Pressure Transmitter Training Fundamentals of Pressure Measurement

Transcription

1 Pressure Transmitter Training Fundamentals of Pressure Measurement

2 Topics To Be Covered Pressure Measurement Absolute, Gauge, & Differential Pressure Pressure Transmitters Basic Functions Transmitter Terminology Span & Range Limits Flow Rate Applications Primary Devices & Square Root Relationship Level Applications Open & Closed Tanks; Dry & Wet Legs Intelligent Transmitters Characterization, Calibration, & Reranging Construction Body Styles, Flanges, & Seals

3 What is Pressure? Pressure is Force applied over a known Area (force per unit area) Force Pressure = Area Some typical units of pressure measurement: Pa, kpa, & MPa psi (lb/in 2 or lbf/in 2 ) kg/cm 2 (kgf/cm 2 ) bar The letter a is added to indicate absolute pressure; for example: kpaa, psia, & bara When the letter a is not added, the measurement is assumed to be gauge and not absolute, unless otherwise stated.

4 Absolute & Gauge Pressure Measurement Absolute Pressure Gauge Pressure Barometric Pressure (Variable) Vacuum Region Zero Absolute Pressure (0 psia, 0 kpaa, 0 bara) Barometric or Atmospheric Pressure varies with location and weather. Gauge Pressure is referenced to current barometric pressure; can be positive or negative. Vacuum measurement in the industries is normally a gauge pressure measurement, referenced to barometric pressure. Absolute Pressure has a fixed reference of zero absolute & there are no negative values. Absolute Pressure is independent of barometric pressure.

5 Pressure Transmitters Basic Functions Getting the measurement to the control room! The manual way The automatic way Analog Communiations: 4 to 20 ma dc loop Transmitter regulates current, proportional to pressure Digital Communications: Fixed loop current with HART, FOXCOM, FOUNDATION FIELDBUS, PROFIBUS, etc. digital signal

6 Span and Range Terminology Range Values: The end points of a measurement range Lower Range Value (LRV): The starting point of the range; corresponds to the 4 ma output for a 4 to 20 ma transmitter. Upper Range Value (URV): The ending point of the range; corresponds to the 20 ma output for a 4 to 20 ma transmitter. Span: The absolute value difference between URV and LRV (URV LRV); examples: Range: 0 to 100; LRV = 0 & URV = 100; Span = 100 Range: -10 to 90; LRV = -10 & URV = 90; Span = 100 Range: 75 to -25; LRV = 75 & URV = -25; Span = 100 Transmitter/Sensor Span Limits: Span must be within transmitter Span Limits. Transmitter Range Limits: Each range value (LRV & URV) must be within transmitter range limits.

7 Span and Range Examples Example 1: Range: 0 to 100 inh2o LRV = 0, URV = 100, & Span = 100 IDP10 Span Code B: Span Limits 3.5 & 200, Range Limits +/- 200 Span of 100 is within limits of 3.5 & 200 Both LRV and URV are within limits of +/- 200 Example 2: Range: 2050 to 2100 mbar LRV = 2050, URV = 2100, & Span = 50 IDP10 Span Code B: Span Limits 8.7 & 500, Range Limits +/- 500 IDP10 Span Code C: Span Limits 70 & 2100, Range Limits +/ Neither B nor C meets both span and range limit requirements, but IDP25 Span Code C: Span Limits 6.25 & 2500, Range Limits +/ Meets both span and range limit requirements

8 d/p Cell Terminology Span and Range Values and Limits: Same as shown on previous slides, including the examples that we examined earlier: Range: 0 to 100; LRV = 0 & URV = 100; Span = 100 Range: -10 to 90; LRV = -10 & URV = 90; Span = 100 Range: 75 to -25; LRV = 75 & URV = -25; Span = 100 But the following is unique for d/p Cell transmitters: Measurement is the difference between the pressure on the High side, marked H and the pressure on the Low side, marked L. A negative range value indicates that the pressure on the Low side is higher than the pressure on the High side d/p Cell range examples

9 d/p Cell Range Examples Example 1 (DP for gauge pressure): DP LRV: Low Side = 0 and High Side = 0 DP = 0 DP URV: Low Side = 0 and High Side = 100 inh2o DP = 100 inh2o Range: 0 to 100 inh2o; Span: 100 inh2o Example 2 (DP for flow): DP LRV: Low Side = 600 psi & High Side = 600 psi DP = 0 DP URV: Low Side = psi & High Side = 600 psi DP = 100 inh2o Range: 0 to 100 inh2o; Span: 100 inh2o (3.6 psi) Example 3 (DP for level): DP LRV: Low Side = 6000 & High Side = 1000 mmh2o DP URV: Low Side = 6000 & High Side = 5500 mmh2o Range: to -500 mmh2o; Span: 4500 mmh2o DP = mmh2o DP = -500 mmh2o Note: DP Span is the difference of the difference in pressures

