Final Report.pdf

PROJECT REPORT ON

STEAM TEMPERATURE CONTROL Submitted in partial fulfillment of the requirement for the award of degree of Bachelor of Technology In

( ECE) (Batch: 2010-2014)

KIIT COLLEGE OF ENGINEERING GURGAON

Under the Guidance of: Mr. Amit Kumar TPO Coordinator

Submitted by: Rahul Kumar Patel (53078) Saddam Hussain (53081) Vishal Dung (53099)

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CERTIFICATE

Certified that Mr. Vishal Dung, Saddam Hussain and Rahul Kumar Patel student of B.Tech sem. 8 th ECE batch (2010-14) KIIT College of Engineering have undertaken the project work entitled Steam System Process Control under the supervision of Mr. Rajeev Verma as per requirement stipulated in the course curriculum . The performance of the student has been satisfactory.

Project Guide: Mr. Amit kumar

HOD : Mr. N.K. Aggarwal

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ACKNOWLEDGEMENT

The project work entitled Steam System Process Control was carried out at KIIT College of Engineering, under the guidance of Mr. Rajeev Verma.

We gratefully acknowledge the

guidance of our project Incharge for the successful completion of the project. The support of head of the department, Director, faculty and the laboratory staff in execution of project is gratefully acknowledged.

Vishal Dung (53099) Saddam Hussain (53081) Rahul Kumar Patel (53078)

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ABSTRACT

Boiler steam temperature in thermal power plant is very complex and hard to control, many control strategies have been applied to it. Direct Digital Control (DDC) is adapted to the control of boiler steam temperature in this paper. The nonlinear model of boiler steam temperature is set up firstly, then the principle of the DDC is briefly introduced and the DDC based boiler steam temperature closed loop control system is designed and the closed loop system is studied with simulation. Compared with the traditional PID, more robust and better disturbance rejection properties are obtained. Steam temperature control is one of the most difficult control problems in a gas power plant. Inadequacies in steam temperature control during low load and cycling operation can result in excessive variation in steam temperature. These steam temperature variation can cause thermal-stress cracking, which decreases the life of the boiler and turbine. A dynamic model of a gas fired, 330MW combined cycle power plant was developed to address steam temperature control problem and potential improvement. The model was validated against the transient plant data and then used to test alternative control strategies. The result of these test lead to two major control improvement: The first method utilizes existing controls with the addition of a variable controller set point, providing improved steam temperature control and reduce energy consumption for feed water pumping; and the second method incorporates a predictive, model reference feed forward signal to the at temperature spray valve control, providing nearly optimal steam temperature control during load cycle. An interface between the existing analog controls and a microprocessor-based system is being developed to test the control the improvements on the unit.

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TABLE OF CONTENTS

Sr. No.

Page Number

Content

1.

Brief Company Profile

6-7

2.

Current Running Project

8-10

3.

Overview of the Plant

11-16

4.

Heat Recovery Steam generation

17-29

5.

Steam System Process Control

30-50

6.

Measurement of Steam Temperature

51-73

7.

Distributed Digital Control System

74-81

8.

Conclusion

82

9.

Bibliography

83

10.

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BRIEF PROFILE OF THE COMPANY

Combined Cycle Pragati Power Station (CCPPS)

Pragati Power Corporation Limited (PPCL) is a power generation company under the Govt. of NCT of Delhi. It is presently having a 330MW gas based, Pragati Power Station. A 1500 MW (N) CCGT, Pragati-III, Power Project is under construction at Bawana in North-West Delhi which shall be commissioned before the Commonwealth Games in Oct.,2010.PPCL is also in the process to set up a 750 MW CCGT power project, Pragati-II, at Bamnauli in south-West Delhi. Land has been acquired and preliminary studies including EIA are in progress.PPCL is regularly making profit & paying dividend to Govt. of NCT of Delhi ever since it‘s commissioning in 2003-04. IPGCL was incorporated on 1st July,2002 and it took over the generation activities w.e.f. 1st July,2002 from erstwhile Delhi Vidyut Board after its unbundling into six successor companies. The main function of IPGCL is generation of electricity and its total installed capacity is 994.5 MW including of Pragati Power Station. Its associate Company is Pragati Power Corporation Limited which was incorporated on 9th January, 2001.To bridge the gap between demand and supply and to give reliable supply to the capital City 330 MW combined cycle Gas Turbine Power Project was set up on fast track basis. This plant consists of two gas based Units of 104 MW each and one Waste heat Recovery Unit of 122 MW. Gas supply has been tied up with GAIL through HBJ Pipeline. Due to paucity of water this plant was designed to operate on treated sewage water which is being supplied from Sen Nursing Home and Delhi Gate Sewage Treatment plants. Their Vision: ―TO MAKE DELHI – POWER SURPLUS‖ • To maximize generation from available capacity •To plan & implement new generation capacity in Delhi •Competitive pricing of our own generation 6

•To set ever so high standards of environment Protection. •To develop competent human resources for managing the company with good standards.

The Power demand in the Capital City is increasing with the growth of population as well as living standard and commercialization. Then restricted power demand in the summer of year 2000 was 3000 MW and increasing every year @ 6 to 7%. In 20052006, it is expected to be4078 MW and by 2009-10 it will reach 5075 MW. Erstwhile DVB's own generation from RPH, I.P. Station and Gas Turbine Power Station had been around 350-400 MW and Badarpur has been supplying 600-700 MW and the balance was met from the Northern Grid and other sources.

To bridge the gap between demand and supply and to give reliable supply to the Capital City, Delhi Govt. had set up 330 MW Pragati Power Project on fast track basis. To cut down the project cycle duration, turn key contract was awarded to M/s BHEL in May 2000 based on similar project executed by BHEL at Kayamkulam (owned by NTPC). To further ensure reliable and smooth operation of the plant, experience of NTPC was utilized by retaining them as engineering consultant and specification of the Kayamkulam Project were adopted.

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RUNNING PROJECT OF PPCL PPCL-III BAWANA 1000 MW PROJECT

Pragati Power Corporation Ltd. (PPCL) propose to establish 1000 MW capacity gas based Combined Cycle Power Project at Bawana, New Delhi. It is a project of Delhi government. To prepare the Feasibility Studies of the proposed Bawana Power Project, PPCL has retained the services of National Thermal Power Corporation (NTPC) Ltd. The proposed capacity of 1000 MW Bawana power plant will be based on any one suitable configuration of three possible alternatives. Alternative 1: Three modules each comprising of one Gas Turbine, one HRSG and one ST. Alternative 2: Three modules each comprising two Gas Turbines, two HRSGs and one Steam turbine. Alternative 3: Two combined cycle modules each comprising three Gas Turbines, three HRSGs and one Steam turbine. The gas based combined cycle power generation is inherently an environmental friendly technology, but as all Industrial developmental activity have some beneficial and adverse impact on environment, this CCPP also may have some or the other impact. Thus the impact of the proposed 1000 MW Bawana power project has also to be analyzed. Therefore National Thermal Power Corporation has entrusted the job of preparation of Comprehensive Environmental Impact Assessment report and Environmental Management Plan for various environmental attributes to M/s Kirloskar Consultants Limited (KICONS), Pune. OBJECTIVE OF THE PROPOSED PROJECT:

Based on demand supply study of Power, Delhi is experiencing severe power shortages during the 10th Five Year Plan and it is expected to persist in the 11th plan. Demand for electricity in Delhi increases about 10% each year. Presently the requirement hovers around 3700 MW. 8

