SIS (Safety Instrumented Ysytems) in Basic Fundamentals

Basic Fundamentals of Safety Instrumented Systems SIS

The operation of many industrial processes involve inherent risks due to the presence of dangerous material like gases and chemicals. Safety Instrumented Systems SIS are specifically designed to protect personnel, equipment and the environment by reducing the likelihood (frequency) or the impact severity of an identified emergency event.

Explosions and fires account for millions of dollars of losses in the chemical or oil and gas industries each year. Since a great potential for loss exists, it is common to employ Safety Instrumented Systems SIS to provide safe isolation of flammable or potentially toxic material in the event of a fire or accidental release of fluids.

This online training tutorial will explain the basic concepts, definitions and commonly used terms in Safety Instrumented Systems SIS and provide a basic understanding of related concepts.

Basics of Safety and Layers of Protection

Protection Layer

Instrumentation Amplifier

Instrumentation amplifier, it is beneficial to be able to adjust the gain of the amplifier circuit without having to change more than one resistor value, as is necessary with the previous design of differential amplifier. The so-called instrumentation builds on the last version of differential amplifier to give us that capability:

Differential Amplifiers

Reduce Measurement Noise

Ensuring measurement accuracy often means going beyond reading raw specifications in a data sheet. Understanding an application in the context of its electrical environment is also important for securing success, particularly in a noisy or industrial setting. Ground loops, high common-mode voltages, and electromagnetic radiation are all prevalent examples of noise that can adversely affect a signal.
There are many techniques for reducing noise in a measurement system, which include proper shielding, cabling, and termination. Beyond these common best practices, however, there is more you can do to ensure better noise immunity. The following five techniques serve as guidelines for achieving more accurate measurement results.

1. Reject DC Common-Mode Voltage

Making highly accurate measurements often starts with differential readings. An ideal differential measurement device reads only the potential difference between the positive and negative terminals of its instrumentation amplifier(s). Practical devices, however, are limited in their ability to reject common-mode voltages. Common-mode voltage is the voltage common to both the positive and negative terminals of an instrumentation amplifier. In Figure 1, 5 V is common to both the AI+ and AI- terminals, and the ideal device reads the resulting 5 V difference between the two terminals.

 An ideal instrumentation amplifier completely rejects common-mode voltages.

Isolation Technology in Industrial Measurements

Overview

Voltage, current, temperature, pressure, strain, and flow measurements are an integral part of industrial and process control applications. Often these applications involve environments with hazardous voltages, transient signals, common-mode voltages, and fluctuating ground potentials capable of damaging measurement systems and ruining measurement accuracy. To overcome these challenges, measurement systems designed for industrial applications make use of electrical isolation. This white paper focuses on isolation for analog measurements, provides answers to common isolation questions, and includes information on different isolation implementation technologies.

Understanding Isolation

Isolation electrically separates the sensor signals, which can be exposed to hazardous voltages¹, from the measurement system’s low-voltage backplane. Isolation offers many benefits including:

• Protection for expensive equipment, the user, and data from transient voltages
• Improved noise immunity
• Ground loop removal
• Increased common-mode voltage rejection

Isolated measurement systems provide separate ground planes for the analog front end and the system backplane to separate the sensor measurements from the rest of the system. The ground connection of the isolated front end is a floating pin that can operate at a different potential than the earth ground. Figure bellow represents an analog voltage measurement device. Any common-mode voltage that exists between the sensor ground and the measurement system ground is rejected. This prevents ground loops from forming and removes any noise on the sensor lines.

Hazardous voltages are greater than 30 V_rms, 42.4 V_pk, or 60 VDC.

Measuring with Strain Gages

What Is Strain?

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure bellow.

 Definition of Strain

Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10^-6.

When a bar is strained with a uniaxial force, as in Figure 1, a phenomenon known as Poisson Strain causes the girth of the bar, D, to contract in the transverse, or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's Ratio. The Poisson's Ratio n of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force) to the strain in the axial direction (parallel to the force), or n = e_T/e. Poisson's Ratio for steel, for example, ranges from 0.25 to 0.3.

High-Voltage Isolation and Measurements

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.

