Pressure Measurement - ISA.ppt

1、Pressure Measurement,Mark Murphy, PE Technical Director, Fluor Corp,2,3,Pressure Measurement,Pressure is the force exerted per unit area Pressure is the action of one force against another force. Pressure is force applied to, or distributed over, a surface. The pressure P of a force F distributed ov

2、er an area A is defined as P = F/A,4,Pressure Measurement Terms,Absolute Pressure Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure. Atmospheric Pressure The pressure exerted by the earths atmosphere. Atmospheric pressure at sea level is 14.696 psia. The v

3、alue of atmospheric pressure decreases with increasing altitude. Barometric Pressure Same as atmospheric pressure. Gauge Pressure The pressure above atmospheric pressure. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by addin

4、g actual atmospheric pressure value. Differential Pressure The difference in magnitude between some pressure value and some reference pressure. In a sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gauge pressure (

5、defined above) could be considered as Differential Pressure with atmospheric pressure as the reference,5,Pressure Measurement,Pressure Units,6,Types of Pressure Instruments Pressure Gauges (Vacuum, Compound, Absolute, Gauge) Differential Pressure Gauge Pressure Switch (Vacuum, Absolute, Gauge) Diffe

6、rential Pressure Switch Pressure Transmitter (Vacuum, Absolute, Gauge) Differential Pressure Transmitter,Pressure Instruments,PRESSURE GAUGE,PRESSURE SWITCH,DIFFERENTIALPRESSURE TRANSMITTER,7,Pressure Gauge,PRESSURE GAUGES: A Pressure Gauge is used for measuring the pressure of a gas or liquid. A Va

7、cuum Gauge is used to measure the pressure in a vacuum. A Compound Gauge is used for measuring both Vacuum and Pressure. Pressure Gauges are used for Indication only,8,Pressure Gauge,9,Measuring Principle Bourdon tube measuring element is made of a thin-walled C-shape tube or spirally wound helical

8、or coiled tube. When pressure is applied to the measuring system through the pressure port (socket), the pressure causes the Bourdon tube to straighten itself, thus causing the tip to move. The motion of the tip is transmitted via the link to the movement which converts the linear motion of the bour

9、don tube to a rotational motion that in turn causes the pointer to indicate the measured pressure,Coiled Bourdon,Helical Bourdon,Pressure Gauge,C” Type Bourdon,10,Differential Pressure Gauge,Measuring Principle: Differential pressure gauges have two inlet ports, each connected to one of the volumes

10、whose pressure is to be monitored. In cases where either input can be higher or lower than the other, a bi-directional differential range should be used,11,Differential Pressure Gauge,Unidirectional and Bidirectional DP Gauges,12,Pressure Gauge - Accessories,Pulsation Damper (Adjustable Snubber) Thr

11、eads on to a gauge socket and provides a restriction by means of a pin, which may be placed in either of five different sized holes, and thus allows the user to vary the amount of dampening to suit requirements. The pulsating pressure moves the pin up and down, providing self cleaning action,Safety

12、Glass Front Safety Glass is normally used to prevent the glass shattering in the event of the bourdon tube rupturing,Liquid Filled Gauge The liquid filling is used to dampen any vibration/pulsation in the bourdon, either silicone oil or glycerin is used,Snubber Used for dampening and filtering and r

13、educing the damaging effects of pulsation on a gauge. The snubber has a metal disc available in standard grades of porosity,13,Pressure Gauge - Accessories,COIL PIPE,PIG TAIL,Pressure Limit Valve Protects pressure instruments against surges and pulsations. Provides automatic positive protection and

14、accurate, repeatable performance. Automatic pressure shut-off, built in snubber enhances instrument protecting performance,Siphon Tubes Used to dissipate heat by trapping condensed liquid to keep high temperature steam or condensing vapor from damaging the pressure gauge,14,Diaphragm Seals,Diaphragm

15、 seals, also known as chemical seals, isolate pressure measuring instruments from the process media. The system pressure is transmitted to a fill fluid in the upper housing of the diaphragm seal, and from there to the pressure-measuring instrument itself. The use of diaphragm seals should be discuss

16、ed with and approved by the Client. Diaphragm sealed gauges should be considered for: Process fluids that would clog the pressure elements. Process fluids that are toxic, corrosive, slurried and viscous. Process fluids that could crystallize or polymerize. Materials capable of withstanding the proce

17、ss fluids that are not available as a pressure element, such as high temperature. Process fluid that might freeze due to change in ambient temperature and damage the element. Hydrocarbon services having a Reid vapor pressure (RVP) of 18 psig and over. (RVP is the absolute vapor pressure exerted by a

18、 liquid at 100F. The higher this value, the more volatile the sample and the more readily it will evaporate). Auto-ignitable hydrocarbon services,15,Diaphragm Seals,16,When selecting a Pressure Gauge, care should be given to a number of parameters which have an effect on the gauges accuracy, safety,

19、 and cost. Accuracy required Dial size Operating pressure range Chemical compatibility with gauge construction materials Operating temperature range Vibration, pulsation, and shock Pressure fluid composition Mounting requirement,Pressure Gauge Selection Guideline,17,Pressure Gauge Ranges,Since the a

20、ccuracy of most pressure gauges is better in the middle portion of a gauge, you should always select a gauge with a range that is about double your normal anticipated pressure. The maximum operating pressure should not exceed 80% of the full pressure range of the gauge. Standard pressure ranges are

