DEAERATOR 1.0 Introduction:

Modern Power Plants are operated at high pressures and temperatures

and the presence of oxygen in dissolved form in feed water results in harmful

corrosive attack. Of the many measures adopted to contain corrosion in

boilers and associated plant, the removal of oxygen from feed water is the

most important one. Incorporating a deaerating unit in the feed system does

this removal of air or deaeration.

2.0 Functions Of The Deaerator: In addition to the primary function i.e., ‘Deaeration', the Deaerator also

serves for the following purposes:

1. The Deaerator functions as a surge tank of feed water capable of

meeting the variable demands of the plant

2. The Deaerator also forms a part of the regenerative heating system

improving the temperature of condensate

3. It also acts as a heat-conserving unit.

3.0 Sources Of Oxygen Entry Into The System: 1. The DM water stored in the condensate storage tank is exposed to the

atmosphere and hence the air gets dissolved in water. When this water

is added at the condenser Hot well as make up water, oxygen makes

entry into the systems.

2. The condenser itself is the source of air entry since it can not be made

to work as a fully air tight vessel

4.0 Effect Of Oxygen In Feed Water: Most frequently encountered problem in any power station is

electrochemical corrosion and it is intensified by the presence of dissolved

oxygen in the feed water. It occurs when the metal of the heating surfaces is

attacked by electrolytic solutions (i.e. feed water here) and hence the flow

of current from one part of the metal to another through the electrolyte.

Water, the electrolyte, contains equal numbers of positively charged

hydrogen cations (H+) and negatively charged hydroxyl anions (OH-).

Different areas of the same metal (e.g.: Iron) may themselves act as an

anode or a cathode. The electrolyte (water) couples the anode and the

cathode areas to form a galvanic cell. Due to the difference in potential

between the two areas, the corrosion current flows between the anode and

the cathode areas through the electrolyte (water). Deterioration (corrosion)

takes place where the current leaves the metal i.e., at the anode area. The

electrons (negative charges, i.e., e-) leave the anode and flow to the

cathode areas. Since these electrons are released by the metal (iron) atoms

at the anode surface, the metal atoms become a positively charged ion.

Fe ↔Fe ++ + 2e-

Ferrous ions formed at the anode area are attacked by negatively

charged hydroxyl ions (OH -) and tend to unite to form an unstable

compound called Ferrous hydroxide Fe (OH) 2

Fe ++ + 2OH - → Fe (OH) 2

Due to the presence of dissolved oxygen in the feed water, oxidation of

unstable ferrous hydroxide takes place resulting in the formation of Ferric

hydroxide (Common rust)

4Fe(OH) 2 + O2 +2H2O → 4Fe(OH) 3

Ferric hydroxide on heating in the boiler becomes ferric oxide i.e., rust.

4Fe(OH) 3 → 2Fe2O3 + 6H2O.

The electrons arriving at the cathode area neutralizes some of the

hydrogen cations (H+) collected there.

2H+ + 2e- → H2↑

Cathode area is subjected to only a protective reaction. Anode and

cathode processes occur simultaneously supporting each other. However,

the potential difference between the two areas is a necessary condition.

As the corrosion products build up at the anode and the cathode areas,

the voltage difference tends to dip slowing down the rate of corrosion. This

change is called ‘Polarisation’. The process which reduces Polarisation i.e.,

intensifies corrosion is called ‘Depolarization’. The substances those

contribute to depolarization are called as ‘Depolarisers’.

Oxygen dissolved in feed water acts as a depolariser. Hydrogen gas (H2)

formed after neutralization by the electrons can blanket the cathode surface

thereby reducing the rate of corrosion. Hydrogen blanket on the cathode

surface may be removed by combining it with oxygen as a depolarization

process.

4H+ + O2 + 4e- = 2H2O

Thus, the dissolved oxygen in the feed water promotes the corrosion on

the heating surface by

1. Helping in the process of oxidation and

2. Acting as a depolariser so as to intensify the process at corrosion.

Concentration of dissolved oxygen in the feed water for high-pressure

boilers should be maintained at less than 0.005 ppm.

5.0 Principle Of Deaeration: Deaeration is based on the following two principles:

1. The solubility of a gas in a definite mass of a liquid is inversely proportional

to the temperature of the liquid.

If water is heated up to its saturation temperature (boiling temperature)

and kept at this temperature for a sufficient period, the gases can be

removed and vented to atmosphere due to their reduced solubility. This

process of heating takes place in the LP heaters and inside the Deaerator

itself with an aim to reduce the solubility of the gases.

2. The mass of a gas, which a definite mass of a liquid will dissolve at a given

temperature, is directly proportional to the partial pressure of the gas in

contact with the liquid. This holds within close limits for any gas, which does

not unite chemically with the solvent.

The term, “the partial pressure of a gas “can be explained by studying

“Dalton’s Law of Partial Pressure”. If it is considered that there is a

homogeneous mixture of inert ideal gases at a temperature ‘T’, a pressure ‘P’

in a volume ‘V’, it follows that

P = P1 + P2 + _ _ _ _ _ Pn

Where P = the total pressure

P1, P2, _ _ _ Pn = Partial pressures of the gases concerned.

(Since there is no chemical reaction, the mixture is in a state of equilibrium).