There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask is whether the system will be safe. Making high-voltage measurements can be hazardous to the equipment, to the unit under test, and to you and your colleagues. To ensure that the system is safe, you should provide an insulation barrier, using isolated measurement devices, between the user and hazardous voltages.

Table of Contents

1. What Is Isolation?
2. Isolation Considerations
3. Typical Applications Requiring Isolation

Insulator Cleaning

Problem

It is widely known that dirty or defective insulators and bushings are the cause of most flashovers, resulting in down-time and loss of production.

Flashed Over Fuse-Holders

  Flashed Over Standoff

Atmospheric contamination build-up on electrical apparatus generated from industrial plants and mills as well as some residential and commercial locations contribute heavily to black-outs and brown-outs. This contamination builds up on insulators, bushings, lightning arrestors, current-transformers, potential-transformers, stress cones, potheads, fuse holders, switches, terminators, trificators, and guy-wire insulators. Depending on the resistance values associated with said contamination, tracking, and in extreme circumstances, arcing takes place usually during periods of light to moderate rainfall. When this contamination forms of a bridge of conductive material over your porcelain non-conductor (insulator) the high voltage electricity finds a path to ground (phase-to-ground fault) resulting in a number of possible inconveniences for your firm including, but not limited to: insulator explosion, insulator etching, insulator chipping, circuit-trip, exposed live-lines, damage to transformers and breakers, loss of production, injury to personnel, exposure to electrocution risks to personnel, expensive and time-consuming repairs, and fires.

Harmonic Compensation

An Active Harmonic Compensator (or Active Filter) has been developed by the University of Canterbury and  Metalect Industries (NZ) Ltd . An active filter removes harmonics from the supply current by injecting the opposite harmonics that are produced by the load. An example of the active filter system is shown in the next graphic.

The Harmonic Filter

Harmonic Filters correct poor power factor, while avoiding potential dangerous resonance points, particularly associated with the 5th and 7th harmonic. They also can help to elimiate or reduce power factor penalties levied by the local utility.

Fixed Passive Harmonic Filters
Cutler-Hammer Fixed Harmonic Filter Banks are applicable when accurate power factor correction is required in an environment where considerable harmonic distortion exists. The capacitor cells are connected with a tuning reactor so that dangerous resonance conditions are avoided. Fixed capacitor banks are tuned to the 4.7th harmonic and will reduce dangerous levels of the 5th harmonic, helping your facility meet the IEEE 519 standard.

• UL Listed / CSA listed
• Reduces harmonics and corrects and maintains desired power factor
• Robust design
• Ideal for 6 pulse drive environments
• Standard ratings available 150-600 kvar, up to 600 VAC; other ratings available

Lamps

Function and Construction

Lamps emit light when an electric current passes through them. All of the lamps shown on this page have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with a high melting point such as tungsten and it is usually wound into a small coil. Filament lamps have a shorter lifetime than most electronic components because eventually the filament 'blows' (melts) at a weak point.

Circuit symbols

There are two circuit symbols for a lamp, one for a lamp used to provide illumination and another for a lamp used as an indicator. Small lamps such as torch bulbs can be used for both purposes so either circuit symbol may used in simple educational circuits.

 Lamp used for lighting

 

Lamp used as an indicator  

Variable Resistors

Construction

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.
Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.
Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.
The resistance and type of track are marked on the body:
    4K7 LIN means 4.7 k linear track.
    1M LOG means 1 M logarithmic track.
Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.
 

Electromagnetic Coils Work

Electromagnetic coils, most commonly known as inductors, are amongst the simplest electric and electronic components. They basically consist of a simple metal electrically insulated wire looped into a cylindrical, toroidal or even disk-like shape, with the role of providing inductance in an electric circuit. Inductance is an electrical property characteristic to electromagnetic coils opposing the flow of current through the circuit.

How it works

Cables

 Cable... flex... lead... wire... what do all these terms mean?

• A cable is an assembly of one or more conductors (wires) with some flexibility. 
• A flex is the proper name for the flexible cable fitted to mains electrical appliances.
• A lead is a complete assembly of cable and connectors.
• A wire is a single conductor which may have an outer layer of insulation (usually plastic). 