To surmount the power shortage Delhi Government intends to install 1000 MW (Nominal) Bawana CCPP through Pragati Power Corporation Limited which is expected to start yielding benefits to Delhi in 2006-2007 and beyond. GEOGRAPHICAL LOCATION OF THE PROPOSED PLANT:

The Proposed Power Project would be located at Bawana in the Bawana Industrial Area of Delhi having a Latitude and Longitude of 28o 47‘ 30" North and 77o 4‘ 22" East respectively. The PPCL/DTL are in possession of 40 Hectares of land at Bawana, which is about 38 Km by road from Delhi City. The site is well connected through 4track carriageway with the Delhi City. PROCESS DETAILS:

The combined Cycle power plant with Natural Gas as primary fuel has been considered for the Bawana Power Project of 1000 MW (Nominal) capacity. The proposed Bawana power plant will be based on any one of the following configurations. Configuration Considering the plant capacity and various Gas turbine modules available, including advance class Gas turbines, the three possible alternate configurations are as follows: Alternate I 3 x (1GT + 1 HRSG + 1 ST) i.e. Three modules each comprises of one GT, one HRSG and one ST. The Gas turbines for this alternate shall be advance class machines. Since open cycle operation is not envisaged for the project Single shaft modules are considered for this alternative. Alternate II 3 x (2GT + 2 HRSG + 1 ST): i.e. Three modules each comprising two Gas turbines, two HRSGs and one Steam turbine. Alternate III

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2 x (3GT + 3 HRSG + 1 ST): i.e. Two combined cycle modules each comprising three Gas turbines, three HRSGs and one Steam turbine.

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OVERVIEW OF THE PLANT

PLANT LAYOUT

The Combined Cycle Power Plant or combined cycle gas turbine, a gas turbine generator generates electricity and waste heat is used to make steam to generate additional electricity via a steam turbine. The gas turbine is one of the most efficient one for the conversion of gas fuels to mechanical power or electricity. The use of distillate liquid fuels, usually diesel, is also common as alternate fuels.

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More recently, as simple cycle efficiencies have improved and as natural gas prices have fallen, gas turbines have been more widely adopted for base load power generation, especially in combined cycle mode, where waste heat is recovered in waste heat boilers, and the steam used to produce additional electricity. This system is known as a Combined Cycle. The basic principle of the Combined Cycle is simple: burning gas in a gas turbine (GT) produces not only power – which can be converted to electric power by a coupled generator – but also fairly hot exhaust gases. Routing these gases through a water-cooled heat exchanger produces steam, which can be turned into electric power with a coupled steam turbine and generator.

This type of power plant is being installed in increasing numbers round the world where there is access to substantial quantities of natural gas. A Combined Cycle Power Plant produces high power outputs at high efficiencies (up to 55%) and with low emissions. In a Conventional power plant we are getting 33% electricity only and remaining 67% as waste. By using combined cycle power plant we are getting 68% electricity. It is also possible to use the steam from the boiler for heating purposes so such power plants can operate to deliver electricity alone or in combined heat and power (CHP) mode.

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MECHANISM:

Combined cycle power plant as in name suggests, it combines existing gas and steam technologies into one unit, yielding significant improvements in thermal efficiency over conventional steam plant. In a CCGT plant the thermal efficiency is extended to approximately 50-60 per cent, by piping the exhaust gas from the gas turbine into a heat recovery steam generator. However the heat recovered in this process is sufficient to drive a steam turbine with an electrical output of approximately 50 per cent of the gas turbine generator. The gas turbine and steam turbine are coupled to a single generator. For startup, or ‗open cycle‗ operation of the gas turbine alone, the steam turbine can be disconnected using a hydraulic clutch. In terms of overall investment a single-shaft system is typically about 5 per cent lower in cost, with its operating simplicity typically leading to higher reliability.

WORKING PRINCIPLE OF PLANT:

First step is the same as the simple cycle gas turbine plant. An open circuit gas turbine has a compressor, a combustor and a turbine. For this type of cycle the input temperature to turbine is very high. The output temperature of flue gases is also very high. This is therefore high enough to provide heat for a second cycle which uses steam as the working medium i.e. thermal power station.

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AIR INLET:

This air is drawn though the large air inlet section where it is cleaned cooled and controlled. Heavy-duty gas turbines are able to operate successfully in a wide variety of climates and environments due to inlet air filtration systems that are specifically designed to suit the plant location. Under normal conditions the inlet system has the capability to process the air by removing contaminants to levels below those that are harmful to the compressor and turbine. In general the incoming air has various contaminants. They are: In Gaseous state contaminants are: • Ammonia • Chlorine • Hydrocarbon gases • Sulfur in the form of H2S, SO2 • Discharge from oil cooler vents In Liquid state contaminants are: • Chloride salts dissolved in water (sodium, potassium) • Nitrates • Sulfates • Hydrocarbons In Solid State contaminants are: • Sand, alumina and silica • Rust • Road dust, alumina and silica • Calcium sulfate • Ammonia compounds from fertilizer and animal feed operations • Vegetation, airborne seeds

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Corrosive Agents: Chlorides, nitrates and sulfates can deposit on compressor blades And may result in stress corrosion attack and/or cause corrosion Pitting. Sodium and potassium are alkali metals that can combine with Sulfur to form a highly corrosive agent and that will attack portions of the hot gas path. The contaminants are removed by passing through various types of filters which are present on the way. Gas phase contaminants such as ammonia or sulfur cannot be removed by filtration. Special methods are involved for this purpose.

Turbine Cycle: The air which is purified then compressed and mixed with natural gas and ignited, which causes it to expand. The pressure created from the expansion spins the turbine blades, which are attached to a shaft and a generator, creating electricity. In second step the heat of the gas turbine‘s exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG) with a live steam temperature between 420 and 580 °C.

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HEAT RECOVERY STEAM GENERATOR(HRSG) In Heat Recovery Steam Generator highly purified water flows in tubes and the hot gases passes a around that and thus producing steam .The steam then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG at around 140 degrees centigrade and are discharged into the atmosphere. The steam condensing and water system is the same as in the steam power plant.

HRSGS CONSIST OF FOUR MAJOR COMPONENTS: The economizer, evaporator, superheater and

water

preheater.

The

different

components are put together to meet the operating requirements of the unit. See the attached illustration of a Modular HRSG General Arrangement. Modular HRSGs can be categorized by a 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 17

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 superheaters to raise the temperature beyond the one at the saturation point.

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TYPICAL SIZE AND CONFIGURATION OF PLANT: The combined-cycle system includes single-shaft and multi-shaft configurations. The single-shaft system consists of one gas turbine, one steam turbine, one generator and one Heat Recovery Steam Generator (HRSG), with the gas turbine and steam turbine coupled to the single generator on a single shaft. Multi-shaft systems have one or more gas turbine-generators and HRSGs that supply steam through a common header to a separate single steam turbine-generator. In terms of overall investment a multishaft system is about 5% higher in costs. The primary disadvantage of multiple stage combined cycle power plant is that the number of steam turbines, condensers and condensate systems-and perhaps the cooling towers and circulating water systems increases to match the number of gas turbines.