“The partial pressure of a gas “is the pressure that the gas would exert if it

occupies the entire volume ‘V’, at the temperature ‘T’. Dalton’s Law of

partial pressure states that the total pressure of a mixture of ideal gases is

equal to the sum of the partial pressures exerted by the individual gases.

The solubility of oxygen (air) in feed water can be reduced by decreasing

the partial pressure of oxygen. Decreasing the partial pressure of oxygen in

contact with feed water may be achieved by any one of the following

methods.

1. Decreasing the total pressure

2. Using another gas (whose pressure is higher than the partial pressure of

gas dissolved) to scrub the liquid.

In the case of the Deaerator, the total pressure cannot be decreased, as

it will affect the other system parameters. Use of steam to remove the oxygen

in the feed water can be done. When steam is used for removing the oxygen

in the feed water, it preheats the feed water further. It also acts as a carrying

medium to take the liberated gases to the atmosphere.

Since the process of Deaeration is carried out along with heating, it is

called as ‘Thermal Deaeration”.

To increase the efficiency of heating and deaerating and to increase the

rate of Deaeration, ‘the area of contact’ and ‘the duration of contact’

between the steam and the feed water have to be sufficiently increased.

Hence, a water distribution system comprising of spray nozzles and

perforated trays is to be provided in addition to an efficient distribution

arrangement for steam inside the Deaerator.

6.0 Chemical Deaeration: In order to remove the traces of oxygen present in the feed water after

thermal Deaeration, chemical means is employed. Either sodium sulphite or

hydrazine hydrate can be used for oxygen removal. Hydrazine is found to

possess distinct advantages over sodium sulphite as a de-oxygenating

chemical.

1. Hydrazine is a powerful reducing agent and is effective in removing

even the last traces of oxygen in water

N2H4 + O2 2H20 + N2↑

2. Its decomposition and reaction products do not increase the solid

contents in the water. The reaction products are only nitrogen and

water.

3. Small quantity of hydrazine is required compared to the requirements

of sodium sulphite. (Removal of 1 ppm of oxygen requires 1 ppm of

Hydrazine/ 8 ppm of Sodium Sulphite)

4. Hydrazine and the decomposition products of the excess hydrazine

are not acidic. They help in maintaining an alkaline pH, which is good

for the power station operation.

5. It also removes other oxidizing impurities like Cl2.

6. Hydrazine also reacts with oxides of iron and copper and removes

them from the system. The black magnetic, oxide of iron viz.magnetite

(Fe3O4) obtained as the product of interaction between rust (Fe2O3)

and hydrazine forms a thin coating on the metal surface and provides

a protective layer preventing any further attack on the metal.

Hydrazine is dozed slightly in excess such that the trace of oxygen present

after thermal Deaeration is removed. The hydrazine dosed in excess remains

as hydrazine until the feed water temperature touches 270 °C. When the

feed water temperature goes beyond 270 °C, the hydrazine decomposes as

given below:

3N2H4 4NH3 + N2↑

The above reaction leads to the formation of Ammonia, which is alkaline

in nature.

If ammonia is present in excess in the feed water for prolonged periods, it

attacks non-ferrous metals like copper and its alloys. This can cause severe

corrosion in condenser and low-pressure heaters provided with the tubes

made of copper alloys. The ammonia concentration in the main condensate

water should never exceed 10 ppm.

Dosing of hydrazine is hence regulated such that the concentration of

ammonia in the main condensate water is between 0.2 to 0.5 ppm.

7.0 Description Of Deaerator The Deaerator of units IV to VII of Thermal Power Station – II / Stage –II is of

combined spray and tray type. It consists of two elements viz a feed water

storage tank and a deaerating header.

The feed water storage tank is a horizontal cylindrical vessel with dished

ends at either end. The horizontal deaerating header is connected at the

upper part of the feed water storage tank. The details shown in figure - (1)

7.1 Storage Tank: The feed water storage tank is fabricated from boiler quality steel plates.

There are three supports in the bottom of the storage tank. The central

support is a fixed one. The end supports are of sliding nature to

accommodate the thermal expansion of the storage tank. The end supports

are rollers supports with two rollers in each. Movement of roller support from

cold to hot condition is about 8 mm. The storage tank is provided with two

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In order to increase the efficiency of deaeration, the area of contact and

also the duration of contact between the main condensate and deaerating

steam are increased by breaking the main condensate into fine particles (in

the form of spray) and also making the condensate flow down through

perforated trays.

The main condensate, which is pre-heated in 3 nos. of LPHs, is making its

entry into the Deaerator at the top of the deaerating header. Then the

condensate is sprayed with the help of 40 nos. of spray nozzles provided at

the distributor. (Ref. Fig: 1). The condensate is sprayed over 2 stacks of

perforated trays. Each stack has 5 nos. of trays in it. The duration of contact

gets increased as the water flows down through the trays. The deaerating

steam admitted at the storage tank rises up in a counter flow direction to the

main condensate flow. The process of deaeration takes place at the header.

Most of the steam quantity is recovered as feed water on condensation. The

remaining quantity of steam along with air removed leaves the deaerating

header through Deaerator vents. By this, the main condensate is also pre-

heated before it reaches the storage tank.

Tray removal doors are provided on both sides of the deaerating header

8.0 Deaerator Connections: (Ref. Fig: 2 and 3) The following inflow and outflow connections of steam and water

provided for the Deaerator.

