1. INTRODUCTION TO IPGCL & PPCL
Life depends on energy, energy is a source that can neither be created nor destroyed. It merely changes its shape and form, when captured energy generates power.To supply power to the citizens of Delhi, earlier it was Delhi Electric Supply Undertaking(DESU) which was formed in 1958 which was subsequently converted into Dehi Vidyut Board (DVB) wef 24.02.97.DVB was split into 6 companies,viz., BSES Rajdhani Power Limited, BSES Yamuna Power Limited, North Delhi Power Limited, Delhi Transco Limited, Indraprastha Power Generation Company Limited, and Delhi power Company Limited .
To bridge the gap between demand and supply and to have reliable supply to capital city a 330 MW combined cycle gas turbine power project was set up on fast track basis. This plant consist of 2 *104 MW frame 9-E gas turbine units commissioned in 2002-03 and 1 *122 MW steam gas units commissioned in 2003-04. Gas supply has been tied up with GAIL through HBJ pipeline.
BRIEF HISTORY of THE POWER PLANT
A contract was signed with BHEL for installation of 330 MW gas based power plant in the vicinity of 220V,I.P extension. The station is comprised of 2*104MW gas turbine of GT- frame 9E and 1*122MW steam turbine. The waste heat emanating from gas turbine is being utilized to generate 122MW power through steam turbine. The hot gases of 560 degree C with a mass flow of approx. 14000 metric ton per hour is passed through 0.2 nos waste heat recovery boiler to generate steam.
The environmental friendly quality power generation through this station is pumped to 220KV sub station of DELHI Transco limited and entire power is being utilized by citizen of DELHI.
combine cycle plants produced higher energy conversion efficiency than gas or steam alone plants. In PPCL a gas turbine generator generates electricity and waste heat is used to make the steam to make additional electricity via steam turbine, the last step enhances the efficiency of electricity generator. Typically, combine cycle power plants utilizes heat from the gas turbine to generate steam. In combine cycle power plant ,the heat of the gas turbine exhaust is used to generate steam by passing it through heat recovery steam generator(HRSG) with live temperature between 420 degree C and 580 degree C. In case of gas turbine Brayton cycle is used and Rankine cycle is used in case of steam turbine. Electronic mark-5 processor is used in case of both GT-1 and GT-2.
CAPACITY OF GT 1 = 104MW CAPACITY OF GT 2 = 104MW CAPACITY OF STG = 122MW TOTAL CAPACITY= 330MW
The primary fuel for gas turbine is natural gas supplied by GAIL through HBJ pipeline. The gas is received at GAIL terminal installed in the vicinity of power station. The GAIL is committed to supply 1.75 MCMD gas on daily basis. The calorific value of natural gas received for power generation is in the band of 8200-8500 kilocalories.
The secondry fuel of gas turbine is HSD/NAPTHA which is to be used only in the case when no supply of gas is available. De-mineralized water is injected to control NOx while machine is operated in liquid fuel.
Raw water requirement is met through sewage treated water being drawn mainly from DELHI gate sewage treated plant. the DM water requirement for steam generation is met up through sewage treated water by treating this through reverse osmosis (RO) de mineralized process.
2. INTRODUCTION TO GAS TURBINE
GAS TURBINE PLANT
The gas turbine is a common form of heat engine working with a series of processes consisting of compression of air taken from atmosphere, increase of working medium temperature by constant pressure ignition of fuel in combustion chamber, expansion of SI and IC engines in working medium and combustion, but it is like steam turbine in its aspect of the steady flow of the working medium. It was in 1939, Brown Beaver developed the first industrial duty gas turbine. The out put being 4000 KW with open cycle efficiency of 18%. The development in the science of aerodynamics and metallurgy significantly contributed to increased compression and expansion efficiency in the recent years.
At PPCL, the GE-Alsthom make Gas Turbine (Model 9E) has an operating efficiency of 31% and 49% in open cycle and combined cycle mode respectively when natural gas is used as fuel. Today gas turbine unit sizes with output above 250 MW at ISO conditions have been designed and developed. Thus the advances in metallurgical technology have brought with a good competitive edge over conventional steam cycle power plant.
PPCL Gas Turbine Plant
The modern gas turbine plants are commonly available in package form with few functional sub assemblies.
The 9E model GEC-Alsthom package consists of Control compartment
Accessory compartment Turbine compartment Inlet exhaust system Load package
Generator excitation compartment CO2 fire protection unit
Each station component is a factory assembled pre-tested assembly & is housed in all weather & acoustic proof enclosure
Combine cycle power plant integrates two power conversion cycle-Brayton cycle (Gas turbine) and Rankine cycle (Steam turbine) with the principal objective of increasing overall plant efficiency.
Gas turbine plants operate on this cycle in which air is compressed (process 1-2, in P-V diagram of figure-1B). This compressed air is heated in the combustor by burning fuel, where plant of compressed air is used for combustion (process 2-3) and the flue gases produced are allowed to expand in the turbine (process 3-4), which is coupled with the generator. In modern gas turbines the temp. of the exhaust gases is in the range of 500 °C to 550 ° C
The conversion of heat energy to mechanical energy with the aid of steam is based on this thermodynamic cycle.
