Modbus Configuration in Schnieder

1. Configure your PLC according to rack arrangement.
2. Configure the PLC modbus port; make sure that all the parameters are same as that of controller to be communicated on modbus.
3. For communication use ‘XXMIT’ block.
4. Configure the parameters of the block as described below.

Steel Making

The steel makers are divided into two categories namely:

1.      The mini mills steel makers, who produce steel by melting scrap.

2.      The integrated steel mills produce steel from iron-ore.

The following block diagram focuses on the processes involved in steel making.

The process starts with the introduction of iron-ore or scrap, depending on the segment belonging of steel making, to the blast furnace. There is an addition of impurities including coke and lime to the blast furnace and the heating is carried out resulting in molten iron. The molten iron is passed to BOF where molten iron is converted to molten steel at 1600degC. The process can be also proceed by the Induction Furnace. The molten steel is transferred to ladle refining furnace so that the temperature is maintained and impurities are removed. Then the molten steel is shaped to the end product.

LADLE FURNACE

LADLE FURNACE is employed to heat up, to hold and to finish all kinds of metal melts.

The main physical principle of an electric arc furnace is the transfer of energy from electric line to the furnace through heat radiation and conduction generated by an electric arc.

The Ladle furnace employs two systems namely:

1. Arc Regulatory System- incorporates Marcos, which is a control system for 3-phase arc furnace.

2. Auxiliary systems- this system includes temperature control, cooling systems, energy optimization, voltage and current control. The parameters and process control is commenced by automation systems (PLC based).

The focus of ladle furnace is to keep the molten iron scrap at constant temperature of 1640.38 degree Celsius.

 CONSTRAINTS:

1. Impurities.
2. Distance of electrode from the molten scrap iron.

The distance between the electrodes and molten iron scrap plays a vital role as only at particular arc length the electrodes produce the perfect temperature of arc, which is required to maintain the constant temperature for the molten iron. The arc length depends on the current and voltage.

Let us now take two extreme conditions of arc length.

1. When the electrodes and the molten scrap iron is on contact then it carries the      maximum current, as the air resistance is negligible. Therefore it doesn’t give us the required energy.

2.  When there is infinite gap between the electrode and the molten iron scrap then the      resistance becomes infinite and there is negligible current flowing.

So, we see taking in account of the two extreme conditions that for optimization of electrode energy we are required to maintain certain required air gap (arc length).

Location of measurement displays

The measurement is displayed for observation by plant personnel.  Typically, the display uses analog principles, which means that the display presents the measurement as a position in a graphical format, which could, for example, be the height of a slide bar or the position of a pointer.  Often, the value is displayed as a line on a trend plot that provides the values for some time in the past.  In addition, the measurement can be displayed as a digital number to provide more accuracy for calibration.  Finally, measurements that are transmitted to a digital control system can be stored in a historical database for later recall and for use in calculating important parameters useful in monitoring process behavior, for example, reactor yields or heat transfer coefficients.


The engineer must ensure that the measurements are displayed where needed by personnel.  Several common approaches are briefly summarized in the following.

·                      Local display - A sensor can display the measurement at the point where the sensor is located.  This information can be used by the people when monitoring or working on the equipment.  A measurement that has only local display involves the lowest cost, because the cost of transmission and interfacing to a digital system are not required.  Note that no history of these measurements is available unless people record the values periodically.

·                     Local panel display - Some equipment is operated from a local panel, where sensors associated with a unit are collected.  This enables a person to startup, shutdown and maintain the unit locally.  This must be provided for units that require manual actions at the process during normal operation (loading feed materials, cleaning filters, etc.) or during startup and shutdown.  Usually, the values displayed at a local panel are also displayed at a centralized control room.

·                     Centralized control room - Many processes are operated from a centralized control room that can be located a significant distance (e.g., hundreds of meters) from the process.  The measurement must be converted to a signal (usually electronic) for transmission and be converted to a digital number when interfaced with the control system.  A centralized control system facilitates the analysis and control of the integrated plant.