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EFFICENCY OF CCGT PLANT: Roughly the steam turbine cycle produces one third of the power and gas turbine cycle produces two thirds of the power output of the CCPP. By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. The efficiency of the cycles adds, because they are powered by the same fuel source. To increase the power system efficiency, it is necessary to optimize the HRSG, which serves as the critical link between the gas turbine cycle and the steam turbine cycle with the objective of increasing the steam turbine output. HRSG performance has a large impact on the overall performance of the combined cycle power plant. The electric efficiency of a combined cycle power station may be as high as 58 percent when operating new and at continuous output which are ideal conditions. As with single cycle thermal units, combined cycle units may also deliver low temperature heat energy for industrial processes, district heating and other uses. This is called cogeneration and such power plants are often referred to as a Combined Heat and Power (CHP) plant. The efficiency of CCPT is increased by Supplementary Firing and Blade Cooling. Supplementary firing is arranged at HRSG and in gas turbine a part of the compressed air flow bypasses and is used to cool the turbine blades. It is necessary to use part of the exhaust energy through gas to gas recuperation. Recuperation can further increase the plant efficiency, especially when gas turbine is operated under partial load.

Schematic of Combined Cycle (CCGT) power plant. Overall efficiency (hCC) is a combination of the efficiency of the Brayton (hB) gas turbine cycle and the Rankine steam turbine cycle (hR). Total combined cycle efficiency is hCC =hB + hR –(hB * hR) .

The combine cycle efficiency (hCC) can be derived by the equation 1 Langston.

hCC = hB + hR - (hB * hR) (1) 20

Equation (1) states that the sum of the individual efficiencies minus the product of the individual efficiencies equals the combine cycle efficiency. This simple equation gives significant insight to why combine cycle systems are successful. For example, suppose the gas turbines efficiency hB is 40% (a reasonable value for a toady‘s gas turbines) and that the steam turbine efficiency hR is 30% (a reasonable value for Rankine Cycle steam turbine). Utilizing equation (1) would lead to the following conclusion. hCC = 0.4 + 0.3 – (0.4 * 0.3) hCC = 0.58 5 hCC = 58%

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FUELS FOR CCPT PLANTS: The turbines used in Combined Cycle Plants are commonly fuelled with natural gas and it is more versatile than coal or oil and can be used in 90% of energy applications. Combined cycle plants are usually powered by natural gas, although fuel oil, synthesis gas or other fuels can be used.

EMISSIONS CONTROL: Selective Catalytic Reduction (SCR): To control the emissions in the exhaust gas so that it remains within permitted levels as it enters the atmosphere, the exhaust gas passes though two catalysts located in the HRSG. One catalyst controls Carbon Monoxide (CO) emissions and the other catalyst controls Oxides of Nitrogen, (NOx) emissions. Aqueous Ammonia – In addition to the SCR, Aqueous Ammonia (a mixture of 22% ammonia and 78% water) is injected into system to even further reduce levels of NOx.

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BRAYTON CYCLE : The '''Brayton cycle''' refers to the energy cycle that includes power generation from the operation of the gas turbine generator (GTG). During GTG operation, combustion air is compressed in the GTG compressor section. The air is mixed with fuel in the combustion section and ignited. The resulting hot gases are expanded through the GTG turbine section, driving the generator and converting rotational energy to electrical power. This is referred to as a "simple cycle" arrangement. In a simple cycle machine, a large amount of energy (hot exhaust gases) is lost to the atmosphere.

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RANKINE CYCLE :

The '''Rankine cycle''' refers to a typical steam/water cycle. Heat input to the steam/water cycle is derived from the hot exhaust gases discharged from the GTG. The HRSG transfers the exhaust heat to feedwater, which is converted to steam. The steam is directed to the steam turbine generator (STG) where it is expanded, driving a generator and converting rotational energy to electrical power. The exhausted steam is condensed in the condenser and the condensate pumps return the condensate to the HRSG. Heat is removed from the cycle in the condenser by the circulating water system.

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MERITS OF CCPP FUEL EFFICIENCY: In conventional power plants turbines have a fuel conversion efficiency of 33% which means two thirds of the fuel burned to drive the turbine off. The turbines in combined cycle power plant have a fuel conversion efficiency of 50% or more, which means they burn about half amount of fuel as a conventional plant to generate same amount of electricity.

LOW CAPITAL COSTS: The capital cost for building a combined cycle unit is two thirds the capital cost of a comparable coal plant.

COMMERCIAL AVAILABILITY: Combined cycle units are commercially available from suppliers anywhere in the world. They are easily manufactured, shipped and transported.

ABUNDANT FUEL SOURCES: The turbines used in combined cycle plants are fuelled with natural gas, which is more versatile than a coal or oil and can be used in 90% of energy publications. To meet the energy demand now a day‘s plants are not only using natural gas but also using other alternatives like bio gas derived from agriculture.

REDUCED EMISSION AND FUEL CONSUMPTION: Combined cycle plants use less fuel per kWh and produce fewer emissions than conventional thermal power plants, thereby reducing the environmental damage caused by electricity production. Comparable with coal fired power plant burning of natural gas in CCPT is much cleaner.

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POTENTIAL APPLICATIONS IN DEVELOPING COUNTRIES: The potential for combined cycle plant is with industries that requires electricity and heat or stem. For example providing electricity and steam to a Sugar refining mill.

DEMERITS 1. The gas turbine can only use N atural gas or high grade oils like diesel fuel. 2. Because of this the combined cycle can be operated only in locations where these fuels are available and cost effective.

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INTRODUCTION TO PROJECT STEAM SYSTEM PROCESS-CONTROL This Best Practices Steam Technical Brief was developed to provide a basic understanding of the different process-control schemes used in a typical steam system. This brief provides a fundamental overview, and the reader should be aware that more in-depth knowledge is required to achieve the best process-control results.

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This brief will cover the following process-control schemes: • Feedback • Feed-forward • Backpressure • Ratio • Cascade • Differential.

The above control schemes can be applied to the following generic applications: • Temperature • Flow • Level • Pressure.

A control system will use one or more of the above schemes to achieve process control. The various control schemes are detailed in the typical application examples defined in this brief. In any control scheme that is applied, the user must define three elements for the control process: • Process Variable (Sensing device) – Flow transmitter – Level transmitter – Pressure transmitter – Differential transmitter – Temperature transmitter • Controller – Self-contained – Proportional and integral (PI) 32

– Proportional, integral, derivative (PID) • Output control signal (Final controlling mechanism) – Control valve – Actuator – Another device. This review focuses on control valves, which are generally used as the final element. The control valve has several classifications: • Regulating design valve – Self contained – External pilot operated • Pneumatic actuated valve – Globe design – Caged trim – Ball. In any process-control selection, understanding the advantages and disadvantages of each selection is important. The regulating control valve, or regulator, is a device that has a 20 to 1 turndown and limited selections of flow-trim characteristics. The globe-style control valve has 30 to 1 turndown and is a device that can provide a limited number of selections of flow-trim characteristics. Flow-trim characteristics can be linear, non-linear, or modified equal percentage. Flow-trim selection can enhance control of steam flow at varying load demands. The cage-trim control valve is the most flexible and may be the most commonly used for precise steam process control. This valve provides the largest selection of different flow-trim characteristics, and the highest turndown capabilities, with a 40 to 1 turndown. The ball valve has a number of different flow characteristics. The flow profile can be changed by the design of the ball (for example, standard, V-ball, etc.). The ball-valve turndown can be as high as 25 to 1. 33

• Turndown summary: – Regulating valve: 20 to 1 – Globe style valve: 30 to 1 – Cage trim valve: 40 to 1 – Ball valve (with special trim): 25 to 1

SYMBOL DEFINITIONS CS = Cascade CT = Controller FT = Flow transmitter PT = Pressure transmitter PV = Process Variable R =Ratio SP = Set Point TT = Temperature transmitter

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FEEDBACK CONTROL One of the simplest process-control schemes that steam applications use is the feedback-control scheme (Figure 1). The advantage of this control scheme is that it is simple; however, it depends on a single transmitter sensing a change in flow, pressure, or level to provide the feedback response to the controller or valve. This control scheme does not take into consideration any of the other variables in the process.