Single core equipment wire

This is one solid wire with a plastic coating available in a wide variety of colours. It can be bent to shape but will break if repeatedly flexed. Use it for connections which will not be disturbed, for example links between points of a circuit board. Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A. 

Stranded wire

This consists of many fine strands of wire covered by an outer plastic coating. It is flexible and can withstand repeated bending without breaking. Use it for connections which may be disturbed, for example wires outside cases to sensors and switches. A very flexible version ('extra-flex') is used for test leads. Typical specifications:
10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A.
7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A.
16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A.
24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A.
55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads.

Connector

Battery clips and holders

The standard battery clip fits a 9V PP3 battery and many battery holders such as the 6 × AA cell holder shown. Battery holders are also available with wires attached, with pins for PCB mounting, or as a complete box with lid, switch and wires.
Many small electronic projects use a 9V PP3 battery but if you wish to use the project for long periods a better choice is a battery holder with 6 AA cells. This has the same voltage but a much longer battery life and it will work out cheaper in the long run.
Larger battery clips fit 9V PP9 batteries but these are rarely used now.

Terminal blocks and PCB terminals

Microcontroller

Basically, a microcontroller is a device which integrates a number of the components of a microprocessor system onto a single microchip and optimised to interact with the outside world through on-board interfaces; i.e. it is a little gadget that houses a microprocessor, ROM (Read Only Memory), RAM (Random Access Memory), I/O (Input Output functions), and various other specialized circuits all in one package.

On the other hand, a microprocessor is normally optimised to co-ordinate the flow of information between separate memory and peripheral devices which are located outside itself. Connections to a microprocessor include address, control and data busses that allow it to select one of its peripherals and send to or retrieve data from it. Because a microcontrollers processor and peripherals are built on the same silicon, the devices are self-contained and rarely have any bus structures extending outside their packages.
So a microcontroller incorporates onto the same microchip the following:

• The CPU core
• Memory (both ROM and RAM)
• Some parallel digital I/O
  
  microcontroller component

Assemble and Programming Sensor Color

After my last article that discusses the basic concepts of color sensor, will now be discussed on how to assemble itself and to program a simple color sensor. To detect the number of colors. All it takes is a single LED and a sensor detector sensors like infrared transmitter, can be seen in the figure LED LED nodes and black (which is actually the phototransistor).

Photo Transistor

If desired photoresistor is used as in the picture then use a green LED with a photoresistor which was given a protective black solasi.

Photoresistor

One advantage of using a bright LED for sensing robot is a robot can see very clearly when the light dies. Here are pictures taken from societyofrobots where the robot uses sensors to detect the line color.

Color Sensor Pikachu

Measure Electrical Unit

The standard SI units used for the measurement of voltage, current and resistance are the Volt [ V ], Ampere [ A ] and Ohms [ Ω ] respectively. Sometimes in electrical or electronic circuits and systems it is necessary to use multiples or sub-multiples (fractions) of these standard units when the quantities being measured are very large or very small. The following table gives a list of some of the standard units used in electrical formulas and component values.

Standard Electrical Units

Parameter	Symbol	Measuring Unit	Description
Voltage	Volt	V or E	Unit of Electrical Potential V = I × R
Current	Ampere	I or i	Unit of Electrical Current I = V ÷ R
Resistance	Ohm	R or Ω	Unit of DC Resistance R = V ÷ I
Conductance	Siemen	G or ℧	Reciprocal of Resistance G = 1 ÷ R
Capacitance	Farad	C	Unit of Capacitance C = Q ÷ V
Charge	Coulomb	Q	Unit of Electrical Charge Q = C × V
Inductance	Henry	L or H	Unit of Inductance VL = -L(di/dt)
Power	Watts	W	Unit of Power P = V × I
Impedance	Ohm	Z	Unit of AC Resistance Z2 = R2 + X2
Frequency	Hertz	Hz	Unit of Frequency ƒ = 1 ÷ T

Three Phase and Single Phase in Simple Terms

Electrical service is transmitted in the form of alternating current whose magnitude and direction reverse cyclically (60 times per second). That is, if you put a voltage meter on a single phase AC line, (and magically slowed it down so that you could see it) the voltage would peak in one direction and then reverse polarity sixty times per second.

