Learning Objectives

Armstrong International, Inc.Welcome to todays lesson--Superheated Steam.

Click the start button to begin.

1 Learning ObjectivesWhat superheated steam is and how it is generatedRecommended optimizations for superheated steamCommon applications for superheated steam

Typical system components used with superheated steamBenefits and drawbacks of superheated steamIn this course you will learn:What superheated steam is and how it is generatedThe benefits and drawbacks of superheated steamThe common applications for superheated steamThe typical steam system components that are used with superheated steam, andRecommended optimizations for superheated steam

Please reserve the next 40 minutes of your time to complete this course.3Course Prerequisites

Prior to beginning this course we recommend that you complete the following courses on Armstrong University:

Steam QualityBasic Heat TransferSteam TrapsPressure Reduction Stations, andPressure Reduction Troubleshooting4SH Glossary

Click each definition in the list to learn more about the terms used in this course. Click the next slide button in the upper right when you are ready to continue.

Drip Leg: Vertical piping taken from the bottom of steam distribution piping to drain condensate toward a steam trap.Enthalpy: A measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. Typical units are Btu, GJ or Kcal.Saturated Steam: The state in which steam contains only the amount of heat energy required to maintain the vapor state. Any loss of heat energy from saturated steam will cause it to begin condensing.Sensible Heat: The amount of heat energy required to raise the temperature of a liquid to its boiling point (or of a vapor above its boiling point), at a given pressure. In the case of steam the liquid is water. Also known as the heat of the saturated liquid. Superheated steam (vapor stage) also has sensible heat.Superheat: The number of degrees steam has been heated above saturation temperature.Superheated steam: Steam at a temperature higher than water's pressure specific boiling point. Normally produced by adding heat from the boiler to saturated steam. Saturated steam can also be superheated by electrical heaters or through a higher pressure steam-to-steam heat exchanger.5What Is Superheated Steam?Click play to continue

Lets define superheated steam. Click play to continue.6Superheated Steam (SH) and Its UsesSH used for steam distribution linesSH is created by adding extra coils inside boiler, exhaust area of boiler, or superheater

SH is also called Superheat and High Pressure/High Temperature (HPHT) steamAdditional superheating tubes are place in the boiler sectionHeat energy elevates the steam temperature without increasing the steam pressureSH is high-temperature steam used by turbines to generate electricityIn applications where dry steam is a necessity, additional superheating tubes are placed in the boiler section of the furnace. More heat energy is added to the saturated steam to vaporize any carryover. The additional heat energy elevates the steam temperature without increasing the steam pressure and is referred to as superheated steam.Superheated steam is high-temperature steam that is used for a number of purposes, such as driving steam turbines to generate electricity. Superheated steam is also used to maintain the temperature required to deliver steam long distances from the boiler, as in district heating where superheated steam is delivered underneath city streets for use in numerous buildings. Superheat can be created by adding extra coils inside the boiler or in the exhaust area of the boiler, or by means of a dedicated superheater.Superheated steam is also referred to by other names: Superheat, and High Pressure/High Temperature (HPHT) steam. 7Superheated Steam (SH) CreationSuperheat created by use of a superheater

Superheat can also be created by the addition of a superheat chamber after the boiler, attached to the steam main. A schematic diagram of a steam generator with a superheater section is shown here.

8Superheated Steam Created by ThrottlingSuperheat can also be created by throttling high pressure saturated steam to lower pressure.Throttling is a constant enthalpy process with low-pressure steam having approximately the same heat value as high-pressure steam.Downstream piping from the PRV should include a minimum of 20 outlet pipe diameters of straight pipe run before the first turn.Low-pressure steam can be superheated as well as saturated or wet steam. Actual pressure reduction determines final condition.

