DEAERATOR ( PART 2 )

DEAERATOR ( PART 2 )

OPERATION PROCEDURES

** Pre-start checks and operations

(1) Vent valve shall be wide open.

(At low load start, it may be necessary to throttle valve slightly to increase the operating pressure, but vent valve not to be fully close in any case.)

(2) Steam, condensate or water and feed water suction valve shall be closed.

(3) All block valve of instrument is to be open.

(4) All control, alarm and switch is on proper function.

(5) Temperature pressure gauge and instrument shall be calibrated.

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** Initial state (STARTUP)

Before water and steam flow into the equipment, check below contents.

(1) Open the vent completely. In case of very low pressure operation vent throttle valve is slightly closed to maintain the pressure of tank But don't close vent completely at any case.

(2) Check the safety valve.

(3) Check to see that all local instruments are operation and indicating correctly.

(4) Check the degree accuracy of temperature gage, pressure gage and all measurement instruments.

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**Normal Start-up

(1) Open slowly the condensate or water supply valve.

(2) Fill-up water to storage tank up to 60-70% level.

(3) Stop the condensate or water supply.

(4) Open slowly the sparger steam supply valve to heat the water within storage tank.

(5) After water temperature is increased to proper temperature, close the sparger steam supply valve and stop and valve operation until vapor fume disappear from vent pipe.

(6) Above step 1- 5 may be omitted if feed water can be recirculated through minimum flow line.

(7) Open slowly the condensate or water supply valve to 15-30% load flow rate.

(8) Open step by step the main steam supply valve until operating pressure will be rate.

(Caution: Do not fill the deaerator with steam first and then supply the condensate or water. This operation will cause the damage of internals and the partial collapse of shell.)

(9) Adjust the steam supply valve to Auto mode.

(10) Open minimum flow valve of feed water pump discharge.

(11) Start feed water pump.

(12) Recirculate feed water through minimum flow line until feed water temperature is increased to near operating temperature.

(Step 10 & 12 can be omitted in case that water is heated by steam of step 1-5)

(13) During recirculation operation, drain valve shall be open to protect the increase of water level and the suspension of eater supply.

(14) Adjust the water supply valve to Auto mode when water level is near normal level.

(15) Open slowly the discharge valve of feed water pump and supply the feed water to boiler or its related equipment.

(16) Load of deaerator will be increased as the discharge valve of feed water pump is more open.

(17) After normal operation is maintained with proper load than 30minutes, vent valve can be closed to one or two turns.

(Normally it indicates sufficient venting if steam fume rises approximately one meter above the end of vent pipe.)

(18) Now deaerator is ready for continuous operation.

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**Normal Shutdown

(1) Close slowly the boiler feed water pump discharge valve.

(2) Close the condensate or water supply valve.

(3) Isolate the main steam supply valve.

(4) Open the atmospheric vent valve fully to expel any remaining steam in tank.

(5) If it should be necessary to drain the tank. Start drain after wide opening of vent valve.

(Draining without sufficient venting will cause the undue stress on shell by vacuum.)

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**Emergency Shutdown

(1) Isolate the main steam supply valve.

(2) Close the condensate or water supply valve.

(3) Close the boiler feed water pump discharge valve.

(4) Open the atmospheric vent valve fully to expel any remaining steam in tank.

(5) If it should be necessary to drain the tank.

Start drain after wide opening of vent valve.

(Draining without sufficient venting will cause the undue stress on shell by vacuum.)

“This article for, Dearator & Storage tank O&M manuals , SPC. LTD “

DEAERATOR ( PART 1 )

DEAERATOR ( PART 1 )

FUNCTION

-The function of the deaerating heater is to remove dissolved non-condensable gases and to heat boiler feed water. A deaerating heater consists of a pressure vessel in which water and steam are mixed in a controlled manner. When this occurs, water temperature rises, and all non-condensable dissolved gases are liberated and removed and the effluent water may be considered corrosion free from an oxygen or carbon dioxide standpoint. Free air or other non-condensable gases should be vented prior permitting the fluid to enter the deaerator.

