Date post: | 18-Jan-2017 |
Category: |
Engineering |
Upload: | engr-sahadat-hussain |
View: | 116 times |
Download: | 1 times |
Steam Engineering Basics
Engr. Sahadat HussainSr. Consultant, Environmental & Process
Engineering Resources International (ERI)
What is steam?
Steam is water in the gas phase, which is formed when water boils.
What is steam? (Cont...)
ENTHALPY IN KCal/kgGauge
pressure bar
SpecificEnthalpy
of evap'tion
(hfg)
VolumeDry Sat. m /kg
Temp.ºC
Water(hf)
Steam(hg) 3
1 120 120 526.2 646.2 0.88123456
134144152159165
133144152159166
517.6509.7504498.9493.5
650.6653.7656657.9659.5
0.6030.4610.3740.3150.272
7 170 171 489.8 660.8 0.24
0 100 100 540 640 1.673
What is steam?(Cont.…)
What is so different about Steam?
Fixing one parameter fixes all others.Steam at 1 barg pressure will have -
1200CTs - Saturation Temp.h -
L - V -
Sensible HeatLatent heatSpecific Volume
120 Kcal/kg.526.2 Kcal/kg.0.902 m3/kg. (steam)
0.0011 m3/kg. (cond.)Exercise -What will be the above properties at a pressure of 3.5 Kg/cm2g.
What is so different about Steam - contd.
Steam quality is a changing parameterbe measured on-line.It is indicated by a term called Dryness
and can’t
Fraction.Dryness fraction ‘x’ indicates fraction of dry steampresent in given total massx=0.9Means 90% steam with SH+LH and 10% water(only SH)Exercise - What will be total heat in steam at 3 kg/cm2g pressure and 0.85 dry?
What is so different about Steam? - contd.Steam Quality depends on type of boiler and
effectiveness of steam distribution.
A coil type boiler will deliver wet steam at loadvariation as it has no steam space.
A shell type boiler will ensure better qualityunder all load conditions
steam
Saturated steam will never be 100% dry
What is so different about Steam?-contd.
Steam has higher Specific VolumeVolume of steam at 1 kg/cm2gVolume of steam at 7 kg/cm2g
at low pressure0.902m3/kg0.24m3/kg
376% more volume to be handled at 1 kg/cm2g(lower pressure) as compared to 7 kg/cm2g.
Always advisable to reduce steam pressure nearthe equipment and not in Boiler House.
What is so different about Steam - contd.
Saturated steam is a condensing vapor withinstant heat release on phase change.Latent heat is released when steam condenses
Superheated steam is a gas with very slow releaseof superheat before latent heat can be released.
It is ideal to use saturated steam for any indirectheat transfer.
80%HP
steam15% PRVFlue
losses3%gases
Fuel100%
2%losses
Blowdown
5%Condensate
20%Standard Steam Circuit - Energy Balance
ProcessPlant
55% to process
BoilerPlant
Steam Circuit
Actual Enthalpy of Wet Steam; Dryness fraction is the proportion of dry steam within a given sample.; The ‘actual’ enthalpy of evaporation of wet steam is the product of the dryness fraction (x) and the specific enthalpy (hfg) from the steamtables. This in short means the lower the dryness fraction more
amount of steam would be needed for the providing the same heatinput.Enthalpy of evaporation
Drynessfraction
Actualenthalp
y
; What will be the actual enthalpy of 0.9 dry steam at 3 barg pressure?
Advantage Steam
; High heat content, Easily available, Very cheap in comparison.
;
;
Steam lines are relatively light in weight.
Steam flows in response to the pressure drop along the line and there is no need for circulating pumps.Steam is flexible and loads can be added or subtracted at will,;within the limitations of the supply.
Heat transfer coefficients from steam are often twice as high as those from water.
Constant temperature gradient will occur along the heat emitting surfaces.Relatively easy plant maintenance
;
;
;
Energy Equivalent Flow rate for 1KW
Hot Water
Flow rate kg/s = 1000W = 0.0217 kg/s78 Kg/hr.(BDT 10/hr.)4187J/kg K (82ºC - 71ºC)
Hot oilFlow rate kg/s = 1000W = 0.091 kg/s
550J/kg K (300ºC - 280ºC) 328 Kg/hr. (BDT 9/hr.)SteamTaking steam at 6 bar g. Enthalpy of evaporation per kilogram = 2066 kCal/kg
= 1 = 0.00048 kg/s1.7 Kg/hr.(BDT 1.25/hr)2066
Cost of equivalent electrical energy will be BDT 6.74/hr.The comparison in flow rates are well in favour of steam !
Engineering involved with Steam
• Quality of steam is a constant variable.Dry steam at boiler end may become wet at equipment.
Process steam demand is fluctuating with timeOn an average a process boiler works at 50% load
•
• Steam may not get due importance at ‘Project’ stage.Sizing and component selection lacks engineering touch.
The problems start during normal operationsHigh pressure drops, Water-hammer, Air binding, Longer batch timings, Poor product quality are very common
•
Common operational problems
In an average process plant steam load is variablepeaks occurring for short periods.
with
Steam has major ingress of air whenever used forintermittent heating applications
Equipments use steam intermittentlyPlantPlantPlant
works only for one/two shifts in a dayhas seasonal operationsworks for five days or six days in a week.
Steam Should Be Supplied At TheDesired Time Demand Satisfied by
a drop in Boiler
Demand NotSatisfied
PressurePeak
Boiler Maximum Continuous Rating
Average Steam Flow RateSteamFlowRate Actual Steam
Flow Rate
Time
Layers That Reduce HeatTransfer Efficiency
Scale Film
Air FilmCondensate Film
Stagnant Water
SteamMetal Wall Water
Being Heated
Steam Engineering - Related Problems
• Incorrect Basis For Boiler Capacity
• Lack Of Info On Boiler Efficiency
• Flue Gas Not Monitored• Improper Loading Of Boiler Plant
No Relation With Process Demand
Confusion On Relation BetweenBlow-down & Boiler LoadBlow-down & Hardness
•
Common Problems in Steam Distribution
High And Abnormal Pressure Drops
Manufacturing Operations Limited To Only Few
Longer Processing Timings’
Lower Productivity On Machines
Steam Starvation At End Points
• No Modification In Piping System To ReflectChanges In Equipment, Etc.
.• Misconceptions Regarding Flash Steam
• Hot Condensate Cooled Because Pump - MotorSet Cannot Handle It
• Operating Staff Unaware Of Cost Of Steam AndIts Components
Common Problems In Steam Distribution cont...
• Bypass Of Traps/Control Valves Open
• Wrong Trap Selection (Open Bypass) AndInstallation
• Use Of Traps For Lifting Condensate
• No Air Vents Except On Headers/JacketedVessels. Mostly Manual Venting
Common Problems In Steam Distribution cont...
High - Pressure Steam Necessary For Fast Heating.Required
PRV Not
Improper Sizing Of PRV Assembly
Absence Of External Pressure - Balancing
Pressure Reduction/Control By Blowing Rather ThanControl Valve
Using
Common Problems In Steam Distribution cont...