Throttling Governor on SH TurbineSuperheated steam can be created by throttling high pressure saturated steam to a lower pressure in certain conditions. Throttling is a constant enthalpy process with the lower pressure steam having approximately the same heat value as high pressure steam. In all practical aspects, superheat dissipates quickly after throttling. However, the potential of superheat needs to be considered in regard to how piping is done after the pressure reducing valve. Downstream piping from the PRV should include a minimum of 20 outlet pipe diameters of straight pipe run before the first turn. Low-pressure steam can be superheated as well as saturated or wet steam, depending on operating conditions. Actual pressure reduction determines final steam conditions. 9Superheat ConsiderationsSH is unsuitable for heat transferbut ideal for work and mass transferPressure and temperature of SH are independent, unlike saturated steamHeat transfer coefficient is different between SH and Saturated SteamSH temperature must be reduced to saturation temperature for process use

Superheated steam has properties that make it unsuitable to use as a heat exchange medium, but is ideal for work and mass transfer. Unlike saturated steam, the pressure and temperature of superheated steam are independent of each other. Superheated steam is formed by increasing the temperature and specific volume while maintaining a constant pressure. Superheat temperature must be reduced to saturation temperature before the steam can condense and give up the latent heat energy that can be used for process heat. 10Superheat Considerations (cont.)SH has low BTU value, making it a poor heat transfer mediumU value of SH can be very lowSH helps minimize condensation formationSH can lose heat through radiation without forming condensateSH increases the performance of rotating equipment

Since superheated steam has such low btu value, similar to trying to heat with hot air, it makes a poor heat transfer medium. The heat transfer coefficient is significantly different between superheat and saturated steam. The U value of superheated steam is significantly lower than saturated steam.

Superheated steam helps to minimize the potential for condensation to form in large distribution systems seen in large petrochemical plants and district heating systems. Its important to remember that superheated steam can lose heat through radiation without forming condensate. Superheated steam also increases the performance of rotating equipment, such as turbines. The additional specific volume of superheated steam as well as the delayed condensing of the motive steam increases the operating performance as well as minimizes the erosion of the turbine blades (buckets), adding to the overall life of the equipment.

11Benefits of Superheated Steam100% Steam QualityNo liquid to corrode piping or equipment and no entrained water to damage turbine bladesHigher specific volume to provide additional rotational work in turbinesLess potential for condensate to form in extensive distribution systems

Benefits of superheated steam include: 100% steam quality No liquid to corrode piping or equipment, and no entrained water droplets to damage turbine bladesHigher specific volume to provide additional mechanical, rotational work in turbines, and Less potential for condensate to form in extensive distribution systems

12Drawbacks of Superheated SteamLow heat transfer coefficientSevere duty for steam traps and valvespotential reduction in service lifeWill maintain dry supply systemcan hinder proper operation of some equipmentWorker safety is critical due to high temperatures and pressuresIf superheated steam is used for process heating, steam will give up superheat at a slow rate before condensing Decrease in effective surface area reduces heat transfer and can greatly affect overall productivityDrawbacks of superheated steam include: Low heat transfer coefficientCan be severe duty for steam traps and valves in terms of service life Superheated steam will maintain a dry supply system and can hinder the proper operation of equipment that requires a water seal to operate, such as steam traps, mechanical pumps, and liquid drainersWorker safety issues due to high pressures and temperatures If superheated steam is used for process heating, steam will give up its superheat at a very slow rate before it is allowed to condense. The decrease in effective surface area reduces heat transfer and can greatly affect overall productivity13Properties of Superheated SteamClick play to continue

In order to see the benefits of superheat, its helpful to review a few basic steam concepts and then incorporate superheat into our understanding.

Click Play to continue.14Types of Heat Sensible Heat--heat energy required to raise temperature of liquid to boiling pointSaturated steamsteam contains only the amount of heat energy required to maintain vapor stateCondensation--the process by which a gas or vapor changes to a liquidLatent heat--energy required to cause a change of phase of a liquid at its boiling point to vapor

Sensible heat is the amount of heat energy required to raise the temperature of a liquid to its boiling point (or of a vapor above its boiling point), at a given pressure. In the case of steam, the liquid is water. Also known as the heat of the saturated liquid. Superheated steam (vapor stage) also has sensible heat.

Latent heat is the amount of energy required to cause a change of phase of a liquid at its boiling point to a vapor.

Condensation is the process by which a gas or vapor changes to a liquid.

Saturated Steam is the state in which steam contains only the amount of heat energy required to maintain the vapor state. Any loss of heat energy from saturated steam will cause it to begin condensing.15Sensible Heat and Latent HeatComes from Steam Quality, Screen 7

Saturated Water Saturated SteamWaterWater and SteamSuperheated SteamThe heat contained in steam can be represented on this diagram.The vertical axis represents the temperature, while the horizontal axis represents the Enthalpy.Any point on the left side of the curve represents water.