In its simplest form the cycle works as follows:
The initial stage of working fluid is water, which at a certain temperature is pressurized by a pump (process 3-4) and fed to the boiler, In the boiler the pressurized water is heated at constant pressure (process 4-5-6-1).Superheated steam (generated at point-1) is expanded in the turbine (process1-2),which is coupled with generator. Modern steam power plants have steam temperature in the range of 500°C to 550°C at the inlet of the turbine.
COMBINING TWO CYCLES TO IMPROVE EFFICIENCY
We have seen in the above two cycles that exhaust is at temperature of 500-550 °C and in Rankine cycle heat is required to generate steam at the temperature of 500-550 °C. Therefore gas turbine exhaust heat can be recovered using a waste heat recovery boiler to run a steam turbine on Rankine cycle. If efficiency of gas turbine cycle (when natural gas is used as fuel) is 31% and the efficiency of Rankine cycle is 35%, then over all efficiency comes to 49%. Conventional fossil fuel fired boiler of the steam power plant is replaced with a heat recovery steam generator (HRSG). Exhaust gas from the gas turbine
is led to the HRSG where heat in exhaust gas is utilized to produce steam at desired parameters as required by the steam turbine.
ADVANTAGES OF GAS TURBINE PLANT
Some of the advantages are quite obvious, such as fast operation, minimum site investment. Low installation cost owing to standardization, factory assembly and test. This makes the installation of the station easy and keeps the cost per installed kilowatt low because the package power station is quickly ready to be put in operation. Site implementation includes one simple and robust structure to get unit alignment. Transport: Package concept makes easier shipping, handling, because of its robustness. Low standby cost: fast start up and shut down reduce conventional stand by cost. The power requirements to keep the plant in standby condition are significantly lower than those for other types of prime movers.
Maximum application flexibility: The package plant may be operated either in parallel with existing plants or as a completely isolated station. These units have been used, widely for base, peaking and even emergency service. The station can be equipped with remote control for starting, synchronizing & loading.
Control reliability: the microcomputer based control, with an integrated temperature system (ITS) provides accurate control, quick protection and complete sequential start up & shut down & operation. Maintenance Cost is comparatively low.
3. PPCL POWER PLANT:CONTROL SYSTEM3.1
BRIEF DESCRIPTION OF THE POWER PLANT SYTEM
The basic functional sub assemblies of GT Poer Plant Control system are:
The control compartment contains the equipment needed to provide control indication and protection functions. Arrangement can be made for manual operation or for remote unattended operation. The control compartment is located at central control room with control interface of turbine control panel, generator control panel, batteries and battery charger.
The accessory compartment, contains the mechanical and control elements necessary to allow the gas turbine to be a self, contained operational station. The major components located in the accessory compartment are the lubricating oil system and reservoir, lube oil cooler, starting means, accessory gear fuel system, turbine gauge panel, hydraulic system and atomizing air system, water system, cranking motor exhaust frame blowers (88TK- 1, 88 TK-2.)
The gas turbine has a 17 stage axial compressor. The compressor rotor consists of individual discs for each stage, and is connected by through bolts to the forward and aft stub shafts. The turbine rotor consists of three stages, with one wheel for each bucket stage. The turbine rotor wheels are assembled by through bolts similar to the compressor, and with two spacers, one between the first and second stage wheels, and the other between the second and the third stage wheels.
The entire stator stages utilize precision cast, segmented nozzles, with the 2nd and 3rd stage segments supported from the stationary shrouds. This arrangement removes the hot gas path from direct contact with the turbine shell.
The turbine rotor stages also have precision cast, long shank buckets (air foils on the compressor wheels are called blades, those on turbine wheels are called buckets) and this feature effectively shields the wheel rims and bucket dovetails from the high temperature of the main gas steam. The gas turbine unit and shells are split and flanged horizontally for convenience of disassembly. Compressor discharge air is contained by the discharge casing, combustion wrapper, and turbine shell. The 14 combustion liners are mounted completely inside the combustion wrapper, which eliminates the need for combustion cans.
Inlet and exhaust system
protect the compressor from debris. The inlet arrangements generally comes out from the back of the inlet air house, over the control and accessory compartments, and down to the inlet plenum, which is mounted on the turbine base. The exhaust arrangement includes the ducting, silencing, and necessary expansion joints. The exhaust gases exit from the side to exhaust plenum, which is mounted separately on its own base, and are directed straight out to the exhaust arrangement.
The load package consists of an air-cooled, synchronous generator and associated equipment. The generator also has roof-mounted terminals for out going leads. An air-cooled open ventilation of generator and associated equipments can be used in the load compartment
Fire protection unit
The fire protection system consisting of on base piping, detectors etc. capable of distributing a fire extinguishing agent (CO2, or Halon) in all the compartments of the gas turbine and local control room. The bulk of fire extinguishing agent stage unit is located near gas turbine with one main CO2 skid.
The package plant has been designed to provide maximum operational flexibility and simplicity. The actual operating sequence can be best understood by considering the four basic operating modes: Stand By, Start, Run and Shutdown.
During stand by, each component must be maintained in a state, which allows for immediate start up operation if needed.
All the station components that are affected by low temperature or moisture are fully protected during stand by. The lubricating oil and the control compartment are maintained at a minimum temperature. The batteries are kept fully charged and heated. Turbine
compartment is also maintained hot.