Graph of single phase single pole current

Transformator Measurements

You may want to determine the various parameters of a transformer without having any manufacturer's information available. In the early stages of experimentation, you may be working with equipment and components that have been "acquired" from other projects as surplus. Or, you may be at a surplus yard, and want to determine if that transformer you can pick up cheap is going to work for your needs. I own a lot of very heavy scrap now, having bought a number of transformers on spec, hoping they would work for my needs.
You'll need a variable voltage AC source (like a variac), a voltmeter, an ammeter, and for some measurements either a watt meter or an oscilloscope. A couple of big power resistors in low resistances (100 ohms, 100 W) will also be useful.
Often, the nameplate or markings on the transformer don't tell you everything you would like to know, particularly in the case of surplus.
Per Unit system of ratings - commercial power distribution transformers have fairly complete data on their nameplates, but it is often stated in terms "per unit". Per Unit measurements are essentially percentages of the rated capacity. For instance, a power transformer might be rated at 10 kVA, and have a rated loss of .03 per unit. 10 kVA multiplied by the .03 gives a rated loss of 300 Watts. Impedances are also usually stated in per unit values
First, you need to identify the windings. An ohmmeter can help you determine which wires are connected to which windings, by looking for continuity. Sometimes, you can determine the order of taps by the resistances. However, the resistance is affected by the size of the wire, as well as the number of turns, so you can't use the ohmmeter to determine voltages or currents. In general, the heavy wires or terminals are for the low voltage windings. A notable exception is some power supply transformers that have some medium voltage windings to provide isolated bias supplies at low currents.
Almost all the parameters of the transformer can be determined by making a series of measurements with the secondary open circuited and then shorted. The choice of which winding is primary and secondary is arbitrary for measurement purposes. For commercial distribution transformers, these measurements are often made at 115 Volts, as measurement equipment is readily available for that voltage. In the case of a transformer with a HV winding and a LV winding, the 115 Volts is applied to the HV winding for safety reasons. In the lab, lower voltages are convenient: using 1.15 Volts or 11.5 Volts allows simple scaling of the results.
Real transformer model

 Transformer Model

Wind Formula

How much power is in the wind?


    P = .5 * AD * (D^2*.7854) * V^3

Where:  P = power in watts
            AD = air density ( typically 1.22 at sea level )
            D = Diameter of prop ( in meters )
            V = Velocity of the wind ( in meters/sec )

So we could say in a 20mph (8.9 m/s) wind and a 6 ft dia ( 1.8 m) prop there is ...

    P = .5 * 1.22 * (1.8^2*.7854) * 8.9^3   or
    P= 1094 watts passing through the prop

Simple Color Sensor

Make a color sensor? We can make a color sensor sederhana.Cukup loh use Super Bright LED and LDR (Light Dependent Resistor) or can be replaced with photodioda / phototransistor. Value constraints LDR / photodioda / phototransistor is dependent on the intensity of light on its surface. The greater the intensity of light about it, the LDR resistance is getting smaller and applies also vice versa. So, with the working principle is quite "unique" this component can be utilized as an object color detection. Because of the intensity of light emitted from the LED Superbright and pemantulnya color different for each color, the value of the barrier was also varied. Strand color detection is quite simple, ie, by utilizing strand voltage divider as shown below.

LDR diagram

 

The Electrical communication signals

The output signals from most control systems are low power analogue signals but there is a growing use of digital systems such as 'Fieldbus®' or 'Profibus®'.

An analogue system provides a continuous but modulating signal whereas a digital system provides a stream of binary numeric values represented by a change between two specific voltage levels or frequencies.

A comparison between digital and analogue systems can be made using Example a and Example b:

Example a

Imagine two people, person A and person B, each on opposite hilltops and each with a flag and a flag-pole. The aim is for person A to communicate to person B by raising his flag to a certain height. Person A raises his flag half way up his pole. Person B sees this and also raises his flag halfway. As person A moves his flag up or down so does person B to match. This would be similar to an analogue system.