8.1 Steam Inlets: 1. The deaerating steam for Deaerator enters at one end of the storage

tank from a common junction formed by the following 3 sources of steam.

a) Normal source of steam: The normal source of deaerating steam is

made available from the 4th Extraction steam of Turbine. It is tapped from IP

cylinder exhaust steam, is from cross around pipes, after 20 stages of IP

casing. It is admitted through an electric motor driven isolation valve EX-014.

b) 1ST standby source of steam (CRH Steam): In order to feed the

Deaerator during the occasions in which 4th extraction steam of Turbine is

not sufficient, a stand by source of steam is warranted. The steam drawn

from CRH lines is used as 1ST standby source of steam for the Deaerator. The

supply of CRH steam is made available through a pressure control station

comprising of an electric motor driven inlet isolation valve (EX-001), and a

pneumatic control valve (PCV-2) as shown in Fig: 2. The Deaerator can also

be fed from CRH lines during the occasions in which the Turbine is not in

service and the Boiler is in service with HP and LP bypass systems.

c) 2nd standby source of steam (Pegging/Auxiliary steam): In order to

feed the Deaerator during initial start-up periods and also during the period

after shut-down, where both 4th extraction steam and CRH steam may not

be sufficient, another source of steam is made available from Auxiliary steam

bus. This steam also admitted through a pressure control station PCV-1.

All the above 3 sources of steam are made available to the junction with

provision of NRVs in order to prevent any back flow. Four numbers of line

Safety valves are also provided to relieve the increase in pressure in case that

any pressure control valve malfunctions.

2) Preheating steam from the auxiliary steam system:

Initial preheating of feed water in the storage tank can be done with the

help of steam drawn from the auxiliary steam bus.

3) CBD (continuous Blow Down) tank steam to the storage tank:

Continuous blow-down from boiler drum is resorted to keep the salt

concentration of boiler water not only within limits i.e., not to cross the critical

value which will result in salt carry over along with steam but also to maintain

a steady salt concentration irrespective of time. This is achieved by making

the salt input to the cycle through the make up water supplied in the

condenser equal to the salt output from the cycle through the continuous

blow-down.

Since practically there is no salt carry over along with steam from the

boiler drum, all the salts remain trapped in the boiler drum water. This highly

salt concentrated water from the boiler drum is taken, thereby reducing

considerably the need of make up water.

The CBD water is at high pressure and temperature and consequently has

a high enthalpy. Roughly, one third of this water is flashed into steam in the

CBD flash tank located near the Deaerator. Pressure maintained in this tank is

slightly above that of the Deaerator. The CBD flashed steam is supplied to the

deaerator continuously. This is one of the sources of steam supply to the

deaerator.The quantity of water wasted is roughly two thirds of the water

taken from the boiler drum. This cannot be used as it is highly contaminated.

Further reduction in make –up water need is achieved by this.

Water from CBD flash tank goes to the intermittent blow-down flash tank

where further flashing takes place. Steam is let out to atmosphere and the

water to the drainage.

Thus, the CBD flash tank arrangement not only reduces the make up

water requirement to the cycle considerably but also results in the utilization

to the maximum possible extent of the heat energy of the CBD water.

The CBD flash tank steam is admitted to the storage tank through a gate

valve and a NRV.

4) Shell Vents of HPHs (5) & (6) (2 Nos.) to the Deaerator header:

Heat transfer between the extraction stream and the feed water takes

place in HP heater. Non-condensing gases coming along with the extraction

steam also enter the heaters. If they are allowed to remain, they get

accumulated in the heaters. These gases blanket the surface of the heater

coils and the performance of the heaters will get affected. If the oxygen is

also present in these non-condensing gases, it will promote corrosion of the

tubes since the stream is getting condensed there. Hence continues venting

of non- condensing gases is warranted. The shell vent of HP heaters (5) and

(6) are connected to the deaerating header for evacuating the non-

condensing gases along with a small part of the extraction steam. Thus, these

gases are finally driven out to atmosphere through the Deaerator vent. These

vent lines are provided with isolation valves and non return valves.

8.2 Water Inlets to Deaerator: 1. Main condensate after LP Heater (3):

The main condensate, which is preheated in the three numbers of LP

heaters, is admitted at the top of the deaerating heater. Thereafter it is

sprayed and it is flowing down through perforated trays kept in stakes. It

reaches the storage tank after the process of deaeration and heating.

2. Initial filling line from the condensate storage tank (‘CST’):

When the unit is to be started up, the initial filling of Deaerator can be

done directly by supplying the DM water delivered by condensate transfer

pump. The DM water from the condensate storage tank is admitted through

an electrical motor driven isolation valve ‘DM 20’ in the initial filling line.

3. Boiler feed pump’s recirculation lines (3 nos.):

Boiler feed pump is provided with a recirculation line in order to maintain

a minimum water flow through the pump. This line connects the discharge of

boiler feed pump with the Deaerator through a recirculation valve. The

recirculation line is provided with (nearer to Deaerator) a non-return valve, a

manually operated gate valve and an orifice. All the three lines join the

storage tank.