10 d/p Cell for Flow Rate Measurement LRV = 0 URV = DP across primary element (e.g. orifice plate) at maximum flow rate URV from flow sizing sheet: Bore calculated for design maximum DP or Maximum DP calculated for design bore size d/p Cell Transmitter Orifice Plate & Flange Union Pipeline Flow Schematic Illustration of d/p Cell Transmitter used for Flow Rate Measurement

11 DP Transmitters for Flow Rate Advantages: Wide Range of Materials Large Pipe Diameters High Pressure Rating Variety of Primary Devices Standards for Custody Transfer, e.g. Natural Gas Low Cost Quick Delivery Limitations: Not suitable for viscous fluids and sediment (unless using seals) Square law affects accuracy and limits flow turndown Effects of turndown

12 Flow Error in % of Reading Effect of Square Root Read Read Read Read Read Read Read Flow Enter DP Enter DP Operating Sq. Rt. of Oper. DP Sq. Rt. of Oper. % of Max Flow Turndown Error in Error Value URV DP Value Oper. DP W ith Error DP w/error Flow Ratio % of Reading Guideline: Limit flow turndown ratio to approximately 3:1 to 5:1, depending on the required accuracy

13 d/p Cell Level Measurement - Vented Tank Example: LRV = 0 URV = Maximum Level X SG Range: 0 to 1.8 mh2o 2.25 m X 0.8 = 1.8 mh2o SG = m maximum level Zero Level Schematic Illustration of d/p Cell Transmitter used for Open Tank Level

14 Closed Tank Dry Leg Example: LRV = 0 URV = Maximum Level X SG 2.25 m X 0.8 = 1.8 mh2o Range: 0 to 1.8 mh2o SG = 0.8 Pressure 2.25 m maximum level Zero Level Schematic Illustration of d/p Cell Transmitter used for Closed Tank Level with Dry Leg

15 Closed Tank Wet Leg Example: LRV = (0) (0.8) - (3) (1.2) = -3.6 mh2o URV = (2.25) (0.8) - (3) (1.2) = -1.8 mh2o Pressure Wet Leg SG = 1.2 Range: -3.6 to -1.8 mh2o SG = m maximum level 3 m Zero Level Schematic Illustration of d/p Cell Transmitter used for Closed Tank Level with Wet Leg

16 Closed Tank Dual Seals Example: LRV = (0.2) (0.8) - (3.0) (1.76) = mh2o at minimum level URV = ( ) (0.8) - (3.0) (1.76) = mh2o at maximum level Range: to mh2o SG = 0.8 Pressure 3 m 2.25 m max. level Capillary Fluid SG = 1.76 Zero Level 0.2 m Schematic Illustration of d/p Cell Transmitter used for Closed Tank Level with Dual Remote Seals

17 What is an Intelligent Transmitter? Input Outputs Previously: Measured Pressure Analog Transmitters 4 to 20 ma Only 4 to 20 ma Output Now: Measured Pressure Intelligent Transmitters Digital Value of Pressure LCD Indicator, Engr. Units Digital Value of Pressure, Engr. Units 4 to 20 ma Signal Intelligent Transmitters - Internal Digital Value of Pressure is key: It exists from Lower Range Limit (LRL) to Upper Range Limit (URL). It can be sent digitally to local indicator or to remote system, eliminating need for reranging. Selected pressure ranges can also be converted to an analog ma signal.

18 Behind the Scenes Questions to be answered What is Characterization? How are the coefficients determined? Where are the coefficients stored & why? How does Calibration affect the transmitter? What is One Point & Two Point calibration? What is the effect on accuracy? What is Reranging? How can reranging be done with or without pressure? How are LRV & URV used? Why is reranging not required with digital communications?

