STEAM - PowerPoint Presentation

Slide1

STEAM & WATER ANALYSIS SYSTEM

AN OVERVIEW

PRESENTED BY :

DEPARTMENT OF IPC & CHEM. LAB - SANTALDIH TPS - WBPDCL

10-Jan-2011

Page 1 of 21

A K SARKARSlide2

WHY SWAS ?

Most of the Power generation in India is

Thermal Power. That means Coal or Gas is used as fuel to heat water in the boiler. This water becomes steam and this steam runs the turbine to produce electricity

by means of Generator connected with the turbine. The turbine

is a masterpiece of engineering. Its design is very compli- -cated and intricate. The turbine is very

costly and is very carefully manufactured and installed in a power station. Boiler, various

tubes

and

pipes

that make the Power Plant are also important. All this is dependent on good quality of steam. If the steam produced is of bad quality, it starts damaging this equipment by means of corrosion. The turbine/boiler and various tubes/pipes in the power plant start getting damaged and eventually fail.

It is therefore essential to arrest all the elements that cause the corrosion effects of various kinds. It calls for modern methods that keep a 24-hours-a-day vigil on these elements, however small these may be. This can be precisely done by an On-line Steam and Water Analysis System, popularly termed as SWAS.

Page 2 of 21Slide3

THE IMPORTANCE OF MONITORING

BOILER PARAMETERS

The power plants using steam need some water source. As all of us know, water is no more a free

resource. Further, the quality of water available from rivers, dams or underground sources is deteriorating every day. If we use such contaminated

water for generating steam, it will have an immediate impact on the complete plant and machinery in the power plant, as there will be a lot of erosion and

corrosion that will take place.

In any power plant running on steam, the

purity

of boiler

feed water and steam is absolutely crucial; especially to steam turbine, steam boiler, super heater, condenser and other steam equipment. To prevent

damage of steam turbine, steam boiler and other apparatus due to scaling and corrosion, on line steam and water analysis of critical parameters is inevitable. A well-engineered SWAS can measure all these parameters accurately and reliably. As we know, anything that can be measured accurately can be controlled accurately.

Page 3 of 21Slide4

WHAT ARE THESE CRITICAL PARAMETERS ?

pH

CONDUCTIVITY HYDRAZINE SILICA DISSOLVED OXYGEN

SODIUM OTHER PARAMETERS

Monitoring of other parameters such as alkalinity, hardness, calcium, chloride,

phosphate, dissolved ozone is also required, depending on the size of the plant and the quality of water / steam equipment.

Let us see the

significance

of each of these parameters :

pH In a soln

. pH approximates but is not equal to p[H], the –ive logarithm (base10) of the molar concentration of dissolved hydronium ions(H3O+); a low pH indicates

high concentration

of

hydronium

ions& a

high pH

indicates

low

concentration

.It

is

a measure of the acidity or basicity of a soln. The pH scale ranges from 0 to 14.

Page 4 of 21

Ultra pure water

has a

neutral

pH value of 7. A pH less than 7 is

acidic

and

greater than 7 is

basic

or

alkaline

. In the steam circuit the normal practice is

to

keep the pH value of feed water at slightly alkaline levels

. Accurate

pH

analysis

can therefore help in

preventing the corrosion

of pipe work and

other equipment.Slide5

CONDUCTIVITY Conductivity (G), the inverse of Resistivity (R) is the ability of a material to

conduct electric current. Since the charge on ions in solution facilitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration.

Page 5 of 21

Conductivity in water is affected by the presence of inorganic dissolved solids such as chloride, nitrate, sulfate and phosphate anions (ions that carry a -ive charge)

or sodium, magnesium, calcium, iron and aluminium cations (ions that carry a +ive charge). Organic compounds like oil, phenol, alcohol and sugar do

not conduct electrical current very well and therefore have a low conductivity when in water.

Conductivity is also

affected by temperature

:

the warmer the water, the higher the conductivity. For this reason, conductivity is reported as

Conductivity at 25 Degrees Celsius (250C). The basic unit of measurement of conductivity is the mho(Ω) or siemens(s). Conductivity is measured in micromhos per centimeter(µΩ/cm) or microsiemens per centimeter(µs/cm). Distilled water has a conductivity in the range of 0.5 to

3.0µs/cm. Industrial waters can range as high as 10000µs/cm.