-A deaerating heater is the watch dog of boiler plant as it protects the feed pumps, piping, boiler and any other piece of equipment that is in the boiler feed and return cycle from the effects of corrosive gases, i.e. oxygen and carbon dioxide, to a level where they are no longer a corrosion factor.

Principles of Deaeration

-There is physical law which states that the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. Another law states, the solubility of a gas in a liquid decreases with an increase in temperature of the liquid. Experience has shown that more rapid and more complete removal of non-condensable gases from a liquid is obtained when the liquid is vigorously boiled or scrubbed by condensable or carrier gas bubbles.

-Therefore, essentially the deaerating heater must first heat the feed water to as high a temperature as possible, i.e. to the temperature corresponding to the steam pressure, It must vigorously boil and scrub the heater water with fresh steam, which can carry to the liquid surface any traces of oxygen and carbon dioxide.

-The partial pressure of the oxygen and carbon dioxide in the steam atmosphere must be maintained as low as possible, particularly at the point where the deaerated water separates from the steam. Non-condensable gases must be continually withdrawn from the heater at the rate at which they are being liberated.

Deaerator Heater

-A deaerating heater utilizes steam by spraying the incoming water into an atmosphere of steam in the preheated section (first stage). It then mixes this water with fresh incoming steam in the deaerator section (second stage).

-In the first stage the water is heated to within 1.1C of steam saturation temperature and virtually all of the oxygen and free carbon dioxide are removed. This is accomplished by spraying the water through self-adjusting spray valves which are designed to produce a uniform spray film under all conditions of load and consequently a constant temperature and uniform gas removal is obtained at this point.

- From the first stage the preheated water, containing minute traces of dissolved gases, flows into the second stage. This section consists of either a distributor or several assemblies of trays,

Here the water is in intimate contact with an excess of fresh gas-free steam. The steam passes into this stage and it is mixed with the preheated water. Deaeration is accomplished at all rates of flow if conditions are maintained in accordance whit design criteria. Very little steam is condensed here as the incoming water has a high temperature caused by the preheating. The steam then rises to the first stage and carries the small traces of residual gases. In the first stage most of the steam is condensed and remaining gases pass to the vent where the non condensable gases flow to the atmosphere, A very small amount of steam is also discharged to the atmosphere which assures that the deaerating heater is adequately vented at all times.

- The water which leaves the second stage falls to the storage tanks where it is stored for use. At this time the water is completely deaerated and is heated to the steam saturation temperature corresponding to the pressure within the vessel.

VENT

-Efficient removal of the non-condensable gases from the deaerating heater requires that the vent valve be opened sufficiently to allow complete discharge of the gases passed to the vent condenser outlet pipe. The maximum concentration of the non-condensable gases such as oxygen, carbon dioxide passing out the vent depends on the degree of condensation produced by the steam and gas mixture passing through and around the spray created by the special spray valve.

“This article for, Dearator & Storage tank O&M manuals , SPC. LTD “

On/Off Control

On/Off Control

- Suppose a process operator has the task of holding the temperature, T, near the desired temperature, Td, while making sure the tank doesn't overflow or the level get too low. The question is how the operator would cope with this task over a period of time. He or she would manually adjust the hot water inlet valve (HV-1) to maintain the temperature and occasionally adjust the outlet valve (HV-2) to maintain the correct level in the tank.

- The operator would face several problems, however. Both indicators would have to be within the operator's view, and the manual valves would have to be close to the operator and easy to adjust.

- To make the operator's work easier, suppose we installed electrically operated solenoid valves in place of the manual valves, as shown in the next figure. We can also install two hand switches (HS-1 and HS-2) so the solenoid valves can be operated from a common location. The valves can assume two states, either fully open (on) or fully closed (off). This type of control is called two-position or on/off control.

