes.

Water hammerA shock that results from the sudden collapse of a steam pocket in a two phaseline such as a condensate return line. Also theimpingement of a water slug carried by relativelyhigh velocity steam.



Appendix B: Steam Tip Sheets

Appendix B: Steam Tip Sheets

Some improvement opportunities are available to many different systems. To increase industry awareness of several fundamental improvement opportunities, the U.S. Department of Energys Industrial Technologies Program has developed the following tip sheets through its BestPractices activities.

1. Inspec

Documents

Steam System Basics.pdf