A point inside the curve represents a mix of water and steam.

While a point on the right side of the curve represents superheated steam.16Sensible Heat and Latent Heat (cont.)Steam Quality, Screen 8

Stage 1212F (100C)32F (0C)Stage 2Stage 3Stage 4Stage 5The curve shown here illustrates the evolution of water at atmospheric pressure, at 32F (0C), when heat is added to it. The initial stage is represented on the chart as Stage 1.When heat is added to the water, the temperature will rise. If sensible heat is added, the water will heat to the saturation temperature. This is indicated on the chart as Stage 2.If more heat is added, water will start vaporizing. This vaporization occurs at constant temperature. This is indicated on the chart as Stage 3.The more heat that is added, the higher percentage of water vaporization. Water will be completely vaporized if the heat added in stage 2 corresponds to the latent heat. Saturated steam results once all the water has been vaporized. This is represented on the chart as Stage 4.If more heat is added, the temperature will increase still farther, resulting in superheated steam. This is indicated on the chart as Stage 5.17Saturated SteamSteam Quality, Screen 12

212F (100C)32F (0C)Saturated SteamThe arrow indicates the point corresponding to saturated steam at atmospheric pressure.This is the intersection between the horizontal line corresponding to 212F (100C), and the saturated steam curve.Saturated Steam will have a total latent heat which is the sum of the sensible heat and the latent heat.

18Superheated SteamSuperheated steam is steam that is heated to a temperature higher than the boiling point of water corresponding to its pressureUnlike saturated steam, there is no temperature-pressure relationship with superheated steamSuperheated steam cannot exist in contact with water, nor contain water, and resembles a perfect gas

Superheated steam is steam that is heated to a temperature higher than the boiling point of water corresponding to its pressure.Superheated steam cannot exist in contact with water, nor contain water, and resembles a perfect gas.Unlike saturated steam, there is no temperature-pressure relationship with superheated steam.

19Superheated Steam CurveSteam Quality, Screen 12

212F (100C)32F (0C)Superheated SteamSuperheated steam and water cannot coexistHere, the line shows the evolution of steam at atmospheric pressure when it is heated above the saturation temperature.

At this point (the red line) there is no longer water present and you have superheated steam. Superheated steam and water cannot coexist.

20Superheated Steam CharacteristicsSH = Steam temp higher than waters specific boiling pointPressure reduction of superheat will produce saturated steamSH and condensate cannot exist in thermodynamic equilibriumSH is designated by the number of degrees steam is heated above saturationAdding energy to saturated steam at constant pressure produces SH

Superheated steam is steam whose temperature is higher than waters specific boiling point. Keep in mind the following points:

If saturated steam is further heated at constant pressure, its temperature will rise producing superheated steam.

Superheated steam is designated by the number of degrees it is heated above the saturation temperature.

Superheated steam and condensate cannot exist together in thermodynamic equilibrium.

A pressure reduction of superheated steam will produce saturated steam.

21Superheated Steam RemindersSteam flows only if there is pressure differentialSteam only condenses if it is at saturationIf flow stops, radiation losses will be enough to produce condensatePressure reduction of saturated steam will produce superheated steamSaturated steam is better for heat transferSuperheat increases operating efficiency of turbinesSuperheat minimizes condensate load in large distribution systems

Additional condensate can flood traps and cause water hammerAnd remember: steam flows only if there is a pressure differential. Condensation of steam causes a pressure drop. Steam can only condense when it is at saturation temperature. A stagnant or inactive supply main or drip leg will dissipate the superheat, leaving only saturated steam.

Keeping the superheated steam flowing is critical. You can reduce the superheat temperature and yet condensate will not form. But if the flow stops, the radiation losses will quickly be sufficient to produce condensate. The additional condensate can flood traps and potentially cause unwanted water hammer.

Key points:A pressure reduction of saturated steam will produce superheated steam. Saturated steam is better for heat transfer processes.Superheated steam increases the operating efficiency of turbines, andSuperheated steam helps to minimize the condensing load in large distribution systems.