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FEEDBACK CONTROL (BACKPRESSURE APPLICATION) Feedback control for a steam-system backpressure-control scheme utilizes another parameter to provide the controller with information on process changes (Figure 2). Backpressure control is used to maintain inlet-steam pressure above a predetermined setpoint. Pressure transmitters are located on the inlet and outlet piping, which will notify the controller that changes are occurring. Consequently, backpressure control work in conjunction with feedback control. The most common application for a steam system is the elimination of instant, high demand for steam from a process that will affect the boiler operation.

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FEED-FORWARD CONTROL Feed-forward control uses a secondary input from another variable to assist or provide the controller with the knowledge that various changes are occurring in the process (Figure 3). Steam flow measurement in pressure-reducing applications adds instant identification that a change is occurring. This allows the controller to make corrective actions before a significant temperature or steam-pressure change has occurred. Consequently, feed-forward control is used in conjunction with feedback control. The feedback loop is used to maintain setpoint control, and feed forward is used to compensate for any errors and unmeasured disturbances. One of the most common applications is a pressure transmitter that is used on a shell-and-tube heat exchanger to sense and feed-forward a change in steam pressure on the shell (steam side). The steam pressure change on the shell side is the first indication that the temperature, or process variable, will change in a short period of time.

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RATIO CONTROL Ratio control is a duplex form of feedback control that has two sets of variables, for which the controller calculates a setpoint from the two variables for the control scheme (Figure 4). The object of a ratio-control scheme is to keep the ratio of two variables at different values, depending on the final objective of the control system. As Figure 4 indicates, on a pressure-control system the control output to the different valves is a ratio that depends on the percentage of travel, 0 to 100%, and the pressure transmitter. This type of control scheme is applied when two or more control valves occur in a pressure-reducing application.

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CASCADE CONTROL Cascade control is widely used within steam-process industries (Figure 5). The conventional cascade scheme has two distinct functions with two control loops. Cascade control is used to improve the response of the single-feedback strategy. A heat exchanger that varies process flow will have different steam requirements depending on the flow. Cascade control ―understands‖ the requirements and adjusts the output to the control valve according to process flow. The main objective is to achieve the desire output temperature of the process, which is the lead process variable. The idea is similar to that of the feed-forward control scheme.

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DIFFERENTIAL CONTROL Differential control is typically used on rotating-cylinder dryers because differential pressure is required across the siphoning joint to assist in evacuating the condensate (Figure 6). The use of rotating cylinders is the only instance where gravity drainage of condensate is not possible from the process. Therefore, using differential control identifies the parameters of inlet (P1) and outlet (P2) process pressures and maintains a lower outlet steam pressure (P1>P2), thus achieving the differential. Other gravity-limited heat-transfer applications will use differential control for condensate evacuation.

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CONTROL ACTIONS The controller‘s output to the final control element, the valve or actuator, is accomplished in different ways: • On/off – Simplest – Least accurate • PI (Proportional and integral) – Medium cost factor – Medium accuracy • PID (Proportional, integral and derivative) – Highest cost – Highest accuracy.

On/Off Control Control schemes using a feedback control parameter can use on/off control. On/off control is the simplest control scheme with the highest degree of inaccuracy. The controller has a set point with high- and low-control action points, similar to a home air conditioning or heating system. The thermostat has a desired setpoint (SP) and the system is actually operated between two temperature points: on/off. The desired outlet temperature is 180°F (SP) and the on/off control activates the steam valve to heat the product to 185°F. At 185°F, the steam valve deactivates and this allows the process to cool down to 175°F, a lower set point. The steam is activated and deactivated between the high and low process setpoints.

Proportional and Integral (PI) Control PI control uses an algorithm that is proportional to the difference between a setpoint (SP) and a process variable (PV), and integral time-function algorithms, which provide a continuous-control process output to meet the desired setpoint. This is similar to a residential light dimmer switch versus an on/off light switch. The dimmer 41

mechanism provides a light variable from off to full brightness, or anywhere in between. PI controls the steam flow from zero to full flow, or anywhere in between, on a continuous basis.

Proportional, Integral, and Derivative (PID) Control PID control has proportional, integral, and derivative algorithms available to maintain the setpoint of the process. Steam applications use the proportional and integral part of ―PID;‖ the derivative algorithm is seldom used, and then only by experts who are experienced in control algorithms. If the heat-transfer equipment, control valve, and the controller are properly selected, then proportional and integral are the only parameters required to maintain a highly accurate process result.

APPLICATIONS OF CONTROL SCHEMES

Steam Pressure Control The majority of industrial steam systems will have a pressure-reducing valve application. High-pressure steam is reduced to lower-pressure steam for a process or heating application. Used throughout all types of industries, some plants will have from one to more than one hundred different pressure-control valves. The feedback control scheme is simply a pressure transmitter and a controller, or a sensing line coming back to a pilot on a valve. In a simple regulator-type control system for pressure control, a sensing line is providing the feedback to the external pilot, which is the controlling device (Figure 7). The main valve is the final controlling element.

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A control-valve layout uses a pressure transmitter as the feedback-sensing device with the controller providing the correct control action (Figure 8). The pneumatic valve is the final controlling element.

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applications require the use of one or more valves to achieve the necessary turndown. Control valves that are used in any type of control scheme should utilize a secondary pressure drop if the control valve is in a sub-critical flow operation. Figure 9 shows the use of a simple orifice plate, placed after the control valve, to provide a secondary pressure drop. This type of installation has been used for more than 60 years. The inlet pressure to the control valve is P1, the pressure between the control valve and the orifice is P2, and the final control point or outlet pressure to the control valve is P3. Orifice plates, when properly sized and installed, prevent the valve from operating at a sub-critical flow and causing premature failure

.

Backpressure Control The backpressure control is a type of feedback-control scheme (Figure 10), typically used on smaller boilers without large steam reservoir capabilities for instant steam load demands. High instantaneous demands for steam can cause unwanted shutdowns of the boiler. Using backpressure control prevents the shutdown. A transmitter sensing the inlet pressure to the valve identifies a reduction of pressure beyond the predetermined set point, and the valve begins to close down to maintain the steam set

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pressure on the inlet of the valve. This action overrides any pressure requirements or needs on the downstream side of the valve.

Feedforward Control Figure 11 shows a feedforward/feedback control system. The orifice steam flow meter is providing the feedforward information to the controller. The pressure transmitter is providing the feedback to the controller. The pneumatic control valve is the final element.

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Ratio Control Another way to accomplish the goal of meeting large steam-flow requirements is the use of multiple valves (Figure 12). Multiple valves can provide better control in meeting the process requirements. In a two-stage pressure-control scheme, the stages use a feedback-control scheme and then ratio the controller output to the valves. As shown in Figure 12, the system is the ratio or position of the primary and secondary valve depending on the required flow rates. Parallel positioning valves are quite commonly used in process-heating applications where load conditions vary greatly from the coldest part of the season to the warmest part of the season.

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Differential Control Differential control provides the condensate removal of a rotating cylinder dryer (Figure 13). This goal is accomplished by maintaining a lower steam pressure (pressure transmitter no. 2) than the inlet steam pressure (pressure transmitter no. 1).