21、measured in PSI, Bar, Pa or kPa and most of the gauges have dual PSI/metric scales. Very low pressure gauges have scales that measure in Inches of H2O, mm H2O, and Inches of Hg. Vacuum gauges have scales in inches of mercury, while compound gauges have scales that measure in both vacuum and pressure

22、,18,Pressure Gauge Installation,Gas Service,Liquid Service,Top connection preferred for gas installations & side connection preferred for liquid installations. The pressure gauge can be connected to the pipe by individual block and bleed valves or a two way manifold,19,Pressure Switch,Measuring Prin

23、ciple: The device contains a micro switch, connected to a mechanical lever and set pressure spring. The contacts get actuated when process pressure reaches the set pressure of the spring. It can be used for alarming or interlocking purposes, on actuation. It can be used for high / high-high or low /

24、 low-low actuation of pressure in the process . The set range can be adjusted within the switch range. The sensing element may be a Diaphragm or a piston,20,Pressure Switch,Pressure/Vacuum Switch - A device that senses a change in pressure/vacuum and opens or closes an electrical circuit when the se

25、t point is reached. Pressure switches serve to energize or de-energize electrical circuits as a function of whether the process pressure is normal or abnormal. The electric contacts can be configured as single pole double throw (SPDT), in which case the switch is provided with one normally closed (N

26、C) and one normally open (NO) contact. Alternately, the switch can be configured as double pole double throw (DPDT), in which case two SPDT switches are furnished, each of which can operate a separate electric circuit,21,Pressure Switch,The switch housings can meet any of the NEMA standards from Typ

27、e 1 (General Purpose) to Type 7 (Explosion Proof), or Type 12 (Dust Proof) or Type 4 (Water Proof). Pressure switches are also available in hermetically sealed enclosures. Gold plated contacts are available for reliability . Pressure Switches are not as commonly used today, since they contain mechan

28、ical moving parts and moving parts are significantly more likely to fail than transmitters and the failures can go undetected,22,Pressure Transmitter,23,Pressure Transmitter,A Pressure Transmitter is used where indication and/or record of pressure is required at a location not adjacent to the primar

29、y element. A Pressure Transmitter is used for both indication and control of a process. A Pressure Transmitter is used where overall high performance is mandatory. Both Electronic and Pneumatic Transmitters are used. These can be either Gauge, Absolute or Differential Pressure Transmitters,24,Transm

30、itter Measuring Principle,The diagram shows an electronic differential pressure sensor. This particular type utilizes a two-wire capacitance technique. Another common measuring technique is a strain gauge. Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a

31、sensing diaphragm. The sensing diaphragm is a stretched spring element that deflects in response to the differential pressure across it. The displacement of the sensing diaphragm is proportional to the differential pressure. The position of the sensing diaphragm is detected by capacitor plates on bo

32、th sides of the sensing diaphragm. The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 420 mA or 1-5 VDC signal. For a gauge pressure transmitter, the low pressure side is referenced to atmospheric pressure,25,Pressure Transmitter,Typi

33、cal Outputs 4 to 20 milliamp (mA). analog signal Smart HART digital signal (superimposed on analog signal) Fieldbus digital signal 3 to15 psi pneumatic signal,26,Diaphragm Seal System,A diaphragm seal system consists of a pressure transmitter, diaphragm seals, a fill fluid, and either a direct mount

34、 or capillary style connection. During operation, the thin, flexible diaphragm and fill fluid separate the pressure sensitive element of the transmitter from the process medium. The capillary tubing or direct mount flange connects the diaphragm to the transmitter. When process pressure is applied, t

35、he diaphragm transfers the measured pressure through the filled system and capillary tubing to the transmitter element. This transferred pressure displaces the sensing diaphragm in the pressure-sensitive element of the transmitter. The displacement is proportional to the process pressure and is elec

36、tronically converted to an appropriate current, voltage, or digital HART output signal,27,Diaphragm Seal System,WHY USE DIAPHRAGM SEALS? Diaphragm Seal systems provide a reliable process pressure measurement and prevent the process medium from contacting the transmitter diaphragm. Transmitter/ Diaph

37、ragm Seal systems shall be used for: For process fluid that would clog the pressure elements. For process fluids that are toxic, corrosive, slurry and viscous. For process fluids that could freeze or solidify. For process temperatures outside the normal operating range and cannot be brought to those

38、 limits by impulse piping. For process that needs frequent cleaning. For processes that need replacement of wet legs, to reduce maintenance,28,When selecting a pressure transmitter care should be given to a number of parameters which have an effect on transmitter accuracy, safety, and utility. Accur

39、acy required Power supply Operating pressure range Operating temperature range Body Material Pressure fluid composition and Internal parts Mounting requirement Process connection size,Pressure Transmitter Selection Guideline,29,Pressure Transmitter installation,Mounting above tap is typical for gas

40、service and mounting below tap typical for liquid and steam services. Direct mount is possible for low temperature services,30,Diaphragm Seal installation,31,Pressure Instruments,Selection of Pressure Instruments: Rules of Thumb: Application: Understand your application. Examine the particulars of your application. Is it necessary to know if the pressure is negative or positive? Do you need to know the difference in pressure between two points? Answering these questions about your