22Saturated versus Superheat Properties

Click play to continueHeat value of steam is in the latent heatSuperheat generally isnt usable steam except at a turbine or for transport to other locationsAdding energy to saturated steam results in superheatThe heat value of steam is in the latent heat. As you add energy to saturated steam, you get superheat. As noted earlier, however, this is not a 1:1 ratio as it is in heating water to the saturated steam point. Superheat generally isnt usable steam except at a turbine, or for transport to other locations.

In this table, superheat values are calculated by subtracting sensible and latent heat from total heat. As you can see, for a given pressure there is more heat value in superheat than in saturated steam.

Click play to continue.

23Superheated Steam ApplicationsClick play to continue

As we proceed through our understanding of superheated steam, it's important to look at real-world examples. In this next section we will look at detailed superheated steam use in turbines and heat exchangers, piping considerations for superheat, and how to reduce the temperature of superheat for process use. Click play to continue.24Superheated Steam Applications

Superheat is used in a number of applications. Click each tab to see more information.

Turbines. Superheated steam is used to supply completely dry gas to drive power-generating equipment, such as turbines. The generated power helps minimize plant operational cost, while generating potential revenue as overages which can be sold back to the grid. Turbine blades rotate at extremely high speeds and any entrained moisture can erode the blades and cause premature turbine failure. In addition to power generation, turbines are used for gas compressors, air compressors, pumps, fans, and most rotating equipment. As you can see, steam trap operation is critical in order to drain any condensate from a turbine.

District Heating. Superheat is also used where steam distribution systems are extensive. Many major cities use a distribution of steam mains that run underneath the streets, and are used for heating buildings as well as additional power generation. This system is known as District Heating.Industries. Industries that use superheat include paper processing, chemical processing, and hydrocarbon processing such as coking, cracking, and distillation. It is also used in gas synthesis processes in fertilizer plants. Superheat is also used in steam ejectors for maintaining high vacuum in typical applications such as evaporators and digesters.Solar/electrical power plants can produce superheat by using the sun to heat thermal fluids, such as oil or chemical salts to high temperatures (1000F / 537C). The thermal fluid is then circulated through a reboiler to generate saturated steam.25Turbine Using Superheated Steam vs Saturated SteamSATURATED STEAMTurbine operating performance: 30lb/hphr (water rate), 650 psig @ 497 F (18,22 kg/kWh, 45bar @ 259C)Pump power requirement:100 hp (74,57kW)Total steam consumption requirement: 3000 lb/hr (1360 kg/h)SUPERHEATED STEAMTurbine operated performance: 23.76 lb/hphr (water-rate), 650psig @ 700F (14,4 kg/kWh, 45bar @ 371C)(SUPERHEATED STEAM)Pump power requirement 100hp (74.57 kW)Total steam consumption requirement 2376 lb/hr (1074 kg/h)

Using Superheat 20.8% Less Steam Consumed 3.8% Energy SavingsClick play to continueHere is an example of a steam turbine using saturated steam versus a turbine using superheated steam. While the recommendation is to use superheat for turbine use, there are still many plants around the world that use saturated steam to drive turbines. As you can see from the difference between saturated and superheated steam, the use of superheat results in 20.8% less steam consumed, and a 3.8% energy savings.

Click play to continue26Heat Exchanger Using Superheated and Saturated Steam650psig@497F(saturated) vs. 650psig@700F(superheated) (45 bar @ 258C)Heat exchanger requirement: 3,000,000 lb/hr (45 bar @ 371C)Heating oil from 300F to 400F (149C to 204C)Heat transfer coefficient (U value) for steam to oil: 3.5 btu/fthrf (SH) 26.5 btu/fthrf (SAT) 542,12 J/m.h.C Calculate surface area required Superheat (17.7% of total steam energy)Q = UAT(LMTDlogarithmic mean temp. difference)(3,000,000)(0.177) = (3.5 btu/fthrf )(A)(210LMTD) A = 722 ft required to absorb superheated steam energy(3,000,000)(0.823) = (26.5 btu/fthrf)(A)(140LMTD) A = 665 ft required to condense saturated steam energy3 167 610 [kJ/h] = 542,12 [kJ/m.h.C] . A . 78,3 [C] A= 75 m required to condense saturated steam energyTOTAL SURFACE AREA REQUIRED 1387 ft (128.8 m)Click play to continueHere is another example, this time showing heat exchangers using superheated and saturated steam, comparing total surface area required. Note that more than half of the surface area is required just to dissipate the superheat, while only accounting for 17% of the heat transfer.