4. Drain condensate from HP heaters (5) & (6) to the Deaerator Storage Tank:

The drain condensate collected in the HP heaters (5) & (6) is sent to the

Deaerator through the respective heater level control stations. For HPH - 5

level control station, a pneumatic level control valve ‘DR 11’ and a motor

driven bypass valve ‘DR 43’ are provided. Similarly for HPH – 6, the valves ‘DR

6’ and ‘DR 42’ are provided as shown in the fig- (3). The drain condensate

from the HPHs is also preheating the feed water in the Deaerator.

8.3 Water Outlets from the Deaerator: 1. Feed water suction bus for the Boiler Feed Pumps:

The suction for the boiler feed pumps is taken at the bottom of the

storage tank at its one end. Dozing of Hydrazine Hydrate is done at the

suction bus of the boiler feed pumps.

2. Deaerator over flow line:

Deaerator overflow water line is at one end of the Deaerator storage

tank and is connected to Blow down Tank. This line is provided with a

pneumatic “Open/Close” (ON/OFF) valve ‘DR 33’. The valve ‘DR 33’ gets

opened /closed automatically according to the Deaerator water level.

3. Deaerator Drain:

Two drain line connections are provided at the bottom of the Deaerator

storage tank on both ends. These two drain lines join together before finally

joining with Deaerator overflow line, which is connected to blow down tank.

8.4 Steam Outlets From Deaerator: Deaerator vents are provided for the deaerating header as detailed

below. They are useful for venting the air removed along with a part of

deaerating steam.

1. One vent with provision of motor operated valve ‘SM 20’.

2. One vent with provision of hand operated isolation valve.

3. Two nos. of vents provided with an orifice in each line and without any

valve.

9.0 Deaerator Operating Parameters: The operating parameters of Deaerator working at 100% MCR conditions

are indicated below.

(i) Operating pressure : 6.6 ata

(ii) Operating temperature : 161.9 °C

(iii) Normal working water level)

in the storage tank :1625 mm (UCB Instrument)

585 mm above geometric center line

9.1 Deaerator Pressure: Mode of operation of the Deaerator is “VARIABLE PRESSURE MODE”.

The pressure in the Deaerator is allowed to vary according to the operating

status of the turbine. Fourth extraction steam, the normal source, is feeding

the steam to Deaerator at a pressure, which is varying along with operating

load on the turbine. Since the load on the turbine is getting varied during

normal operation of the units, the operating pressure of the Deaerator will

also vary.

Since the very low pressure of the Deaerator can lead to failure of Net

Positive Suction Head (NPSH) for Boiler Feed Pumps resulting in cavitation, the

pressure in the Deaerator should not be allowed to go down to very low

values. This type of situation may arise during some occasions as detailed

below.

a) During the period of start-up, the stream will not be available in both

fourth extraction steam line and the cold reheat steam lines.

b) During low load operation of turbine, the steam for Deaerator may

not sufficiently feed the incoming main condensate temperature is

also low.

c) Sudden unloading of turbine leads to a pressure drop in the

Deaerator.

During the above occasions, the reserve steam sources namely (i). Cold

Reheat Steam lines and (ii). Auxiliary Steam bus is provided with respective

pressure control valves in order to feed the deaerating steam.

9.2 Deaerator Water Level: The water level in the Deaerator storage tank is maintained by regulating

the make-up water flow.

Out of the three water storage facilities available in the thermal cycle,

the condenser hot well water level is maintained by regulating the main

condensate flow to Deaerator. The boiler drum water level is maintained by

regulating the feed water flow towards the drum. Since the water levels in

the hot well and the drum are maintained as stated above, water losses in

the thermal cycle will lead to water level drop in the Deaerator. Hence, the

Deaerator water level needs to be maintained by regulating the make-up

water flow.

The DM water stored in the condensate storage tank is pumped to a

surge tank of capacity 50 m3 with the help of Condensate Transfer Pumps

(CTP) through a motor operated valve ‘DM 23’.The Surge Tank is located

adjacent to deaerator at 27.0 m level. (Ref. Fig.2). The details of make-up

water system are shown in fig. (4).

From the surge tank, the make up water is added at the condenser hot

well through a set of control valves viz CD 64 and CD 66 located in parallel

lines. The regulation of the flow of makeup water is done by these valves

based on the Deaerator water level. The water level in the surge tank is in

turn made up by adding the DM water through the DM 23. In case of

emergency requirements, the make up water can also be added to the hot

well through another line provided with the motor operated valve ‘DM 6’. This

additional line can receive the DM water either directly from the condensate

storage tank by gravity or from the discharge of Condensate Transfer pumps

as shown in fig. 4.

In case of high water level in the Deaerator, a provision is also made

available to send a certain quantity of main condensate back to the surge

tank. In order to bring down the water level in the Deaerator, a line called

‘excess dump line’ with a control valve ‘CD 23’ is provided. This control valve

will regulate the flow of main condensate to the surge tank with reference to

the Deaerator water level. A motor operated by pass valve ‘CD 60’ is also

provided for this control valve.

Additionally, the overflow valve (pneumatic valve) ‘DR 33’ can also

reduce the Deaerator water level by sending the water in the Deaerator to

the Blow down Tank.

10.0 DEAERATOR LOGICS &CONTROLS:

10.1 Deaerator Level Switches: Totally nine level switches are provided for Deaerator storage tank. The details are indicated below. These switches are located in

two standpipes as shown in Fig. (3).