·                      Remote monitoring - In a few cases, processes can be operated without a human operator at the location.  In these situations, the measurements are transmitted by radio frequency signals to a centralized location where a person can monitor the behavior of many plants.  Typical examples are remote oil production sites and small, safe chemical plants, such as air separation units.

Major issues for selecting sensors

The major issues in sensor selection are summarized in the following.  The relative importance of each issue depends upon the specific application; for example, one application might require excellent accuracy, while another might require only moderate accuracy, but high reliability.  Generally, we find that the greater the requirements for good performance, the higher the cost for purchase and maintenance. Therefore, we must find the proper balance of performance and cost, rather than always specify the best performing sensor.

ISSUE		COMMENTS
·           Accuracy - Accuracy is the degree of conformity of the measured value with the accepted standard or ideal value, which we can take as the true physical variable.  Accuracy is usually reported as a range of maximum inaccuracy.  These ranges should have a significance level, such as 95% of the measurements will be within the inaccuracy range. Accuracy is needed for some variables, such as product quality, but it is not required for others such as level in a large storage tank.  See Section 24.3 in Marlin (2000) for a discussion on the needs of sensor accuracy.		Accuracy is usually expressed in engineering units or as a percentage of the sensor range, for example: Thermocouple temperature sensor with accuracy of ± 1.5 K. Orifice flow meters with accuracy of ±3% of maximum flow range.
·           Repeatability – The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions, approaching in the same direction.		The term “approaching in the same direction” means that the variable is increasing (decreasing) to the value for all replications of the experiment.
·           Reproducibility – The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions over a period of time, approaching from both directions.  This is usually expressed as non-reproducibility as a percentage of range (span). Often, an important balance is between accuracy and reproducibility, with the proper choice depending on each process application.		The period of time is “long”, so that changes occurring over longer times of plant operation are included. Reproducibility includes hysteresis, dead band, drift and repeatability.
·          Range/Span - Most sensors have a limited range over which a process variable can be measured, defined by the lower and upper range values.  Usually, the larger the range, the poorer the accuracy, and reproducibility.  Therefore, engineers select the smallest range that satisfies the process requirements. We select ranges that are easily interpreted by operating personnel, such as 100-200 °C, but not 100-183 °C.		If a chemical reactor typically operates at 300 °C, the engineer might select a range of 250-350 °C. Since the reactor will be started up from ambient temperature occasionally, an additional sensor should be provided with a range of -50 to 400 °C.
·          Reliability – Reliability is the probability that a device will adequately perform (as specified) for a period of time under specified operating conditions.  Some sensors are required for safety or product quality, and therefore, they should be very reliable.  Reliability is affected by maintenance and consistency with process environment.  Also, some sensors are protected from contact with corrosive process environment by a cover or sheath (e.g., a thermowell for a thermocouple), and some sensors require a sample to be extracted from the process (e.g., a chromatograph).		If sensor reliability is very important, the engineer can provide duplicate sensors, so that a single failure does not require a process shutdown.  See Chapter 22 in Marlin (2000) for the use of duplicate sensors in process control.
