Industrial Instrumentation

INDUSTRIAL INSTRUMENTATION (BASIC INSTRUMENTS)

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Measurement of pressure

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges A manometer is a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. It is often used to refer specifically to liquid column hydrostatic instruments. A vacuum gauge is used to measure the pressure in a vacuum, which is broadly divided into two categories: high and low vacuum (and sometimes ultra-high vacuum). Many of the different techniques used to measure these categories have an overlap at some point in the pressure range. By combining several different types of gauge it is possible measure system pressure from 10 mbar down to 10e-11 mbar. The SI unit of pressure is the pascal (abbreviation Pa). Atmospheric pressures are usually stated using its decimal multiple kilopascal (kPa), where 1 kPa is close to 1.0% of Earth's atmospheric pressure at sea level. In meteorologic reports, hPa or mbar are the commonly used units (being by definition 1 bar = 100 kPa). In vacuum systems, the equivalent units torr and millimeter of mercury (mmHg) are also used, with 1 torr equaling 133.3223684 Pa above an ideal vacuum. Other vacuum units occasionally encountered in the literature include micrometers of mercury, the barometric scale, or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is measured in the United States also in inches of mercury (inHg) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the atmospheric pressure (29.92 inHg) minus the vacuum pressure in inches of mercury. (This is effectively a gauge pressure.)Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 29.92 inHg − 26 inHg = 3.92 inHg.

BOURDEN TUBE TYPE PRESSURE GUAGE

A Bourdon gauge uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm connected to the tube. A combination pressure and vacuum gauge (case and viewing glass removed)

Indicator Side with card and dial

Mechanical Side with Bourdon tube

The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed. Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings. In the following pictures the transparent cover face has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis: 

the left side of the face, used for measuring manifold vacuum, is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.

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the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/cm² on its inner scale and pounds per square inch on its outer scale.

Mechanical details

Stationary parts:    

A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case. B: Chassis Plate. The face card is attached to this. It contains bearing holes for the axles. C: Secondary Chassis Plate. It supports the outer ends of the axles. D: Posts to join and space the two chassis plates.

Moving Parts: 1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block. 2. Moving end of Bourdon tube. This end is sealed. 3. Pivot and pivot pin. 4. Link joining pivot pin to lever (5) with pins to allow joint rotation. 5. Lever. This an extension of the sector gear (7). 6. Sector gear axle pin. 7. Sector gear. 8. Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle. 9. Hair spring to preload the gear train to eliminate gear lash and hysteresis.

DIAPHRAGM TYPE PRESSURE GAUGE

A pile of pressure capsules with corrugated diaphragms in an aneroid barograph. A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined using by calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used. Useful range: above 10-2 torr [5] (roughly 1 Pa) For absolute measurements, welded pressure capsules with diaphragms on either side are often used. Shape:    

Flat corrugated flattened tube capsule

PRESSURE TRANSMITTER @

OPERATION OF PRESSURE TRANSMITTER:

1. P TO F CONVERSION: The process fluid applied to capacitive sensor change the value of the sensor’s Cs capacitor, generating a sense frequency Fs by the Enhanced Oscillator (EMO) that is directly proptional to the pressure.

pressure thereby Mode applied

2. F TO DIGITAL CONVERSION: The first of the three frequencies (Fr, Fs+r, Fs) generated by the BMO is applied to the ASIC. When all three frequencies are store, the micro-controller shifts the data into its serial port. The micro-controller uses a specially developed algorithm that cancels the effects of parasitic capacitance Cp and calculates the true ratio Cr/Cs. When the ratio is equal to 1 the pressure diff between the two capacitor is known to be zero. A ratio less than I corresponds to a +ve pressure diff. And a ratio greater than a –ve pressure diff. 3. D/A CONVERSION AND CURRENT SIGNAL TRANSMISSION: The pressure signal recived by the micro-controller is applied through the ASIC to a DAC. The DAC translates the digitalized pressure signal into a PWM signal, whose PW is proportional to the magnitude of the process pressure. The pulse are filtered and applied to an operational amplifier, which drives V4 converter, whose output is Darlington transistor pair acting as a pass transistor, that outputs a std. 4 to 20mA current signal to the n/w. A transmitter can be configured to operate in either an analog or a digital mode, for a point-to-point or multi drops n/w res. (A) Analog mode: A single transmitter is connected to a controller recorder or other field device, a loop known as point to point n/w interconnects the instruments. The transmitter `s output is the process variable and it is sent to a controller or recorder using astd.4-20mA analog currnt. The HART protocol is used to send all process variable information to a HART compatible controller, recorder or other device. (B) Dgital Mode:

Twisted pair cable is used to wire up to 15 parallel connected transmitter to the multidrop ii/w. HART protocol is used to send all process variable information to a HART compatible controller, recorder or other device.