SETTINGS

Sl With reference to Instr.Level Purpose

No Description Tag No. Location Horizontal center Line

1. HHWL-High High LS-2020 Stand Pipe 1010 mm above 2100 mm of WC 1.Dea.level very high-Alarm

Water Level No.1 2. EX-014 (in extraction line) closes

3.EX 108 opens

2. HHWL-High High LS-2021 Stand Pipe 1010 mm above 1820 mm of WC ‘Dea Level High’ alarm.

No.2

3. HWL-High water LS-2033 Stand Pipe 835 mm above

Level No.1

4. NWL-Normal LS-2034 Stand Pipe 585 mm above 625 mm of WC

Water Level No.1

5. LWL- Low water LS-2017 Stand Pipe 85 mm above 1080 mm of WC 1.’Dea Level Low’-alarm.

3.Start up permissive for BFP.

9. OVO-Over flow LS-2018 Stand Pipe 660 mm above 1530 mm of WC 1.’Dea.O/F Valve Opens’-Alarm resets

Water Level No.2 2.Makeup line valves CD-64&CD66

8. OVO-Over flow LS-2019 Stand Pipe 935 mm above 1930 mm of WC 1.’Dea.O/F Valve Opens’-Alarm

Valve Close. No.2 2. 2. Deaerator over flow valve.

Water Level No.1 ‘Dea Level Very Low’ – alarm.

Valve Open No.2 2. Deaerator over flow valve

6. LLWL-Low Low LS-2015 Stand Pipe 1050 mm below 100 mm of WC Protection for all BFPs

Protection open

‘DR-33’ closes.

‘DR-33’ opens.

7. LLWL-Low Low LS-2016 Stand Pipe 1050 mm below 100 mm of WC

Water Level No.2

10.2 CRH Steam to Deaerator Valve EX001 – Logics: 1. When the CRH steam pressure becomes greater than 8 Ksc, the valve

Ex 001 opens automatically.

2. When the CRH steam pressure becomes less than 8 Ksc, the valve

Ex 001 closes automatically.

3. Manual opening and closing operations are also possible from the

control desk.

4. When CRH steam pressure is less than 8 Ksc, manual opening of Ex 001

is not possible.

10.3 FCNRV In Extraction ‘IV’ – Check Valve A4 Logic: a) Open Logic:

When the differential pressure (DP) across the check valve ‘A4’ becomes

greater than 300 mm of water column (W.C), the check valve ‘A4’ will open

automatically after 2 sec. on energisation of Solenoid operated valve SOV

‘MAX 51 AA 027’.

b) Close Logic:

When the differential pressure (DP) across the check valve ‘A4’ becomes

less than 100 mm of water column (W.C), the check valve ‘A4’ will be

protection closed automatically on de-energization of SOV.

10.4 Deaerator Pressure Control: Control scheme for ‘Deaerator Pressure Control’ is shown in fig. 5 and the

details of control desk are shown in fig. 6.

The Deaerator operates normally on variable pressure mode. Purpose of

this control loop is to maintain at least a minimum pressure for the Deaerator

in all occasions during which the fourth extraction steam from IP casing is not

sufficiently available. Cold reheat steam and Auxiliary steam are the reserve

source of steam for deaeration. This control loop actuates Auxiliary Steam to

Deaerator pressure control valve ‘PCV1’ and Cold Reheat Steam to

Deaerator pressure control valve ‘PCV 2’.

Actual Pressure: A dual measurement system is employed for measuring the Deaerator

pressure. The actual pressure in the Deaerator is measured by two pressure

transmitters PT-0339 A and PT-0339 B. The purpose of the dual measurement

system is to provide 1 out of 2 redundancies in measurement in order to

achieve a reliable measurement system.

Set Pressure (Set Point): The set point for the Deaerator pressure control is formed at a ‘MAX ‘gate

as shown in fig.5. Three input signals are made available for the ‘MAX ‘gate

and the maximum of the following three inputs becomes the set point.

1. The measured values (after subtracting a small negative bias).

2. The set point adjustable from the control desk between 0 & 10 Ksc.

3. A fixed set point of 3.5 ata.

a) The measured value of the actual pressure itself becomes the input

signal. Since a small negative bias is subtracted from it, this input signal value

is slightly less than the measured value.

This set point is intended to serve during normal service of the unit. The

Deaerator operates normally on variable pressure mode with the steam

made available from the fourth extraction of Turbine. Since the set point is

less than the measured value, both the control valves (Aux. Steam to

Deaerator ‘PCV 1’ and CRH steam to Deaerator ‘PCV 2’) remains closed.

b) Whenever the steam from fourth extraction of turbine is not sufficiently

available for the Deaerator, the Deaerator pressure is maintained at the

adjustable set point, which can be modulated at the setter in the desk. This is

done by regulating the CRH steam pressure to Deaerator ‘PCV 2’ when the

CRH pressure is above 8 ata. When the CRH steam pressure is greater than 8

ata, the auxiliary steam to deaerator ‘PCV 2’ remains in closed position. This is

achieved by applying a small negative bias (approximately 2%) to the

control deviation (set value – actual value) of PI controller (as shown in fig.5).

