presentation on steam

7/31/2019 Presentation on Steam

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BASICS OF STEAM What is steam?

-Gaseous phase of H2O At atmospheric pressure,

- Evaporates at 100 deg. C

- Solidifies at 0 deg.C

When pressure increases,- Temperature of evaporation increases.

- Specific volume reduces

Will steam be completely gaseous?

No. It will carry water droplets. ie: wet steam

What is dry steam?

- Steam with no water droplets.


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Enthalpy hf - Liquid enthalpy or sensible heat

Heat energy required to raise the temperature from a datum of 0 deg. C to

current temperature.

hfg Enthalpy of evaporation

Amount of heat required to change the state f water at its boiling

temperature into steam

hg Total enthalpy of saturated steam

Total energy required to change the state to saturated steam from a datumof 0 deg.C

hg = hf+ hfg

Dryness fraction

What is dryness fraction?

Fraction of mass of dry steam out of total mass- Actual enthalpy =hf+hfg.X

- Actual specific volume = Vg.X


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Flash steam

Proportion of flash steam=(hf at P1)-(hf at P2)

hfg at P2


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Disadvantages of wet steam

- High line losses

- Cavitations in turbine blades

- Less thermodynamic efficiencies

( Carnot cycle, Rankine cycle)

Solution?

Superheated steam


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SUPERHEATED STEAM

Less line losses Less cavitations in turbine blades

Higher thermodynamic efficiencies

Advantages of superheated steam

How superheated steam is generated ?

- Can it be done in a boiler?


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Disadvantages of superheated

steam

Poor heat transfer

Temperature drop in heat transfer


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What is entropy?

Can it be measured with a gauge?

No

A calculated value from the things that can be

measured

Entropy at the latter condition is always high

Entropy = Enthalpy / Absolute temperature


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Sat water line Sat. steam line

Constant

pressure lines

Constant dryness fraction


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Steam flow metering


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Important parameters

Repeatability-indicating same value for identical flow

Uncertainty

- Deviation of true value Accuracy

-Indication of correct flow rate against the true value

obtained by extensive calibration procedure

Turn down- Range of flowrates over which the meter will work within

the accuracy and repeatability


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Example


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Example

Maximum flow rate = 1000 kg/hr

Turndown =4:1

= 1000/4=250 kg/hr


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Turn down ratios of different

flowmeters


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Types of flow meters

Orifice plate flowmeter

Pitot tube

Turbine flowmeter Variable area flowmeter

Vortex shedding flowmeter


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Orifice plate flow meter


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Theory

Assume

and

By substituting flow, Q in V1 and V2,

and

Then,


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Theory ..

Substituting,

Where,

C: Coefficient of discharge

e: Coefficient of expansion


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Theory.

As per ISO 5167,

Where,Beta= D2/D1 L1 and L2 are functions on tap type

and it is:

L1=L2=0 for corner taps

L1=1 L2=0.47 for D & D/2 taps

L1=L2=0.0254/D D[m] for 1" taps


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Theory


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Computer Program for calibrating

orifice plates

- Based on ISO5167

- Need to give data on Fluid properties

- Fluid

- Density

- Viscosity- Pressure

- Temperature

Pipe properties

- Pipe diameter

- Roughness

- Material Orifice plate properties

- Diameter

- Type of tapping

- Material


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Sizing the orifice plate

Select 70% of usual full scale flow for 50% dp.

- gives more accurate reading

- Protect from dp reading exceeding the limit.

Select suitable full scale dp Select suitable beta value

- Values between 0.4 and 0.7 is preferred

- Smaller values give higher pressure drops

- Larger values reduces the accuracyCalculate dp for different flows


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Typical installation of a orifice plate

flow meter


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Guide for installing an orifice plate

Use small bore pipes as tappings

Do not tap from the bottom of the line

Keep sufficient straight lengths in up and down streams

Keep the orifice plate exactly concentric with the pipe

Prevent from packing gasket misalignments

Place the orifice plate in the correct orientation

Mark the flow direction on the plate handle

Mark the size of the opening on the handle


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Guide for installing an orifice plate.....


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Guide for installing an orifice

plate.....


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Advantages of orifice flowmeters

Simple and rugged

Good accuracy

Low cost No calibration is required provided

calculations, tolarances and installation

comply with ISO 5167


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Disadvantages of orifice plate

flowmeters

Turndown is limited to 4:1

Orifice plate can buckle due to water

hammer

Erosion of the square edge with time

High unobstructed straight length is

required


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POWER GENERATION


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Efficiency


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Types of turbines based on output

conditions Non condensing or back pressure

Exhaust steam is used for processes

Controlled by a back end pressure regulator

Condensing Used when primary objective is power

Dryness fraction of exhaust is more than 90%

Reheat Steam is extracted at the middle and supplied to the next stage after re-

heating

Used for power generation plants

Extracting Steam is extracted in intermediate stages

Induce Low pressure steam is introduced at intermediate stages

To get more power

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