- Assume for the moment that the level is holding steady and that the main concern is controlling temperature. The operator has been told to keep the temperature of the fluid in the tank at 100°F. He compares the reading of the temperature indicator with the selected set point of 100°F. The operator closes the hot water valve when the temperature of the fluid in the tank rises above the set point. Because of process dead time and lags the temperature will continue to rise before reversing and moving toward the set point. When the temperature falls below 100°F, the operator opens the hot water valve. Again, dead time and lags in the process create a delay before the temperature begins to rise. As it crosses the set point, the operator again shuts off the hot water, and the cycle repeats.

- This cycling is normal for a control system that uses on/off control. This limitation exists because it’s impossible for the operator to control the process exactly with only two options.

This on/off type of control can be expressed mathematically as follows:

e = PV – SP

In the on/off control mode, the valve is open valve when the error (e) is positive (+), and the valve is closed when e is negative (–).

General Requirements of a Control System

General Requirements of a Control System

The primary requirement of a control system is that it be reasonably stable. In other words, its speed of response must be fairly fast, and this response must show reasonable damping. A control system must also be able to reduce the system error to zero or to a value near zero.

SYSTEM ERROR

The system error is the difference between the value of the controlled variable set point and the value of the process variable maintained by the system. The system error is expressed in equation form by the following:

e(t) = PV(t) – SP(t)

where:-

e(t) = system error as a function of time (t)

PV(t) = the process variable as a function of time

SP(t) = is the set point as a function of time

SYSTEM RESPONSE

-The main purpose of a control loop is to maintain some dynamic process variable (pressure, flow, temperature, level, etc.) at a prescribed operating point or set point. System response is the ability of a control loop to recover from a disturbance that causes a change in the controlled process variable.

-There are two general types of good response: underdamped (cyclic response) and damped. (Figure 1) shows an underdamped or cyclic response of a system in which the process variable oscillates around the set point after a process disturbance. The wavy response line shown in the figure represents an acceptable response if the process disturbance or change in set point was large, but it would not be an acceptable response if the change from the set point was small.

- (Figure 2) shows a damped response where the control system is able to bring the process variable back to the operating point with no oscillations.

Control Loop Design Criteria

- Many criteria are employed to evaluate the process control’s loop response to an input change. The most common of these include settling time, maximum error, offset error, and error area.

- When there is a process disturbance or a change in set point, the settling time is defined as the time the process control loop needs to bring the process variable back to within an allowable error. The maximum error is simply the maximum allowable deviation of the dynamic variable. Most control loops have certain inherent linear and nonlinear qualities that prevent the system from returning the process variable to the set point after a system change. This condition is generally called “offset error” . The error area is defined as the area between the response curve and the set point line as shown by the shaded area in (Figure 3).

- These four evaluation criteria are general measures of control loop behavior that are used to determine the adequacy of the loop’s ability to perform some desired function.

REFERENCES , ( THOMAS HUGHES , MEASUREMENT & CONTROL BASICS )

Elements of a Process Control System

Elements of a Process Control System

INTRODUCTION

-The term automatic process control came into wide use when people learned to adapt automatic regulatory procedures to manufacture products or process material more efficiently. Such procedures are called automatic because no human (manual) intervention is required to regulate them.

-All process systems consist of three main factors or terms: the manipulated variables, disturbances, and the controlled variables. Typical manipulated variables are valve position, motor speed, damper position, or blade pitch. The controlled variables are those conditions, such as temperature, level, position, pressure, pH, density, moisture content, weight, and speed, that must be maintained at some desired value. For each controlled variable there is an associated manipulated variable. The control system must adjust the manipulated variables so the desired value or “set point” of the controlled variable is maintained despite any disturbances.

  

Elements of a Process Control System

-The following figure illustrates the essential elements of a process control system. In the system shown, a level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank. The purpose of this control system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.

