What is a deaerator

What is a deaerator?

Deaerator is one of the key equipment of boiler and heat supply system. If the deaerator has poor deaeration capacity, it will cause serious losses to the corrosion of boiler feed water pipeline, economizer and other auxiliary equipment, and the resulting economic losses will be dozens or hundreds of times of the cost of deaerator. Therefore, the Ministry of electric power has put forward some standards for the oxygen content of deaerator, That is, the oxygen content of feed water of atmospheric deaerator shall be less than 15% цɡ/L. The oxygen content of feed water of pressure deaerator shall be less than 7 цɡ/L.

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When the condensate flows through the negative pressure system, air will leak into the condensate from the place where it is not tightly sealed. In addition, the condensate make-up water also contains a certain amount of air. Under certain conditions, this part of gas will not only corrode the equipment in the system, but also reduce the heat exchange force of the heater and boiler and reduce the economy of the unit. In order to reduce the corrosion of water supply system, economizer and water wall pipe, the main method is to reduce dissolved oxygen in water supply, or appropriately increase dissolved oxygen under certain conditions to alleviate oxygen corrosion, appropriately increase pH value of water supply and eliminate CO2 corrosion.
There are two kinds of deaeration methods: chemical deaeration and thermal deaeration. Thermal deaeration is commonly used in power plants, supplemented by chemical deaeration.
In the chemical deoxidation method, some interactive agents that are easy to react with oxygen are used to make it react with the oxygen dissolved in water to produce substances that do not corrode metals to achieve the purpose of deoxidation. Chemical deaeration can only completely remove the oxygen in the water, but can not remove other gases. At the same time, the generated oxide will increase the content of soluble salts in the feed water, and the reagent is expensive. Therefore, chemical deaeration is only used as an auxiliary deaeration means.
Deaerator is a hybrid heater that works based on the principle of thermal deaeration, which can analyze and remove the dissolved gas in feed water; It can also store a certain amount of feed water to alleviate the flow imbalance between condensate and feed water; The regenerative extraction steam can also be used to heat the feed water to improve the thermal efficiency of the unit. In the design of thermal system, deaerator is also used to recover high-quality drainage and valve rod leakage.
During normal operation of the unit, the combined water treatment mode of ammonia and oxygen addition (i.e. CWT working condition) is adopted. At this time, the deaerator completes the function of heater and removes other water-soluble gases; In the start-up stage or in case of abnormal water quality, feed water is treated with ammonia and hydrazine (i.e. AVT working condition) to reduce the oxygen content in the water and slow down the oxygen corrosion. At this time, the deaerator not only completes the function of heating feed water, but also plays the role of deaeration.

Characteristics of deaerator

It can not only remove dissolved oxygen in boiler feed water, but also remove free CO2, NH3, H2S and other corrosive gases in water.
After deaeration, the salt content and other impurities in the water will not increase.
High efficiency packing is adopted, with high deaeration efficiency, and the water supply can also be at room temperature.
When the water temperature is at normal room temperature, the oxygen content of the effluent can still meet the specified requirements.

Structure and principle of deaerator

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The deaeration equipment is mainly composed of deaeration tower head, deaeration water tank, connecting pipe and external parts. The main comp1nt deaerator (deaerator head) is composed of shell, steam water separator, new rotating membrane device (film lifting pipe), water spraying grate, heat storage packing liquid steam network and other components. Next, we will focus on the structural principle of deaerator.