Example  b

Now assume that person A does not have a pole but instead has two boards, one with the figure '0' and the other with the figure '1', and again wants person B to raise his flag half way, that is to a height of 50% of his flag-pole. The binary number for 50 is 110010, so he displays his boards, two at a time, in the corresponding order. Person B reads these boards, translates them to mean 50 and raises his flag exactly half way. This would be similar to a digital system.

It can be seen that the digital system is more precise as the information is either a '1' or a '0' and the position can be accurately defined. The analogue example is not so precise because person B cannot determine if person A's flag is at exactly 50%. It could be at 49% or 51%. It is for this reason, together with higher integration of microprocessor circuitry that digital signals are becoming more widely used.

Digital addressing

Sensors

In this Section the subject of temperature measurement will be covered more broadly. There are a wide variety of sensors and transducers available for measuring pressure, level, humidity, and other physical properties. The sensor is the part of the control system, which experiences the change in the controlled variable.

The sensor may be of a type where a change in temperature results in a change of voltage or perhaps a change in resistance.

The signal from the sensor may be very small, creating the need for local signal conditioning and amplification to read it effectively. A small change in resistance signalled by a sensor in response to a change in temperature, may, for example, be converted to an electrical voltage or current for onward transmission to the controller.

The transmission system itself is a potential source of error.

Wiring incurs electrical resistance (measured in ohms), as well as being subject to electrical interference (noise). In a comparable pneumatic system, there may also be minute leaks in the piping system.

The term 'thermostat' is generally used to describe a temperature sensor with on/off switching. 'Transducer' is another common term, and refers to a device that converts one physical characteristic into another; for example, temperature into voltage (millivolts).

An example of a transducer is a device that converts a change in temperature to a change in electrical resistance.

With pneumatic devices, the word 'transmitter' is frequently encountered. It is simply another description of transducer or sensor, but usually with some additional signal conditioning.

However, the actual measuring device is usually termed as the sensor, and the more common types will be outlined in the following Section.

Filled system sensors

With pneumatic controllers, filled system sensors are employed. Figure bellow illustrates the principles of such a system.

Liquid filled system sensor and gas filled or vapour pressure system

The Controllers

It is important to state at the outset that not all control applications need a sophisticated controller.

An on/off valve and actuator, for example, can be operated directly from a thermostat. Another example is the operation of high limit safety controls, which have a 'snap' action to close valves or to switch off fuel supplies.

However, when the control requirements become more sophisticated, a controller is needed to match these requirements.

The controller receives a signal, decides what action is needed and then sends a signal to the actuator to make it move.

In the age of the microchip, integrated circuits and computers, the functions performed by the controller can be very complex indeed.

However, since an analogy between the human brain and controllers/computers has been made in previous Tutorials, the renowned IBM motto can be paraphrased:

Computer - Fast, accurate and stupid
Human being - Slow, slovenly and brilliant

Basic Control Valves

Introduction to Electric / Pneumatic Controls

A basic control system would normally consist of the following components:

• Control valves.
• Actuators.
• Controllers.
• Sensors.

All of these terms are generic and each can include many variations and characteristics. With the advance of technology, the dividing line between individual items of equipment and their definitions are becoming less clear. For example, the positioner, which traditionally adjusted the valve to a particular position within its range of travel, can now:

• Take input directly from a sensor and provide a control function.
• Interface with a computer to alter the control functions, and perform diagnostic routines.
• Modify the valve movements to alter the characteristics of the control valve.
• Interface with plant digital communication systems.

However, for the sake of clarity at this point, each item of equipment will be considered separately.

Control Valves

Whilst a wide variety of valve types exist, this document will concentrate on those which are most widely used in the automatic control of steam and other industrial fluids. These include valve types which have linear and rotary spindle movement.

Linear types include globe valves and slide valves.

Rotary types include ball valves, butterfly valves, plug valves and their variants.

The first choice to be made is between two-port and three-port valves.

• Two-port valves 'throttle' (restrict) the fluid passing through them.
• Three-port valves can be used to 'mix' or 'divert' liquid passing through them.

Two-port valves

Globe valves

Globe valves are frequently used for control applications because of their suitability for throttling flow and the ease with which they can be given a specific 'characteristic', relating valve opening to flow.