Starting the unit
Start-up can be ordered either remote or from the control compartment. (LCR) The starting sequence is given below: The starting system consists of an induction motor and torque converter coupled to the accessory gear. The staring system is energized and connected to the turbine up to the value from which Turbine becomes self-sustaining. At about 12% normal speed, fuel is injected and ignited. To avoid thermal shocks in hot parts of turbine, the unit is accelerated under acceleration mode after a short Warm-up period. When the turbine becomes self-sustaining, the gas turbine speeding up continues, but the starting system (Cranking motor) is automatically made off at 60% speed.
The operator at either the local or remote station has the option of holding the station at spinning reserve, or loading to a point, or running under maximum load exhaust temperature control. The load can be varied manually over the entire load range.
Upon initiation of a normal shut down signal, either locally or remotely, the following events occur:
1. The generator load is gradually reduced to zero. 2. The generator breaker is opened.
3. The fuel supply is reduced & then is shut off. 4. The gas turbine coasting down to rest.
The starting system components also provide slow speed rotation of the turbine for cool down purposes after shut down. A crank and restart can be initiated at any time below 10% speed & can also be started above 95% speed.
GAS TURBINE EQUIPMENT DATA SUMMARY
Number of compressor stages
Compressor type Axial flow, heavy duty
Inlet guide vanes Modulated
Number of turbine stages Three (3)
Casing splits Horizontal
Nozzles Fixed area
Type Fourteen (14) multiple combustors, reverse flow design
Fuel nozzles One (1) per combustion chamber i.e. (one for gas & one for liquid)
Spark plugs Two-(2) electrode type, spring-injected self-retracting. Flame detectors Four (4),ultra-violet type
Quantity Three (3) Lubrication Pressure lubrication No.1 bearing assembly (Located in inlet casing assembly) Active and inactive thrust and journal, all contained in one assembly Journal Elliptical Active thrust Tilting pad, self-equalizing Inactive thrust Tapered land No.2 bearing assembly (Located in the compressor discharge casing) Elliptical journal No.3 bearing assembly (Located in the exhaust frame) Journal, tilting pad.
Starting device Electrical starting motor 1 MW drive Torque converter Hydraulic with adjustor drive Fuel pump Accessory gear-driven, Continuous out put screw type pump Gas stop ratio & control valve Electro hydraulic servo-control
SPEEDTRONIC MARK IV control system
3.3 COMPRESSOR SECTION
The axial-flow compressor section consists of the compressor rotor and the casing. Included within the compressor casing are inlet guide vanes, the 17 stages of rotor and stator blading, and the exit guide vanes.
In the compressor, air is confined to the space between the rotor and stator blading where it is compressed in stages by a series of alternate rotating (rotor) and stationary (stator) airfoil-shaped blades. The rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters in the following rotor stage at the proper angle. The compressed air exits through the compressor-discharge casing to the combustion chambers. Air is extracted from the compressor for turbine bearing cooling sealing, and for pulsation control during start-up (to avoid surging). Since minimum clearance between rotor and stator provides best performance in a compressor, parts have to be assembled very accurately.
The combustion system is of the reverse flow type with 14 combustion chambers arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles, spark plug ignition system, flame detectors, and crossfire tubes.
Hot gases, generated from burning in combustion chambers, are used to drive the turbine. High-pressure air from the compressor discharge is directed around the transition pieces and into the combustion chambers inlets. This air enters the combustion zone through metering holes for proper fuel combustion and through slots to cool the combustion liner. Fuel is supplied to each combustion chamber through a nozzle designed to disperse and mix the fuel with the proper amount of combustion air Orientation of the combustion around the periphery of the compressor is shown on figure CS-
1.Combustion chambers are numbered counter-clockwise when viewed looking down-stream and starting from the top of the machine. Spark
plug and flame detectors
locations are also shown.
The three-stage turbine section is the area in which the energy in the hot pressurized gas produced by compressor and combustion sections is converted into mechanical energy. The MS 9E major turbine section components include: the turbine rotor, turbine shell, exhaust frame, exhaust diffuser, nozzles and diaphragms, buckets & shrouds, and No.3 (aft) bearing assembly, spacers.
4. STEAM TURBINE4.1
Governing system is an important control system in the power plant as it regulates the turbine speed, power and participates in the grid frequency regulation. For starting, loading governing system is the main operator interface. Steady state and dynamic performance of the power system depends on the power plant response capabilities in which governing system plays a key role. With the development of electro- hydraulic governors, processing capabilities have been enhanced but several adjustable parameters have been provided. A thorough understanding of the governing process is necessary for such adjustment.
In this paper an overview of the steam turbine governing system is given. The role of governing system in frequency control is also discussed.
BASIC GOVERNING SCHEME
Need for governing system
The load on a turbine generating unit does not remain constant and can vary as per consumer requirement. The mismatch between load and generation results in the speed (or frequency) variation. When the load varies, the generation also has to vary to match it to keep the speed constant. This job is done by the governing system. Speed which is an indicator of the generation – load mismatch is used to increase or decrease the generation.
Governing system controls the steam flow to the turbine in response to the control signals like speed error, power error. It can also be configured to respond to pressure error. It is a
closed loop control system in which control action goes on till the power mismatch is reduced to zero.