  • 1. Shell: it is made by welding the cylinder body and stamping oval head. The medium and small low-pressure deaerator is equipped with a pair of flanges connecting the upper and lower parts for assembly and maintenance, and the high-pressure deaerator is equipped with manhole for maintenance
  • 2. Steam water separator: this kind of device replaces the original structural design of straw hat cone in the old deaerator, so that the deaerator eliminates the phenomenon of steam exhaust with water.
  • 3. Membrane spinner group: it is composed of water chamber, steam chamber, membrane spinner, condensate connecting pipe, make-up water connecting pipe and primary steam inlet connecting pipe. Condensate, chemical make-up water and water are sprayed spirally through the membrane spinner at a certain angle to form a water film skirt, and heat exchange with the heating steam introduced by the primary heating steam connecting pipe to form primary deaeration, The feed water contacts with the rising secondary heating steam through the drenching grate, is heated to the saturation temperature close to the working pressure of the deaerator, that is, 2-3 ℃ lower than the saturation temperature, and carries out rough deaeration. Generally, about 90-95% of the oxygen content in the feed water can be removed through this rotating membrane section
  • 4. Drenching grate: it is made of several layers of staggered angular steel. The feed water roughly deaerated by the rotating membrane section is distributed twice here and falls on the liquid steam network installed under it in a uniform rain shape
  • 5. Liquid steam network of heat storage packing: it is composed of mutually spaced flat steel strips and a cylinder, which is internally equipped with a certain height of special stainless steel wire mesh. Here, the feed water is in full contact with the secondary steam, heated to saturation temperature and deeply deaerated. The low-pressure atmospheric deaerator is lower than 10ug/L and the high-pressure deaerator is lower than 5ug/L (the ministerial standards are 15ug/L and 7ug/L respectively)
  • 6. The deaerated feed water from the water tank is collected into the lower container of the deaerator, i.e. the water tank. The deaerated water tank is equipped with the latest scientifically designed strong heat exchange reboiler. The device has the advantages of strong heat exchange, rapid increase of water temperature, deeper deaeration, reduction of tank vibration and noise, improves the service life of the equipment and ensures the safety and reliability of equipment operation.

Safety protection system

Safety valve

In order to prevent overpressure of deaerator, 2 A48Y-25 spring full open safety valves are equipped with an opening pressure of 1.4MPa, and the total steam exhaust capacity of the two safety valves is about 126.876t/h. When the working pressure in the deaeration equipment reaches 1.4MPa, the safety valve will automatically open for pressure relief to play a role in safety protection.

Overflow and emergency drain pipe

Too high water level of deaerator may cause overpressure of deaerator. When the water level of deaerator is out of control or even full of water, water may enter the turbine and cause malignant accidents. Therefore, the deaerator is provided with deaerator overflow and water outlet, and the high water level limit is set in the sequence control (see the figure below). The two water outlets are above the normal water level.

Deaerator overflow and emergency drain pipe

When the water level rises to a higher value, first open the overflow valve to discharge part of the feed water; When the water level of deaerator rises to the high value, the emergency drain valve opens. Drain water from both channels to the condenser.

Shutdown drain pipe

When the deaerator needs to drain water during shutdown, the shutdown drain valve at the bottom can be opened to drain water to the pressurized drain header.

Steam supply mode of deaerator

Operation mode of the equipment: constant sliding constant pressure operation. Auxiliary steam is used for constant pressure operation during deaerator startup; When the unit is loaded to a certain load, the heating steam is switched to the reheat cold section, and then switched to the steam extraction of section 4 of the steam turbine for sliding pressure operation; When the unit load drops to a certain value and the steam extraction of section 4 cannot meet the operation requirements of deaerator, switch to reheat cold section or auxiliary steam again for constant pressure operation. During sliding pressure operation, the pressure changes with the load of the unit. There are only electric shut-off valves and check valves on the steam extraction pipeline to prevent water inlet and steam inlet of the turbine, and there is no regulating valve.
Sliding pressure operation range: 0.147MPa-1.314MPa, auxiliary steam: pressure: 0.8MPa, temperature: 250 ℃; Demineralized water: pressure 3.0MPa, temperature ~ 20 ℃.

Deaerator measuring device

  • 1) The deaerator is equipped with three sets of single chamber balance vessels, which are respectively installed on both sides of the deaerator.
  • It is used to obtain high and low water level signals of deaerator. When the water level of the deaerator is high or lower than a certain value of the normal water level, the water level signal sent by the balance vessel is transformed into an electrical signal through the differential pressure transmitter and sent to the automatic water level regulation device for regulation.
  • 2) Two sets of magnetic flap level gauges are installed on the left and right sides of the deaerator to monitor the water level in the deaerator locally.
  • 3) Two pressure gauges are installed on the left and right sides of the deaerator to monitor the pressure in the vessel locally.
  • 4) Two bimetallic thermometers are installed on the left and right sides of the deaerator to monitor the medium temperature in the deaerator locally.

Deaerator layout

In order to prevent cavitation of the feed pump, the feed booster pump is arranged at zero meter and the deaerator is arranged at 24.0m to increase the effective NPSH of the feed pump.

Deaerator water level protection

The high water level protection of deaerator is divided into 3 grades:

  • First gear – high water level: alarm (open the overflow valve when the operator thinks it is necessary);
  • Second gear – high high water level: open the emergency drain valve to discharge water in emergency;
  • Third gear – dangerous high water level: forcibly close the extraction check valve and extraction electric valve. When the water level reaches the normal value, the emergency drain valve and overflow drain valve can be closed together.