PRESSURE SWITCH

The device is used to control any associated parameter during the rise or full of pressure above or below a required level. A pressure switch turns and electric circuit “on” or at a preset pressure. This pressure is called the “set point “ of the switch.construction wise a bottom tube, diaphragm or bellows acts as the sensing elements which actuates the switch, which may be a micro switch or mercury switch. As shown in fig. There is a bellow which recives the pressure to be measured which is in contact with a flapper balancing spring whose tension could be adjusted with the help of range screw and its indication by a pointer on a scale calibrated in terms of pressure. Thus, the force due to bellows and spring tension acts against each other on the other side of flapper tjere is a micro switch. When the process pressure increase it expands the bellows which moves the flapper end in the direction shown. When the force due to bellows exceeds that due to the spring at the same value, the other side of the flapper comes in contact with the micro switch as a result of which activating the “no” or “nc” contact. The change over to the contacts could be used to actuate a relay which controls a final control element like a control value of pressure switch has a “dead band” which is the range of value of pressure during which the changing of the contacts does not take place.

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MEASUREMENT OF TEMPRETURE THERE ARE MANY INSTRUMENTS ARE USED FOR MEASURE MENT OF TEMPERATURE SOME OF THEM ARE BELOW.

LIQUID IN METEL TYPE TEMP. GUAGE

This is most often used tempreture gauge. In this type liquid is filled in the metal tube.most generally used liquids are mercury and alchohol. OPERATION PRINCIPLE: As above, the liquid filled in tube if there is any change in tempareture occurred then the level of liquid will be increase or decrease. The tube connected with liquid filled bourden tube. so, as pressure increase or decrease there will be change in the bourden tube. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is

presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. The difflection of tube connected with pointer by link.the movement of pointer will indicat the value of tempratureThis is easy to install. this kind of guage has lower cost.

THERMOCOUPLE

In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is precision; system errors of less than 1 °C can be difficult to achieve.

@ Principle of operation when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit will have a different voltage generated, leaving a small difference voltage available for measurement, which increases with temperature. This difference can typically be between 1 and about 70 microvolts per degree Celsius for the modern range of available metal combinations. Certain combinations have become popular as industry standards, driven by cost, availability, convenience, melting point, chemical properties, stability, and output.

` is important to note that thermocouples measure the temperature difference between two points, not absolute temperature. In traditional applications, one of the junctions — the cold junction — was maintained at a known (reference) temperature, while the other end was attached to a probe. For example, in the image above, the cold junction will be at copper traces on the circuit board. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated. Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature. Thus, the voltages of the individual thermocouple add up, which allows for a larger voltage.

Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They incorporate into their circuits an artificial cold junction using some other thermally sensitive device (such as a thermistor or diode) to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation. Additionally, cold junction compensation can be performed by software. Device voltages can be translated into temperatures by two methods. Values can either be found in look-up tables or approximated using polynomial coefficients.

Usually the thermocouple is attached to the indicating device by a special wire known as the compensating or extension cable. The terms are specific. Extension cable uses wires of nominally the same conductors as used at the thermocouple itself. These cables are less costly than thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distances - typically as flexible insulated wiring or multicore cables. They are usually specified for accuracy over a more restricted temperature range than the thermocouple wires. They are recommended for best accuracy. Compensating cables on the other hand, are less precise, but cheaper. They use quite different, relatively low cost alloy conductor materials whose net thermoelectric coefficients are similar to those of the thermocouple in question (over a limited range of temperatures), but which do not match them quite as faithfully as extension cables. The combination develops similar outputs to those of the thermocouple, but the operating temperature range of the compensating cable is restricted to keep the mis-match errors acceptably small.Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and RTDs are more suitable.

RESISTANCE TEMPERATURE DETECTRS Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C.

@ General description There are two broad categories, "film" and "wire-wound" types. 