As shown in the basic scheme given in Fig. 1, the inlet steam flow is controlled by the control valve or the governor valve. It is a regulating valve. The stop valve shown in the figure ahead of control valve is used for protection. It is either closed or open. In emergencies steam flow is stopped by closing this valve by the protective devices.
Grid Reference ST : stream turbine G : generator SV : stop valve CV SV Steam Speed Power GOVERNING SYSTEM N
STCV : control valve
The governing process can be functionally expressed in the form of signal flow block diagram shown in Fig.2. The electronic part output is a voltage or current signal and is converted into a hydraulic pressure or a piston position signal by the electro- hydraulic converter (EHC). Some designs use high pressure servo valves. The control valves are finally operated by hydraulic control valve servo motors.
The steam flow through the control valve is proportional to the valve opening in the operating range. So when valve position changes, turbine steam flow changes and turbine power output also changes proportionally. Thus governing system changes the turbine mechanical power output.
In no load unsynchronized condition, all the power is used to accelerate the rotor only (after meeting rotational losses) and hence the speed changes. The rate of speed change is governed by the inertia of the entire rotor system. In the grid connected condition, only power pumped into the system changes when governing system changes the valve opening.
When the turbine generator unit is being started, governing system controls the speed precisely by regulating the steam flow. Once the unit is synchronized to the power system grid, same control system is used to load the machine. As the connected system has very large inertia (‘infinite bus’), one machine cannot change the frequency of the grid. But it can participate in the power system frequency regulation as part of a group of generators that are used for automatic load frequency control. (ALFC).
As shown in the block diagram, the valve opening changes either by changing the SPEED
+Valve Position SET POINT -Mechanical Power
reference setting or by the change in speed (or frequency). This is called primary regulation. The reference setting can also be changed remotely by power system load frequency control. This is called secondary regulation. Only some generating units in a power system may be used for secondary regulation.
A heat recovery steam generator or HRSG is a heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone. Another application for an HRSG is in diesel engine combined cycle power plants, where hot exhaust from a diesel engine is fed to an HRSG to generate steam which in turn drives a steam turbine. The HRSG is also an important component in cogeneration plants. Cogeneration plants typically have a higher overall efficiency in comparison to a combined cycle plant. This is due to the loss of energy associated with the steam turbine
HRSGs consist of three major components. They are the Evaporator, Super heater, and
Economizer. The different components are put together to meet the operating requirements of the unit. See Modular HRSG GA.
Modular HRSGs can be categorized by number of ways such as direction of exhaust gases flow or number of pressure levels. Based on the flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In horizontal type HRSGs, exhaust gas flows horizontally over vertical tubes whereas in vertical type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum
and steam is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through super heaters to raise the temperature and pressure past the saturation point.
Packaged HRSGs are designed to be shipped as a fully assembled unit from the factory. They can be used in waste heat or turbine (usually under 20MW) applications. The packaged HRSG can have a water cooled furnace which allows for higher supplemental firing and better overall efficiency.
Some HRSGs include supplemental, or duct firing. These additional burners provide additional energy to the HRSG, which produces more steam and hence increases the output of the steam turbine. Generally, duct firing provides electrical output at lower capital cost. It is therefore often utilized for peaking.
HRSGs can also have diverter valves to regulate in the inlet flow into the HRSG. This allows the gas turbine to continue to operate when there is no steam demand or if the HRSG needs to be taken offline.
Emissions controls may also be located in the HRSG. Some may contain a Selective Catalytic Reduction system to reduce nitrogen oxides (a large contributor to the formation of smog and acid rain) and/or a catalyst to remove carbon monoxide. The inclusion of an SCR dramatically effects the layout of the HRSG. NOx catalyst performs
best in temperatures between 650°F and 750°F. This usually means that the evaporator section of the HRSG will have to be split and the SCR placed in between the two sections. Some low temperature NOx catalysts have recently come to market that allows for the SCR to be placed between the Evaporator and Economizer sections (350°F-500°F).
Heat recovery can be used extensively in energy projects.
In the energy-rich Persian Gulf region, the steam from the HRSG is used for desalination
Universities are ideal candidates for HRSG applications. They can use a gas turbine to produce high reliability electricity for campus use. The HRSG can recover the heat from the gas turbine to produce steam/hot water for district heating or cooling.
A deaerator is a device that is widely used for the removal of air and other dissolved
gases from the feed water to steam generating boilers. In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). It also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. Most deaerator’s are designed to remove oxygen down to levels of 7 ppb by weight (0.0005 cm³/L) or less.
There are two basic types of deaerator , the tray-type and the spray-type:
The tray-type (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feed water storage tank.
The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feed water storage tank
With reference to a thermal power station, the word deaerator generally implies not only the deaerator but also the feed water tank below where deaerated water is stored and fed to the suction of boiler feed pumps. The description herein is mainly with reference to its use in thermal power stations. See also feed water heating.
Necessity for Deaeration
Practical considerations demand that in a steam boiler/steam turbine/generator unit the circulating steam, condensate, and feed water should be devoid of dissolved gases, particularly corrosive ones, and dissolved or suspended solids. The gases will give rise to
corrosion of the metal in contact thereby thinning them and causing rupture. The solids will deposit on the heating surfaces giving rise to localized heating and tube ruptures due to overheating. Under some conditions it may give rise to stress corrosion cracking.