The low water level protection of deaerator shall be divided into 2 levels: the setting diagram of deaerator water level protection is shown in the figure below.

  • First gear – low water level: alarm;
  • Second gear – Dangerous low water level: stop the feed pump and other protective devices.

Technical specification for deaerator

Deaerator Structural parameters

Structure of deaerator Table:

Serial number

Project

Parameter

1

Deaerator type

Horizontal, headless, spray type

2

Deaerator model

SSD-2010/235

3

Total volume of deaerator

357 m3

4

Effective volume of deaerator

235 m3

5

Maximum output of deaerator

2010t/h

6

internal diameter

3800mm

7

length

32668mm

8

wall thickness

28mm

9

Net weight

124215kg

Technical parameters of deaerator

Technical parameters of deaerator:

Serial number

Term order

Parameter value

1

Design pressure

1.48MPa

2

Design temperature

381℃

3

Maximum working pressure

1.314MPa(a)

4

Maximum operating temperature

400.5℃

5

Rated output of single nozzle

100t/h

6

Safety valve seating pressure

1.4MPa

7

Flow rate of safety valve

126.876t/h

8

Oxygen content of condensate at outlet

≤5 μg/l

9

Condensate temperature at inlet

140.8℃

10

Condensate temperature at outlet

187.9℃

Water quality requirements

Water supply quality requirements of deaerator:

Serial number

Project

Parameter value

1

Cation conductivity (25 ℃)

≤0.15 μs/cm

2

PH value

8~9

3

hardness

≈ 0 mmol

4

Fe Fe ion

≤ 10 μg/l

5

Cu ion

≤ 3 μg/l

6

SiO 2

≤ 15 μg/l

7

dissolved oxygen

30~200 μg/l

Working principle of deaerator

The condensate and make-up water first enter the water chamber of the internal rotating membrane unit of the deaerator head, and under a certain water level differential pressure, it is sprayed obliquely from the small hole of the membrane tube to the inner hole to form a jet. Because the inner hole is filled with rising heating steam, the water will suck in a large amount of heating steam during the jet movement (the test shows that the jet movement has the effect of entrainment); In a very short time and a very small stroke, it produces in10se mixed heating, and the water temperature increases greatly, while the rotating water continues to rotate downward along the inner hole wall of the membrane tube to form a layer of rolling water film skirt (turbulent tumbling occurs when the critical Reynolds number of water decreases a lot during rotating flow). At this time, the heat and mass transfer effect of water in turbulent state is the most ideal, and the water temperature reaches the saturation temperature. The oxygen is separated. Because the oxygen cannot diffuse freely in the inner hole, the rising steam can only be discharged to the atmosphere from the steam exhaust pipe (although the old deaerator heats the water and separates the oxygen, the specific gravity of the oxygen is greater than the heating steam, and part of the oxygen is brought into the water tank by the downflow water, which is also a reason for the poor deaeration effect). The feed water subjected to rough deaeration in the film lifting section and the drain introduced by the drain pipe are mixed here for secondary distribution, which falls into the liquid steam network installed under it in a uniform rain shape, and then flows into the water tank after deep deaeration. The oxygen content of water in the water tank is high pressure 0-7 цɡ/L. Low pressure less than 15 цɡ/L meet the operation standards issued by the Ministry.
Because the rotating membrane deaerator keeps the water in a turbulent state during operation and has a large enough heat exchange surface area, the heat and mass transfer effect is better and the steam exhaust volume is small (that is, the amount of steam used and heated is less, The economic benefits brought by small energy loss are also considerable) the surplus generated by good deaeration effect can make the deaerator operate under overload (usually 50% of the rated output in a short time) or under full make-up water at low water temperature to meet the operation standard.

The principle of thermal deaeration is based on Dalton’s law and Henry’s law.

Henry’s law

At a certain temperature, when the gas dissolved in water is in equilibrium with other separated in water, the amount of gas dissolved in unit volume of water is directly proportional to the partial pressure of the gas on the water surface. The partial pressure of a gas in equilibrium is called equilibrium pressure. If the century partial pressure of a gas on the water surface is less than the equilibrium pressure corresponding to the dissolved gas in the water, the gas will be separated from the water under the action of unbalanced pressure difference until a new equilibrium is reached. If the gas can be completely removed from the water surface and the actual partial pressure of the gas is zero, the gas can be completely removed from the water. This is the basic principle of thermal deaeration.