-Thus, a process control system consists of four essential elements: process, measurement, evaluation, and control. A block diagram of these elements is shown in the following figure. The diagram also shows the disturbances that enter or affect the process. If there were no upsets to a process, there would be no need for the control system. The figure also shows the input and output of the process and the set point used for control.

PROCESS

In general, a process consists of an assembly of equipment and material that is related to some manufacturing operation or sequence. In the example presented, the process whose liquid level is placed under control includes such components as a tank, the liquid in the tank, and the flow of liquid into and out of the tank, and the inlet and outlet piping. Any given process can involve many dynamic variables, and it may be desirable to control all of them. In most cases, however, controlling only one variable will be sufficient to control the process to within acceptable limits. One occasionally encounters a multivariable process in which many variables, some interrelated, require regulation.

MEASUREMENT

- To control a dynamic variable in a process, you must have information about the entity or variable itself. This information is obtained by measuring the variable.

- Measurement refers to the conversion of the process variable into an analog or digital signal that can be used by the control system. The device that performs the initial measurement is called a sensor or instrument. Typical measurements are pressure, level, temperature, flow, position, and speed. The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequence.

EVALUATION

- In the evaluation step of the process control sequence, the measurement value is examined, compared with the desired value or set point, and the amount of corrective action needed to maintain proper control is determined. A device called a controller performs this evaluation. The controller can be a pneumatic, electronic, or mechanical device mounted in a control panel or on the process equipment. It can also be part of a computer control system, in which case the control function is performed by software.

CONTROL

The control element in a control loop is the device that exerts a direct influence on the process or manufacturing sequence. This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process. In most cases, this final control element will be a control valve that adjusts the flow of fluid in a process. Devices such as electrical motors, pumps, and dampers are also used as control elements.

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Process and Instrumentation Drawings

-In standard P&IDs, the process flow lines, such as process fluid and steam, are indicated with heavier solid lines than the lines that are used to represent the instrument. The instrument signal lines use special markings to indicate whether the signal is pneumatic, electric, hydraulic, and so on. two types of instrument signals are used: double cross-hatched lines denote the pneumatic signals to the steam control valve and the process outlet flow control valve, and a dashed line is used for the electrical control lines between various instruments. In process control applications, pneumatic signals are almost always 3 to 15 psig (i.e., pounds per square inch, gauge pressure), and the electric signals are normally 4 to 20 mA (milliamperes) DC (direct current).

- A balloon symbol with an enclosed letter and number code is used to represent the instrumentation associated with the process control loop. This letter and number combination is called an instrument identification or instrument tag number.

- The first letter of the tag number is normally chosen so that it indicates the measured variable of the control loop. In the sample P&ID shown, T is the first letter in the tag number that is used for the instruments in the temperature control loop. The succeeding letters are used to represent readout or passive function or an output function, or the letter can be used as a modifier. For example, the balloon in Figure marked TE represents a temperature element and that marked TIC is a temperature-indicating controller. The line across the center of the TIC balloon symbol indicates that the controller is mounted on the front of a main control panel. No line indicates a field-mounted instrument, and two lines means that the instrument is mounted in a local or field-mounted panel. Dashed lines indicate that the instrument is mounted inside the panel.

- Normally, sequences of three- or four-digit numbers are used to identify each loop. In our process example, we used loop numbers 100 and 101. Smaller processes use three-digit loop numbers; larger processes or complex manufacturing plants may require four or more digits to identify all the control loops.

- Special marks or graphics are used to represent process equipment and instruments. For example, in our P&ID example in Figure two parallel lines represent the orifice plate that is used to detect the discharge flow from the process heater.

