Boiler

BOILER

The boiler is divided into a ’furnace’ section and a ’second pass’ by a division membrane wall. The furnace section is made of tube walls and a refractory wall. The furnace comprises the furnace side-wall, roof, floor & rear walls and the front refractory wall. The furnace side, roof and floor, rear walls are of membrane panel construction. The furnace front wall is of refractory construction. The second pass comprises the Super heater, convection bank tubes. The second pass is enclosed by the rear wall & boiler side-wall. Entire array of tubes in the furnace & second pass is connected to the steam and water drums. The Super heater is designed for convective heat transfer and is fully drainable. Feed water from plant is admitted to the economizer through a feed water control station. The feed water is then led to the steam drum. Steam is generated in the convection bank tubes. In the riser tubes partial evaporation takes place due to the heating. The resulting water-steam mixture returns to the steam drum where the separation of steam from water takes place. The saturated steam is led to the Superheater and then through the main steam stop valve to the process plant. Combustion of the fuel takes place in the furnace with the help of the burners mounted on the furnace front refractory wall. Combustion air is sucked from the plant environment by the FD fan. The burners are capable of firing Associated Gas, Diesel Oil safely and efficiently. Flue gases generated from combustion pass through the convection bank and are led to economizer through flue gas duct and finally into the atmosphere through steel stack. Two burners have been provided on the furnace front wall in two elevations for burning fuel oil. The starting, stopping and safe shutdown of the burner are monitored by a PLC based Burner Management System (BMS). DCS based controllers are provided by contractor for the control loops. The operation of the boiler is envisaged from the DCS operator stations. Three long-retractable & three rotary soot blowers are provided in the super heaters, boiler bank and Economizer surfaces, when the burners are in service. Soot blowing is done to keep up the heat transfer efficiency at the maximum level. Safety valves have been provided in the Drum and in the main steam line of the Boiler. Suitable

insulation around the drum, the membrane panels, steam lines, feed water lines, hot air and flue ducts have been provided to minimize heat loss and for Operators safety. Pressure, Temperature, Level transmitters, conductivity, pH, oxygen analyzers etc., for local indication as well as input to the DCS has been provided. Microprocessor based automatic (as well as manual) control of Drum level, combustion, steam temperature etc., has been provided. Operation of the boiler as well as Data Acquisition has been envisaged through the DCS.

A boiler’s function can be simply understood as mentioned below

“It is a process of conversion of water to steam by burning a fuel inside the boiler burner to produce fire. The produced steam is thus used to rotate a turbine.”

For a fire to take place we need fuel, heat and oxygen as per fire triangle.

The fuel trains (1st section) supplies a fuel like diesel, methane gas or biogas etc.

The air and flue gas (2nd section) gives the air supply for the fire.

Burner (3rd section) has an igniter which acts as heat for the fire to take place.

The fire in this burner is maintained for the required time and is used to convert the water in boiler to dry steam. Thus high temperature steam is produced in the steam and water section (4th section).

This high temperature steam is passed to specially designed High Pressure, Intermediate Pressure and Low Pressure blades of the turbine and rotates the turbine (5th section) which produces electricity.

The final dosing section is used to inject different chemical for removing corrosion, air from water etc for chemical treatment of different section.

The boiler sections can be classified as follows:

1.FUEL TRAINS SECTION

2.AIR AND FLUE GAS SECTION

3.BURNER SECTION

4.STEAM AND WATER SECTION

5.TURBINE SECTION

6.DOSING SECTION

“FAB-STD”

1.FUEL TRAINS SECTION

This sub-section describes the burners installed in the boiler and also the fuel lines with their valves and instruments for burner light up, monitoring, control and shut down.

 

(supply, piping, BMS Local panel, operational control)

a.Fuel Gas, Oil & Pilot Gas Supply Pipe to the Burner

b.Burner Front Piping

c.Burner Management System (BMS)

d.Local Burner Panel

e.Operational Control.

 

a.Fuel Gas, Oil & Pilot Gas Supply Pipe to the Burner

The gas is obtained from the plant gas VRU unit or supplied from outside govt organisation or gas vendors. Fuel gas flow control station is used to control the fuel gas. The fuel gas flow control station consists of a pneumatic operated spring opposed globe type control valve with isolation valves. A bypass self acting pressure control completes the flow control station. The bypass control valve is for maintaining the minimum pressure required for start-up of the burner. The start-up pressure control signal is obtained from pressure transmitter.

 

Fuel oil supply

 

In case the fuel gas is found to be insufficient or gas station encounters a problem, there will be a diesel oil tank which acts as a spare to supply the necessary fuel to the burner. Fuel oil is supplied using a flow oil control station: The fuel oil flow control station consists of a pneumatic operated spring opposed globe type control valve. The control valve is provided with isolation valves. A bypass self acting pressure control valve completes the flow control station. The bypass control valve is for maintaining the minimum pressure required for start-up of the burner. The start-up pressure control signal is obtained from pressure transmitter. 

 Pilot Gas Supply 

Pilot gas for the igniter torch is taken from the natural gas supply piping during initial start up of burner ignition using diesel oil (fuel oil supply specified above). An additional NRV is also provided in Pilot line in tandem with main fuel line connection tapping in order to isolate the pilot line when main fuel shall be used as pilot fuel instead of LPG

Atomizing Steam and Air Supply

 Atomizing steam is obtained from the steam drum (4750 kPa (g) through an isolating valve which provides the air supply required for the burning of fuel. 

b.Burner Front Piping 

The burner front piping involves the piping that connects the fuel oil, pilot gas and combustion air to the burner. The major components include the safety shut off valves, damper. They are described below.

The fuel gas burner front piping is provided with the following

  • Isolation valve

  • Block & bleed arrangement for burner safety shut off valves

  • Two-safety block shut off valves are provided with a vent shut off valve in between them. This arrangement of two shutoff valves and an intermediate vent valve is called a block & bleed arrangement. All the shut off valves are connected to the BMS and interlocked for safe operation of the burner.

  • Pressure indicator for local indication of the fuel gas pressure.