Heat-Transfer Feedback Control One of the most simple and most common control scheme used in heat transfer is simplefeedback control (Figure 14). This is a simple system, but this control scheme does not account for any upsets, disturbances, or unknown factors that might occur in the system and affect the heattransfer process.

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Heat-Transfer Feed Forward Control The heat-transfer feed forward control scheme uses feedback to control temperature, and uses pressure as the feedforward (Figure 15). A disturbance or change in process immediately causes a pressure drop to occur in the heat transfer because of the collapsing of the steam. The steam-pressure feed-forward anticipates the temperature change in process-flow stream before the change actually occurs. This provides anticipation of process changes.

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Heat-Transfer Feedback, Feedforward, Cascade Control The control system in heat transfer can have feedback (temperature), feed-forward (steam pressure), and cascade product flow into the control scheme (Figure 16). Using all possible variables, heat-transfer control can provide the highest degree of accuracy.

Feedback-Condensate Control A condensate feedback-control scheme is used in condensate removal and is typically a simplelevel transmitter, controller, and control valve (Figure 17). This system is typically used on a heat transfer application. In process-flow operations where condensate flow rates are 8,000 pounds per hour (lbs/h) or higher, steam traps are not advised. Instead, use a level transmitter, controller, and control valve with a feedbackcontrol scheme. This feedback-control scheme gives you the ability to remove the condensate from the heat-transfer process on a continuous basis. In these high-flow rates, a control valve provides a high degree of accuracy and control in the removal of condensate from heat transfer.

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CONTROL VERSUS COST The best process-control scheme is one that provides the system with the most information possible; to do this, the user must evaluate cost factor. The more information that is provided to the control scheme, the higher the cost for the field devices—and the more complex the controller, control strategy, and wiring. Therefore, the user must consider cost justification to identify the correct control scheme for the process application.

CONCLUSIONS AND RECOMMENDATIONS Control strategies, when evaluated from basic inputs and outputs, are simple and straightforward. Control schemes should be determined based on the level of control necessary, the cost, and the process. When determining control-scheme selection: • • •

Outline objectives and goals clearly before starting the selection process Select the correct control scheme for the process Select the proper equipment for the application. 50

STEAM TEMPERATURE MEASUREMENT INTRODUCTION An RTD is a device which contains an electrical resistance source (referred to as a ―sensing element‖ or ―bulb‖) which changes resistance value depending on it‘s temperature. This change of resistance with temperature can be measured and used to determine the temperature of a process or of a material.

RTD sensing elements come in two basic styles, wire wound and film Besides the sensing element which we have previously discussed, the measuring circuit also consists of a combination of lead wires, connectors, terminal boards and measuring or control instrumentation. The exact make-up of the measurement circuit is dependent on many factors including: • Temperature in the sensing area as well as the environmental conditions expected to exist between the sensor and instrumentation. • Distance between the sensor and instrumentation. • Type of interconnections the customer prefers. • What type of wiring system is currently in place (if not new).

2-wire construction is the least accurate of the 3 types since there is no way of eliminating the lead wire resistance from the sensor measurement. 2-wire RTD‘s are mostly used with short lead wires or where close accuracy is not required.

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Measured resistance Rt = R1 + R2 + Rb The 3-wire construction is most commonly used in industrial plant applications where the third wire provides a method for removing the average lead wire resistance from the sensor measurement. When long distances exist between the sensor and measurement/control instrument, significant savings can be made in using a three-wire cable instead of a four-wire cable.

(R 1+2+R b ) - (R 2+3) = (R b ) The 3 wire circuit works by measuring the resistance between #1 & #2 (R 1+2) and subtracting the resistance between #2 & #3 (R 2+3) which leaves just the resistance of the RTD bulb (R b). This method assumes that wires 1,2 & 3 are all the same resistance

4-wire construction is used primarily in the laboratory where close accuracy is required. In a 4 wire RTD the actual resistance of the lead wires can be determined and removed from the sensor measurement.

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The 4-wire circuit is a true 4-wire bridge, which works by using wires 1 & 4 to power the circuit and wires 2 & 3 to read. This true bridge method will compensate for any differences in lead wire resistances. Although RTD‘s are typically ordered as 100 Ohm Platinum sensors, other resistance‘s (200 Ohm, 500 Ohm, 1000 Ohm, etc.) and materials (Nickel, Copper, Nickel Iron) can be specified. Since RTD‘s are a resistor, they will produce heat when a current is passed through them. The normal current limit for industrial RTD‘s is 1 mA. Thin film RTD‘s are more susceptible to self-heating so 1 mA should not be exceeded. Wire wound RTD‘s can dissipate more heat so they can withstand more than 1 mA. The larger the sheath or the more insulation there is the better chance there will be an error caused by self heating. Temperature coefficient for RTD‘s is the ratio of the resistance change per 1 deg. change in temperature over a range of 0 - 100 deg. C. This ratio is dependent on the type and purity of the material used to manufacture the element. Most RTD‘s have a positive temperature coefficient which means the resistance increases with an increase in temperature. The temp. coeff. for pure platinum is .003926 ohm/ohm/deg. C. The normal coefficient for industrial RTD‘s is .00385 ohm/ohm/deg. C per the DIN std. 43760 1980 & IEC 751 - 1983. Ni120 RTD‘s are more commonly used in the Power industry, specifically coal-fired plants. It is important to ensure that transmitters that are being used have the curves/linearization data built-in to the memory for the specific RTD without the need for any custom programming.

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THERMOCOUPLES :

Base metal thermocouples are known as Types E,J,K,T and N comprise the most commonly used category of Thermocouple. The conductor materials in base metal thermocouples are made of common and inexpensive metals such as Nickel, Copper and Iron.

Type E: The Type E thermocouple has a Chromel (Nickel-10% Chromium) positive leg and a Constantan (Nickel- 45% Copper) negative leg. Type E has a temperature range of -330 to 1600°F, has the highest EMF Vs temperature values of all the commonly used thermocouples, and can be used at sub-zero temperatures. Type E thermocouples can be used in oxidizing or inert atmospheres, and should not be used in sulfurous atmospheres, in a vacuum or in low oxygen environments where selective oxidation will occur. 55

TYPE J: The Type J thermocouple has an Iron positive leg and a Constantan negative leg. Type J thermocouples can be used in vacuum, oxidizing, reducing and inert atmospheres. Due to the oxidation (rusting) problems associated with the iron leg, care must be used when using this thermocouple type in oxidizing environments above 1000°F. The temperature range for Type J is 32 to 1400°F.

Type K: The Type K thermocouple has a Chromel positive leg and a Alumel (Nickel- 5% Aluminum and Silicon) negative leg. Type K is recommended for use in oxidizing and completely inert environments. Type N: The Type N thermocouple has a Nicrosil (Nickel-14% Chromium- 1.5% Silicon) positive leg and a Nisil (Nickel- 4.5% Silicon- .1% Magnesium) negative leg.

