Steam Engineering basics

transcript

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

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

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.

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

; 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.

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

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

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