  • Flexible hose connects the front piping to the burner.

 

The Diesel Oil burner front piping is provided with the following –

  • Isolation valve

  • One shut off valve cum ramp up valve is provided. This valve is connected to the BMS and interlocked for safe operation of the burner. This valve is having the BMS interlock with a function block for ramp up of burner for smooth light up.

  • A manual three way valve is provided after shut off valve

from which two lines are connected to main and auxiliary gun of burner. A three way valve is having limit switches to indicate the open position in BMS whether main gun or auxiliary gun is in line for running burner.

  • The limit switches for main gun and auxiliary gun are provided to indicate the inserted position of main gun and auxiliary gun respectively.

  • An interlock for main and aux. gun is provided in such a way that, for light up of burner with recommended main gun, a three way valve limit switch , main gun inserted limit switch and atomizing media three way valve limit switch feedback should be available.

  • The auxiliary gun can be fired in the event of chocking of main gun by ensuring auxiliary gun at inserted position with

availability of atomizing media at auxiliary gun and by placing the three way control valve towards auxiliary gun.

  • Pressure indicator for local indication of the fuel gas pressure.

  • Flexible hose connects the front piping to the burner.

 The Atomizing Steam burner front piping is provided with the following –

  • Isolation Valve

  • A Shut off valve with limit switches and interlocks with BMS.

  • A minimum flow orifice as bypass.

  • A NRV to restrict the back flow.

  • A three way valve with limit switches to indicate the position (Main gun or Aux gun is provided).

  • Pressure gauges for Main gun and Auxiliary gun are provided.

  • Flexible hose connects the front piping to the burner.

  • A tapping is routed to main gun and auxiliary gun from first isolation valve for gun clearing purpose with isolation valves.

 The pilot gas front piping is provided with the following –

  • Isolation valve

  • Block & bleed arrangement for pilot gas burner safety shut off valves – Two-safety block shut off valves are provided with a bleed shut off valve in between them.

This arrangement is similar to that of the main burner. All the shut off valves are connected to the BMS and interlocked for safe operation of the pilot burner.

  • Pressure indicator for local indication of the pilot gas pressure

  • Flexible hose connects the front piping to the burner

The burner is provided with a damper on its combustion air side. This is used for isolating the burner air side during non-operation of an individual burner. The damper is provided with a position feedback arrangement on it

Open and close positions and interlocked to the BMS.

1

figure- boiler fuel supply

c.Burner Management System (B.M.S) 

The Burner Management System (B.M.S) is a programmable logic controller (PLC) that controls the permissive for burner start, stop and trip functions. The PLC acquires the status from the field instruments. The PLC and its hardware are installed in independent PLC panel. The PLC system consists of a CPU, power supply unit, I/O modules & communication modules. The BMS PLC is provided for the burner safety interlocks/ monitoring of related shutoff valves & alarm generation. The burners can be started / stopped either from local burner panel Installed on the operating platform near the burners or from the DCS operating station placed in control room. The local burner panel is provided with necessary lamps and push buttons for operation & Indications. PLC is provided with necessary software and hardware for communicating with DCS. 

d.Local Burner Panel 

The following indicating lamps / push buttons are provided on the local panel.

Emergency stop

Lamp test

Furnace purge start

Diesel burner start

Gas burner start

Diesel burner stop

Gas burner stop

Reset

Main interlock satisfied

Pre interlock satisfied

Diesel ready to start

Gas ready to start

Pilot on

Diesel firing on

Gas firing on

Furnace purge required

Furnace purge ready to start

Furnace purge running

Furnace purge finished

FD fan running.

e.Operational Control

A brief overview of operational control points is

described below. The under mentioned factors contribute towards good combustion.

Correct fuel gas to combustion air ratio. The combustion control must be fine tuned not only during commissioning but also once every six months, by checking the flame and analyzing the flue gas at economizer exit for O2, CO & CO2.

The local operator must be encouraged to view the flame in the boiler two or three times in a shift and report for any irregularity. As the spark device, gas nozzle and UV flame scanners must be maintained in good condition by following the vendor instructions. The scanner cooling air must be continued at least for eight hours after a boiler shut down to shield the scanner from the boiler heat.For the safety of the equipment (and personnel) protections envisaged in BMS must not be by-passed. Any malfunction of protections or nuisance trips must be analyzed and corrective actions initiated.

Possible fire risks in handling pressurized and heated fuel gas must be recognized. To avoid accidents the following are suggested –

  • Fuel gas lines and valves must be accessible.

  • Gas leakage, if any, noticed must be promptly attended.

  • Portable fire fighting equipment in working condition should be readily available at an accessible location.

2.AIR AND FLUE

The air and flue gas system covered in this sub-section describes the supply of combustion air to the boiler and forcing of the flue gas through the boiler to the stack. The components that form part of the system are

a.FD fan

 

b.Air duct

 

c.Wind box

 

d.Furnace

 

e.Flue ducts

 

f.Chimney

 

a.FD fan

The FD fan supplies combustion air to the burners. FD fan is of centrifugal type, with aerofoil backward curved blades. FD fan is equipped with motor drive, driven by an electrical motor.

 b.Air duct 

The FD fan is provided with a suction duct for sucking the combustion air from the plant environment. FD fan Discharge Duct connects the FD fan with wind box 

c.Wind box 

The air duct terminates in the wind box. Burners are accommodated in the wind box. Wind box facilitates proper air distribution around the burners. The other functions of wind box are

Combustion Air Flow Control

Oxygen Trimming

Excess Air Adjustment 

d.Furnace 

The construction of the furnace is such that a totally gas sealed enclosure suitable for forced draft operation is available. The furnace floor tubes are covered with refractory bricks to maintain natural circulation of water inside the tubes. The base frame supports the entire boiler. The base frame anchored to the concrete foundation. The base frame anchors the water drum at front side so that during operation the Boiler is allowed to expand vertically upwards and on rear side.

e.Flue ducts 

Flue gases which are generated after combustion of fuel in the furnace; pass through the convection bank tubes in the second pass to the economizer and then to stack. Flue ducts are provided to connect the boiler to the economizer and the economizer to the stack.

f.Chimney 

 The stack exhausts the flue gases from the economizer to the atmosphere. The stack is a cylindrical steel chimney fabricated out of carbon steel plates.

