The 0-10 V signal has tendency to drop because of line resistance. If the distance between sensor and input card is more the signal will not properly represent the field value. The 4-20 mA will travel a long distance without dropping signal value.
Following specifications are used for control valve:
A feedback device which converts mechanical motion into electronic signals. Usually an encoder is a rotary device that outputs digital pulses which correspond to incremental angular motion. The encoder consists of a glass or metal wheel with alternating clear and opaque stripes that are detected by optical sensors to produce the digital outputs.
Process pressure is trmitted through isolating diaphragms and oil fill fluid to a sensing diaphragm. The sensing diaphragm is a stretched spring element that deflects in response to differential pressure across it. The displacement of the sensing diaphragm, a maximum deflection of0.004 inch (0.10 mm), is proportional to the applied pressure. Capacitor plates on both sides of the sensing diaphragm detect the position of the diaphragm. The trmitter electronics convert the differential capacitance between the sensing diaphragm and the capacitor plates into a two- wire, 4-20 mA dc signal and a digital output signal.
Suitable restriction placed in flow stream causes a differential pressure across it. As flow depends upon differential pressure (Head) & area, so any of them or both can be varied for varying flow.
A trmitter is an electronic device that is generally mounted in the field in close proximity to a sensor. The sensor (also known as a trducer) measures a physical variable such as temperature or pressure and outputs a very low level electronic signal. The basic function of the trmitter is to provide the correct electrical power to turn on (or excite) the sensor then to read the low level sensor signal, amplify it to a higher level electrical signal and send that signal a long distance to a control or read-out device.
Since low-level electrical signals do not trmit long distances with great accuracy, installing a trmitter generally gives a tremendous improvement in the accuracy of the information delivered to a larger control system. Typically the output form the trmitter is 4-20 mA or 0-10 V
Actuator, Body, Trim, Diaphragm, Diaphragm plate, Actuator stem, Actuator spring, Seat, Travel Indicator, Valve stem, Gaskets, Yoke, Hand wheel
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In a closed-loop control system, the part of the final control element that trlates the control signal into action by the control device.
An actuator is a component of a machine that is responsible for moving or controlling a mechanism or system. An actuator requires a control signal and a source of energy. The control signal is relatively low energy and may be electric voltage or current, pneumatic or hydraulic pressure, or even human power.
The components of automation system include
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In PC based control system, the CPU of computer acts as processor, the PCI based cards are used for connecting Input and Output. The RAM acts as memory. Hard disk is used as storage device. Currently this systems are very useful when the large data is to be proceed with very high speed. In many cases for greater accuracy we can use the real-time operating system.
ON – OFF SERVICES :- Gate, Ball, Diaphragm, Plug, Butterfly valves.
THROTTLING SERVICES :- Globe, Butterfly, Diaphragm, Pinch valves.
NON – REVERSE FLOW :- Check valves.
Resistance Temperature Device works on the principles that the resistance of the material changes as its temperature changes. Temperature is determined by measuring resistance and then using the RTD Resistance vs Temp characteristic to detect temperature.
Typical elements used for RTD are Nickel, Copper and Platinum. Platinum is widely used in RTDs because of accuracy. PT 100 me at 0 deg temp 100 ohms resistance. A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a protective coating (glass or ceramic).
Thermocouple consists of two strips or wires made up of different metals and joined at one end. The temperature at that juncture induces an electromotive force (emf) between the other ends. As the temperature goes up the emf also increases. Through standard charts and tables the corresponding temperature can be found out.
The relationship between the thermocouple output and the temperature is quite non linear. Different metallurgies produce different outputs. The different metallurgies and different lineararities result in different thermocou0le designations such as “J”, “K,”, “N”, “L”, etc.
Fieldbus is a general term for a digital only, high speed communications protocol. The key attribute to Fieldbus communications is higher speed communications with the possibility of addressing multiple trmitters all on the same field wiring. The Foundation Fieldbus is a specific digital protocol that is often shortened to just be called Fieldbus. Other digital only communications such as Profibus are also Fieldbus protocols
PID (Proportional Integral Derivative) is the algorithm widely used in closed loop control. The PID controller takes care of closed loop control in plant. A number of PID controller with single or multiple loop can be taken on network.
PID Controllers are widely for independent loops. Although some logic can be implemented but not much of sequential logic can be implemented in PIDs.
For creating differential pressure : Orifice plate, Venturi Tube, Flow Nozzle , pitot tube
For measuring pressure : U-Tube Manometers, Ring–Balance Manometer, D.P. Cell
The control valve, commonly named the final control element of control contains a pneumatic device that converts the control signal from the controller in action, regulating the flow.
A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
The industry accepted standard for the temperature at open end is 0 deg C. Therefore most tables and chart make the assumption that the temp at open end is 0 deg C. In industry the open ends are always at actual room temperature and not 0 deg C. The emf adjustment because of difference between the actual temp and 0 deg C is referred as Cold Junction Correction (CJ Correction)
DCS: The system uses multiple processors, has a central database and the functionality is distributed. That is the controller sub system performs the control functions, the history node connects the data, the IMS node gives reports, the operator station gives a good HMI, the engineering station allows engineering changes to be made.
PLC: The system has processor & I/O’s and some functional units like basic modules, communication modules and so on. Uses a SCADA for visualization. Generally the SCADA does not use a central database.
DCS is often used in the big plants where the redundancy level needed is more and the analog input used are high.
With 0- 20 mA you can not distinguish between minimum field value and connection break. With 4-20 mA, internal circuit can distinguish between connection break of minimum value. Normally when the value is minimum the trmitter will give you 4 mA while in case of connection breakage it will give 0 mA.
It is the flow of water (G=1, T= 6 to 34 deg. C) through the valve at full lift in U.S gallon per minute with a pressure drop across the valve of 1 psi.
The thermocouple work on broad temperature range ie –270 to 2300.
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Automation is delegation of human control functions to technical equipment for increasing productivity, better quality, reduced cost & increased in safety working conditions.
The RTD work on temperature range between–250 to 850 deg C.
No. The T/C signal is a very low-level millivolt signal, and should only be connected to one device. Splitting to two devices may result in bad readings or loss of signal. The solution is to use a “dual” T/C probe, or convert one T/C output to a 4-20 mA signal by using a trmitter or signal conditioner; then the new signal can be sent to more than one instrument