Africa Looks to Nuclear for Future Generation

“THIS ARTICLE GIVE US FACTS ABOUT NUCLEAR DEVELOPMENTS AT AFRICAN COUNTRIES LIKE EGYPT , NAMIBIA , SOUTH AFRICA,”

-Africa is emerging as a prominent voice in calling for a global nuclear renaissance. Driven by chronic shortages from population explosions, decades of drought, and dependence on hydropower — and spurred by discoveries of significant uranium reserves on the continent — several countries are considering nuclear power as a viable option.

-Only one nuclear plant exists in Africa today—Eskom’s 1,800-MW Koeberg Station in the Western Cape region of South Africa. Several other countries on the continent are looking to nukes for future power generation, including Egypt, Namibia, and Eastern African nations.

SOUTH AFRICA

-South Africa has a significant advantage over the continent’s other nations because it houses the only nuclear power plant in Africa — the 1,800-MW Koeberg Station, which was built in 1984 on the Western Cape. Stricken with a power shortage that has taken a toll on the continent’s largest economy; the country is convinced nuclear power will be key to battling the crunch.

-It is considering the construction of three new nuclear power stations on its coastline. The new stations, provisionally known as Nuclear 1, 2, and 3, would each be able to deliver twice as much electricity as Koeberg. The government wants to build Nuclear 1 starting in 2012, next to the existing Koeberg plant, and at the same time begin work on Nuclear 2 at Bantamsklip, southeast of Pearly Beach. It would later build Nuclear 3 at Thyspunt near Cape St. Francis in Eastern Cape, South African newspaper Die Burger reported this July, citing a study carried out on the government’s behalf by engineering consultancy group Arcus Gibb.

-No decisions on reactor technology have been made yet, but experts agree that the choice must be economically viable. Some even assert that the government is considering the homegrown Pebble Bed Modular Reactor, of which an 80-MW pilot plant is expected to come online by 2018, with initial costs estimated at 27 billion rand ($3.45 billion).

EGYPT

-Egypt is quickly moving forward to revive the civilian nuclear program it froze 20 years ago, following the accident at the Chernobyl plant in Ukraine. The nation has contracted Australian firm WorleyParsons to select a site, technology, and design, and then build a proposed 1,200-MW plant. The El-Dabaa site on the Mediterranean coast, which had been selected as the preferred site for a new reactor in 1983 before the program was scrapped, remains the preferred site, according to media reports.

-More recently, the nation was considering six bids from international firms to provide support and advice on setting up the country’s nuclear safety regulatory framework. The shortlist includes companies from Canada, Germany, France, South Korea, the UK, and the U.S.

NAMIBIA

-Namibia is Africa’s top uranium producer, followed by Niger, and then South Africa. Two significant uranium mines in the nation are capable of providing 10% of world mining output: the Rössing Uranium Mine, one of the largest open pit uranium mines in the world, and Langer Heinrich. But the country imports more than 60% of its power from other southern African countries—and it is suffering shortages as those nations are being forced to cut supply because of rising demand and stagnant generation capacity. For that reason, Namibia is considering setting up a nuclear program and recently made a deal with India for training and nuclear expertise in exchange for uranium. Courtesy: University of the Orange Free State

-Blessed with two significant uranium mines capable of providing 10% of world uranium mining output, Namibia has been quietly realizing plans to generate its own power with nuclear energy by 2018. In a significant development, this September the country signed an agreement with India to trade uranium for expertise in designing atomic plants.

- Namibia already has deals for uranium prospecting with French state-owned nuclear giant AREVA, and it has reportedly discussed prospecting agreements with Russia and Japan — all countries well-versed in nuclear power know-how. The country is also en route to developing a nuclear regulatory framework; last June, the Namibian cabinet tasked the Ministry of Mines and Energy with that responsibility.

‘This article for, SONAL PATEL, Editor “

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Generator Temperature Rise Test

Generator Temperature Rise Test

From a few days, we have made a test on the Generator’s windings (Stator and Rotor), this test accomplish the temperature rise of the windings at full load is within the Normal range.