2

figure-boiler air and flue gas system.

 

3.BURNER

 

The main parts of burner are

a.Front plate

b.Air Register assembly

c.Sleeve

d. Swirler

e.Gas chamber and spuds

f.Fuel oil atomizer (gun) assembly

g.Igniter

h.Flame scanners

i.peep hole / view ports

34567figure-burner details

 

a.Front plate 

The front plate forms the structural attachment basis of the burner components. Front plate is mounted directly to the windbox front face by frames with series of equi-spaced bolts along four sides of the front plate. 

b.Air Register assembly 

The air register consists of a series of vanes arranged circumferentially in the air passage from the wind box to the oil gun, axially surrounding the impeller of the gun. The air register vanes provide a (rotary) swirling motion to the combustion air, which it imparts to the mist of atomized fuel oil being sprayed by the oil gun, enabling the air and the oil mist to mix intimately for quick and complete combustion. Air registers are not to be used for control of air quantity to the burners. (Air quantity adjustments are only by varying the FD fan output). 

c.Sleeve

Burner sleeve makes the air distribution and balancing across the cross section of the burner. The design assures equal air distribution and acceleration of air to the required velocity for complete mixing with fuel. Air from windbox enters into the burner through para flow damper, is then accelerated to a velocity in multiple of the windbox velocity and also diverted axially into the burner. The converging cone of sleeve further accelerates the air and also distributes it equally across the cross section.

d. Swirler

 Air passing through the swirler is referred to as primary air, which gets further accelerated due to the pressure drop across swirler. The primary air is imparted with a swirl, which is a function of the swirler blade angle. This additional acceleration and swirling gives the air the strength to penetrate and mix with the fuel. The air distributed around the swirler is termed as secondary air. The secondary air provides the oxygen for complete combustion process. 

e.Gas chamber and spuds 

Fuel gas is introduced through eight equally spaced gas spuds located in the secondary air annulus. Gas for the spuds is introduced through the gas chamber mounted on the burner front plate. The gas spuds are drilled with injection orifices, properly sized to provide the correct velocity to the exiting gas. In addition, the spuds are angled such that the gas is directed for improved mixing with air to give proper flame shape and minimum emissions.

f.Fuel oil atomizer (gun) assembly

The gun uses the energy of expanding steam to atomize the fuel oil, that is to divide the oil into very fine particles for spray as a mist, for complete mixing with air for easy combustion. The fuel oil gun is composed of two concentric pipes to lead steam and oil up to the atomizer. Atomizer comprises of mixing chamber and sprayer cap. Fuel Oil flows through the inner pipe and reaches to the atomizer at the end. Steam/Air, which flows outside the inner tube, enters the atomizer through tangential slots in the mixing chamber of the atomizer, mixes with oil and atomizes it.

 g.Igniter 

Igniter is a gas (pilot gas/LPG) fired lighter with an electric sparking device, which produces a pilot flame to light the fuel air mixture of the main fuel. Establishing the pilot flame and sensing it by a scanner is a pre-requisite for admission of main fuel to the burner. The Igniter consists of a gas nozzle to admit pilot gas to a combustion chamber where the gas mixes with the combustion air. The gas nozzle is connected to the pilot gas supply line. A spark device is an electrode with 3-mm gap fitted on an insulator. The electrode is connected to a High – tension transformer in the burner local control box through an H.T. cable. The sequence of operations for placing in service the igniter of a burner is controlled by BMS.

 h.Flame scanners.

Each Burner is provided with two flame scanners (One Ultra Violet (UV) flame scanner & another one is Infrared flame scanner) in the scanner view pipes for continuous monitoring of the flame and provide an input to BMS. The ‘detector’/ UV cell of the scanner is a sealed, gas filled, ultra-violet transmitting envelope containing two  electrodes, connected to an A.C. voltage. The detector is aligned to view the flame whereas IR flame scanner is based on photon transistor. When the flame is there the UV rays/radiation striking the electrodes makes the gas between the electrodes conductive and a current flow from one electrode to the other in the form of pulses. Number of pulses emitted per second are characteristic of the flame and differs from other radiation (for example from the hot refractories) striking the scanner. The pulses are amplified and compared with a pre-set value to detect the presence of flame.

 In general IR flame scanner justify presence of oil flame as it is having more infrared spectrum in its flame front whereas UV flame scanner justify the gas flame.

 i.Peep hole / view ports

The Burners are provided with peepholes with an inner glass shield and a tiltable outer cover on burner front plate to monitor the flame by operator during burner start up / running. The burner peepholes are used to the view the flame for carrying out adjustments

Fassgfifp

 

4.STEAM FROM WATER

a. Deaerator

b.Boiler feed water pumps and control station

c.Economizer

d.Boiler assembly

e.Super Heater

f. Attemperator

g.Main steam piping

h.Operational Control

i.Sample System.

A brief explanation is below

a. Deaerator

Deaeration removes the corrosive gases such as dissolved oxygen and free carbon dioxide from the boiler feed water. This ensures protection of the feed water lines, steam lines, boiler tubes and other pressure parts of the boiler against corrosion and pitting, saves costly boiler re-tubing and expensive plant shutdowns. Further as the temperature of feed water is raised from ambient to Deaerator operating temperature of 130 °C [which corresponds to the operating pressure of 169 kPa (g) ] and then fed to boilers, the overall boiler thermal efficiency also increases. Deaeration is done by heating the feed water with steam. Vigorously scrubbing the water with this steam removes the last traces of non-condensable gases and brings down well below the recommended level in feed water.

8

figure-boiler deaerator

b.Boiler feed water pump and control station 

These pumps are connected from the deaerator outlet piping, providing necessary suction to the pump at exit of de-aerator. This ensures that during the operation of the pump there will always be a minimum flow across the pump even when there is no discharge into the boiler.