When the CRH steam pressure is less than 8.0 ata, the inlet gate valve

‘EX 001’for CRH to Deaerator ‘PCV 2’ remains in closed position on logic.

Hence, the auxiliary steam to Deaerator ‘PCV1’ is modulated to maintain a

Deaerator pressure corresponding to a value slightly less than the adjustable

set point.

c) A fixed value of 3.5 ata (approximately) is set in the cabinet. This

effectively limits the above two set values (detailed in (I) & (II) to a minimum

of 3.5 ata (approximately).

Controllers: The difference between the ser value and the actual value becomes

‘control deviation’ and it is applied to

1. PI controller for ‘CRH steam to Deaerator’ pressure control directly and

2. PI controller for ‘Aux.steam to Deaerator’ pressure control after

applying a small negative bias (nearly 2%).

Auto/Manual stations are provided for these controllers individually.

Normally both automatic and manual controls take through the controller

itself.

Each controller’s output signal is taken to respective Electro pneumatic

converter (E/P) for converting the electrical control signal into a pneumatic

signal air. The signal air pressure is communicated to the valve positioner for

finally driving the pneumatic actuator. The valve positions are indicated in

the control desk.

Set Point Control Module (SPCM): Sudden load rejections on turbine can lead to a rapid decay in the

pressure of Deaerator. Sudden drop in Deaerator pressure causes flashing of

water in the Deaerator leading to cavitation of boiler feed pumps. Hence, a

sudden decay in Deaerator pressure is to be avoided. For this purpose, a ‘set

point control module’ (SPCM) function block is provided in the path of the set

point generated from the measured value of the actual pressure (detailed in

sec. (a) above ref Fig.5).

The measured actual Pressure signal becomes input signal for the module

‘SPCM’ and output signal from this module later becomes input signal for

“MAX” gate after subtraction of a small negative bias. When the actual

pressure in the Deaerator decreases faster than 0.5ata per min, the decrease

in output signal of ‘SPCM’ module is limited to only 0.5ata per min. Hence the

set point reduced at the rate of 0.5 ata per min, i.e. slower than the rate of

actual pressure drop. Hence, the set point becomes greater than the actual,

forcing the CRH steam to Deaerator PCV to open and supply steam to

Deaerator. This builds up the pressure in the Deaerator thereby arresting the

decay.

10.5 Deaerator Water Level Controller: Control scheme for ‘Deaerator water level control’ is shown in fig 7.

Deaerator water level control loop maintains the water level in the Deaerator

by modulating makeup water flow to the hot well. For normal level control, to

control valves viz. CD-64 & CD-66 are provided in parallel in the make water

line from surge tank to hot well. For high-level control, the control valve CD-23

in the excess dump line connecting the main condensate system with the

surge tank is provided.

Two independent level controllers viz. Normal Level Controller and High

Level Controller are provided to modulate the above said control valve.

10.5.1 Normal Level Control: The actual water level in the Deaerator is measured by a level transmitter

‘LT-0662.1’ and the set point is adjustable between 0 to 100mm (1650-

1750mm) at a set pt station is provided in the desk (Ref Fig. 6.)

The set value and the actual value are compared and the control

deviation is fed to the ‘Normal Level PI controller’. This controller modulates

both the control valves ‘CD-64 and CD-66’ in parallel as a ‘SPLIT RANGE

OPERATION’. The output of the controller taken to an electro pneumatic

converter (E/P) for converting the electrical control signal into a pneumatic

signal air. The signal air pressure is communicated to valve positioner of the

actuators of both the valves CD-64 and CD-66. The valve positions are also

indicated in the control desk. Their position feed back signals are added and

fed to the controller to improve the overall response of the control loop. An

auto/manual station is provided for this PI controller for executing

manual/auto control functions.

In addition, interlocks as detailed below are also provided.

1. When the water level in the Deaerator goes down to 1080 mm both

the control valves CD-64and CD- 66 are protection opened by the

controller in order to increase the make up water flow, provided that

the surge tank level is not low.

2. When the water level in the surge tank goes down to very low level

(700 mm), the controller protection closes both the control valves CD-

64 and CD-66.

10.5.2 High Level Control: The actual water level in the Deaerator is measured by another level

transmitter ‘LT-0662.2’ and the set point is adjustable between 100 to 400 mm

(1750 – 2150 mm) at a set point station provided in the desk. The set value

and the actual value are compared and the control deviation is fed to the

‘High Level Controller’. This controller modulates the control valve CD-23. An

auto/ manual station is provided for this PI controller for executing manual /

auto control functions.

 

11.0 Operation Of Deaerator:

11.1 Preparation Of Deaerator For Starting ‘Deaeration Of Water’: A). Steam Inlets:

1. IV Extraction steam to Deaerator – Extraction Isolation Valve ‘EX 14’ is

to be kept in ‘CLOSE’ position. Drain pot drain valve EX 108 has to be

kept in ‘OPEN’ position. It should be closed after opening EX 14.

2. CRH steam to Deaerator Inlet Isolation Valve EX 001 and bypass valve

to ‘PCV’ are to be checked for ‘CLOSE’ position. Outlet Isolation

Valve is to be checked for ‘OPEN’ Position.

3. CRH steam to Deaerator PCV is to be kept in ‘CLOSE ‘position with it’s

A / M Station on Manual position.