TYPE N : It is very similar to TYPE K but is less susceptible to selective oxidation effects. Type N should not be used in a vacuum or in reducing atmospheres in an unsheathed condition. The temperature range is 32-2300 deg F. Type T: The Type T thermocouple has a Copper positive leg and a Constantan negative leg. Type T thermocouples can be used in oxidizing, reducing or inert atmospheres, except the copper leg restricts their use in air or oxidizing environments to 700°F or below. The temperature range for Type T is -330 to 700°F. Noble Metal Thermocouples are another category of thermocouples and are made of the expensive precious metals Platinum and Rhodium. There are three types of noble metal thermocouples: Type B (Platinum/Platinum-30% Rhodium) Type R (Platinum/Platinum-13% Rhodium) Type S (Platinum/Platinum-10% Rhodium)

Types R and S have temperature ranges of 1000 to 2700°F and Type B thermocouples have a temperature range of 32 to 3100°F. Types E, J, and T they find widest use at temperatures above 1000°F. Type K, like Type E should not be used in sulfurous atmospheres, in a vacuum or in low oxygen

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environments where selective oxidation will occur. The temperature range for Type K is -330 to 2300°F.

RTD’s vs T/C’s Characteristics RTD Strengths:RTD‘s are commonly used in applications where accuracy and repeatability are important. Common instrumentation wire is used to couple the RTD to the measurement and control equipment making them more economical to install as compared to thermocouples which must use special extension wire, much like the composition of the thermocouple itself, to extend the wiring to the control equipment.

RTD Weaknesses: An RTD in the same physical configuration as a thermocouple will typically be 3 to 7 times the cost. RTD‘s are more sensitive to vibration and shock than a thermocouple and are limited to temperatures of approximately 800°F.

Thermocouple Strengths: A thermocouple can be used to temperatures as high as 3100° F. They generally cost less than RTD‘s and can be made smaller. TC‘s will respond faster to temperature changes and are more durable allowing use in high vibration and shock applications.

Thermocouple Weaknesses: Thermocouples are less stable than RTD‘s when exposed to moderate or high temperature conditions. Thermocouple extension wire must be used in hooking up thermocouple sensors to measurement instruments. Summary: RTD‘s and thermocouples are widely used in power plant temperature measurement. Each has its advantages and disadvantages. The application will determine which sensing element is best suited for the job. An RTD will provide higher accuracy and more stability than thermocouples. They also use standard instrumentation wire to couple the sensor to the measurement device. Thermocouples are less expensive than RTD‘s, are more durable in high vibration and mechanical shock applications and tolerate higher temperatures than RTD‘s. They can be made smaller than RTD‘s, generally, and can be formed to fit specific applications.

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Over half of the temperature applications in the United States, and most often in power plants involve direct wiring a temperature sensor to the controls system.

TRANSMITTERS VS DIRECT WIRING For temperature measurement, engineers must decide whether they wire the sensors directly to the control system (PLC, DCS, recording system…) or if they use transmitters. Nowadays, many engineers still wire direct because they mistakenly believe this is a cheaper and easier solution. The reality however, is different: transmitters allow a engineer to save time and money, improve the measurement reliability and facilitate maintenance.

Why use transmitters? Reduce wiring costs If you do not use a transmitter, you need sensor extension wires to the control system for a precise temperature measurement. These wires are expensive and sometimes fragile. By using a transmitter, you only need inexpensive copper wires. The greater the distance between the sensor and the control system, the more money that can be saved! For applications involving, using a Pt100 4-wire, only a pair of wire is needed to run from the transmitter to the control system.

Basic information about thermocouple wiring:

There are 4 typical wiring setups: Using extension wires to the transmitter Using compensation wires to the transmitter Using the thermocouple wires to the head mounted transmitter Wiring direct to the control system

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Extension wires:

They are manufactured as stranded or solid conductors with various insulating materials and armoring. The conductors (the flexible strands or solid wires) consist of substitute materials. When a relatively flexible cable is required, flexible conductors are used. These conductor materials and the corresponding thermocouples have the same nominal structure and chemical composition.

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Compensation cables: They are manufactured as solid conductors with various insulating materials and armoring. The conductors (the flexible strands or solid wires) are made of substitute materials and therefore their chemical composition differs from the corresponding thermocouple material. Different alloys may be used for the same thermocouple type. The substitute material and the corresponding thermocouple have the same thermoelectric characteristics within the allowed temperature range.

Example: Type K thermocouple. Assumptions : Extension wires (twisted and shielded): $1.15 / foot Compensation wires (twisted and shielded): $0.78 / foot Copper connection wires : $0.10 / foot

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Breakeven with extension wires: PC Programmable DIN-rail transmitter (A): $175 Breakeven at 130 feet HART head transmitter: $245 (B) Breakeven at 200 feet HART field transmitter: $670 (C) Breakeven at 500 feet

Breakeven with compensation wires:

PC Programmable DIN-rail transmitter: $135 Breakeven at 270 feet HART head transmitter: $205 Breakeven at 410 feet HART field transmitter: $510 Breakeven at 1020 feet

Eliminate plant noise

Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are present in almost all types of plants, not just power. Their effects on the extension wires are important and obviously affect the measured value. By using transmitters, the temperature measurement can be made immune to EMI/RFI problems. EMC compliance to IEC61326 is necessary for use in noisy environments

Make maintenance easier / Advanced diagnostics You can save long and unnecessary trips to the field. The smart diagnostics capabilities of the sensor indicate (via HART® and upscale/downscale output signals) if the sensor is broken or if there is corrosion on the sensor input loop.

Increase accuracy

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Temperature transmitters not only accept RTD inputs with 2, 3 or 4 wires. There are over two dozen different types of RTD‘s or Thermocouples that can be connected to a transmitter without the need for special programming. As can be seen from the calculations below (Fig 2), a 2-wire RTD would produce the largest error because the measured resistance is the combination of the sensor and the wires.

Reduce control systems costs If you wire directly to the control system, you need several different input cards for different sensor types. Price for a 4-channel RTD input card: $399.00 Price for a 4-channel TC input card: $399.00 Price for a 4-channel 4-20mA Analog input card: $225.00

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This also makes things simpler for Power Engineers since only one input type would be used (same 4-20mA card for flow, pressure, level… inputs). What about routine maintenance? You can switch from a thermocouple to a Pt100, simply reconfigure the transmitter and send the output to the control system.

Allow sensor flexibility You need a new sensor type? Just replace it and use the same transmitter since transmitters accept universal inputs (12 different thermocouple types, 6 different RTD, mV and Ohms). You can switch to another sensing element without worrying about the installation and wiring changes.

Avoid ground loops In applications where fast response time is needed, customers use grounded thermocouples. This thermocouple type may cause a ground loop. This will be avoided by using transmitters with superior galvanic isolation (upto 2kV galvanic isolation). Ground is an elusive and often misunderstood electrical concept. Its very name implies that the soil we walk on is the place to which all currents and voltages are somehow referred. In an electric power distribution system, a rod driven into the earth or a buried metal pipe is ‗ground.‘ Unfortunately, that is not the entire the story. The local ‗ground‘ where you are now located can be several volts above or below that at the nearest building or structure. If there is a nearby lightning strike, that difference can rise to several hundreds or thousands of volts. This arises not only from the resistance of wiring but its inductance. If the currents change very rapidly, the voltage drops in the ground system will approach several hundred volts for short periods of time. Some of the instrumentation installed in today‘s industrial plant is wired in close proximity to power wiring. For example, studies conducted by power companies have shown that the operation of an oil burner igniter can produce transient differences up to 2000 volts routinely. 64

Imagine the potential for similar voltage spikes in other industrial environments. The voltages themselves can obviously provide a great source of interference for a measurement loop, but the currents which cause them can also induce significant currents and voltages in the signal wires located nearby. Circulating currents in ground loops may also be periodic in addition to being transient events.