·          Linearity - This is the closeness to a straight line of the relationship between the true process variable and the measurement.  Lack of linearity does not necessarily degrade sensor performance.  If the nonlinearity can be modeled and an appropriate correction applied to the measurement before it is used for monitoring and control, the effect of the non-linearity can be eliminated.  Typical examples of compensating calculations are the square root applied to the orifice flow sensor and the polynomial compensation for a thermocouple temperature sensor.  The engineer should not assume that a compensation for non-linearity has been applied, especially when taking values from a history database, which does not contain details of the measurement technology.		Linearity is usually reported as non-linearity, which is the maximum of the deviation between the calibration curve and a straight line positioned so that the maximum deviation is minimized. See ISA (1979) for further details and several alternative definitions of linearity.
·         Maintenance - Sensors require occasional testing and replacement of selected components that can wear.  Engineers must know the maintenance requirements so that they can provide adequate spare parts and personnel time.  Naturally, the maintenance costs must be included in the economic analysis of a design.		On-stream analyzers usually require the greatest amount of maintenance.  The cost associated with maintenance can be substantial and should not be overlooked in the economic analysis.
·         Consistency with process environment - Most sensors will function properly for specific process conditions.  For example, many flow sensors function for a single phase, but not for multi-phase fluid flow, whether vapor-liquid or slurry.  The engineer must observe the limitations for each sensor. Some sensors can have direct contact with the process materials, while others must be protected.  Three general categories are given in the following. Direct contact - Sensors such as orifice plates and level floats have direct contact with process fluids. Sheath protection - Sensors such as thermocouples and pressure diaphragms have a sheath between the process fluid and the sensor element. Sample extraction - When the process environment is very hostile or the sensor is delicate and performs a complex physiochemical transformation on the process material, a sample can be extracted. Naturally, the parts of the sensor that contact the process must be selected appropriately to resist corrosion or other deleterious effects.		A float can indicate the interface for a liquid level.  However, a float is not reliable for a “sticky” liquid. Also, a turbine flow meter can be damaged by a rapid change in flow rate or liquid entrained in a vapor stream. Sensors in direct contact must not be degraded by the process material. The sheath usually slows the sensor response. Samples must represent the fluid in the process.
·         Dynamics - The use of the sensor dictates the allowable delay in the sensor response.  When the measured value is used for control, sensor delays should be minimized, while sensors used for monitoring longer-term trends can have some delay.		A greater delay is associated with sensors that require a sample to be extracted from the process. On-stream analyzers usually have the longest delays, which are caused by the time for analysis.
·         Safety - The sensor and transmitter often require electrical power.  Since the sensor is located at the process equipment, the environment could contain flammable gases, which could explode when a spark occurs.		Standards for safety have been developed to prevent explosions.  These standards prevent a significant power source, oxidizing agent and flammable gas from being in contact.
·         Cost - Engineers must always consider cost when making design and operations decisions.  Sensors involve costs and when selected properly, provide benefits.  These must be quantified and a profitability analysis performed. In some cases, a sensor can affect the operating costs of the process.  An example is a flow sensor.  In some situations, the pumping (or compression) costs can be high, and the pressure drop occurring because of the sensor can significantly increase the pumping costs.  In such situations, a flow sensor with a low (non-recoverable) pressure drop is selected.		Remember that the total cost includes costs of transmission (wiring around the plant), installation, documentation, plant operations, and maintenance over the life of the sensor. See a reference on engineering economics to learn how to consider costs over time, using principles of the time value of money and profitability measures.