First, we increased the load to full load (350 MW), then increased the reactive power and the terminal voltage at about (19.7 KV).

1) Procedure for stator winding.

-Apply the following formulae, after you collect the data every 30 minutes.

T_s = ( T_st – T_s1 ) * ( I_n / I_b )²  + T_s1

T_{st =}   Actual stator winding temp. – Avg. cold gas temp

T_s1=   Factory test value = 4.4

I_{n =}      Factory test value = 13.37

I_{b =}      Max. Current value

2) Procedure for rotor winding.

-Apply the following formulae, after you collect the data every 30 minutes.

T_f =  T_ft * ( I_fn / I_ft )²

T_{ft =}    Actual rotor winding temp. – Avg. cold gas temp

I_{fn =}     Factory test value = 2945

I_{ft =}      Actual Current value

T_s and T_f , should be at the normal levels , at full load

Generator Temperature Rise Test

Generator Temperature Rise Test

From a few days, we have made a test on the Generator’s windings (Stator and Rotor), this test accomplish the temperature rise of the windings at full load is within the Normal range.

First, we increased the load to full load (350 MW), then increased the reactive power and the terminal voltage at about (19.7 KV).

1) Procedure for stator winding.

-Apply the following formula, after you collect the data every 30 minutes.

T_s = ( T_st – T_s1 ) * ( I_n / I_b )²  + T_s1

T_{st =}   Actual stator winding temp. – Avg. cold gas temp

T_s1=   Factory test value = 4.4

I_{n =}      Factory test value = 13.37

I_{b =}      Max. Current value

2) Procedure for rotor winding.

-Apply the following formulae, after you collect the data every 30 minutes.

T_f =  T_ft * ( I_fn / I_ft )²

T_{ft =}    Actual rotor winding temp. – Avg. cold gas temp

I_{fn =}     Factory test value = 2945

I_{ft =}      Actual Current value

T_s and T_f , should be at the normal levels , at full load

Boiler STEAM SYSTEM

Boiler STEAM SYSTEM

-The boiler Steam system consists of SUPERHEATER, REHEATER AND BLOWDOWN SYSTEM

1) SUPERHEATER

-The saturated steam from the drum flows into three stages of superheaters. Steam is superheated up to rated temperature through primary, secondary and tertiary superheaters. Auxiliary steam and sootblower steam are extracted from the primary superheater outlet. Two stages of attemperators are provided between each superheater to control the boiler outlet main steam temperature. The boiler main steam stop valve is provided for the maintenance purpose such as hydrostatic test. Finally, superheated steam is supplied to the HP turbine from the boiler through the Main Steam Piping.

-Superheater spray water is supplied from the HP feedwater heater inlet. The superheater spray water piping distributes spray water to superheater first and second stage attemperators.

2) REHEATER

-The Cold-Reheat Piping conveys HP turbine exhaust steam from the HP turbine outlet to the reheater. The steam is reheated up to rated temperature through the reheater. The reheater outlet steam temperature is controlled by flue gas recirculation flow regulated by the Gas Recirculation Fans. Also an attemperator is provided at the Cold-Reheat Piping to control the reheater outlet steam temperature in abnormal condition. Then, reheated steam is supplied to the IP/LP turbine from the reheater through the Hot-Reheat Piping.

-Reheater spray water is supplied from middle stage of boiler feedwater pumps. The reheater spray water piping supplies spray water to reheater attemperator.

3) BLOWDOWN & DRAIN SYSTEM

-Continuous blowdown from the drum flows into the blowdown flash tank and is separated into steam and drain. Flashed steam is collected to the deaerator through the vent line. And separated drain is discharged into the atmospheric blowdown tank. The blowdown flash tank level control valve controls the blowdown flash tank water level. Safety valve is installed at the blowdown flash tank to prevent the tank from the overpressure.