Dalton’s law

The total pressure of the gas mixture is equal to the sum of the partial pressures of the constituent gases. In the deaerator, the amount of steam on the water surface continues to increase, the partial pressure of steam gradually increases, and the gas is discharged in time. Accordingly, the partial pressure of various gases on the water surface continues to decrease. When the water is heated to the saturation temperature under the deaerator pressure, a large amount of water evaporates, and the partial pressure of steam will approach the full pressure on the water surface. With the continuous discharge of gas, the partial pressure of various gases on the water surface will approach zero, so the gas in the water will escape from the water and be removed.
Thermal deaeration is a heat and mass transfer process. To ensure ideal deaeration effect, the following conditions must be met:

  • (1) The water must be heated to the saturation temperature under the deaerator pressure to ensure that the water vapor pressure on the water surface is close to the full pressure on the water surface;
  • (2) The gas escaping from the water must be discharged in time to reduce the partial pressure of various gases on the water surface to zero or minimum;
  • (3) The deaerated water and heated steam shall have sufficient contact area and flow in reverse, which not only enhances heat transfer, but also ensures large unbalanced pressure difference to separate the gas.

The process of gas separation from water can be basically divided into two stages:

The first stage is the initial deaeration stage. At this time, because there are many gases in the water and the unbalanced pressure difference is large, the gas escapes in the form of small bubbles to overcome the viscous force and surface tension of water. At this stage, 80% – 90% of the gas in the water can be removed.
The second stage is the deep deaeration stage. At this time, because there is still a small amount of gas in the water and the corresponding unbalanced pressure difference is very small, the gas does not have enough power to overcome the escape of water viscosity and surface tension, and it is slowly separated by the diffusion of a single molecule. At this time, the contact surface area of steam and water can be increased to form a water film and reduce its surface tension, so that the gas can be easily diffused. The bubbling effect of steam in water can also be used to make gas molecules adhere to the bubbles and escape from the water.

Working steps of deaerator

  • (1) confirm that the start-up exhaust electric valve and continuous exhaust bypass valve of deaerator are in the open position.
  • (2) when the condensate system flushing is qualified, open the flushing drain valve of the deaerator and fill the deaerator with water for flushing
  • (3) after the water quality of deaerator is qualified, reduce the water level to – 900mm and close the flushing drain valve of deaerator.
  • (4) turn on the auxiliary steam of deaerator for heating, open the isolation valves before and after the regulating valve from auxiliary steam to deaerator, slowly open the pressure regulating valve from auxiliary steam to deaerator, control the feed water temperature rise rate of deaerator not greater than 4.26 ℃/min, and pay attention to the vibration of deaerator during heating. If the vibration is large, slow down the heating speed
  • (5) during the heating process of the deaerator, continue to use the condensate pump to fill the deaerator to the normal water level.
  • (6) when the water temperature of deaerator reaches 100 ℃, close the start-up electric exhaust valve, put the pressure regulating valve from auxiliary steam to deaerator into automatic mode, check that the temperature rise rate of deaerator is not greater than 4.26 ℃/min, and the pressure of deaerator gradually rises to 0.147MPa.
  • (7) during auxiliary steam heating, the water level of deaerator shall be controlled. If vacuum is not established in the condenser, it is forbidden to open the electric valve for overflow and water discharge to the condenser
  • (8) after the condensate system is started, the water level of deaerator shall be adjusted automatically as required.
  • (9) when the four extraction pressure reaches 0.147MPa, check that the pressure and water level of the deaerator are normal, open the electric valve from the four section extraction to the deaerator, switch the deaerator from the auxiliary steam to the four extraction steam supply, close the pressure regulating valve from the auxiliary steam to the deaerator, and change the deaerator from constant pressure operation to sliding pressure operation.
  • (10) after the check valve behind the electric steam extraction valve of Section IV has been opened, check that the pneumatic drain valve in front of the electric valve from section IV steam extraction to deaerator is closed.
  • (11) adjust the continuous exhaust electric valve of deaerator according to the oxygen content of feed water.