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Film thermometers have a layer of platinum on a substrate; the layer may be extremely thin, perhaps 1 micrometer. Advantages of this type are relatively low cost and fast response. Such devices have improved in performance although the different expansion rates of the substrate and platinum give "strain gauge" effects and stability problems. Wire-wound thermometers can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift.

The current international standard which specifies tolerance and the temperature to electrical resistance relationship for platinum resistance thermometers is IEC 751:1983. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392 ohm/°C are also available.

@ How do resistance thermometers work? Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance and require a small power source to operate. The resistance ideally varies linearly with temperature. Resistance thermometers are usually made using platinum, because of its linear resistancetemperature relationship and its chemical inertness. The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. Commercial platinum grades are produced which exhibit a change of resistance of 0.385 ohms/°C (European Fundamental Interval) The sensor is usually made to have a resistance of 100Ω at 0 °C. This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ω/°C. Resistance thermometers require a small current to be passed through in order to determine the resistance. This can cause resistive heating, and manufacturers' limits should always be followed along with heat path considerations in design. Care should also be taken to avoid any strains on the resistance thermometer in its application. Lead wire resistance should be considered, and adopting three and four wire connections can eliminate connection lead resistance effects from measurements. Advantages of platinum resistance thermometers:    

High accuracy Low drift Wide operating range Suitability for precision applications

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@ Resistance thermometer elements Resistance thermometer elements are available in a number of forms. The most common are: Wire wound in a ceramic insulator - wire spiral within sealed ceramic cylinder, works with temperatures to 850 °C

Wire encapsulated in glass - wire around glass core with glass fused homogenously around, resists vibration, more protection to the detecting wire but smaller usable range Thin film - platinum film on ceramic substrate, small and inexpensive to mass produce, fast response to temperature change

@ Resistance thermometer construction

These elements nearly always require insulated leads attached. At low temperatures PVC, silicon rubber or PTFE insulators are common to 250 °C. Above this, glass fibre or ceramic are used. The measuring point and usually most of the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process. Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical attack and offer convenient process attachment points.

TEMPERATURE SWITCHES :Temperature switches generally operates on one of four principles: 1. 2. 3. 4.

thermal expanion differantial thermal expantion vapoure pressure thermocouple

he thermal expansion device can be mercury filled bulb some what like an ordinary laboratory glass thermemeter. In the industrial temperature switch, this would normally be murcury filled or gas filled system, consisting of a copper or stainless steel bulb, connected to a pressure switch throu a small capillary tube. Expansion of the mercury or gas with an increase in temperature would increase the filled system pressure that then actuyates the switch. The vapoure pressure switch is a filled system.having the same configuration as the mercury thermal expansionType.

Thermal expantion and vapoure pressure temprature switch cover a wide range of temperature and provide sample force to operate a dependable switch mechanism. Bimatalic temperature switchs are made in many configuration. They operate on differential thermal expansion between two metal pats that are thermally coupled to the process stream. The motion of the differential expansion is transferred to the switch through and adjustable mechanism. Bimatalic switches are generally small, low cost and useful over a wide range of tempreture. Thermocouple can be used asw a temperature measurement element because they generate a minute voltage that is a function of the temperature these sensores good for a wide range of temperature are very rugged and inexpensive. The measurement voltage for a thermocouple is the difference between the voltage at the point of measurement and the referance junction voltage at some other point in the circuit.

FLOW MEASUREMENT

Orifice,venturies,pitot tubes, annular and nozzle are the main primary elements. The are sued to create a restriction in the flo9w and create a differential pressure across them which are sended by the secondary elements. So construction and design of these elements are of critical importance. Orifice plates are used frequently than other types. They are actually obstruction within the pipe and made from stainless steel 1/8” to ½” thick.

ORIFICE PLATE

An orifice plate is a device which measures the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation.

VENTURI TUBES: The venturi tube is highly accurate instrument for measuring pressure diffrential. It is used for medium and high quantity fluid flow.

The short venturi is particularly adopted to install in pipelines , not having long Unobstructed runs. The flow of fluid through the venturi tube, esteblised the pressure differential , which can then be measured and releted to the flow rate.