-The atmospheric blowdown tank collects drain from the boiler pressure parts, startup blowoff line of the drum, the soot blowing system and the blowdown flash tank. Atmospheric blowdown tank separates the above drains into steam and water, Flashed steam is discharged to atmosphere through the vent line with silencer, And separated drain is discharged to the blowdown sump through attemperator.

 “This description is FOR   HITACHI Steam Generator “

Boiler Water SYSTEM

Boiler Water SYSTEM

We talked before about HITACHI Boiler Air and Flue gas system, we mentioned the characteristics of the boiler Air and flue gas & Parameters of operating Steam and water

At this article we will make a focus on Water system at boiler.

 -The boiler Water system consists of ECONOMIZER  AND DRUM & FURNACE WATER WALL.

1) ECONOMIZER

-Feedwater is supplied from the feedwater system to the boiler and is preheated by the economizer, Feedwater quality shall be properly controlled by the feedwater chemical treatment system. The economizer is situated at downstream of the reheater located in back end of the boiler.

-Since sufficient feedwater is not supplied into the economizer though the heat is absorbed from flue gas at the boiler start-up and low load operation, steaming may occur in the economizer. Therefore the economizer recirculation line is provided between the furnace downcomer and the economizer inlet feedwater piping. This line provides the recirculation flow between the drum, the downcomer and the economizer so as to prevent steaming in the economizer. Economizer recirculation valve is only opened at initial stage of the boiler start-up.

 

2) DRUM & FURNACE WATER WALL

-Feedwater preheated in the economizer is supplied to the drum. At the drum, water flows downward through the downcomer by gravity, and then flows into the furnace water wall. Steam is generated at the furnace water wall and water/steam mixture flows upward to the drum because of its small gravity. In this way, natural circulation is formed at drum, downcomer and furnace water wall.

-Drum water is treated by the phosphate injection system. And the drum has the continuous blowdown valve to discharge suspended solid from the drum water during normal operation. The drum also has cyclone separators and scrubber to separate the water from saturated steam. The startup blowoff valves are provided to the drum to handle the swelling phenomenon during the boiler start-up period.

“This description is FOR   HITACHI Steam Generator “

Turbine Supervisory Instruments

Turbine Supervisory Instruments

The following supervisory instruments are furnished with the unit and are to be observed during start-up, operation and shutdown. We will explain the following instruments in detail as follows.

1- Casing Expansion

As a unit is taken from its cold condition to its hot and loaded state, the thermal changes in the casings will cause it to expand.

The Casing expansion scale measures the movement of the force pedestal relative to a fixed point (the foundation). It indicates expansion and contraction of the casings during starting and stopping period, and for changes in load, steam temperatures.

Should it fail to indicate during these transient conditions, the situation should be investigated? The relative position of the fore pedestal should be essentially the same for similar conditions of load, steam conditions, vacuum, etc.

2- Rotor Position

Rotor Position instrument measures the relative axial position of the turbine rotor thrust collar with respect to the first bearing support. The thrust collar exerts a pressure against the thrust shoes, which are located on both sides of the thrust collar. Wear on the thrust shoes results in an axial movement of the rotor and is indicated on these instruments.

This instrument is equipped with an alarm relay which activates if the rotor moves beyond a predetermined distance. Continued movement beyond a second predetermined distance activates rotor position trip relay which trip the turbine via the emergency trip system.

3- Differential Expansion

When steam is admitted to a turbine, both the rotating parts and the casings will expand. Because of its smaller mass, the rotor will heat faster and therefore expand faster than the casings.

Axial clearances between the rotating and the stationary parts are provided to allow for differential expansion in the turbine, but contact between the rotating and stationary  parts may occur if the allowable differential expansion limits are exceeded. The purpose of the differential expansion meter is to chart the relative motion of the rotating and stationary parts. It gives a continuous indication of the axial clearance while the turbine is in operation.

The instrument is equipped with alarm relay which activates if the value reaches the alarm point. As the rotating and stationary parts become equally heated after a transient condition, the deferential expansion will decrease.