Operation of deaerator

Steam source switching of deaerator

When the four extraction pressure is higher than the pressure of the deaerator, open the four extraction electric isolation valve, maintain the pressure slightly higher than 0.147MPa, and switch the steam source of the deaerator to the four extraction belt. After the auxiliary steam source is out of operation, the drain valve on the steam supply pipe shall be opened to keep the auxiliary steam supply pipe in hot standby state.
After switching, the deaerator enters the sliding pressure operation stage. When the unit load is greater than 20%, confirm that the drain valve on the four extraction pipeline is closed after the electric isolation valve of the four extraction steam supply deaerator is opened.

“Return oxygen” and “regeneration boiling” of deaerator

No matter the deaerator with constant pressure or sliding pressure operation is used, the phenomenon of “oxygen return” or “reboiling” may occur when the load changes, especially the deaerator with sliding pressure operation is more likely to occur.
When the load rises, the pressure in the deaerator rises, and the change of water temperature in the deaerator lags behind the change of pressure and cannot rise immediately, but becomes undersaturated water. Since the solubility of gas in unsaturated water is greater than that in saturated water, the precipitated gas returns to the feed water again, reducing the deaeration efficiency, which is the phenomenon of “reoxygenation”.
The occurrence of “oxygen return” phenomenon will not cause cavitation of feed pump. During operation, the possibility of pressure surge of deaerator is small, while sudden pressure drop often occurs. At this time, the “reboiling” phenomenon of deaerator is easy to occur. The mechanism of reboiling of deaerator is that the saturation temperature of water under different pressure is different, and the higher pressure corresponds to the higher saturation temperature. When the pressure of deaerator drops suddenly, the feed water temperature is greater than the saturation temperature of feed water under this pressure, which makes the feed water vaporize, i.e. “reboiled”. According to the principle of thermal deaeration, the deaeration effect is better when the feed water reboiles, but the possibility of cavitation of the feed pump increases at this time. Therefore, special attention should be paid to avoid sudden pressure drop for the deaerator operating under sliding pressure.

Regulation of steam exhaust volume of deaerator

The amount of steam exhaust of deaerator is directly related to deaeration effect and economy. If the oxygen exhaust valve is opened too large, the steam exhaust loss increases; If it is too small, the deaeration capacity will be reduced, and its opening must be determined after on-site operation and adjustment.
The deaerator of our plant only deaerates the feed water in the unit startup stage. When the load is high, the oxygen exhaust valve is fully closed for continuous exhaust to the condenser.

Abnormal and accident handling of deaerator

Typical accidents in deaerator operation mainly include abnormal pressure, water level, deaerator vibration, etc.

Abnormal deaerator pressure

The abnormal pressure of deaerator is characterized by sudden rise and drop of pressure.
The reasons for the sudden rise of pressure may be the sudden drop of water inflow of deaerator, overload operation of unit, large drainage of HP heater, failure of pressure regulating valve of deaerator, etc. In case of sudden pressure rise, immediately check the cause and deal with it accordingly. If necessary, manually adjust the pressure of deaerator to avoid continuous overpressure operation of deaerator.
When the deaerator pressure drops suddenly, immediately check whether the water inflow, pressure and load of the deaerator are suitable; If the heating steam source is auxiliary steam, pay attention to monitor whether the action of auxiliary steam pressure regulating valve is normal, and adjust it manually if necessary.

Abnormal water level of deaerator

The abnormal change of water level of deaerator is mainly caused by the imbalance of inlet and outlet water and the sudden change of internal pressure of deaerator; At this time, find out the main factors and deal with them. Do not adjust blindly to prevent the deaerator from being full of water.

Deaerator shutdown

During normal shutdown, with the decrease of unit load, the pressure, temperature and water inflow of deaerator gradually decrease. When the load decreases to 20%, the steam source of deaerator is switched to auxiliary steam to maintain the constant pressure of deaerator at 0.147MPa. Monitor that the water level, pressure and temperature of the deaerator adapt to the unit load, reduce the water supply of the deaerator to zero as required, and exit the heating device of the deaerator.

Shutdown and maintenance of deaerator

If the shutdown is within one week, the standby steam source can be opened slightly, and other steam and water inlet and outlet valves can be closed for thermal protection, and the internal pressure can be maintained at 0.02MPa.
If it is shut down for a long time (more than one week), drain the internal ponding for nitrogen filling protection, maintain the nitrogen filling pressure of 0.02MPa or take other protective measures (blow dry the device, put preservatives, etc.). To prevent the inner wall of deaerator from being eroded by oxygen or other harmful gases.

Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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