4. Pegging steam to Deaerator (Aux.steam) - both inlet and outlet

isolation valves to PCV are to be checked for ‘OPEN’ position with

its bypass valve in ‘CLOSE’ position.

5. Aux. Steam to Deaerator PCV is to be kept in ‘CLOSE’ position with it’s

A /M station on manual position.

6. CBD steam to deaerator isolation valve should be kept in ‘CLOSE’

position.

7. Deaerator pressure set point shall be kept at 3.5 Kg/cm2.

8. Preheating steam (Aux.steam) to Deaerator should be checked for

‘CLOSE’ position.

9. HP Heaters (5) & (6) shell vents are to be checked for ‘OPEN’ position.

B. Water Inlets:

1. Main condensate inlet isolation valve to deaerator is to be checked

for ‘OPEN ‘position.

2. Recirculation line isolation valves (3. nos.) for Boiler feed pumps are to

be checked for ‘OPEN’ position. Three line vent lines are to be in

‘CLOSE’ position.

3. HP heaters -6 to Deaerator – Drain condensate line – Heater Level

Control Valve DR 6 should be kept on auto with its inlet and outlet

isolation valves in ‘OPEN’ position. Bypass valve DR 42 should be

checked for ‘CLOSE’ position.

4. HP Heater – 5 to Deaerator – Drain Condensate line – Heater Level

control valve ‘DR 11 should be kept on auto with its inlet and outlet

isolation valves in ‘OPEN’ position. Bypass valve DR 43 should be

checked for ‘CLOSE’ position.

5. Deaerator initial filling line inlet valve (from CT Pump) ‘DM 20’ should

be checked for ‘CLOSE’ position.

C. Water Outlets:

1. Deaerator drain valve to Blow down Tank should be checked for

‘CLOSE’ position.

2. Deaerator overflow line- Inlet and outlet isolation valves to overflow

valve ‘DR 33’ should be checked for ‘OPEN’ position. The valve ‘DR 33’

should be kept in ‘CLOSE’ position.

D. Steam Outlets:

Deaerator manual air vent and motor operated air vent SM 20 should be

kept ‘OPEN’ initially.

E. Deaerator Level Control:

1. DM Water make up to surge tank inlet isolation valve to ‘DM 23 ‘should

be checked for ‘OPEN’ position.

2. ‘SLC of DM 23’ should be kept ‘ON’.

3. Deaerator Level control valves CD 64 and CD 66 – inlet and outlet

isolation valves are to be checked for ‘OPEN ‘position. Deaerator

Normal Level control auto / manual station should be kept in ‘Manual’

position.

4. Main condensate excess dump line to surge tank- Level control valve

‘CD 23’ should be kept in ‘CLOSE’ position with its inlet and outlet

isolation valves in ‘OPEN’ position. Its bypass valve (MOV) ‘CD

60’should be checked for ‘CLOSE’ position. Deaerator High Level

Control auto / manual station should be kept in manual position.

5. Deaerator Normal Level set point should be kept at 1650 mm of WC

(Corresponding to ‘0’ in the set point indicator).

6. Gravity make up (to hot well) valve ‘DM 6’ should be kept in ‘CLOSE’

position along with its bypass valve. Its inlet and outlet isolation valves

are to be checked for ‘OPEN’ position. ‘SLC of DM 6’ should be kept in

‘ON’ position.

11.2 Starting Of Deaeration: 1. The water level in the Deaerator storage tank is to be checked. If it is

low, it can be filled either by opening ‘DM 20’ (from CT pump) or by

making up through hot well if CEP is already in service.

2. Open the atmospheric drain in preheating line and close it after warm

up. The gate valve in preheating line is to be opened slowly to preheat

the water by feeding the stream through the perforated pipes.

Hammering, if any, is to be observed. The temperature of the water is

thus raised until it reaches 80 °C.

3. Cold charging of Boiler feed pumps is to be started along with

Deaerator as all the three Boiler feed pumps need to be warmed up

along with raising of Deaerator temperature.

4. One of the Boiler feed pumps is to be started for feeding the boiler and

kept in recirculation.

5. After raising the Deaerator water temperature up to 80 °C, the auxiliary

steam is admitted through PCV by manual opening and the Deaerator

is brought under pressure. Preheating steam line valve is to be closed.

6. The Deaerator pressure has to be raised along with raise in water

temperature by admitting steam through PCV. The Deaerator pressure

is thus raised upto 3.5 Kg / cm2 gradually in steps.

7. Deaerator vent valve SM 20 shall be closed.

8. After CEP is started and the main condensate system is charged upto

the Deaerator, the following Level Controllers shall be brought to auto.

i. Deaerator Normal Level Control.

ii. Deaerator High Level Control.

iii. Hotwell Level Control.

9. When the boiler is being fed from running Boiler feed pump, the

Deaerator water level will be maintained by the make up water valve

CD 64 & CD 66.

10. When the boiler is lighted up and the CRH stem pressure becomes

greater then 8.0 Ksc, the inlet isolation valve EX 001 will open

automatically. This may occur at approximately 25 % Load condition

(55 MW) in Cold Start- Up or at 15 % Load Condition in Hot Start- UP.