Feed water is supplied continuously to maintain normal water level in the steam drum. The Feed water Control station installed in the feed water line modulates the water flow to maintain the level in steam drum. Water flows from the flow control station through the economizer before entering into steam drum.

 c.Economizer

A continuous finned tube economizer is located on the flue gas duct from the boiler to recover economically feasible heat from the flue gas before discharging to the atmosphere. The recovered heat increases the temperature of feed water entering the steam drum. The direction of feed water flow (inside the tubes) and flue gas (outside the tubes) are opposed cross flow for optimum heat transfer. Flue gas flows vertically through the Economizer. During operation, feed water from control station outlet flows to the economizer inlet header and through the coils absorbing heat from the flue gas. Then it flows into the steam drum from the outlet header.

 d.Boiler assembly. 

The boiler is of bi-drum construction. Upper drum of the boiler is the steam drum and the lower drum is the water drum All the boiler tubes are connected to these two drums except the rear wall panel tubes, which are connected to the steam drum through riser pipes and through down comers to the water drum. The tubes enter the drums radially and have been expanded mechanically inside. The tubes support the steam drum, whereas water drum rests on the saddle supports. The boiler is divided into a furnace section & a second pass by a baffle wall made of membrane panel. The flue gases that are generated from combustion in the furnace, pass to the second pass through screen tubes at the rear side. Heat transfer takes place in furnace by radiation and in second pass by convection. Furnace is formed with the furnace side, roof & floor wall made out of single membrane panel and at rear it has flat studded wall construction. In the second pass, the superheater and the convection bank tubes are located. In between the baffle wall and the studded boiler sidewall the superheater and convection bank are accommodated. Combustion products (flue gas) from the furnace pass over the superheater and then to the convection bank tubes. The flue gas exits the boiler at the front of the boiler sidewall. Steam drum receives feedwater from the economizer. A perforated feed pipe inside the steam drum distributes feedwater across the length. Water flows to the water drum through portion of the convection bank tubes, which are not baffled at steam drum from the water level, acting as down comer tubes. Boiler tubes absorb the heat from combustion and convert water into water-steam mixture and this conversion keep the tubes cool, within their safe permissible operating temperature. Steam drum receives steam-water mixture from the furnace tubes, rear wall tubes and a portion of the convection bank tubes after evaporation. This mixture is routed to the cyclone separators through the baffles inside the steam drum. The mixture flows tangentially through the cyclone separators. While flowing through the cyclones, water that is heavier gets separated from steam and tickle down to mix with the water in the steam drum. Steam rises upward to flow through the scrubbers provided at the top of the steam drum. Scrubbers provide a tortuous path to the Steam and during this passage; last traces of water are stripped out from steam. Saturated dry steam collects at the saturation chamber above the scrubber and then passes to the superheater through the steam supply pipes to increase the temperature of steam.

9

figure-boiler steam water production

e.Super Heater 

Saturated steam from the steam drum flows to the superheater assembly to increase the steam temperature. Saturated steam from the steam drum internals flows through the supply pipes to the superheater primary header, from where it is distributed to other superheater tubes. Superheater tubes are placed in between the Superheater primary (inlet) and outlet headers. The headers are baffled to form multiple passes for effective heat pickup to attain the desired steam temperature. In order to maintain the final steam temperature a spray type attemperator is installed in-between the passes. Superheater is placed at the flue gas entry of Boiler second pass. Continuous and sufficient flow of the steam through the Superheater ensures the metal temperature of the coils does not exceed the design value. Superheater headers are provided with drains, which are connected to the blowdown tank through the drain header. Superheater has to be drained before starting the boiler and after shutdown to remove the condensate. Superheated steam from superheater final header flows to the main steam line.

9a

figure-superheater boiler steam water section

f. Attemperator 

An inter-stage attemperator is provided in the superheater to maintain the final steam temperature. Spraying a controlled quantity of feedwater into the superheated team lowers its temperature as it loses some heat in vaporizing the sprayed water. The attemperator is a header, which accommodates an inner sleeve shaped like a venturi. A spray nozzle is fixed at the entrance to the restricted venturi section. The sleeve is held in position firmly by the locating pins welded to the header at the steam entry side.

The sleeve is free to expand at the steam exit side. Water is sprayed through the spray nozzle. The steam passes through the venturi picks up the spray, which completes the evaporation and thoroughly mixes the steam. The connection of the inlet to the spray nozzle embodies a thermal sleeve construction to protect the steam line from temperature differential between the spray water and the steam. A drain connection is provided at the exit of the attemperator. 

g.Main steam piping 

The main steam line connects the steam outlet from super heater to the plant steam main at the terminal point main steam stop valve.

 h.Operational Control 

A brief overview of operational control points is described below

Steam drum control

  • Maintain feedwater, boiler water quality, and chemical concentration as prescribed.

  • Maintain water level in the steam drum within permissible low and high levels. The protection system envisages boiler trip at very low levels, which should not be bypassed.

 i.Sample System

The sample system includes the sample cooler and the associated analyzers and the cooling water sub-system There are four sample coolers provided in the boiler to get Saturation steam, boiler feed water, drum sample and main steam sample. One sample cooler is provided for drum water sample emerging out from the continuous blow down line. Sample cooler two is provided for the superheated steam sample from main steam line. Sample cooler three is provided saturation steam before steam entering in to superheater. Sample cooler four is provided to feed water line sample water before entering in to drum

 5.TURBINE ROTATION

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. It modern protocol was invented by Charles Parsons in 1884.

Principle of operation is

“Steam at high pressure and temperature expands through nozzles forming high velocity jets”

9b

figure-steam nozzle

Many such nozzles are mounted on inner wall of cylinder or stator casing. The rotor of the turbine has blades fitted around in circular array. Steam jet from static nozzles impinges and imparts its momentum on to rotor blades

This makes the rotor to rotate. A set of one array of stator and rotor blade is called a ‘stage’. Number of stages is arranged one after another and thus thermodynamic energy is converted into kinetic energy. Number of rows of static and rotating blades (stages) produces the requisite torque.