Position in the turbine cycle
The diagram shows the construction of a typical deaerator and feed tank of about 250 MW unit.
The above diagram shows the deaerator's position in the turbine cycle diagram.
The actual construction details may vary from manufacturer to manufacturer. It also depends on the size of the unit and their own design to suit the system including the steel structures erection.
This is generally a horizontally mounted cylindrical steel vessel with dished ends and with internal and external fittings. The size of the same depends on the unit capacity it is associated with. The cylindrical vessel portion acts as storage for boiler feed water supplying to the suction of the boiler feed pumps from a pipe connected to the bottom of the tank, generally in the mid portion.
During cold start of the unit, it is possible the water in the feed tank may be cold. At that time the water has to be heated to bring it up to normal operating temperature to expel the dissolved gases. For this, a provision of a heater pipe inside the tank longitudinally and at the bottom level is provided. A few vertical pipes on this line are provided with holes to distribute the heating steam uniformly to avoid water hammer in the initial stages of heating. For this normally a connection from auxiliary steam header is provided, since the
auxiliary steam is available first after startup of the boiler.
A small bore connection with a pipe line to the full length of the feed tank at the bottom is also provided for injection of chemical liquids.
Generally a direct reading gauge glass is provided on each end for absolute level indication.
Since the feed tank is always hot, sufficient insulation covering (known also as lagging) is provided to minimize the heat loss.
At the top and in the mid portion of the feed tank an inverted domed vessel of sufficient size as dictated, is attached which is called the deaerator. This portion has internals something like a perforated tray to breakdown the down flow of condensate water from the top into fine globules to separate dissolved gases. The heating steam, which is fed at the lower level of the dome, passes upwards to give good intermixing. A small vent pipe at the topmost point of this dome is provided for venting out the dissolved gases. Some designs of smaller sizes may have a vent condenser to trap and recover any water particles escaping through this vent.
The deaerator dome therefore has connections for condensate water inlet (at one side of the dome near the top end) from previous LP feed heater and also a connection for the deaerating steam from the bottom of the dome (which also incidentally heats the feed water). This steam is generally from an extraction point of the turbine to improve the cycle efficiency. The deaerator therefore is also termed as one of the feed water heaters in the turbine cycle. Since the deaerator is always hot, sufficient insulation is provided to minimize the heat loss.
The feed tank is mounted horizontally at a sufficient height above boiler feed pump level to give the necessary positive head (NPSH) to the boiler feed pumps under all conditions of the system operation. The mounting arrangement is such that one end of the dished end is able to move or expand due to hot boiler feed water storage where as the other end is
The moving end is supported on steel rollers to give it frictionless movement whereas the other end is bolted to the girder support underneath.
Controls and monitoring
Normally all the control and monitoring equipment for startups, normal operation and alarms for out of parameter operations are provided at the operators' console. Deaerator level and pressure must be controlled by adjusting control valves - the level by regulating condensate flow, and the pressure by regulating steam flow. If operated properly, most deaerator vendors generally guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).
Cooling towers are heat removal devices used to transfer process waste heat to the
atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power plants
and building cooling. The towers vary in size from small roof-top units to very large
hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.
Classification by use
Cooling towers can generally be classified by use into either HVAC (air-conditioning) or industrial duty.
An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.
HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is the removal of 12,000 Btu/hour
(3517 W). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (4396 W) due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10 °F (5.56 °C), which amounts to 15,000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.
Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and other industrial facilities. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute) and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).
require about 100,000 cubic metres an hour and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.
5. SPEEDTRONIC™ MARK V GAS TURBINE CONTROL
The SPEEDTRONIC™ Mark V Gas Turbine Control System is the latest derivative in the highly successful SPEEDTRONIC™ series. Preceding systems were based on automated turbine control, protection and sequencing techniques dating back to the late 1940s, and have grown and developed with the available technology. Implementation of electronic turbine control, protection and sequencing originated with the Mark I system in 1968. The Mark V system is a digital implementation of the turbine automation techniques learned and refined in more than 40 years of successful experience, over 80% of which has been through the use of electronic control technology. The SPEEDTRONIC™ Mark V Gas Turbine Control System employs current state-of-the-art technology, including triple-redundant 16-bit microprocessor controllers, two-out-of-three voting redundancy on critical control and protection parameters and Software-Implemented Fault Tolerance (SIFT). Critical control and protection sensors are triple redundant and voted by all three control processors. System output signals are voted at the contact level for critical solenoids, at the logic level for the remaining contact outputs and at three coil servo valves for analog control signals, thus maximizing both protective and running reliability. An independent protective module provides triple redundant hardwired detection and shutdown on over speed along with detecting flame. This module also synchronizes the turbine generator to the power system. Synchronization is backed up by a check function in the three control processors.