Click play to continue.27Heat Exchanger Using Only Saturated Steam650psig@497F(saturated) (45 bar @258C)Heat exchanger requirement 3,000,000 btu/hr (3 167 610 kJ/h)Heating oil from 300F to 400F (149C to 204C)Heat transfer coefficient(U value) for steam to oil 26.5 btu/fthr(SAT) 542,12 J/m.h.C Calculate surface area required Saturated Steam

Q = UAT(LMTD)3,000,000 = (26.5 btu/fthrf)(A)(140LMTD)3,167,610 [kJ/h] = 542,12 [kJ/m.h.C] . A . 78,3 [C] A = 809 ft (75 m) required to condense saturated steam energy

TOTAL SURFACE AREA REQUIRED 809 ft (75 m) 42% less when using saturated steam onlyClick play to continueHere is the same example, this time showing the heat exchanger using only saturated steam, comparing total surface area required.

Youll see that there is a 42% reduction in total surface area required using saturated steam only, compared with using both saturated and superheat.

Click play to continue.28Superheat Limitations with Heat ExchangersSuperheat is of limited value when used for heat transferSuperheat requires a larger heat transfer area and larger diameter pipingCan result in process control problems or fouling of heat exchanger surfacesSuperheat creates uneven temperature gradientsSuperheat gives up little heat energy until it has cooled

As previously demonstrated, superheated steam does have its limitations in certain applications, particularly in heat exchangers. Superheated steam is of little value when used in heat transfer applications.Given the high temperatures and pressures normally associated with superheat, we must be aware of potential special piping considerations to meet the additional design and operating requirements.

Superheat gives up little heat energy until it has cooled to saturation temperature and can utilize the latent heatSuperheat requires a larger heat transfer area and sometimes requires larger diameter distribution piping, andSuperheat creates uneven temperature gradients potentially causing process control problems, or fouling of heat exchanger surfaces29Steam Main SizingSaturated versus Superheated Steam650psig@497F (45 bar@258C) Saturated650psig@700F (45 bar@371C) Superheated

Design Requirements:100,000 lb/hr flow @10,000ft/min 45 400 kg/h @ 50 m/sSaturated Steam requires a 5 Schedule 80 pipe (DN125 Schedule 80)Superheated Steam requires a 6 Schedule 80 pipe (DN150 Schedule 80)

The only way to counter the larger pipe size required for superheated steam is to design for a higher velocity. For example, in design requirements for:100,000 lb/hr flow @10,000ft/min (50m/s) for saturated steam 100,000 lb/hr flow @15,000ft/min (75m/s) for superheated steam

Both conditions would be correct using a 5 or 6 (DN125 or DN150) Schedule 80 pipe.

Click play to continueIn comparing steam main sizing of saturated vs superheated steam, lets look at this example:

As demonstrated, superheated steam requires a larger diameter (5 vs 6) steam main due to the increased specific volume. The only way to counter the larger pipe size required for superheated steam is to design for a higher velocity.

Click play to continue.

30Reducing the Temperature of Superheated SteamSuperheat temperature must be reduced using a desuperheater

Desuperheater injects water or other coolant into superheated steamLocation of sensors on desuperheater is importantImproperly functioning desuperheater can cause poor steam qualityInjection DesuperheaterNow, lets look at how we can reduce the temperature of superheated steam for process use.The temperature of superheated steam must be reduced for process use, usually through the means of a desuperheater, so that latent heat is available for heat transfer.A desuperheater injects cooling water into the flow of superheated steam. The vaporizing cooling water absorbs the superheat temperature bringing the steam temperature close to saturation. The quantity of cooling water required is controlled by measuring the desuperheater discharge temperature. The target temperature is 10-30F (3-10C) above saturation.Location of pressure and temperature sensors on a desuperheater is critical. If the sensor is too close or too far from the discharge, an incorrect signal might be sent to control the cooling water flow. A straight piping run of 20-30ft (6-9m) is usually recommended before the steam temperature sensor. The most common problem is over injection of cooling water resulting in a flooded steam main.31Condensate Injection Desuperheater In UseTask: Desuperheat 10,000 lbs/hr (4500 kg/hr) of superheated steam650psig@700F (45 bar @370C)Enthalpy 1348 btu/lb (3130 kJ/kg)

