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# Ch. 2 pressure measurement

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### Ch. 2 pressure measurement

1. 1. CHAPTER 2 PRESSURE MEASUREMENT
2. 2. PRESSURE DEFINITION PRESSURE is force exerted per unit area of surface. P = F/A FORCE P = pressure F = force A = surface area exposed to the force Pressure Units: Pound / Square Inch = P / SI = PSI Newton / Square Meter = N / m2 100,000 100 000 N / m2 = 1 Bar 14.5 psi = 1 Bar Inches of mercury “inch Hg”, “mm Hg” inch. Hg mm.Hg AREA
3. 3. TYPES OF PRESSURE The pressure at point X can be expressed as: o 10” of mercury absolute o 20” of mercury 20 vacuum o -20 i h of 20 inches f mercury gauge
4. 4. PRESSURE SENSING ELEMENTS A basic instrument system consists of three elements: 1. SENSOR or INPUT DEVICE 2. 2 SIGNAL PROCESSOR 3. RECEIVER or OUTPUT DEVICE
5. 5. The block diagram of a sensor is shown below.  Pressure sensors either convert the pressure into mechanical movement or into an electrical output. h i l t i t l t i l t t  Complete gauges not only sense the pressure but indicate them on a dial or scale. 6
6. 6. 1) Bourdon Tube Bourdon tube is a flattened metal tube has an oval cross-section with one end of the tube connected to the process pressure. The other end is sealed and connected to the pointer or transmitter mechanism. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. , g C-type bourdon o Range as low as 0 - 15 psi up to 0-1500 psi. o They are simple and accurate, but they are bulky and get damaged with over-ranging. Helical bourdon o Range as low as 0 - 200 psi up to 0 - 6000 psi. i o Heavy-duty helical bourdons can sometimes tolerate as high as ten times the maximum range pressure. h i Spiral bourdon o Range as low as 0 -10 psi up to 0 100 000 10 0-100,000 psi. 7
7. 7. Designed by Rao KV 16/03/2006 8
8. 8. 2) Bellows sensor  A bellows sensor is an axially flexible, flexible cylindrical enclosure with folded sides. When pressure is applied through an opening, the closed end extends axially.  A bellows sensor can accurately measure much t l h lower pressures than a bourdon tube tube.  Bellows elements can measure absolute pressure, gauge pressure, vacuum, or differential pressure. 9
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10. 10. 3. Diaphragm sensor o Diaphragm is usually metallic and comes in two different configurations; single and capsular. fi ti i l d l o The single diaphragm is either flat or with concentric corrugations. o Th capsular diaphragm consists of two diaphragms welded together at The l di h i t ft di h ld d t th t their perimeters. p pressure reference and single g o Evacuated capsules are used for absolute p diaphragms for very sensitive measurements. 11
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12. 12. ELECTRONIC PRESSURE SENSORS This table shows the ranges of pressures and vacuums that various sensor types are capable of detecting detecting. 13
13. 13. 4. Strain-Gauge Pressure Sensor o Strain-gauge pressure sensors are Strain gauge used in most types of electronic pressure transmitters. When metallic conductors or semiconductors are subjected to mechanical strain, there is a change in resistance This resistance. resistance is then electrically converted into a 4-20 mA signal proportional to the pressure. o There are many different designs of strain-gauge pressure sensors. The most common designs use a metallic diaphragm to i l t th process di h t isolate the fluid and exert a force on a force bar. o Most of the strain elements in 14 current use are semiconductor type.