9c

figure-turbine flow diagram

Major Constructional features of a Turbine include:

 a.Cylinders (HP, IP, LP) and their assembly

b.Steam admission valves and interconnecting pipes

c.Gland Sealing Arrangements

d.Bearings Thrusts and their lubrication system

e.Expansion, locking and guide

f.Barring arrangements

g.Governing system.

a.Cylinders (HP, IP, LP) and their assembly. 

Large steam turbines have many number of stages very high pressure & temperature of inlet steam, large volume expansion of steam. Therefore instead of single turbine cylinder, multiple turbines are used. Steam flow cascade from one turbine cylinder to another arranged in series &/or parallel. Depending on inlet pressure of these multiple turbine cylinders, they are termed as HP (High pressure), IP (Intermediate pressure), and LP (Low pressure).

9d9efigure- turbine sections

Above is a sectional view of turbine. The HP, IP and LP turbines are as shown below

9f9g9h

figure: hp-ip-lp turbine

The rotor Turbine rotors are machined out of a single steel forging.Grooves are similarly machined to mount rotor blades

 Rotors are additionally featured with dummy piston, gland, bearing journal and coupling flange.

9i9j

 

figure turbine rotor.

b.Steam admission valves and interconnecting pipes

Pipes carrying steam from the boiler arrives at the “steam chest” near to the turbine. Steam Chest is a thick walled casing divided into compartments that house the emergency stop valve and governor valves. Steam chests and are also fitted with strainers to trap objects that may be swiped by steam into the turbine. Outlet pipes from Governor Valves enter into the turbine emergency stop valve and Governor valves are powered to open hydraulically against spring force. The hydraulic systems can be of low pressure or high pressure types. Hydraulic pressure derived from pumping arrangement of lubricating oil is usually low pressure type of system. High pressure systems have separate pumping units independent of lubricating system and can use special fire resistant fluid instead of hydraulic oil. High pressure systems are lesser in size and faster in action compared to low pressure version. Outlet pipes from Governor valves enter into the turbine

Entry of steam into the turbine can be controlled by two basic means

A) Throttle Control

B) Nozzle Control

Schematic arrangements of throttle and nozzle governing are shown in Figure 1 & 2 shown below.

9k

fig-throttle nozzle governing

 

c.Gland Sealing Arrangements 

Either ends of rotor shaft that come out from stator need such seals. Both the stator and rotor have typical arrangements known as ‘glands’. Conditioned Steam provides excellent sealing medium in modern turbines, while some old turbines use this in combination to carbon glands. The glands of HP IP & LP differ from one another in construction.

 d.Bearings Thrusts and their lubrication system. 

Comprises of pumps that pressurize oil & supply to bearing Shaft bearing

– Return oil from the bearings is collected back to the sump

– Usually the pumping system includes, pressure control feature, filters and coolers.

Each of the turbine shaft is supported on either ends by journal bearings and are forced lubricated by low viscous oil

  • The oil is filtered to remove solid particles and centrifuged to remove any water

  • In turbines where two shafts are rigidly coupled the common coupled end can be supported by single bearing

  • A journal bearing consists of two half-cylinders that enclose the shaft

  • They are internally lined with Babbitt, a metal alloy usually consisting of tin, copper and antimony.

9m

figure-bearing lubricating system.

e.Expansion, locking and guide 

  • Heating and cooling takes place in a turbine during start-up, shutdown and load changes

  • Relative expansion and contraction also take place between various parts.

  • Guides and keys are designed to accommodate the movement of cylinder, shaft due to thermal changes

  • While doing so the relative alignment has to be maintained

  • f.Barring arrangements 

Once a running turbine is shut down, the shaft speed starts to “coast down” and it comes to a complete stop. Heat inside a turbine at stationary condition concentrate in the top half of the casing. Hence the top half portion of the shaft becomes hotter than the bottom half. If the turbine shaft is allowed to remain in one position for a long time it tends to deflect or bend. The shaft warps or bends only by millionths of inches, detectable by eccentricity meters. But this small amount of shaft deflection would be enough to cause vibrations and damage the entire steam turbine unit when it is restarted. Therefore, the shaft is not permitted to come to a complete stop by a mechanism known as “turning gear” or “barring gear” that takes over to rotate the shaft at a preset low speed. The barring gear must be kept in service until the temperatures of the casings and bearings are sufficiently low. There are various drive systems to achieve this slow rotation, powered by electric or hydraulic motors.

 g.Governing system.

Governors are associated with all Turbo Generators to control the working fluid entering the prime-mover. The fluid can vary from water (for hydel plants) to steam (for thermal) or fuel (for Gas Turbine).The basic control elements are valves capable of controlling the required quantum accurately. Functioning of governor is basically to interpret signals and power of the valves accordingly. The mechanism has developed from very simple arrangement to a complicated system to meet ever growing needs.

There can be two situations envisaged in understanding the basics of Governors.

a) Single Machine Operation: Under this condition one turbo-generator supplies to a group of load connected to it

b) Parallel operation of Turbo-generators: In this case two or more parallel connected turbo-generator supplies a common Load.

9l

figure-turbine operation type

SINGLE TG OPERATION

A Turbo Generator operating under a steady load condition is assumed to possess:

– Fixed speed (say synchronous speed of Alternator)

– Fixed governor valve opening that admits exact amount of fluid to produce the connected power.

– The state remains in equilibrium if not disturbed

If the equilibrium is disturbed by increment of load from the previous state then:

– The TG will try to meet the excess load demand from its storage of rotational kinetic energy

– The speed of turbine will therefore decelerate.

 Similarly if there is a decrease of connected load then

– The TG will have additional prime moving power to add up to its storage of rotational kinetic energy

– The net effect will be acceleration of TG speed

Either of these situations is unstable and not desirable for the machine and its connected load. Governors are intended to keep the speed of TG at a steady level by regulating the flow into the prime-mover under the aforesaid conditions.