The Mark V Control System is designed to fulfill all gas turbine control requirements. These include control of liquid, gas or both fuels in accordance with the requirements of the speed, load control under part-load conditions, temperature control under maximum capability conditions or during startup conditions. In addition, inlet guide vanes and water or steam injection are controlled to meet emissions and operating requirements. If emissions control uses Dry Low NOx techniques, fuel staging and combustion mode are
controlled by the Mark V system, which also monitors the process. Sequencing of the auxiliaries to allow fully automated startup, shutdown and cool down are also handled by the Mark V Control System. Turbine protection against adverse operating situations and annunciation of abnormal conditions are incorporated into the basic system. The operator interface consists of a color graphic monitor and keyboard to provide feedback regarding current operating conditions. Input commands from the operator are entered using a cursor positioning device. An arm/execute sequence is used to prevent inadvertent turbine operation. Communication between the operator interface and the turbine control is through the Common Data Processor, or <C>, to the three control processors called <R>, <S> and <T>. The operator interface also handles communication functions with remote and external devices. An optional arrangement, using a redundant operator interface, is available for those applications where integrity of the external data link is considered essential to continued plant operations. SIFT technology protects against module failure and propagation of data errors. A panel mounted back-up operator display, directly connected to the control processors, allows continued gas turbine operation in the unlikely event of a failure of the primary operator interface or the <C> module. Built-in diagnostics for troubleshooting purposes are extensive and include “power-up,” background and manually initiated diagnostic routines capable of identifying both control panel and sensor faults. These faults are identified down to the board level for the panel and to the circuit level for the sensor or actuator components. The ability for on-line replacement of boards is built into the panel design and is available for those turbine sensors where physical access and system isolation are feasible. Set points, tuning parameters and control constants are adjustable during operation using a security password system to prevent unauthorized access.
Minor modifications to sequencing and the addition of relatively simple algorithms can be accomplished when the turbine is not operating. They are also protected by a security password. A printer is included in the control system and is connected via the operator interface. The printer is capable of copying any alpha-numeric display shown on the monitor. One of these displays is an operator configurable demand display that can be automatically printed at a selectable interval. It provides an easy means to obtain periodic
and shift logs. The printer automatically logs time-tagged alarms, as well as the clearance of alarms. In addition, the printer will print the historical trip log that is frozen in memory in the unlikely event of a protective trip. The log assists in identifying the cause of a trip for trouble shooting purposes. The statistical measures of reliability and availability for SPEEDTRONIC™ Mark V systems have quickly established the effectiveness of the new control because it builds on the highly successful SPEEDTRONIC™ Mark IV system. Improvements in the new design have been made in microprocessors, I/O capacity, SIFT technology, diagnostics, standardization and operator information, along with continued application flexibility and careful design for maintainability. SPEEDTRONIC™ Mark V control is achieving greater reliability, faster meantime- to repair and improved control system availability than the SPEEDTRONIC™ Mark IV applications. As of May 1994, almost 264 Mark V systems had entered commercial service and system operation has exceeded 1.4 million hours. The established Mark V level of system reliability, including sensors and actuators, exceeds 99.9 percent, and the fleet mean-time-between forced- outages (MTBFO) stands at 28,000 hours. As of May 1994, there were 424 gas turbine Mark V systems and 106 steam turbine Mark V systems shipped or on order.
6. INSTRUMENTS USED IN PPCL6.1 TEMPERATURE SWITCH
A temperature switch is a switch that is responsive to temperature changes. Temperature switches generally are provided with a temperature responsive element which will open or close a switch when a predetermined minimum pressure or temperature is sensed by the responsive element. For protection against thermal overload, semiconductor switches are provided with integrated temperature sensors. The temperature sensors acquire the temperature of the power switch and convert this into a temperature-dependent, analog signal which then can be interpreted in a circuit. Temperature sensitive switches, such as a thermostat, typically comprise a temperature sensor which is used to open or close electrical contacts at specified temperatures. A bimetal strip of dissimilar metals is used as the sensing element for temperature sensitive switches. Temperature sensitive switches are often used for thermal protection purposes. If a device gets too hot, the temperature sensitive switch opens the electrical circuit, thereby eliminating power to the circuit. For example, temperature responsive tip-switches are particularly useful in connection with
Normally there are two temperature sensitive devices used in this plant
a) Resistance thermometer
Resistance thermometers, also called resistance temperature detectors (RTDs), are
temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C
Resistance thermometers are constructed in a number of forms and offer greater stability,
accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance
and require a power source to operate. The resistance ideally varies linearly with temperature.
Resistance thermometers are usually made using platinum, because of its linear resistance-temperature relationship and its chemical inertness. The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications.
Commercial platinum grades are produced which exhibit a change of resistance of 0.385
ohms/°C (European Fundamental Interval) The sensor is usually made to have a resistance of 100Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995) . The American Fundamental Interval is 0.392 Ω/°C, based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field.
Resistance thermometers require a small current to be passed through in order to determine the resistance. This can cause resistive heating, and manufacturers' limits should always be followed along with heat path considerations in design. Care should also be taken to avoid any strains on the resistance thermometer in its application. Lead wire resistance should be considered, and adopting three and four wire connections can eliminate connection lead resistance effects from measurements - industrial practice is almost universally to use 3-wire connection. 4-wire connection need to be used for precise application.
Advantages and limitations
Advantages of platinum resistance thermometers: High accuracy
Wide operating range
RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K), due to the fact that there are very few phonons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.
Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However thermistors have a smaller temperature range and stability.
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor
and can also be used to convert heat into electric power. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; System errors of less than one kelvin (K) can be difficult to achieve.