How: Inject treated condensate at 200F (93C)

Final Steam Conditions: 10,000 lbs/hr superheated steam 650psig@547F (45 bar @286C)Enthalpy 1245 btu/lb (2893 kJ/kg)

(As a rule of thumb, only desuperheat steam to within 25-50F (15-30C) of saturation to minimize potential for wet steam, corrosion, and water hammer)

Here is an example of a condensate injection desuperheater in use. We are reducing superheat by injecting condensate at 200F (93C). The pressure remains constant, but we have reduced the temperature significantly.32Reducing the Temperature of Superheated SteamPressure Reducing Desuperheating Valve (PRDS)

Many plants require Pressure Reducing Desuperheating valves (PRDS), also called steam conditioning valves. In this three-inlet valve, water is injected into the valve vena-contracta area while steam pressure is being reduced. This provides excellent heat absorption by superheated steam, and both pressure and temperature are reduced in a single valve.33Trapping Superheated Steam LinesClick play to continue

At first glance, trapping superheated steam may seem confusing due to the idea that superheated steam produces no condensate, and therefore the steam lines carrying superheated steam should not have any condensate in them. This is true once the system is up to temperature and pressure but, as we have noted previously, condensate removal is necessary during start-up or if the superheat dissipates due to a no-flow condition.

Click play to continue.34Why Trap Superheat Systems?Remove condensate and non-condensable gases while minimizing steam lossCondensate is produced on startup and in the event of superheat lossPrimary reason for traps on superheat is startup load and shutdownHandle emergencies such as superheat loss or bypassDuring emergencies, steam traps are a necessityTrap sizing is critical in order to keep efficiency high and avoid water hammer

A steam trap's job is to remove condensate and non-condensable gases while minimizing steam loss from heat transfer equipment and steam distribution piping. Superheat lines do not theoretically have condensate in them, and therefore should not require the use of a steam trap. However, condensate is produced when starting up the system and in the event of the loss of superheat. As the piping comes up to temperature, condensate is produced mostly due to the heat that is absorbed by the cold pipe.The primary reason for traps on superheat systems is the start-up load, as well as during shutdown. Startup loads can be heavy because of the large size of the mains. On start-up, manual valves will most likely be used since time is available to open and to close the valves. This is known as supervised start-up. A second reason for steam traps is to handle emergencies such as superheat loss or bypass which might require operation on saturated steam. In these unscheduled events, there is no time available for manually opening valves; therefore, steam traps are a necessity. These are the situations for which proper trap sizing is a must. Condensate must be removed as it forms in any steam system to keep efficiency high and to keep damaging water hammer and erosion to a minimum.

35Sizing Superheat Loads to TrapsCondensate loads to a trap vary widely with superheatLarge condensate flows, combined with low pressure, requires large capacity traps

The condensate load to a trap used on superheat will vary widely from severe start-up loads to virtually no load during operation. Consequently, this is a demanding application for any steam trap.

During start-up, very large lines are being filled with steam from cold conditions. At this time, only saturated steam at low pressure is in the lines until the line temperature can be increased. This is done slowly over a long period of time so as not to stress the lines. Large condensate flows to move through the traps, combined with low pressure, is the startup condition that requires the use of large capacity traps. These oversized traps are then required to operate at very high pressures with very low capacity requirements during normal superheat operation.

Lets look at different trap types with recommended (thumbs up) or not recommended (thumbs down) indicators for use on superheat.

36Thermostatic BellowsThermostatic Bellows

SH entering traps will cause vapor in bellows to superheatVolume of vapor in bellows increases greatlyBellows cant contain additional vapor volume, causing possible ruptureThermostatic Bellows

The same physical characteristics which apply to steam when superheated will apply to the fluid within the bellows. Superheated steam entering the traps will cause the vapor in the bellows to superheat also, and the volume of the vapor in the bellows will increase greatly. The bellows will not be able to contain the additional vapor volume without a large pressure rise in the bellows; therefore many forms of bellows will rupture.37Thermostatic WaferThermostatic Wafer

Trap has limited ability to handle dirt and impuritiesCondensate capacity varies with degree of subcoolingShould not be used on drip applications that see saturated steamThermostatic Wafer