4- Rotor Eccentricity

When a turbine has been shut down, the rotor will tend to bow due to uneven cooling if the upper half of the casing enclosing the rotor is at a higher temperature than the lower half. By rotating the rotor slowly on turning gear, the rotor will be subjected to more uniform temperature, thereby minimizing bowing.

This bowing of the rotor is recorded continuously as eccentricity from turning gear speed to approximately 600 rpm. The eccentricity instrument is equipped with an alarm signal which activates when the eccentricity reaches the alarm point.

5-Vibration

The vibration instrument is used to measure and record vibration of a turbine rotor at speeds above 600rpm. Below this speed, the rotor bowing is recorded as eccentricity.

The vibrations are measured on the rotor near the main bearings. Excessive vibrations serve as a warning for abnormal and possible hazardous conditions in the turbine. Each vibration instrument is equipped with alarm and trip relays which activates when excessive vibrations are measured at any one of the bearings.

Boiler AIR AND FLUE GAS SYSTEM

Boiler AIR AND FLUE GAS SYSTEM

"We talked before about HITACHI Boiler Overview (Steam Generator), we mentioned the characteristics of the boiler, Parameters of operating Steam and water.

At this article we will make a focus on air and flue gas system at boiler."

 -The boiler air and flue gas system consists of combustion air system, gas recirculation system and flue gas system.

1) Combustion air system

-The combustion air system consists of two forced draft fans, two steam coil air heaters and two air heaters. Two 60% capacity centrifugal forced draft fans are provided to supply the combustion air to the windbox and after air port with adequate flow rate and pressure for combustion. The forced draft fan draws air from the atmosphere through the inlet silencer, and discharges air to the steam coil air heater, the air heater and the windbox. Modulating inlet vane of the centrifugal forced draft fan controls the combustion air flowing to the boiler wind box and after air port. Air heater inlet air damper creates the suitable backpressure for seal air supply through the forced draft fan outlet connecting air duct.

-Combustion air flow is measured by flow element located at the outlet of air heater. Two 50% capacity, bi-sector, vertical shaft regenerative type air heaters are provided to heat combustion air using flue gas from the economizer. To achieve in-furnace NOx reduction, two-stage combustion system using the after air port dampers is applied. Part of the combustion air is supplied to the after air ports for two-stage combustion. The after air port damper (4 dampers) control shall be modulated to maintain the optimum two-stage combustion at each load.

2) Gas Recirculation system

-The gas recirculation system consists of two gas recirculation fans. Two 60% capacity centrifugal gas recirculation fans are provided to supply gas recirculation flow to the furnace for reheat steam temperature control. The gas recirculation fan draws flue gas from the economizer outlet flue gas duct and discharge gas to the furnace. Modulating inlet damper controls gas recirculation flow rate. The gas recirculation flow set point is derived from the reheat steam temperature control.

3) Flue Gas system

-The flue gas system consists of two air heaters. The combustion gas transfers its own heat to the water wall, the superheaters, the reheater and the economizer, then leaves the boiler to the flue gas ducts. The flue gas goes to the stack through the AH and is exhausted to the atmosphere.

*** Steam Coil Air Heater

-Two 50% capacity steam coil air heaters are installed in the combustion air duct upstream of the air heaters to heat combustion air before air flows into the air heaters. The steam coil air heater raises the air heater cold end temperature to predetermined set point over the entire load range when fuel (Mazout) oil is burnt. The steam coil air heater is a finned, steel tube heat exchanger. Steam Used for heating flows through the tube side, Combustion air is heated as it flows across the outside of the finned tubes.

-Steam coil air heaters are installed vertically at the air heater inlet duct. The steam coil air heater casing and coil section covers the full dimension of the duct. Heating steam is supplied for the steam coil air heater system from auxiliary steam system.

 “This description is about HITACHI Steam Generator “