Any circuit made of dissimilar metals will produce a temperature-related difference of voltage. Themocouples for practical measurement of temperature are made of specific
alloys, which in combination have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges, and to resist corrosion. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires, which are less costly than the materials used to make the sensor. Thermocouples are standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of
cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.
Thermocouples are widely used in science and industry; a few applications would include temperature measurement for kilns, measurement of exhaust temperature of gas turbines
or diesel engines, and many other industrial processes
Principle of operation
In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and can typically be between 1 and 70 micro volt per degree Celsius (µV/°C) for the modern range of available metal combinations. Certain combinations have become popular as industry standards, driven by cost, availability, convenience, melting point, chemical properties, stability, and output. This coupling of two metals gives the thermocouple its name.
Thermocouples measure the temperature difference between two points, not absolute temperature. In traditional applications, one of the junctions—the cold junction—was maintained at a known (reference) temperature, while the other end was attached to a probe.
Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They incorporate into their circuits an artificial cold junction using some other thermally sensitive device, such as a thermistor or diode, to measure the temperature of the input connections at the instrument, with special care being taken to
minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.
Additionally, a device can perform cold junction compensation by computation. It can translate device voltages to temperatures by either of two methods. It can use values from
look-up tables or approximate using polynomial interpolation.
A thermocouple can produce current, which means it can be used to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electric power generated by a thermocouple is a conversion of the heat energy that one must continuously supply to the hot side of the thermocouple to maintain the electric potential. The flow of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the
Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature. The voltages of the individual thermocouples add up, allowing for a larger voltage and increased power output, thus increasing the sensitivity of the instrumentation. With the radioactive decay of transuranic elements providing a heat source this arrangement has been used to power spacecraft on missions too far from the Sun to utilize solar power.
Thermocouples are most suitable for measuring over a large temperature range, up to 1800 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and resistance temperature detectors are more suitable
When to use RTDs or thermocouples
temperature detectors (RTDs) and thermocouples. But when should control engineers use a thermocouple and when should they use an RTD? The answer is usually determined by four factors: Factors: - Temperature, time, size, and overall accuracy requirements.
What are the temperature requirements? If process temperatures fall from -328 to 932°F (-200 to 500°C), then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.
What are the time-response requirements? If the process requires a very fast response to temperature changes--fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec)--then a thermocouple is the best choice. Keep in mind that time response is measured by immersing the sensor in water moving at 3 ft/sec with a 63.2% step change.
What are the size requirements? A standard RTD sheath is 0.125 to 0.25 in. dia., while sheath diameters for thermocouples can be less than 0.062 in. What are the overall requirements for accuracy? If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then an RTD is the only choice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within the first few hours of use.
6.2 PRESSURE SWITCH
A pressure responsive switch senses a change in pressure and responds to such changes by alternately making and breaking an electrical connection. Pressure-sensitive switches are used in a variety of applications where it is desired to switch apparatus on or off at predetermined pressures. These switches are utilized in a wide variety of applications, as in automobiles, aircrafts and in various other environments. Pressure switches include set-point pressure switches that actuate when a specified pressure is reached and pressure measuring switches that are capable of measuring the ambient pressure and reacting accordingly. A pressure responsive switch generally comprises a diaphragm responsive to a pressure change, a rigid ring for securing the diaphragm, and a pair of electrically conductive contacts that break contact based on movement of the diaphragm. Mechanical pressure switches typically provide an output signal in the form of a switch closure in response to application of mechanical or atmospheric pressure. A differential pressure
switch is a device which utilizes differential fluid pressure from low and high pressure sources to actuate an electric switch at a pre-set actuation point. Differential pressure switches are commonly employed to control the operation of snap action switches.
In case of pressure switch used in this plant , a piece of metal is vibrating everytime when no contact is made , indicating switch is open. When there is high air pressure , level of glycerine goes high, contact is made indicating switch is closed, thus circuit is completed. Since PPCL is in auto mode ,at pressure switch transmitter is used which convert the pressure into current, so that we can measure same pressure at control room. At control room w again convert current into pressure.
6.3 VIBRATION SWITCH
A vibration switch is a device that (1) recognizes the amplitude of the vibration to which it is exposed and (2) provides some sort of response when this amplitude exceeds a predetermined threshold value. The switch response is typically an electrical contact closure or contact opening. The electrical contact may be either an electromechanical relay or solid-state triac.
WHY USE A VIBRATION SWITCH?
Vibration switches are primarily used for protecting critical machinery from costly destructive failure by initiating an alarm or shutdown when excessive vibration of the machinery is detected.
Conversely, a vibration switch can be utilized to warn of the absence of vibration, such as when a conveyer ceases to function due to a broken drive belt
6.4 LIMIT SWITCH
Limit Switches & Limit Switch Information: A mechanical limit switch interlocks a mechanical motion or position with an electrical circuit. A good starting point for limit-switch selection is contact arrangement. The most common limit limit-switch is the single-pole contact block with one NO and one NC set of contacts; however, limit switches are available with up to four poles.
Limit switches also are available with time-delayed contact transfer. This type is useful in detecting jams that cause the limit switch to remain actuated beyond a predetermined time interval.