A non-welded wafer design can be compatible with superheat, but trap performance may be affected by the trap's limited ability to handle dirt or impurities in a steam system. Condensate capacity of thermostatic wafer traps will vary with the degree of subcooling, so they should not be used on drip applications that may see saturated steam. 38Thermodynamic DiscThermodynamic Disc

Trap will cycle whether or not there is condensate behind itQuick cycling at high temperatures and pressures causes wear and short lifeWith little SH, any trap opening will blow through live steamThermodynamic Disc

The disc type of trap is a time cycle device. As soon as the steam trapped above the disc is condensed, the trap will cycle whether or not there is condensate behind it. Since very little condensate will be present in superheat applications any trap opening will blow through live steam.

Since the trap is opening and closing with little or no condensate passage, it will cycle quickly with steam passage at high temperatures and pressures causing excessive wear. This will result in steam waste and extremely short life.39Float and ThermostaticFloat and Thermostatic

High pressure float required F&T traps with bimetallic thermostatic elements more suitable for SH than traditional Ruggedness and limited ability to handle dirt and impurities must be consideredFloat and Thermostatic

A high pressure float would be required. Standard balanced pressure bellows air vents would not be compatible with superheat. Those F&T traps with bimetallic thermostatic elements would be more suited to superheat applications than traditional F&T traps. Each cycle of the air vent will discharge a mixture of non-condensables and steam. Ruggedness of the design and limited ability to handle dirt or system impurities must also be considered.

40Thermostatic BimetallicThermostatic Bimetallic

Can be set so it wont open until condensate cools to below saturationAs steam temp rises, pull of bimetallic becomes greater, creating greater sealing forceSH tends to seal valve better, so thermostatic bimetallic is a good choice for SH Proper drip leg sizing and layout is mandatoryThermostatic Bimetallic

A thermostatic bimetallic can be set so it will not open until condensate has cooled to a temperature below saturation. For the existing pressure, it will remain closed whenever steam of any temperature is in the trap. As the steam temperature rises, the pull of the bimetallic element becomes greater, providing a greater sealing force on the valve. Superheated steam would tend to seal the valve better. For these reasons, this trap can be a good choice for superheat.During superheat operation, the condensate in the trap must cool to a temperature below the saturation temperature before the trap can open. The result can be a long back up of condensate behind the trap.Condensate may back-up into the line and cause damage to the lines, valves, and equipment if there is not a sufficient length of drip leg before the trap. For this reason, proper drip leg sizing and layout is mandatory.

41Inverted BucketInverted Bucket

Water seal prevents steam getting to valve, promoting no live steam loss and long lifeValve at top makes it impervious to dirt and permits removal of airNecessity to maintain water seal (prime) is potential issueProper piping necessary to maintain primeInverted Bucket

A water seal prevents steam from getting to the valve, promoting no live steam loss and long life. The valve at the top makes it impervious to dirt and permits removal of air. Large start up loads can be handled and still accommodate small running loads. It appears to be the ideal choice, but there are problems associated with its application on superheat, mostly associated with the necessity to maintain its water seal or prime.The loss of prime in an inverted bucket trap will result in the valve opening and blowing of live steam.There are several ways in which this can happen; likewise, there are means for preventing this occurrence.

42Inverted Bucket OptionsInverted Bucket Specify the following options when ordering inverted bucket traps for superheat:

Inlet tubeCheck valve at inletBurnished seats and valvesUse smallest orifice possible; restricted orifices if necessarya. Size main orifice for pressureb. Size restricted orifice for start-up load5. Avoid insulating the trap and the drip leg if possible

Specify the following options when ordering inverted bucket traps for superheat:Inlet tubeCheck valve at inletBurnished seats and valvesUse smallest orifice possible, restricted orifices if necessary--size main orifice for pressure, and size restricted orifice for start-up load, andAvoid insulating the trap and the drip leg if possible43Sizing Superheat TrapsSize for start-up load with no safety factorSelect main valve for max pressure differentialSpecify a burnished valve and seatSelect body materials on basis of max temp and pressureProperly sized inverted bucket traps are acceptable

Bimetallic thermostatic traps are recommended

Trap load will vary from severe load to virtually no loadThe condensate load to a trap used on superheat will vary widely from severe start-up loads to virtually no load during operation. Consequently, this is a demanding application for any steam trap.In sizing a superheat trap, size for start-up load with no safety factor. Select the main valve on the basis of the maximum pressure differential, but use restricted orifices to suit the trap to the load. Always specify a burnished valve and seat. Body materials should be selected on the basis of the maximum pressure of the steam and the steam temperature, including superheat. Properly-sized inverted bucket traps are acceptable, while bimetallic thermostatic traps are recommended.