19 Characterization Precise pressure is applied in steps from LRL to URL and repeated at different temperatures. Coefficients are determined & stored in sensor. Curve fit algorithm is used to linearize & compensate for temperature. Transmitter can accurately measure any pressure from LRL to URL. Calibration is done later to optimize accuracy over a desired range. Example: +200 inh2o Internal Digital Value of Pressure -200 inh2o -200 inh2o (LRL) Error Band +200 inh2o Input Pressure(URL)

20 Calibration Two Point Calibration using precise applied pressures Trims Internal Digital Value of Pressure (offset & slope) Optimizes accuracy over selected range Example: +200 inh2o 100 Internal Digital Value of 0 Pressure Calibrate for Optimized Accuracy example: 0 to 100 inh2o -200 inh2o -200 inh2o (LRL) Input Pressure +200 inh2o (URL)

21 One Point Calibration & Zeroing Trims the Internal Digital Value of Pressure (offset only) Zeroing can be done with various pressures Just tell the transmitter what pressure is applied Example: +200 inh2o 100 Internal Digital Value of 0 Pressure Zero trims the offset without changing the span -200 inh2o -200 inh2o (LRL) Input Pressure +200 inh2o (URL)

22 CAL LRV & 0 CAL LRV requires LRV pressure applied 0 requires zero pressure (or zero differential pressure) Easy zeroing for non-zero based ranges Can be done from on-board pushbuttons or configurators 0 is the Zero Trim function when using a HART Communicator Example: Internal Digital +200 inh2o Value of Pressure inh2o -200 inh2o (LRL) (URL) Input Pressure Calibrated Range: 40 to 100 inh2o LRV = 40 CAL LRV requires 40 inh2o applied 0 or Zero Trim requires 0 inh2o applied +200 inh2o For Example: Zero With: 40 inh2o Applied (LRV) or 0 inh2o Applied (CAL@0)

23 Reranging (Without Applying Pressure) Example: Calibration inh2o Set LRV & URV values to new desired 4 & 20 ma output points Internal digital values & calibration are NOT affected Example: Desired 4 & 20 ma Points ma inh2o (LRL) inh2o (URL) Internal Digital Value of Pressure For Example: New range desired is 20 to 100 inh2o

24 Reranging (With Applied Pressure) - HART Pressures not known? Let transmitter determine them it s intelligent! Example: Dual Seal d/p Cell for Level exact DP values not known With minimum level in tank, let transmitter determine the DP With maximum level in tank, let transmitter determine the DP Reranging accomplished! New LRV & URV set for 4 to 20 ma output Internal Digital Values & Calibration are NOT affected ma Desired 20 4 & 20 ma Points 4 For Example: Transmitter ordered with range: 0 to 200 inh2o Approximate range needed for dual seal: -95 to -5 inh2o Actual range determined by transmitter: to -2.7 inh2o -200 inh2o?? +200 inh2o (LRL) Internal Digital Value (URL) of Pressure

25 ma Trim Does NOT involve pressure Does NOT affect pressure calibration or reranging Trims the digital-to-analog output stage Actual 4 & 20 ma Output Values 20 ma 4 ma 4 LRV 20 URV Desired 4 & 20 ma Points

26 Foxboro Intelligent Transmitter Family Gauge & Absolute Pressure (Direct Connect) Gauge & Absolute Pressure (Bracket Mtd.) Differential Pressure (d/p) Flanged Level (flush or extended diaphragm) Direct Connect & Remote Seals Sanitary Tri-Clamp

27 New! Low Profile DP Structures Traditional DP (horizontal connections) Low Profile LP1 Low Profile LP2 Low Profile Structures join the Traditional d/p Cell group Low Profile has In-Line Process Connections (facing downward): Similar to Competitive Coplanar Mounting Single-ended Process Covers Fits or Retrofits to Coplanar Installation Design Standards

28 LP1 Low Profile Structure Process Covers & Sensor similar to traditional DP except: Process Covers turned 90 degrees to face down Single-ended Process Covers Single side vent screw standard tangential & offset Ideal for manifold-mount applications Limited bracket mounting bracket would interfere with process connectors Vertical or Horizontal Installation Vent screw can be omitted for vertical mounting above pipeline, self draining into pipeline; option V1 Same price as traditional structure

29 LP2 Low Profile Structure Process Covers are a new design Process Covers turned 90 degrees to face down Single-ended Process Covers Dual vent/drain screws on each side Complete, easy, venting and draining Bracket or Manifold Installation Fits Foxboro or Coplanar Brackets & Manifolds Vertical Installation Use in vertical position Ideal replacement for most Coplanar applications Won t drain & vent well in horizontal position; use LP1 to replace a horizontal Coplanar transmitter

30 I/A Series Intelligent Transmitters Take Advantage of the many Features & Benefits of the I/A Series Pressure Transmitter Family