A simple arrangement of valve control is shown in the figure below

In response to falling speed (i.e. higher load) the fly-balls drop down that opens valve via lever arm to admit more fluid. The additional fluid flow copes with increased demand and retains speed (Vice-verse happens under rising speed).The basic components that accomplish the process are:

a.Speed variation sensor (Fly ball)

b.Control function (Linear movement in proportion to change in ‘rpm’)

c.Actuator drive (Lever arm that powers the movement of valve)

d. Final control element (The valve that regulate flow).

9nfigure-simple governor

From this simple device it is evident that corresponding to every position of fly-ball the control valve has a definite opening.

The two aspects in the process can be interpreted as 

– The position of fly-ball to represent speed of TG

– The opening of control valve to represent steam flow & TG loading

Relationship that exists between these two parameters, known as “Governor Characteristic” is shown in the plot.

9ograph

The regulation characteristic is drooping in nature (i.e. the valve close with rise in speed) that ensures a stable operating point. Every governor has its intrinsic droop character that decides the extent of speed change that can bring about a 100% load change. This is usually expressed as change of frequency as the percentage of rated frequency.

[The co-ordinates characteristic can also be otherwise, viz, speed (x) vs. load (y)]

9p

graph-load vs. rpm

Governing of modern high capacity steam turbines are scaled up version of the simple governing system, but principles are same:

-Speed sensors pick up signals from: Fly balls, Oil pressures, Voltage, Digital Pulses (etc)

-Control function can be obtained from: mechanical or hydraulic arrangements, analog or digital computers

-Actuator drive is mostly: hydraulic drives with mechanical or electronic coordination

-Final control element constitute of steam admission valves.

-Mechanical arrangements has been reliably used to detect speed

-‘Speed’ can be translated into hydraulic signal (relay oil pressure) to control governor valve opening and load thereby

-Change of relay oil pressure is small and is incapable of overcoming large forces of spring / ‘steam pressure’ and move the valves

-The pilot plunger and main valve piston acts as a hydraulic amplifier that operate the steam admission valves

-The basic governing is also provided with some additional features such as:

– Over-speed protecting system known as ‘over-speed Governor’ (in the scheme shown centrifugal action on slight ‘off-centric bolt’ operate a plunger at set speed)

– The hydraulic oil is used in various steps as control oil each assigned to a definite purpose

– ‘Trip oil’ is one such that enables governing only when the system oil pressure is available

– Trip plunger both for ‘over-speed’ and ‘manual trip’ need to be reset for development of pressure

9q

figure-over speed governor

  • Speed can also be sensed as oil pressure (H∞N2) developed by an impeller rotating at shaft speed

  • Governors of modern power plants also incorporate various load limiting functions that arises out of process parameters, such as

– Load restriction due to any shortcoming/ de-rating

– Load restriction due to pressure fall

– Load restriction due to vacuum fall

9r

figure-pressure, vacuum, load governor

PARALLEL TG OPERATION 

  • When a number of Turbo Generators run in parallel the notion of speed governing has no relevance individually

  • All the sets collectively respond to rise or fall of frequency by shutting or opening their governor valves respectively

  • The extent of load change take that place in ideal case is in according to the governor characteristic of individual TG sets

  • Turbo-generators connected to an infinite grid have lot more control inputs other than speed.

(Total connected load is very large compared to TG rating)

  • The simple arrangement depicts how a governor picks up after a load-drop due to frequency rise

– With the rise in frequency the fly-balls move out pulling the sleeve back and uncovering the oil port

– As more oil gets drained the relay oil pressure falls, so does the load

– The port can be arranged to move back into the sleeve that will restore the previous opening as well as load

9s

figure-governor parallel TG

6.DOSING

Chemical dosing system consists of chemical dosing tank with two pumps with motorized agitator interconnecting piping, valves and mountings. The complete assembly is mounted on the skid. Chemical dosing system is required to maintain feed & boiler water quality at desirable levels.

 a.Phosphate Dosing System 

During the boiler operation the impurities in the boiler water keep on getting concentrated. If the boiler feed water is hard the concentration of such chemicals may cause formation & deposition of scales on boiler heat transfer surfaces, which is dangerous. The chemicals dosed (tri sodium phosphate-TSP), react and form insoluble compounds, which prevent scale formation and aid in removal of existing scales. 

b.Amine Dosing System (pH Dosing System) 

Amine dosing is required continuously to keep feed water quantity as per recommended parameters. Feed water pH may reduce due to mixing of condensate at deaerator. In order to boost the pH of feed water, 2.5% concentrated solution of cyclohexyl amine to be prepared for continuous dosing to make up water line to de-aerator vapor tank. Dosing pump stroke are set to get required pH. 

c. O2 Scavenging System (LP) 

Removal of dissolved oxygen/gases from boiler feed water is essential. Presence of dissolve gases can cause corrosion and pitting of feed water lines, steam line or condensate lines, boiler tubes and other pressure parts resulting in prematured failure of pressure parts or other expensive plant shut down. Sodium sulphite (catalyzed or non-catalyzed) or hydrazine is to be used for oxygen removal from boiler feed water. The major amount of dissolved oxygen is removed in deaerator by mechanical deaeration. The remaining traces of oxygen are removed by reacting with chemical (sodium sulphite or hydrazine).