It consist of two small hollow pipes made in the form of cone at the one end . the cones are joined by short parellel pipe. The smallest diameter is called the throat section. Where lower or downstream pressure is measured. In the parallel pipe to the upstream, the upstream cone made steeper, to give smooth approach to the throat section. The downstream cone recovers part of the pressure differential, theus redusing overall pressure loss. Fluid speed at throat is faster. It is made of cast iron or steel. It’s initial cost is high but pressure loss is lower than orificeand less wear and abrasion .

ROTAMETER A Rotameter is a device that measures the flow rate of liquid or gas in a closed tube. It is occasionally misspelled as 'rotometer'. It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross sectional area the fluid travels through to vary, causing some measurable effect. A rotameter consists of a tapered tube, typically made of glass, with a float inside that is pushed up by flow and pulled down by gravity. At a higher flow rate more area (between the float and the tube) is needed to accommodate the flow, so the float rises. Floats are made in many different shapes, with spheres and spherical ellipses being the most

common. The float is shaped so that it rotates axially as the fluid passes. This allows you to tell if the float is stuck since it will only rotate if it is not. Readings are usually taken from the top of the float. Some manufacturers may use a different standard, so it is always best to check the documentation provided with the device. Note that the 'float' does not actually float in the fluid: it has to have a higher density than the fluid otherwise it will float to the top even if there is no flow. Advantages:  

A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate. A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for widespread use in places such as third world countries.

Disavantages:  

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Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upwards. Due to its reliance on the ability of the fluid or gas to displace the float, the graduations on a given rotameter will only be accurate for a given substance. The main property of importance is the density of the fluid, however viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity, however this is seldom verifiable from manufacturers specs. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be utilized. Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries to benign fluids, such as water. Rotameters are not easily adapted for reading by machine: although magnetic floats that drive a follower outside the tube are available.

MEASUREMENT OF LEVEL

SIGHT GLASS @ GENERAL A sight glass (gauge glass) is another method of liquid level measurement.it is used for the continuous indication of liquid level with a tank or vessel. @ CONSTRUCTOIN AND WORKING

A sight glass instrument consist of a graduated tube of toughened glass which is connected to the interior of a graduated tube of bottom in which the water level is required.fig.shows a simple sight matches the level of liquid in the tank. As the level of liquid in the tank rises and falls, the level in the sight glass the level of liquid in the tank is measured. In sight glass, it is not necessary, to use the same liquid as in the tank. Any other desired liquid also can be used.

Then it is desired to measure a liquid with the liquid under pressure or vacuum, the sight glass must be connected to the tank at the top as well at the bottom otherwise, the pressure difference between the tank and the sight glass would cause false reading. In this case, the glass tube is enclosed in a protective housing, and two valves are provided for isolating the gauge from the tank in case of breakage of the glass. The smaller valve at the bottom is provided for blowing out the gauge for cleaning purposes fig. Shows a high pressure sight glass. In which measurement is made by reading the position of the level on the calibrated scale. These safety precautions. The glass tube muses have a small inside diameter and a thick wall

DIFFERENTIAL PRESSURE TYPE LEVEL METER The level also measure by measuring the upper and lower level pressure of liquid.the differential pressure will show the level .

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P

TO

F

CONVERSION:

The process fluid applied to capacitive pressure sensor change the value of the sensor’s Cs capacitor, thereby generating a sense frequency Fs by the Enhanced Mode Oscillator (EMO) that is directly proportional to the applied pressure.

@ F TO DIGITAL CONVERSION: The first of the three frequencies (Fr, Fs+r, Fs) generated by the BMO is applied to the ASIC. When all three frequencies are store, the micro-controller shifts the data into its serial port. The micro-controller uses a specially developed algorithm that cancels the effects of parasitic capacitance Cp and calculates the true ratio Cr/Cs. When the ratio is equal to 1 the pressure diff between the two capacitor is known to be zero. A ratio less than I corresponds to a +ve pressure diff. And a ratio greater than a –ve pressure diff. @ D/A CONVERSION AND CURRENT SIGNAL TRANSMISSION: The pressure signal recived by the micro-controller is applied through the ASIC to a DAC. The DAC translates the digitalized pressure signal into a PWM signal, whose PW is proportional to the magnitude of the process pressure. The pulse are filtered and applied to an operational amplifier, which drives V4 converter, whose output is Darlington transistor pair acting as a pass transistor, that outputs a std. 4 to 20mA current signal to the n/w.