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15. 15. Strain-Gage Based Pressure Cell 16
16. 16. 5. Capacitive Pressure Sensor o Capacitive pressure sensors are also used in electronic pressure transmitters. With these devices the change in capacitance resulting from the movement of an elastic element is proportional to the pressure applied to the elastic element. 17
17. 17. o The elastic element is usually a stainless steel diaphragm. As shown, a high frequency high-frequency oscillator is controlled by the sensing element. Changes in pressure deflect the diaphragm and the resultant change in p g q y capacitance changes the oscillator frequency. o The variation in oscillator frequency is converted to a 4-20 mA signal, proportional to the pressure pressure. 18
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19. 19. PRACTICAL CONSIDERATIONS  C t i applications will b so severe and th Certain li ti ill be d the pressure sensor will not remain functional for reasonable amount of time.  The following protection devices can be used to p protect the pressure sensor. p 20
20. 20. 1) Diaphragm Seals o Di h Diaphragm seals are used to l dt isolate the pressure sensor from the process fluid. o This is done when the fluid is toxic, corrosive, dirty, solidifies at ambient temperature, or is extremely cold and may freeze the instrument instrument.. o The diaphragm seal is a thin, flexible disk, which separates the pressure sensor from the process media. 21
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22. 22. 2) Siphons • Siphons are generally used to isolate a hotp process media from the p pressure sensor. • The siphon is a metal, tubular device shaped in a l loop. It can either b f ll d with a h h h be filled h highboiling-point liquid or process condensate which acts as a barrier to the heat contained in the hot gases or steam. • The path the hot vapor takes to the pressure sensor is relatively long and narrow with a lot of surface area for cooling siphons. 23
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24. 24. 3) Throttling Devices o Throttling devices are commonly used to dampen high-frequency pressure fluctuations by putting a restriction in the inlet to the pressure sensor. a) Throttling Screws o Throttling screws are the simplest means of providing a restriction. o Throttling screws have several orifice sizes. o Th They are inserted into a tapped i t di t t d hole in the base (socket) of the pressure sensor to provide a flow restriction 25
25. 25. b) Pressure Snubbers o Pressure Snubbers are very common for reducing pressure fluctuations and filtering the media. Snubbers have a porous element, which restricts the velocity and filters the fluid. o The pulsation dampener is also sometimes called a pressure Snubbers, but does not Snubbers have a filtering element. o The most common design has a bar-stock fitting. As the pressure pulse comes through the dampener, the piston is forced up and restricts the flow from the large chamber by closing the outlet of th h b b l i th tl t f the chamber. 26
26. 26. c) Pressure-Limiting Valves o Pressure-limiting valves protect the pressure sensor from overpressure by blocking the process fluid at a preset limit. o There are several designs of pressurepressure limiting valves. One has the fluid coming in the inlet, passing through a filter, and out to inlet filter the pressure sensor. 28
27. 27. PRESSURE MEASUREMENT DEVICES 1. 1 MECHANICAL DEVICES In mechanical pressure measurement devices the measuring elements (sensors) can never separate it from the rest of the system (signal p y ( g processor and receiver/output device) as in many / p ) y cases they are all integral (e.g. a mechanical pressure gauge incorporates all of these elements). 29
28. 28. PRESSURE GAUGE Precautions o Tubes for gauges to be used on Acetylene must be b made of steel. d f t l o Associated Fittings for use on oxygen must be kept entirely free of oil. y o Gauges used on Hydrogen plants must be gold plated. 30
29. 29. Gauge errors oA gauge with a zero error will always read high or low by a constant amount. oA gauge with a span error has an internal magnification error A error, where the error increases as long as the measured pressure increases. c eases oA gauge with a linearity error may read correctly at 0 and 100% but will not follow a linear path between these points.
30. 30. PRESSURE TRANSMITTER a) Pneumatic transmitter Supply: 20 Psi OR 1.4 Bar Output: 3 to 15 Psi OR 0.2 to 1.0 Bar 32
31. 31. 33
32. 32. b) Electronic transmitter Supply: 24 V DC Output: 4 to 20 mA 34
33. 33. 1. Pressure is applied to the sensor. 2. A change in pressure is measured by a change in the sensor output. output 3. The sensor signal is conditioned for various parameters. 4. The conditioned signal is converted to an appropriate analogue output (i.e. 4 – 20 mA) 35
34. 34. c) Smart Transmitter HHC (Hand Held ( Communicator). o Smart Transmitter can be programmed with a HHC (Hand Held Communicator). o They come with addressable and communication facility. o Transmitter can be re-ranged and zero checked without re ranged disconnecting from the line. 36
35. 35. Electronic Transmitter Wiring Connections 37
36. 36. PRESSURE SWITCHES o An electric pressure switch senses pressure and opens or closes an electrical switch l l t i l it h element at a set pressure to signal another electrical device. o Most pressure switches trip at a pressure above atmospheric, and are called gauge p g g pressure or simply pressure switches. o Switches can also be manufactured to trip at pressure referenced to a complete vacuum and this is called absolute pressure switches switches. o Pressure switches are set to trip at a certain point with rising or falling pressure. f lli o The difference in the trip point and the set point is called dead band or reset or switch differential. 38
37. 37. Electrical Switch Contacts The electrical switch is usually: y single-pole doublethrow or double-pole doublethrow. 39
38. 38. PRESSURE SWITCHES Switch Terminology 40
39. 39. PRESSURE REGULATOR o Pneumatic regulators are used with every pneumatic Instrument. Instrument o Regulator maintains a steady outlet pressure irrespective of variation in the upstream pressure. o Regulator comes with a filter to remove wet the dust. o A drain valve remove any condensation of air. 41
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