9t9u

Indian Penal Code

https://kishorekaruppaswamy.in/2019/02/26/indian-penal-code-ipc/

RTD

https://kishorekaruppaswamy.in/2019/02/27/temperature-transmitter-rtd/

Hook up diagram

https://kishorekaruppaswamy.in/2019/02/27/instrument-installation-hook-up-diagram/

Calibration of Siemens Sipart PS2

https://kishorekaruppaswamy.in/2019/02/27/calibration-of-siemens-sipart-ps2/

Calibration of Rosemount TT

https://kishorekaruppaswamy.in/2019/02/27/calibration-of-temperature-transmitter-zero-trimming/

Control valve sizing equation

https://kishorekaruppaswamy.in/2019/02/27/control-valve-sizing-equations/

Control valve leakage classification

https://kishorekaruppaswamy.in/2019/02/27/control-valve-leakage-classification/

Bently Nevada VMS continued

https://kishorekaruppaswamy.in/2019/02/27/bently-nevada-vms-continued/

Intrinsically safe barrier

https://kishorekaruppaswamy.in/2019/03/07/intrinsically-safe-barriers/

I/P converter

https://kishorekaruppaswamy.in/2019/03/07/i-p-conveter/

Lapping of a control valve

https://kishorekaruppaswamy.in/2019/03/07/lapping-of-a-control-valve/

Limit switch

https://kishorekaruppaswamy.in/2019/03/09/limit-switch/

Dampers

https://kishorekaruppaswamy.in/2019/03/21/dampers/

Hookup diagram continued

https://kishorekaruppaswamy.in/2019/06/06/hook-up-diagrams-continued/

Instrumentation related to a motor driven pump

https://kishorekaruppaswamy.in/2019/06/07/instrumentation-related-to-a-motor-driven-pump/

Flow transmitter DP type

https://kishorekaruppaswamy.in/2019/07/19/flow-transmitter-dp-type/

Typical foundation fieldbus wiring

https://kishorekaruppaswamy.in/2019/08/02/a-typical-foundation-field-bus-wiring-diagram/

Painting procedures

https://kishorekaruppaswamy.in/2019/09/14/painting-procedure/

Instrumentation cable design specification

https://kishorekaruppaswamy.in/2019/09/15/instrumentation-cable-design-specification/

Standard power supply requirements of instruments

https://kishorekaruppaswamy.in/2019/09/15/standard-power-supply-requirements-for-instrumentation-devices/

Acceptable ranges of Instruments

https://kishorekaruppaswamy.in/2019/09/15/acceptable-accuracy-ranges-of-instruments/

Instrumentation general design requirements part 1

https://kishorekaruppaswamy.in/2019/10/09/general-design-requirements-of-instrumentation-part-1/

Instrumentation general design requirements part 2

https://kishorekaruppaswamy.in/2019/10/09/general-design-requirements-of-instrumentation-part-2/

Instrumentation general design requirements part 3

https://kishorekaruppaswamy.in/2019/10/09/general-design-requirements-of-instrumentation-part-3/

Instrumentation general design requirements part 4

https://kishorekaruppaswamy.in/2019/10/09/general-design-requirements-of-instrumentation-part-4/

Instrumentation general design requirements part 5

https://kishorekaruppaswamy.in/2019/10/09/general-design-requirements-of-instrumentation-part-5/

AS-i

https://kishorekaruppaswamy.in/2020/02/22/as-i-actuator-sensor-interface-protocol/

Boiler

https://kishorekaruppaswamy.in/2020/03/03/boiler/

Thermistors

https://kishorekaruppaswamy.in/2020/04/05/thermistors/

Profibus

https://kishorekaruppaswamy.in/2020/04/05/profibus/

3 element control

https://kishorekaruppaswamy.in/2020/04/05/three-element-control-in-a-boiler/

Ultrasonic flow measurement

https://kishorekaruppaswamy.in/2020/04/05/ultrasonic-flow-measurement-working-principle/

Parameter setting of  E+H LT

https://kishorekaruppaswamy.in/2018/04/29/parameter-setting-of-eh-fmp51-gwr-lt/ Siemens LT parameter setting

https://kishorekaruppaswamy.in/2018/04/29/parameter-setting-of-siemens-probe-lu-ultra-sonic-level-transmitter/

Parameter setting of KTek LT

https://kishorekaruppaswamy.in/2018/04/29/parameter-setting-of-ktek-mt-5100-gwr-lt/ Analysis of Pump Vibration

https://kishorekaruppaswamy.in/2018/06/08/analysis-of-motor-pump-vibration/ HART Communication

https://kishorekaruppaswamy.in/2018/06/17/hart-communication/ Instrumentation tube fittings

https://kishorekaruppaswamy.in/2018/03/28/instrumentation-tube-fittings/

Radar LT non contact type

https://kishorekaruppaswamy.in/2018/03/19/radar-non-contact-level-transmitter-working-and-parameters/

Rosemount GWR parameter setting

https://kishorekaruppaswamy.in/2018/03/15/parameter-setting-of-rosemount-gwr-5300-series-lt/

Offset calculation GWR

https://kishorekaruppaswamy.in/2018/02/14/offset-calculation-for-a-guided-wave-radar-level-transmitter/

Temperature transmitter types

https://kishorekaruppaswamy.in/2018/02/13/temperature-transmitter-classification-tree/

Flow transmitter types

https://kishorekaruppaswamy.in/2018/02/13/flow-transmitter-classification-tree/

Level transmitter types

https://kishorekaruppaswamy.in/2018/02/13/level-transmitter-classification-tree/

Pressure  transmitter types

https://kishorekaruppaswamy.in/2018/02/13/pressure-transmitter-classification-tree/

Piping and Instrument diagram

https://kishorekaruppaswamy.in/2018/02/04/pid/

Smoke detector working

https://kishorekaruppaswamy.in/2018/02/02/smoke-detector-working-principle/

Conductivity transmitter working

https://kishorekaruppaswamy.in/2018/02/02/conductivity-transmitters/

Gas detector working

https://kishorekaruppaswamy.in/2018/02/01/gas-detector-working-point-gas-detector/

Flame detector working

https://kishorekaruppaswamy.in/2018/02/01/flame-detector-working-principle/

Units and conversion

https://kishorekaruppaswamy.in/2018/01/05/units-and-its-conversion/

Specific gravity

https://kishorekaruppaswamy.in/2018/01/05/specific-gravity-of-selected-liquidsgases-and-saturated-waterfor-different-temperatures/