Ultra-pure water or steam

is almost a

bad conductor of electricity

. It becomes

conductive

only when there is some

contamination

. Therefore conductivity measurement can give a quick indication of

steam contamination.Slide6

HYDRAZINE (N2

H4) Hydrazine is an inorganic chemical compound with the formula N

2H4. It is a colourless liquid with an ammonia-like odour. H

However, hydrazine has physical properties that are closer to those of water. Hydrazine is N N highly toxic and dangerously unstable, and is H usually handled while

in solution for safety reasons. H H

Page 6 of 21

The use of hydrazine as an

oxygen scavenger

and a

source of Feed water

alkalinity

has advantages that are well known.

a) It

prevents frothing

in the boiler.

b) It

minimizes deposits

on

metal

surfaces.

c) It removes

the dissolved oxygen in the steam by chemical action.

Hydrazine does not produce corrosive gases at high temperatures and pressures,

and in application, reacts with oxygen to form nitrogen and water:

N

2

H

4

+ O

2

→ 2 H

2

O + N

2

Hydrazine

does’nt contribute solids

to the system, so

boiler blowdown is reduced.

Hydrazine helps to maintain a

protective magnetite

(Fe

3

O

4

)

layer

over steel surface,

and maintain

feed water alkalinity

to

prevent acidic corrosion

. It is

essential

to

doze hydrazine correctly

, which can be done with the help of an

on-line

Hydrazine analyzer

.Slide7

SILICA (SiO2

) Most boilers making steam for turbines rarely have excessive carryover of boiler water in the steam. While, silica deposits in turbine can occur even when boiler water carryover is negligible. The

reason is: steam selectively “picks up” silica from the boiler water, dissolves it, and carries it to the turbines, where it redeposit. The key to minimizing silica carryover is in keeping the boiler water silica content

below certain levels, the concentration depending on operating pressures.Page 7 of 21

Steam is a

solvent for silica. The maximum solubility of silica in steam is a direct function of both the steam density

and

temperature

. As steam temperature or

density decreases, the silica solubility also decreases. Since pressure affects steam density and has a bearing on steam temperature, the solubility of silica in steam increases with pressure.

The presence of silica in steam can lead to deposition in superheater tubes and on the turbine blades. Small deposits on the turbine blades can result in a loss of efficiency, whilst larger deposits can cause permanent mechanical damage. In order that the turbines are operated at maximum performance,

continuous monitoring

of silica in

steam

,

boiler water

and

feed water

is highly recommended. It is also essential to

check silica levels at the source of water input to the plant.Slide8

DISSOLVED OXYGEN ( DO

2 ) At high temperatures dissolved oxygen attacks and causes corrosion

of components and piping. The result is pitting, which may eventually causes puncturing and

failures.

SODIUM (Na) The presence of sodium signals contamination with potentially

corrosive anions, e.g. chlorides

,

sulphates

etc.

Page 8 of 21

Do you Know that 10 ppb of DO2 in a boiler of 450TPH creates 90 Kg. of rust per year ! Dissolved oxygen is also responsible for leakage at joints and

gaskets

.

Hence it is

essential

to

measure and control

it precisely.

Under conditions of

high pressure and temperature

, sodium salts are

responsible for stress corrosion – cracking

of boiler and superheater tubes.

Sodium is available in

abundance

in the environment and it

enters

even

from

small openings

.

Sodium measurement

can thus reveal

possible

leakage

conditions anywhere in the

steam circuit

, including

condenser sections.Slide9

PHOSPHATE (PO

4) Phosphate hideout is a phenomenon that causes boiler water control difficulties. It is defined as the

disappearance, by precipitation or absorp-

tion, of the ionic phosphate (PO4) species, in the boiler water under high heat transfer or high load conditions.

Page 9 of 21

The best way to recognise phosphate hideout is to

graph

load changes

, pH, and phosphate over a period of time. The variations of the graphs will clearly indicate that phosphate increases with decreasing load, and

that the pH decreases with decreasing load. Events which may lead to phosphate hideout include: 1) Start-up after chemical cleaning. 2) Changes in unit load. 3) Changes in burner configuration. & 4) Metal oxide ingress and deposition on heat transfer surfaces.