11. Both ‘Aux. Steam to Deaerator ‘and’ CRH Steam to Deaerator’

pressure controllers shall be put on auto. This will make Aux. Stem to

Deaerator PCV close. The Deaerator pressure will be maintained at 3.5

Kg/ cm2 by CRH steam to Deaerator PCV.

12. When the load on turbine is approximately 55 % i.e., 120 MW, IV

extraction steam pressure will become greater then 3.5 Kg / cm2 and

the extraction steam is admitted by opening the isolation valve EX 14.

Until this period, the line drain EX 108 should be kept in ‘OPEN’ position

(as the extraction line will contain locked up steam) in order to warm

up extraction line.

13. As the load on turbine is raised, the Deaerator pressure will also be

raised resulting in closing of ‘CRH steam to Deaerator PCV2

completely. The Deaerator is further operated on variable pressure

mode.

12.0 Keys To Operation: The Deaerator will quickly respond to any change in operating

conditions. Thus, for any increase in incoming main condensate flow; there

will be a corresponding increase in the steam demand. For any decrease in

incoming main condensate temperature also, there will be a corresponding

increase in the steam demand. For any increase in the steam pressure, there

will be an increase in the deaerated water temperature up to the

corresponding saturation temperature. To ensure continuous satisfactory

performance of the Deaerator, the following keys to operation should be

observed.

1. In variable pressure deaerators, sudden drop in pressure caused by

drop in load has to be avoided since they could damage internal

parts of the deaerator, affect its deaerating capacity and produce

cavitation in boiler feed pumps.

2. Under steady state conditions, the water level in the storage tank must

be stable.

3. It is to be ensured that the Deaerator is sufficiently vented. The vent

valves may be opened further if necessary.

4. Pressure and temperature of Deaerator should be checked against

their design values.

5. The oxygen content in feed water at the Deaerator outlet must be

checked periodically.

6. It is to be ensured that the operation of Deaerator is free from water

hammer.

7. Over loading of Deaerator due to bypassing of low pressure, heaters at

full load and drain inlet quantities result in the high oxygen content at

the Deaerator outlet.

8. Initial admission of main condensate into the Deaerator should be

done only after the Deaerator temperature is raised sufficiently.

13.0 Technical Data:

13.1 Design Data: 1) Type : Spray and Tray

2) Layout :

Deaerating Header : Horizontal (Top).

Deaerator storage tank : Horizontal (Bottom).

3) Code followed in the design

and fabrication :ASME Sec VIII Division I (1986 ).

4. Design and Test parameters for header and storage tank:

Design pressure : 7.4 Kg / cm2 and full vacuum.

Design temperature : 250 °C.

Test pressure : 11.1 Kg / cm2 (1.5 design pressure)

Test temperature : Room temperature.

5. Weight of the deaerator:

Dry weight : 65 tonnes

Operating weight : 197 tonnes.

Flooded weight : 262 tonnes.

6. Deaerator support:

Distance between the

centre and the roller : 7500 mm.

support

Expansion rollers : provided at both ends.

7. Elevation:

Deaerator floor : 27.0 M.

Deaerator centre line : 29.22 M.

13.1.1 Deaerating Header: 1) Dimensions :

Diameter : 2400 mm.

Length : 5800 mm.

2) Spray nozzles:

Material : Stainless steel.

Number : 40

3) Number of vertical stacks : 2.

4) Number of trays in each stack : 5.

5) Main condensate pipes

connecting header and : 2 Nos. (Left and Right).

storage tank

6) Steam pipe connecting

header and storage tank : 1 No. (at the Centre ).

13.1.2 Storage Tank: 1) Capacity : 130 m3.

2) Overall dimensions

(OD x Length ) mm : 3500 x 21400.

3) Safety valve:

(i) Set pressure : 7 Kg / cm2 (g).

(ii) Relieving Capacity : 40,000 Kg / hr . of saturated steam.

(iii) Number : 2.

13.2 Thermal Data: 1. Mode of operation of

deaerator : Variable Pressure

2. Limit down to which oxygen

is removed : 0.005 cc / liter.

3. Operating pressure : 6.6 ata (at 100 % MCR).

4. Minimum pegging pressure : 1.5 ata.

5. Operating temperature : 161.9 °C (at 100 % MCR).

6. Normal source of steam for

Deaeration : IVth Extraction steam of Turbine.

i) Parameter at the point of extraction:

Pressure : 7.2 ata.

Temperature : 319.4 °C.

Saturation temperature : 165.3 °C

Degree of superheat : 154 °C.

ii) Enthalphy of steam : 740.5 Kcal / Kg.

iii) Steam flow : 35.18 T/ hr.

iv) Tapping point of extraction : IP cylinder outlet (from cross around

. pipes) (after 45 stages).

8. Water Parameters: Main condensate Feed water

(at inlet) (at outlet)

1. Enthalpy (Kcal / Kg) : 121.3 162.8

2. Temperature (o C) : 120.8 ` 161.9

3. Pressure : 8.5 ata 6.5 ata

4. Flow : 530 T / hr 660 T / hr.

8. Terminal Temperature Difference oC (TTD) :

=(Saturation Temperature of extracted steam – Out going

feed water temperature) = 3.9 deg C.

9. IV th Extraction steam parameters at lower loads:

Presssure (ata) Flow (T / hr)

1. 80 % load : 5.86 27.64

2. 60 % load : 4.54 19.55