Sensors

Sensors are used for process monitoring and for process control.  These are essential elements of safe and profitable plant operation that can be achieved only if the proper sensors are selected and installed in the correct locations.  While sensors differ greatly in their physical principles, their selection can be guided by the analysis of a small set of issues, which are will be presented in next posts. 

When defining sensor requirements and principles, the engineer should use terminology that has a unique meaning, which is not easily achieved.  Therefore, the engineer should refer to accepted standards and use the terminology provided in the standards.  For instrumentation, standards published by the ISA (formerly, Instrument Society of America) are the most relevant.  This post and the next posts will use terms from the ISA wherever possible.

The 4-2OmA current loop

The 4-2OmA current loop has been with us for so long that it's become rather taken for granted in the industrial and process sectors alike. Its popularity comes from its ease of use and its performance. However, just because something is that ubiquitous doesn't mean we're all necessarily getting the best out of our current loops. A big benefit of the current loop is its simple wiring just the two wires. The supply voltage and measuring current are supplied over the same two wires. Zero offset of the base current (ie. 4mA) makes cable break detection simple: if the current suddenly drops to zero, you have a cable break.
In addition, the current signal is immune to any stray electrical interference, and a current signal can be transmitted over long distances.

You can think of the current loop itself as being analogous to a water system. You have a hose pipe (the wires) and a source tap (the power supply). You have a spray gun that regulates the flow (the transducer). You can have other equipment on the line, but it all has to be connected together in a ring Loop. The more holes (devices) you have on the hose pipe, the higher the pressure will be required from the tap. Relating all that back to the current loop, you see a power supply, a transducer and one or more pieces of instrumentation all connected together in a ring. You'll often hear things referred to as being either active
or passive. Some instruments have an active output which includes both the control of the current in the loop as well as provide the supply voltage. This is typically specified as being a 4-20mA output into 10-750 Ohms, or something similar. A passive input would be a simple resistor input that has a voltage drop to be factored into the equation once the supply voltage is chosen. This is typically specified as a 4-20mA input into 10 Ohm. 
Working out the power supply requirement is a simple matter of adding up all the units in the loop at maximum current of 20mA. As an example, suppose you have a sensor 'regulator' which requires minimum 12V DC and instrumentation of 10 Ohm input:
10 Ohm x 20mA = 0.2V

So, for this circuit, a 12.2V minimum supply is required, the sensor's maximum voltage might be specified at 30V, so a 24V supply would be all the circuit requirements with spare capacity to boot. In order to measure the current loop it is necessary to break the loop and insert a current meter into it. You can also measure the voltage across the various components by in the loop, such as the voltage out of the power supply, the
voltage over a sensor, and the voltages over the various pieces of instrumentation. This information will give you a good picture of what is happening within the loop.

Measuring Instruments- Selection & Application

Introduction
The measurement of flow in an acid plant is important for the control of the process and for troubleshooting problems that may occur.  In some cases, an approximately flow rate is acceptable for control purposes so a relatively inexpensive instrument is acceptable.  In other situations, greater accuracy is required which will require a more sophisticated instrument cost more.  There are a variety of flow measuring devices available but not all are suitable for acid plant service.

Differential Pressure Devices

Flow devices producing a differential pressure signal, have a reputation for accuracy, simplicity and reliability.  Some differential pressure devices are orifice plates, venturi tube, flow nozzles, annubars, wedge flow meters, etc.  The most common types found in sulphuric acid plant application are described.

Annubars

An annubar produces a differential pressure (DP) signal proportional to the square of the flow rate in accordance with Bernoulli's theorem.  This signal has two components; the high pressure (PH) and the low pressure (PL). 

The high pressure is produced by impact of the velocity profile on the sensor.   The velocity profile results in a corresponding impact pressure profile.  Multiple sensing ports located on the front of the sensor, sense the impact pressure profile.  Inside the high pressure chamber, the average velocity pressure is maintained by the proportionality of the sensing port diameters to the chamber cross-sectional area.

The velocity profile continues around the sensor and creates a low pressure profile. The low pressure profile is sensed by ports, located downstream and opposite the high pressure ports.  Working on the same principle as the high pressure side, an average low pressure is maintained in the low pressure chamber.  The resulting differential pressure is transmitted through the instrument head to a differential pressure flow meter.

Orifice Plates

A concentric, sharp-edged orifice plate is the simplest and least expensive of the differential head meters.  The orifice plate constricts the flow of a fluid to produce a differential pressure across the plate.   The result is a high pressure upstream and a low pressure downstream that is proportional to the square of the flow velocity.

The disadvantage of an orifice plate is it usually produces a greater overall pressure loss than other differential pressure devices.  An advantage of this device is that cost does not increase significantly with pipe size.

Elbow Meters

As a fluid passes through a pipe elbow, the pressure at the outside radius of the elbow increases due to centrifugal force.  If pressure taps are located at the outside and inside of the elbow, a reproducible measurement can be made.

Elbow flow meters are inexpensive and do not introduce any obstructions to the flow of liquid.

Magnetic Flow Devices

The operating principle of the magnetic flow tube is based on Faraday's law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field.

Faraday's Law:  E = kBDV

The magnitude of the induced voltage (E) is directly proportional to the velocity of the conductor (V), conductor width (D) and the strength of the magnetic field (B).