Phosphate hideout causes

confusion

, frustration over

uncontrollable

water

chemistry

, and

corrosion

. Some phosphate-related corrosion may be severe

. Slide10

ON-LINE ANALYSERS (AT SANTALDIH TPS)

Line

Nos.Samples taken from

SWAS ON-LINE ANALYSERS

pH

Ks

Kc

N

2

H4SiO2DO

NaPO41.FW at Economiser I/L

2.

CBD from Boiler

Drum

3.

Saturated Steam

4.

Superheated Steam

5.

Condensate bef. LP

Htr

.

6.

CEP Discharge

7.

DM Make-up Water

8.

Deaerator

Feed

Water

9.

DMCW Aux. Header O/L

10.

DMCW (B) HF O/L

Page 10 of 21Slide11

THE NEED FOR SAMPLING

All the analyzers as mentioned, work efficiently if the

temperature, pressure and flow

conditions of sample are maintained properly. THE SAMPLE CONDITIONING SYSTEM

The sample conditioning systems have several functions. The sample

must be:

1. Withdrawn from the process.

2. Transported.

3. Conditioned.

4. Introduced into the analyzer.

& 5. Disposed off.Page 11 of 21

Further, the

sensors

are

capable

of handling the

water

/

steam

samples

at particular pressure and temperature only.

This necessitates the use of the

sample conditioning system

.Slide12

Position of Sample Extract for

Conductivity

measurementPage 12 of 21Slide13

Position of Sample Extraction Probes for

pH

measurementPage 13 of 21Slide14

Position of Sample Extraction for Silica

measurement

Page 14 of 21Slide15

Position of Sample Extraction Probes for DO

measurement

Page 15 of 21Slide16

Position of Sample Extraction for

Sodium

measurement

Page 16 of 21Slide17

Position of Sample Extraction

for

Hydrazine & PO

4measurement

Page 17 of 21Slide18

SAMPLE EXTRACTION PROBES –

For Withdrawal of Sample

Studies by CEGB(Central Electricity Generation Board, UK) suggest a more important factor in obtaining representative particulate samples

may be the maintenance of a sufficiently high transport velocity in the sample line to prevent hideout of the suspended species. Thus proper

sample extraction probe selection is the first step for ensuring

accurate analysis of steam / water.

SAMPLE PIPE WORK

–

For Transport of Sample

Sample lines should be kept as short as possible. Use of unnecessary large line diameters should be avoided so as to avoid delayed response and the damping-out of transients.

Page 18 of 21

Sample

pipework

must be of a

material

, which will

not react chemically

with any species in the sample.

Stainless steel AISI 316 is the standard

.

The

number of bends

and

joints

should be kept to a

minimum

.Slide19

SAMPLE COOLERS –

For Conditioning of sample

The Sample coolers form the heart of the sampling system. These should preferably be of coil-in-shell type design. This should make use of counter flow type

of heat exchange principle. FILTRATION

:

For the majority of on-line analyzers

, the presence of

unwanted

particulate matter can be damaging. To arrest this damage, a small High-pressure Filter with a sintered stainless steel element is necessary to permit

regular cleaning.Page 19 of 21 The coolers with Double helix type coil design give better approach temperature than conventional single coil designs, apart from being compact in size

.

A built-in

shell relief valve

takes care of possible mishap due

to high pressures caused by coil rupture etc.Slide20

PRESSURE REDUCTION & REGULATION

Traditional

Pressure Reducing devices do not offer pressure ‘regulation’. In such cases, any pressure fluctuations in the

inlet conditions can get transmitted to analysers and this is not a healthy condition for

analysis.

CHILLED WATER

Use of chilled water becomes

necessary

when the

cooling water available on site is not capable of cooling the sample to the temperatures required by the

analysers.Page 20 of 21 Pressure Reduction as well as ‘regulation’ is essential to ensure accurate,

reliable

and

stable performance

of on-line analysers.

If available cooling water is upto

40

0

C

, a well-designed sampling system

can ensure sample

outlet temperatures of

450C. Most of the modern analysers can work comfortably with these sample temperatures.

However, if cooling water temperature

exceeds 40

0

C

, use of

chilled water

becomes

necessary

. Of course for some analysers, samples are

always required to be

conditioned

at

25

0

C

by means of

Chillers.Slide21

SANTALDIH THERMAL POWER STATION, WBPDCLBOILER WATER CHEMISTRY REPORTO & M (CHEMISTRY)Date_________ Shift _____________ Time ________

Page 21 of 21Slide22

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Courtesy: M/s. Forbes MARSHALL LIMITED.