Grounded Thermocouples :

Ground loop potentials and currents are a major problem for this thermocouple type. There is one completely satisfactory way to solve these problems — insert an isolation stage between the signal and the rest of the measurement system. Ground loop problems are most likely to occur in industrial plant environments such as: -Aluminum Smelters (High operating voltages in smelters) - Cement Plants, Power plants (High voltages relating to Material Handling equipment) In applications where fast response time is needed, customers use grounded thermocouples. This thermocouple type may cause a ground loop. This will be avoided by using a transmitter with good galvanic isolation (2kV). You might also have this problem with an ungrounded sensor in case of insulation breakage…

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The Need for Galvanic Isolation Isolation is a universal way to eliminate ground loop problems. Isolation simply means using one of a number of electronic techniques to interrupt the connections between two grounds while passing the desired signal with little or no loss of accuracy. Without a path for ground currents to flow, these currents cannot induce signal errors. Isolation also solves the other problem encountered with ground loops — voltage differences which cannot be rejected by the signal conditioner. Galvanic isolation refers to a design technique which will separate signal currents from AC power distribution introduced stray noise currents. Basically this process will provide two separate paths for signal and noise currents which will not allow them to mix or to mix only over short distances, thereby minimizing the effects of noise currents on the signals. An example of Galvanic Isolation method is a transformer. It provides galvanic isolation in that no electrical current can flow directly from one winding to the other as they are not in direct electrical contact. However, a signal can flow via electromagnetic coupling between the two windings.

Galvanically- isolated transmitters for non-grounded RTD applications

This question is often being raised by users of RTD sensors. Since Pt-100 thermal elements are not usually grounded, it is often assumed that they do not require isolated transmitters for proper operation. If the measurement environments were ideal, indeed this assumption could be, at least, partially correct. Unfortunately, industrial environments are often ridden with various types of airborne contaminants in solid, liquid and gaseous forms. These may precipitate and settle inside, and around the instruments‘ and the sensors‘ terminals. Add just a little bit of humidity and you have created several potential parasitic leakage current paths, which could seriously affect the device measurement accuracy as well as the signal integrity.

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Parasitic resistance paths may also be caused as a result of metal migration internal to the sensor structure, but these are not related to environmental conditions and are more common in sensors used at elevated temperatures. Isolated transmitters break the path of the parasitic resistance and prevent a leakage current from flowing through the transmitter‘s circuitry, hence avoiding the errors almost entirely. Galvanically- Isolated transmitters in general also provide for a far superior noise rejection as well as far superior protection from electrical transients and surges in electrically noisy environment or during weather extremes such as lightning or thunderstorms. The current generation of Temperature Transmitters have a galvanic isolation that is atleast about 3-5 times better than any the previous generation units. • Corrosion detection

Corrosion of the sensor connections can lead to corruption of the measured value. Temperature transmitters now offer the option of detecting corrosion on thermocouples and resistance thermometers with a 4-wire connection before measured value corruption occurs. Sensor connection cable corrosion can lead to false measured value readings.

• Sensor backup Sensor backup offers you maximum safety. If sensor 1 fails, the device automatically switches to sensor 2.

• Low Voltage Warning Temperature transmitters now have the capability to provide a low voltage warning if the potential drops below a threshold value. The alternative is to continue reading with some of the older transmitters and get a faulty reading when the voltage levels drop. 67

When voltage falls below 11 V dc, the unit indicates warning for low voltage instead of continuing to send you a false and misleading reading ! With older technology transmitters, when voltage drops, the unit continues to send a signal, although it could be off by as much as 25% or higher from the reading. • Gold-plated terminals (virtually eliminates corrosion of terminals). Customer saves big-time for not having to replace a transmitter if the terminal block goes bad, as with some of the low-end transmitters. 6 large-size terminals for sensor connection. No need to share terminals between sensors - minimize chances of mis-wiring. Transmitters are now available that can take AWG12 wire ; no need to mess with tiny screw drivers. Wiring graphic is laser etched on terminal block - How many times has a technician gone to site without an Instruction Manual ? Well, it's no longer a problem with the transmitters currently available.

Ambient Temperature Monitoring – Transmitters now have built-in RTD at the electronics module that monitors for ambient temperature. When temperature exceeds the limits the units is specified for, the unit gives a warning indication. With older trasnmitters, the unit would just get it's electronics module cooked, by the time the customer comes to know about it. Large and brilliant blue back-lit display. Irrespective of whether you mount the transmitter in a pitch dark location or in path of direct sunlight - you can still get a clear reading from a distance of 8-10 feet. The digits on a new transmitter display are atleast twice the size of any of the older devices. So you can see it from a farther distance. Transmitter displays now also have a bar graph to give you a visual indication from an even farther distance on reading.

Split- ranging function for dual sensor units. Switch reading from sensor 1 to sensor 2 dependent on temperature

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Temperature Measurement Application Guidelines

A. Previously, we stated that a thermocouple signal is a very small voltage (millivolt) and because of this weak signal, thermocouples are very susceptible to electrical noise. These stray voltages can come from many sources such as electric motors, heaters or even 2 way radios. To avoid this problem, use an ungrounded thermocouple and shielded extension wire.

B. There should never be a third metal in the hot junction to create a thermocouple junction, all that is needed is to electrically short the ends together. Butting the wire ends against a metal surface will create a junction. Remember, that the thermocouple signal is generated over the entire length of wire. C. Non-thermocouple materials cannot be used in the thermocouple circuit. It is permissible to use non-thermocouple materials as terminal blocks or splices as long as there is no temperature gradient across these devises.

D. The largest possible extension wire should be used to connect a thermocouple. This phrase used to be true 30 years ago before there was solid state electronics. The old instruments were Voltage based circuits and resistance was critical. The newer solid state electronics are current based so extension wire resistance is not important.

E. Transmitter Grounding : The transmitter will operate with the current signal loop either floating or grounded. However, the extra noise in floating systems affects many types of readout devices. If the signal appears noisy or erratic, grounding the current signal loop at a single point may solve the problem. The best place to ground the loop is at the negative terminal of the power supply. Do not ground the current signal loop at more than one point. The transmitter is galvanically isolated to 2 kV AC ( from sensor input to output), so the inoput circuit may also be grounded at any single point. When using a grounded thermocouple, the grounded junction serves as this point. When installing a transmitter; the shield on the analog output must have the same potential as the shield at the sensor connections. In plants with strong electro-magnetic 69

fields, shielding of all cables with a low ohm connection to ground is recommended. Shielded cable should be used in outdoor installations, due to the danger of lightning strikes.

F. RTD‘s are now specified according to IEC751 curve or calibration standards, with an alpha =0.00385. For lot of the powerplants that were built several years ago with older field instruments, caution is advised while replacing older RTD‘s. The alpha coefficient value for the existing RTD should be checked before ordering a replacement.

G. Temperature Measurement points in a Power plant – - Coal Mill, Oil or natural gas supply - Residue Disposal : Gypsum treatment - Water supply and treatment - Heat Generation : Main firing system, combustion air system, electrostatic precipitator, desulphurization, denitrification - Steam/Water : feedwater system, condensate system - Turbines : Steam, Gas, Lubricant supply

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The graphic above shows a field installation for controlling temperature and pressure of steam in the low-pressure system. These are process inputs for controlling the regulating valve. The head mount temperature transmitter is used in conjunction with an integrated temperature sensor assembly.

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The graphic below shows three temperature instruments (sensor and transmitter) in the steam pipe to the middle-steam header. The temperature sensors are Thermocouples. It is designed with 2 out of 3-measurement choice for reliability and safety.