Field coils placed on opposite sides of the flow tube generate a magnetic field.   As the conductive process liquid moves through the field with average velocity V, electrodes sense the induced voltage.

The flowmeter must be completely flooded at all times to obtain an accurate measurement of flow.  Installation in vertical pipes where the flow is going up is ideal.  If the flowmeter and piping are drained during shutdowns, this should be specified to the vendor so appropriate measures can be taken to avoid damage to the flowmeter due to overheating.

Mass Flow Meters

True mass flowmeters measure the mass flow directly rather than the volumetric flow and density and then calculate the mass flow from the two measured values.

Coriolis Meters

The Coriolis meter uses an obstructionless U-shaped tube vibrating at its natural frequency as the fluid passes through it.  The tube twists as result of the flowing fluid and the angular movement of the tube created by the vibration.  Sensors measure the amount of twist and this is proportional to the mass flow rate through the pipe.   Coriolis meters operate independent of liquid density, pressure and viscosity.

Thermal Meters

Thermal meters are commonly applied to gas streams only where the transfer of heat to and from them stream is a usual element of the metering process.  Measuring the heat transfer supplies data from which a mass flow rate may be calculated.  Thermal meters operate independent of density, pressure and viscosity.

Variable Area Devices

Variable area devices, more commonly known as rotameters, are typically made from a tapered glass tube that is positioned vertically in the fluid flow.  A flow that is the same size as the base of the glass tube rides upward in relation to the amount of flow.  Because the tube is larger in diameter at the top of the glass than at the bottom, the float resides at the point where the differential pressure between the upper and lower surfaces balance the weight of the float.  In most cases, the flow rate is read directly from a scale inscribed in the flow tube but transmitting rotameters are available.

Ultrasonics

Ultrasonic flow meters utilize sound waves to determine the flow rate of fluids.   Pulses from a piezoelectric transducer travel through a moving fluid at the speed of sound and provide an indication of fluid velocity.   Two different methods are currently employed to establish this velocity measurement; transit-time and Doppler methods.

Ultrasonic meters have several advantages, including freedom from obstructions in the pipe and negligible cost sensitivity with respect to pipe diameter.  However, their performance is sensitive to flow conditions.

Transit-Time

The transit-time method uses two opposing transducers which are mounted at a 45o angle to the direction of flow.  The speed of sound from the upstream transducer to the downstream transducer represent the inherent speed of sound plus a contribution from the fluid velocity.  A simultaneous measurement is taken in the opposite direction which represents the speed of sound minus the fluid velocity.  The difference between these two values is representative of the fluid velocity which is linearly proportional to the flow rate. 

This method works well where the fluid is free of entrained gas or solids which can scatter the sound waves traveling between the transducers.

Doppler

This type of ultrasonic meter uses two transducers mounted in the same case on one side of the pipe.  An ultrasonic sound wave of constant frequency is transmitted into the fluid by one transducer.  Solids or bubbles within the fluid reflect the sound back to the receiver element.  The Doppler principle states that there will be a shift in apparent frequency or wavelength when there is relative motion between transmitter and receiver.  Within the Doppler flow meter, the relative motion of the reflecting bodies suspended within the fluid tends to compress the sound into shorter wave length (high frequency).  This new frequency measured at the receiving element is electronically compared with the transmitted frequency to provide a frequency difference which is directly proportional to the flow velocity in the pipe.  In contrast to the transit-time method, Doppler ultrasonic meters required entrained gases or suspended solids within the flow to function correctly.