LEVEL SWITCH

@ OPERATING PRINCIPLE: Diagram illustrated the simple and fool proof magnetron operating principle switching action is obtained, that the use of a magnetic attractive sleeve, actuated by a float and switching mechanism are separated by a non magnetic, pressure fifth enclosing tube. Switching magnets are assembled to a switching arm which operates on precision 35-pivot socket.

@ OPERATION CYCLE: At “normal level” operating of liquid in the chamber the float moves the magnetic attraction sleeve upward in the enclosed tube and into the field of the switch mechanism magnet. As a result the magnet is drawn in tightly to the enclosing tube causing the switch to tilt,” making” on “breaking” and electric circuits. A liquid level reduces the float pulls the magnetic attracting sleeve downward until, at predetermined “low level “ the switch magnet release and is drawn outward away from the enclosing tube by a tension spring. This in turn tilts the switch in an opposite direction, thus reversing switch action. When liquid level returns to normal the float once again moves the magnetic sleeve up the enclosing tube, causing the switch to assume its original position.

CONTROL VALVES Control valves are valves used within industrial plants and elsewhere to control operating conditions such as temperature, pressure, flow, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "setpoint" to a "process variable" whose value is provided by sensors that monitor changes in such conditionsThe opening or closing of control valves is done by means of electrical, hydraulic or pneumatic systems.

GLOBE VALVE Globe valve is a device for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. Globe Valves are named for their spherical body shape with the two halves of the body being separated by an internal baffle. This has an opening that forms a seat onto which a movable plug can be screwed in to close (or shut) the valve. In globe valves, the plug is connected to a stem which is operated by screw action in manual valves. Typically, automated valves use sliding stems. Globe valves have a smooth stem rather than threaded and are opened and closed by an actuator assembly. When a globe valve is manually operated, the stem is turned by a handwheel. Although globe valves in the past had the spherical bodies which gave them their name, many modern globe valves do not have much of a spherical shape. However, the term globe valve is still often used for valves that have such an internal mechanism. In plumbing, valves with such a mechanism are also often called stop valves since they don't have the global appearance, but the term stop valve may refer to valves which are used to stop flow even when they have other mechanisms or designs.

Globe valves are used for applications requiring throttling and frequent operation. For example, globe valves or valves with a similar mechanism may be used as sampling valves, which are normally shut except when liquid samples are being taken. Since the baffle restricts flow, they're not recommended where full, unobstructed flow is required. Globe valves are typically two-port valves, although three port valves are also produced. Ports are openings in the body for fluid flowing in or out. The two ports may be oriented straight across from each other on the body, or oriented at an angle such as a 90° angle. Globe valves with ports at such an angle are called angle globe valves.

BUTTERFLY VALVE

A butterfly valve is a type of flow control device, typically used to regulate a fluid flowing through a section of pipe. The valve is similar in operation to a ball valve. A flat circular plate is positioned in the center of the pipe. The plate has a rod through it connected to a handle on the outside of the valve. Rotating the handle turns the plate either parallel or perpendicular to the flow. Unlike a ball valve, the plate is always present within the flow, therefore a pressure drop is always induced in the flow regardless of valve position.

There are three types of butterfly valve: 1. Resilient butterfly valve which has a flexible rubber seat. Working pressure up to 1.6 megapascals (MPa)/232 pounds per square inch (PSI) 2. High performance butterfly valve which is usually double eccentric in design . Working pressure up 5.0 MPa/725 PSI 3. Tricentric butterfly valve which is usually with metal seated design. Working pressure up to 10.0 MPa/1450 PSI

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Butterfly valves are widely used in water distribution and waste water processing (not recommended, as the debris may block the operation of the disc). Butterfly valves can come in two body types, affecting installation and maintenance: lugged or wafer. Wafer style valves are more common. They are typically installed between two flanges using bolts or studs and nuts. Lug style valves are also installed between two flanges but with a separate set of bolts for each flange. The lug style setup makes it possible to remove one side of the piping while the other remains intact.

An additional application is found within the exhaust system of automobiles. By incorporating a butterfly valve in the exhaust system, it is possible to control the backpressure and noise output from the muffler and catalytic converter. When in a closed position, the valve increases the amount of back pressure produced and suppresses noise. The angle of valve can be controlled in a variety of ways, including manual control, vacuum control, as well as being tired directly to the throttle.

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