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Compact Transmitters Water, pressure, heat and chemicals from wash down procedures combine to creative a highly moist and corrosive environment that can be damaging to traditional temperature sensors and transmitters. Short sensor life, repeated replacementeven potential failures—all can be a direct result from this environment. An integral temperature sensor and transmitter offers a unique solution for

wash down

environments and many others. The compact temperature transmitter consof a 4-wire Pt 100 Class A sensor, available in several different lengths, built-in 4-20mA transmitter. Its integral design offers power plant personnel a low-cost devifor temperature monitoring that is resistant to moisture and corrosion. Waterproof and impervious to water, steam, pressure and chemicals, th design of the compact device uses no external screw connections. Instead, they aavailable with M12 microconnector that easily plugs in to a commercially available cable eliminating any potential for mechanical damage to the temperature device orthe cable. The miniature RTD is hermetically sealed and both the sensor and transmitter are completely potted to withstand the rigors of the process and praccurate and reliable measurement. The Compact Temperature Sensors are commonly used with direct wiring to control system.The benefits of using transmitters however,has led to an increasing trend of their use. The benefits relate not only to cost savings but also reduced downtime, maintenance and advanced diagnostics. Transmitters also offer the option of digitalcommunication protocols such as HART, Foundation Fieldbus and Profibus. Future technology advances would be on the lines of improved software that is intuitive aneasier to program, availability of more diagnostics and Service Tools to facilitate commissioning and maintenance.

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DCS Control System A correctly designed DCS control system is essential for the safe and efficient operation of a process plant. Process Technical Services can provide qualified and experienced instrument and control system engineers and technicians that will review the P&I diagrams in detail to determine if the DCS control system will meet the needs of your operations. Particular attention is to be paid to DCS control system feed-forward, feedback, and cascade control systems. Although final tuning of DCS control system controllers can only be done with the plant in operation, the initial setting of the controller tuning coefficients is essential for a smooth start-up of the process. PTS engineers and technicians utilize results from dynamic model simulations, process flowsheets, and equipment sizes to estimate the impact of dead-time and equipment holdup times on DCS control system tuning coefficients for the various processing units throughout the process. The process variables sent to a DCS control system include pressure, temperature, level, and flow sensors. PTS' qualified and experienced instrument and control system engineers and technicians can provide valuable insights and consultation regarding the reliability and accuracy of a variety of sensors. Their advice may help avoid installation of field instruments of questionable reliability and accuracy during the installation phase of a project and possibly avoid countless hours of poor operation, upsets and downtime after the process is commissioned. PTS‘ experienced instrument technicians can pre-commission DCS controls and provide a preliminary set of parameters that will provide for a smoother and more orderly start-up of process units utilizing accurate P&IDs. Essential DCS control system elements in most modern process control system designs include on-line and in-line process analyzers. An on-line analyzer is one that processes a sample extracted from the process and thus requires an automatic sampling system to extract material from the process for analysis such as a gas 74

chromatograph (GC). Another method is the use of an in-line process analyzer with the sensor inserted directly into the process material. A pH probe is an example of an in-line analyzer. Another type of in-line analyzer might require a side stream that causes process material to flow through a small bypass line. It is important that the process design criteria specified by the DCS control system engineers be strictly followed during the procurement process. There is a strong incentive for the equipment purchasers to buy from the lowest qualified bidder, but one who has not necessarily met all of the design criteria. This can be a particular problem with process control valves. PTS control system quality assurance (QA) technicians can insure that all control system equipment being purchased meets all of the design specifications. Finally, even with a good design, and equipment that meets all the design specifications, there is still one area of activity where problems can develop. That is during the installation phase. Experienced and professional PTS Quality Control (QC) technicians can monitor the installation process from start to finish ensuring that the DCS control system installation follows industrial standards and meets all of the design specification. A well designed DCS control system, implemented with care and skill by PTS can provide years of safe, efficient, and effective control of your process.

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TEMPERATURE CONTROL SYSTEM High pressure and high temperature steam from the water tube boiler is sent to turbine. What happens here is, we all know that the turbine opens its account by means of safts its rotation. Here the pressurised steam is directly passed through the turbine and by doing, so the turbine in safts are moves at its extreme speed, there occurs EMF (Electro Magnetic Force), and from here, we get MW (Mega Watts) power In the power plant system, we have altogether three turbines. Each turbines produce different types of MW power. GET Company manufactures turbine 1 and 2, BHEL Company manufactures turbine 3.

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TEMPERATURE CONTROL BLOCK DIAGRAM

High Pressure [44kg/cm²(440ºC)] steam comes from the water tube boiler and it is sent to turbine by the help of control valves. In turbine, we have two different sections i.e., one starts its shaft movement from the initial stage and the other gets its movement from the final stage. Five different plants activate this constant Low Pressure and Medium Pressure steam by adding or reducing its strength. For instance we have in paper machine I and II [Paper Dry Purpose] ,Pulp Mill [Old and New], SRP[Soda Recovery pant], Deaerator tank. 77

Desuperheater is used to reduce the steam temperature. In order to reduce the steam temperature they use water high-pressured water spray and the temperature sensor calculates the further readings. Temperature sensor means Thermocouples, Thermistors, RTD (Resistive Temperature Detector), Thermopile, etc., .The temperature output is DC mV. Again, this DC mV is shifted to DCS (Distributed Control System) by the help of temperature transmitter. If we want to activate DCS, we have to apply more than 110DC voltage. The output here in DCS is (4-20 mA). Finally, the DCS output changes its account in accordance to the calculation given by the temperature transmitter. 78

Again the current which is produced here in DCS changes into current pressure in order to activate valve. In order to active the target level i.e., the fixed set point, the control valve controls the DCS system until it gets its accurate set point.

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STEAM FLOW DIAGRAM The control valve at Desuperheater temperature control valve by giving corresponding values for I /P converter at DCS. If we have to decide the steam, flow temperature of the corresponding current (4 - 20mA) has to be given to I/P by setting at DCS.

There is a regulator to give 1.2 Kg/cm2 pressure of air and corresponding pressure for me at I/P mingled and tends to positioner. The positioner has the regulator of 2 Kg/cm2 pressure of air to control valve. This control valve constructed as the type of air to open. So that air from positioner passing to control valve, and control valve can be opened for the corresponding air from positioner. The set point for I/P converter is set at the HIS (Human Interface System). The values, which all are going to set by DCS, are set at HIS only, and the corresponding output values are able to see here. HIS goes to FCS (Field Control Station). It connected with Marshalling cabinet, which is having two cards named Analog i/p card, Analog o/p card. Results of the process are taken into the line of Analog i/p card and the set points to control the process variable are taken out through Analog o/p card. Marshalling cabinet is connected with junction box.

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CONCLUSION The project on Desuperheater Temperature Control System is implemented in TamilNadu News Print and Papers Limited. In Industries we cannot control the high temperature manually. This project provides simple way to measure the low temperature parameters. This project is designed using Distributed Control System. The temperature of steam flow in high temperature is measured using thermocouple temperature sensor. In this system, the control valve is automatically controlled and temperature is reduced up to preset valve defined by programmer. In future this DCS system can be updated for more process with the same DCS programs.

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BILIBIOGRAPHY References : 1. Mr. Rajiv Verma (Training guide) 2. K.D Sharma (C & I) 3. Instruments and contro system by W.Bolton 4. A. Nagoor Kani, Control System, First Addition 5. Steam – Its generation and use, 40th Edition by Bobcock and W ilcox 6. Power Plant Engineering (Black & Veatch) 7. Internet

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