Two typical globe valve types are shown in Figure bellow. An actuator coupled to the valve spindle would provide valve movement.

Two differently shaped globe valves

Electrical Actuators

Where a pneumatic supply is not available or desirable it is possible to use an electric actuator to control the valve. Electric actuators use an electric motor with voltage requirements in the following range: 230 Vac, 110 Vac, 24 Vac and 24 Vdc.

There are two types of electrical actuator; VMD (Valve Motor Drive) and Modulating.

VMD (Valve Motor Drive)

This basic version of the electric actuator has three states:

1. Driving the valve open.
2. Driving the valve closed.
3. No movement.

Typical electric valve actuator

 Valve motor drive actuator system

Positioners

For many applications, the 0.2 to 1 bar pressure in the diaphragm chamber may not be enough to cope with friction and high differential pressures. A higher control pressure and stronger springs could be used, but the practical solution is to use a positioner.

This is an additional item (see Figure bellow), which is usually fitted to the yoke or pillars of the actuator, and it is linked to the spindle of the actuator by a feedback arm in order to monitor the valve position. It requires its own higher-pressure air supply, which it uses to position the valve.

Basic pneumatic positioner fitted to actuator pillars (valve not shown)

A valve positioner relates the input signal and the valve position, and will provide any output pressure to the actuator to satisfy this relationship, according to the requirements of the valve, and within the limitations of the maximum supply pressure.

When a positioner is fitted to an 'air-to-open' valve and actuator arrangement, the spring range may be increased to increase the closing force, and hence increase the maximum differential pressure a particular valve can tolerate. The air pressure will also be adjusted as required to overcome friction, therby reducing hysteresis effects.

Example: Taking a PN5400 series actuator fitted to a DN50 valve (see Table in Figure above)

1. With a standard 0.2 to 1.0 bar spring range (PN5420), the maximum allowable differential pressure is 3.0 bar.
2. With a 1.0 to 2.0 bar spring set (PN5426), the maximum allowable differential pressure is increased to 13.3 bar.

With the second option, the 0.2 to 1.0 bar signal air pressure applied to the actuator diaphragm cannot provide sufficient force to move an actuator against the force provided by the 1.0 to 2.0 bar springs, and even less able to control it over its full operating range. In these circumstances the positioner acts as an amplifier to the control signal, and modulates the supply air pressure, to move the actuator to a position appropriate to the control signal pressure.

For example, if the control signal was 0.6 bar (50% valve lift), the positioner would need to allow approximately 1.5 bar into the actuator diaphragm chamber. Figure bellow illustrates this relationship.

 The positioner as a signal amplifier

It should be noted that a positioner is a proportional device, and in the same way that a proportional controller will always give an offset, so does a positioner.

Actuator

One form of controlling device, the control valve, has now been covered. The actuator is the next logical area of interest.

The operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve. The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal.

The actuator accepts a signal from the control system and, in response, moves the valve to a fully-open or fully-closed position, or a more open or a more closed position (depending on whether 'on / off' or 'continuous' control action is used).
There are several ways of providing this actuation. This Tutorial will concentrate on the two major ones:

• Pneumatic.
• Electric.

Pneumatic actuators - operation and optionsPneumatic actuators are commonly used to actuate control valves and are available in two main forms; piston actuators (Figure bellow) and diaphragm actuators (Figure bellow).

  Typical piston actuators

Piston actuators

Valves Used in Water Rupply Pipe Lines

Valves in pipe lines are provided to control supply of water. They are used to stop supply when any repair is carried in pipe line. There are fourteen types of valves for plumbing.

1. Fancy Stop Valve

   

   These valves are available in normal sizes of 15 mm and 20 mm. These are also called open stopcock valves. 

2. Angle Stop Valve

   

   This valve has inlet and outlet at right angles to each other. This is used to stop supply of water to geyser, flushing cistern and wash basin etc. This valve is also used for servicing of water fitting to be carried out without shut off complete water supply of the house.

3. Concealed Stop Valve

   
   It is used to regulate supply of hot and cold water to shower, mixer etc. in concealed plumbing line.