Instrumentation Interview questions

https://kishorekaruppaswamy.in/2018/01/03/instrumentation-interview-questions/

Temperature transmitter calibration

https://kishorekaruppaswamy.in/2017/12/20/temperature-transmitter-calibration/

Pressure transmitter calibration

https://kishorekaruppaswamy.in/2017/12/19/pressure-transmitter-calibration/

Magnectostrictive LT calibration

https://kishorekaruppaswamy.in/2017/12/19/magnetostrictive-level-transmitter/

Hazardous area classification

https://kishorekaruppaswamy.in/2017/12/17/hazardous-area-classification/

Excel short keys

https://kishorekaruppaswamy.in/2017/12/16/excel-short-keys/

Digital electronics

https://kishorekaruppaswamy.in/2017/12/14/digital-electronics/

Thermocouple mV conversion chart

https://kishorekaruppaswamy.in/2017/12/14/thermocouple-temperature-mv-conversion-chart/

Foundation field bus

https://kishorekaruppaswamy.in/2017/11/20/foundation-fieldbus/

Rosemount 3051 series transmitter

https://kishorekaruppaswamy.in/2017/11/20/rosemount-3051-series-transmitters/

Hydrogen sulphide

https://kishorekaruppaswamy.in/2017/11/20/hydrogen-sulphide-h2s/

Root cause Analysis

https://kishorekaruppaswamy.in/2017/11/20/root-cause-analysis/

Shore Key Line up

https://kishorekaruppaswamy.in/2017/09/28/how-to-line-up-interface-level-transmitter/

pH Analyser

https://kishorekaruppaswamy.in/2017/09/27/ph-analyzer-working-principle-and-calibration/

Mathematic Formula

https://kishorekaruppaswamy.in/2017/09/24/important-mathematical-formulas/

India

https://kishorekaruppaswamy.in/2017/08/15/india/

Oil in Water Analyser

https://kishorekaruppaswamy.in/2017/08/13/oil-in-water-analyzer/

BSW Analyser

https://kishorekaruppaswamy.in/2017/08/13/bsw-analyzer/

Capacitive Level Transmitter

https://kishorekaruppaswamy.in/2017/08/12/capacitance-type-level-transmitter/

LRV and URV Level Transmitter

https://kishorekaruppaswamy.in/2017/08/11/lrv-and-urv-of-interface-level-transmitter/

Instrument Loop Diagram

https://kishorekaruppaswamy.in/2017/07/28/instrument-loop-diagrams/

Laser Level Transmitter

https://kishorekaruppaswamy.in/2017/05/20/laser-level-transmitters/

Turbine Flow Transmitter

https://kishorekaruppaswamy.in/2017/05/20/turbine-flow-meter/

Zero Suppression and Elevation

https://kishorekaruppaswamy.in/2017/05/18/zero-suppression-and-elevation/

Bently Nevada 3500 VMS

https://kishorekaruppaswamy.in/2017/04/08/bently-nevada-3500-vms/

Control Valve Servicing

https://kishorekaruppaswamy.in/2017/04/08/smp-of-a-control-valve/

Safety Integrity Level

https://kishorekaruppaswamy.in/2017/02/23/safety-integrity-level-sil/

Instrumentation Working Principle

https://kishorekaruppaswamy.in/2017/02/22/instrumentation-working-principle-continued/

Instrumentation Gland Sizes

https://kishorekaruppaswamy.in/2017/02/15/instrument-gland-sizes/

LRV and URV of DP level transmitter

https://kishorekaruppaswamy.in/2017/01/02/lrv-and-urv-determination-for-d-p-type-level-transmitter/

Instrumentation working principle continued

https://kishorekaruppaswamy.in/2017/01/02/instrumenation-working-principle/

Level measurement using Pressure gauge

https://kishorekaruppaswamy.in/2016/06/10/level-measurement-using-pressure-gauge/

Tips and tricks in field Instrumentation

https://kishorekaruppaswamy.in/2016/04/18/tips-and-tricks-in-field-instrumentation/

Data Communication Protocol

https://kishorekaruppaswamy.in/2016/03/10/data-communication-protocols/

Pressure Unit Conversion

https://kishorekaruppaswamy.in/2016/02/19/pressure-unit-conversion/

Calibration of  GWR level transmitter

https://kishorekaruppaswamy.in/2016/02/19/calibrationtheory-and-initialization-of-gwr-level-transmitter-rosemount-5300-and-ktek-5100/

Control Valves

https://kishorekaruppaswamy.in/2016/02/18/control-valves/

ERS Level transmitter Parameters

https://kishorekaruppaswamy.in/2016/02/18/calibration-and-initialization-of-electronic-remote-seal-level-transmitter-rosemount-3051ers/

Calibration of wet leg tube Level transmitter

https://kishorekaruppaswamy.in/2016/01/16/calibration-and-initialization-of-wet-leg-impulse-tube-rosemount-3051cd-series-lt/

Calibration of capillary type Level transmitter

https://kishorekaruppaswamy.in/2016/01/15/calibration-and-initialization-of-rosemount-3051cd-capillary-type-level-transmitter/

mV conversion chart

https://kishorekaruppaswamy.in/2015/02/27/942/

Gulf JOB

https://kishorekaruppaswamy.in/2013/09/18/contact-details-for-gulf-jobs/

Reynolds Number

https://kishorekaruppaswamy.in/2012/11/28/reynolds-number/

Calibration of displacer type LT

https://kishorekaruppaswamy.in/2012/11/16/calibration-procedure-of-displacer-type-level-transmitter/

My profile

https://kishorekaruppaswamy.in/2012/06/20/click-here-then-photo-and-view-profile/

Chemical hazard pictogram

https://kishorekaruppaswamy.in/2012/04/04/chemical-hazard-pictograms/

RTD conversion chart

https://kishorekaruppaswamy.in/2012/02/17/calibration-of-dp-type-transmitter-with-actual-pressure/

Ingress Protection

https://kishorekaruppaswamy.in/2012/01/18/ingress-protection/

To know more about Instrumentation and Control,

https://www.amazon.in/dp/B079TKRY1N?ref_=k4w_oembed_gQxp7c3jhGhlq0&tag=kpembed-20&linkCode=kpd

or

https://kishorekaruppaswamy.in/2012/09/30/basic-instrumentation-and-calibration/

figure