Other limit switch contact arrangements include neutral-position and two-step. Limit switches feature a neutral-position or center-off type transfers one set of contacts with movement of the lever in one direction. Lever movement in the opposite direction transfers the other set of contacts. Limit switches with a two-step arrangement, a small movement of the lever transfers one set of contacts, and further lever movement in the same direction transfers the other set of contacts.
Maintained-contact limit switches require a second definite reset motion. These limit switches are primarily used with reciprocating actuators, or where position memory or manual reset is required. Spring-return limit switches automatically reset when actuating force is removed.
Centrifugal Limit switches: A centrifugal limit switch is actuated by speed only. Simple types of centrifugal limit switches consist of speed-sensing units that mount directly on a rotating shaft and a stationary-contact switch assembly. The basic control element is a conical-spring steel disc that has centrifugal weights fastened to the outer edge of its circular base. Fingers on the spring are attached to an insulating spool that rides free of the shaft and actuates the movable switch contact. As the rotating sensing unit reaches switching speed, the centrifugal force of the calibrated weights overcomes spring force, resulting in an instantaneous axial displacement of the spring and the contact-actuating spool.
The contacts switch at one speed as speed increases from zero to operating speed, and at a lower speed as rotation slows from operating speed toward zero. The spring decreasingly opposes centrifugal force as rotational speed increases from standstill until the snap-over point is reached. Then, spring force adds to centrifugal force to axially snap the spool and actuate the contacts. As rotational speed decreases from operating speed, spring force overcomes the centrifugal force of the weights at a lower speed where snapback begins.
A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier
Basic Design and Operation
Simple electromechanical relay
A simple electromagnetic relay, such as the one taken from a car in the first picture, is an adaptation of an electromagnet. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a moveable iron
armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-enerzised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the Printed Circuit Board (PCB) via the yoke, which is soldered to the PCB.
When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-enerzised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but
gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.
By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an opto-coupler can be used which is a light-emitting diode (LED) coupled with a photo transistor.
Relays are used to and for:
Control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers, Control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile, Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays), Isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,
Logic functions. For example, the boolean AND function is realised by connecting normally open relay contacts in series, the OR function by connecting normally open contacts in parallel. The change-over or Form C contacts perform the XOR (exclusive or)
function. Similar functions for NAND and NOR are accomplished using normally closed contacts. The Ladder programming language is often used for designing relay logic networks.
Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital computers. See ARRA (computer), Harvard Mark II, Zuse Z2, and
Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation, they are widely used in safety-critical logic, such as the control panels of radioactive waste-handling machinery. Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed.
These water valves are operated by handles.
A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or
liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category.
Valves are also found in the human body. For example, there are several which control the flow of blood in the chambers of the heart and maintain the correct pumping action (see heart valve article).
Valves are used in a variety of contexts, including industrial, military, commercial, residential, and transportation.
Oil and gas, power generation, mining, water reticulation, sewerage and chemical manufacturing are the industries in which the majority of valves are used.
Plumbing valves, such as taps for hot and cold water are the most noticeable types of valves. Other valves encountered on a daily basis include gas control valves on cookers and barbecues, small valves fitted to washing machines and dishwashers, and safety devices fitted to hot water systems.
Valves may be operated manually, either by a hand wheel, lever or pedal. Valves may also be automatic, driven by changes in pressure, temperature or flow. These changes may act upon a diaphram or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water systems or steam boilers. More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an
actuator. An actuator will stroke the valve depending on its input and set-up, allowing the valve to be positioned accurately, and allowing control over a variety of requirements. Valves are also found in the Otto cycle (internal combustion) engines driven by a
camshaft, lifters and or push rods where they play a major role in engine cycle control
A huge variety of valves are available, and valves have infinite applications and sizes ranging from .004" (0.1 mm) to 24" (600 mm). Special valves can be manufactured to have a diameter exceeding 200" (5000 mm).
Valves range from inexpensive, simple, disposable valves to components in exotic items that in some instances cost thousands of dollars (US$) per inch (25 mm) of diameter. Disposable valves may be found inside common household items including liquid or gel mini-pump dispensers and aerosol spray cans. rks
Valves may be classified by how they are operated: manual
7.1. DEMINERALIZED WATER
Purified water is water from any source that is physically processed to remove impurities. Distilled water and deionized water have been the most common forms of purified water, but water can also be purified by other processes including reverse osmosis, carbon filtration, microporous filtration, ultrafiltration, ultraviolet oxidation, or electrodialysis. In recent decades, a combination of the above processes have come into use to produce water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt). Purified water has many uses, largely in science and engineering laboratories and industries, and is produced in a range of purities.
METHODS OF PURIFYING WATER
Distilled water is often defined as bottled water that has been produced by a process of distillation and has an electrical conductivity of not more than 10 µS/cm and total dissolved solids of less that 10 mg/L. Distillation involves boiling the water and then condensing the steam into a clean container, leaving most solid contaminants behind. Distillation produces very pure water but also leaves behind a leftover white or yellowish mineral scale on the distillation apparatus, which requires that the apparatus be frequently cleaned. Distillation does not guarantee the absence of bacteria in drinking water; unless the reservoir and/or bottle are sterilized before being filled, and once the bottle has been opened, there is a risk of presence of bacteria.
For many applications, cheaper alternatives such as deionized water are used in place of distilled water.