44Drip Leg Sizing and InsulationDrip leg sizing should be followed when installing traps on superheat systemsUninsulated drip leg and piping allows condensate to form ahead of the trap

See links menu above for Trapping Superheated Steam Systems To ensure the condensate is removed efficiently, proper drip leg sizing and piping recommendations should be followed on superheat systems. The question arises on whether insulation should be used on the drip leg, piping leading to the trap, and the trap. The answer is no. Unless it is mandatory for safety reasons, this section of the steam system should not be insulated. The reason for this is to ensure that some condensate is continuously being formed ahead of the trap. This will ensure that the trap has some condensate going to it at all times, thus prolonging the traps life.

For more information on superheat traps and drip leg sizing, see the Armstrong publication, Trapping Superheated Steam Systems in the links menu above.

45Optimizations for Superheated SteamClick play to continue

Finally, lets examine some of the recommended optimizations for superheated steam. Click play to continue.

46Turbine Performance with Superheated SteamRemoving condensate only at startupPreventive maintenancetrap surveys at least annuallyInverted bucket trap must be properly sized and appliedPRV will improve dryness of steam due to throttling processNo condensate in properly functioning SH systemSaturated steam turbine may not last as long as SH turbineRecommended type of trap is properly-applied bi-metallicAIM wireless monitoring of traps is recommended

Preventive maintenancetrap surveys at least annuallyInverted bucket trap must be properly sized and appliedPRV will improve dryness of steam due to throttling processRecommended type of trap is properly-applied bi-metallicIs it assumed that steam is 100% at inlet of a PRV

It's important to remember that there is no condensate in a superheated steam system that is properly functioning. When trapping superheat we are removing condensate only during startup. A saturated steam turbine may not last as long as one operating on superheated steam, but saturated turbines can last for years. As a turbine operating on saturated steam starts to wear due to erosion of the blades from entrained moisture in wet steam, the blades (or buckets as they're also known) become less efficient and turbine performance starts to diminish.Armstrong AIM products, which wirelessly monitor traps and send a signal when a trap has an issue, are an alternative to trap surveys, especially for turbines. If a trap goes down on a superheat line supplying a turbine, entrained moisture could damage the turbine. The basic assumption is that steam is 100% dry at inlet of a pressure reducing valve. In practice, it is difficult to achieve 100% dry steam, and a PRV will improve the dryness factor due to the throttling process.It is critical to evaluate all the different operating conditions of a superheated steam system when selecting the proper steam trap.An inverted bucket trap can be used on superheated steam, but it must be properly sized and appliedThe recommended trap is a properly-applied bi-metallicPreventive maintenance must be performed, with trap surveys conducted at least annuallyA pressure-reducing valve will improve the dryness of steam due to the throttling process47ConclusionSuperheat is useful primarily as a motive force for electricity generation and for District HeatingSuperheat requires special considerations and understanding that trapping is still required

Superheat is useful primarily as a motive force for electricity generation, and for District Heating. Superheat requires special considerations and the understanding that trapping is still required during periods of startup, shutdown, and in the event of emergencies such as superheater loss or bypass.48Quiz Instructions

The quiz includes 10 questions. A passing score of 80% is required to pass. An incorrectly answered primary question will require that you answer a follow-up question. A missed follow-up question will result in a non-passing score and youll need to return to the beginning of the course for review before attempting the quiz again. Take your time and read each question and available answers carefully before making your selection. When you pass the quiz, youll have the option to print your personalized certificate from a link on Armstrong University. A copy of the certificate in PDF format will also be sent to you by email.

49Superheated Steam

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Armstrong International, Inc.Thanks for completing Armstrongs Superheated Steam training course. We encourage you to continue expanding your knowledge by completing other courses on Armstrong University.

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