Instruments

Instrument	Accuracy/Repeatability	Installation	References	Remarks
Annubar	± 1% over a flow turndown of greater than 10 to 1, independent of Reynold's number. ± 0.1% repeatability	7 PDU 3 PDD (single elbow flow disturbances)	Dieterich Standard Diamond II Annubar	Relatively low pressure drop
Orifice Plate	± 2-4% of full scale	10-30 PDU 4-8 PDD (dependent on flow disturbances and orifice b ratio)		Relatively high pressure drop
Coriolis	± 0.2% of rate	No requirements for upstream of downstream pipe diameters	Micro Motion Model D65 Rosemount Technical Data Sheet 3031	Can also be used to measure density of fluid
Magnetic	± 0.5% of rate from 1 to 30 ft/s ± 0.005% from low-flow cut-off to 1.0 ft/s	5 PDU 3 PDD butterfly valves not recommended immediately upstream or downstream	Rosemount Model 8701	Grounding rings required with lined or not conductive process piping.  Pipe must be running full.
Elbow	± 5-10% of full scale	30 PDU	Rosemount Technical Data Sheet 3031	Suitable for liquid flow only.  Typically, there will be insufficient pressure differential generated in gas service.
Thermal	± 2% of full scale	No requirements for upstream or downstream pipe diameters	Rosemount Technical Data Sheet 3031	Suitable for gas flow measurement only.
Variable Area	± 2% of full scale	No requirements for upstream or downstream pipe diameters	Rosemount Technical Data Sheet 3031
Ultrasonic	Transit-Time: ± 1% of rate Doppler: ± 3% of rate	5-30 PDU	Rosemount Technical Data Sheet 3031

Applications

The following is a list of typical acid plant services for flow meters and the recommended instrument to measure the flow.

Service	Instrument	Remarks
Sulphuric Acid	Magnetic	Use if accurate measurement of flow rate is required.  Advantage is no additional pressure losses are introduced.  The probe type magnetic flow meters should not be used.  These were replaced by the flowtube type.
	Orifice	Can be used where accuracy is not important (ie. for alarm or interlock function). Additional pressure drop is introduced to the system.  Material of construction is typically Lewmet (Lewis Pumps).
	Ultrasonic	Used primarily as a diagnostic tool for situations where a flowmeter has not been installed. Suitable only for liquid flow measurement.
	Elbow	Can be used where accuracy is not important (ie. for alarm or interlock function).  Does not introduce any additional pressure drop to the system.
Liquid Sulphur	Wedge	Proven reliable and accurate.   Diaphragm seals and a custom steam jacket are required.
Liquid SO2	Coriolis	Suitable for use in commerce (i.e. weighing for sales purposes)
Process Gas	Annubar	Used to measure flow at plant inlet, blower discharge and stack.  Prone to plugging if not maintained properly.
	Thermal	Should be considered as a replacement for annubars in location where annubars continually plug.
Water	Annubar	Used where accuracy is required.
	Orifice	Can be used where accuracy is not important (ie. for alarm or interlock function). Additional pressure drop is introduced to the system.
	Rotameters	For small flows rates, such as acid dilution or make-up water service.
Steam	Orifice	Used in saturated or superheated steam service.  Typically, used as part of a multi-point level control for the steam drum level.

What is the difference between two wire RTD and three wire RTD

Serious lead-wire resistance errors can occur when using a two-wire RTD  especially in a 100&#937 sensor In a two-wire circuit, a current is passed through the sensor.As the temperature of the sensor increases,  the resistance increases. This increase in resistance will be detected by an increase in the voltage (V = I•R). The actual resistance causing the voltage increase is the total resistance of the sensor and the resistance introduced by the lead wires. As long as the lead wire resistance remains constant, it can be offset and not affect the temperature measurement. The wire resistance will change with temperature, however, so as the ambient conditions change, the wire resistance will also change, introducing errors. If the wire is very long, this source of error could be significant. Two-wire RTDs are typically used only with very short lead wires, or with a 1000Ω element.

In a 3-wire  there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current, while L2 acts only as a potential lead. Ideally, the resistances of L1 and L3 are perfectly matched and therefore canceled.The resistance in R3 is equal to the resistance of the sensor Rt at a given temperature—usually the begining of the temperature range. At this point, V out = zero. As the temperature of the sensor increases, the resistance of the sensor increases, causing the resistance to be out of balance and indicated at V out. Resistances L1 and L3 in leads up to tens of feet long usually match well enough for 100 ohm three-wire RTDs. The worst case is resistance offset equal to 10% of single-lead resistance.