Pressure Transmitters. P atm. P head P low. = P head + P atm. = P atm

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Todd Hines
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2 Contents Topics: Slide No: Pressure Transmitter, HTG & Hybrid System Other Technology (Float, Capacitance, Displacer, Servo, Nucleonic, Laser & Ultrasound) Exercise

Pressure Transmitters 3 Open Vessel Level Measurement In open vessel a pressure transmitter mounted near the bottom of the tank will measure the pressure corresponding to

3 Pressure Transmitters 3 Open Vessel Level Measurement In open vessel a pressure transmitter mounted near the bottom of the tank will measure the pressure corresponding to the height of the fluid above it. P atm P high = P head + P atm P head P low = P atm P high - P low P atm XMTR = P head What happen to Atmospheric pressure? L H Cancelled Off

100% XMTR 0% h L H g 0% The sensor will not be able to sense any pressure

4 Pressure Transmitters Transmitter mounted above the tap in an open vessel What happens when the liquid level drop below the sensor? 100% XMTR 0% h L H g 0% The sensor will not be able to sense any pressure change!! 4 The 0% has to be at least at the same level as the transmitter sensor or below the tapping point..

below the vessel connection. At min. level the High side pressure is high than the low side pressure 20mA Max.

5 Pressure Transmitters Zero Suppression is often applied to compress the range of the transmitter OR to cancel the effects of the liquid head in the pipe connecting the transmitter to a tank when the transmitter is mounted below the vessel connection. At min. level the High side pressure is high than the low side pressure 20mA Max. Level 20mA Max. Level 4mA Min. Level Actual Zero Suppressed XMTR Actual Zero Suppressed 4mA XMTR Min. Level L H L H 5

measuring level in a pressurized vessel. 20mA Max.

6 Pressure Transmitters Zero Elevation is often applied to cancel the effects of the head caused by the seal fluid in the reference leg (low side) of a transmitter measuring level in a pressurized vessel. 20mA Max. Level 4mA Actual Zero Elevated XMTR Min. Level L H At min. level the low side pressure is high than the high side pressure 6

7 Pressure Transmitters 7 Closed Tank Level Measurement (Dry Leg) Dry leg: no fluid in low side impulse piping, or leg If the gas above the fluid does not condense, the piping for the low side of the transmitter will remain empty. Ullage or Vapor P head P high = P top +P head XMTR P low = P top L H P high - P low = P head

8 Pressure Transmitters 8 Closed Tank Level Measurement (Wet Leg) Wet leg pressure is additive to pressure on low side of the transmitter. If the gas above the liquid condenses, the piping for the low side of transmitter will slowly fill with liquid. To eliminate this potential error, the pipe is filled with a convenient reference liquid. P wet P top = Ullage P head P high =P head +P top L XMTR H P low =P wet leg +P top P high - P low = P head - P wet leg

9 Pressure Transmitters Limitations Bottom-mount technology: potential leakage Often requires 2 taps Variable density creates errors Temperatures beyond 600 F High vacuum applications are tricky Highly corrosive processes limit life Abrasive processes can damage diaphragms Liquids Only 9

10 Pressure Transmitters Bubbler System Consists of air supply, pressure regulator, flow meter, transmitter & extended tube. Can be used for very corrosive applications. Tank vented. Pressure to maintain flow = P head TXR 10 P in (flow=const) Air is bubbled through the tube at a constant flow rate. The pressure required to maintain flow is determined by the vertical height of the liquid above the tube opening times the specific gravity. P head = H * S.G f H S.G f

11 Pressure Transmitters Gauge Pressure Transmitter Application of Bubbler system: Allows dp to be a top down measurement No process contact with transmitter Open or low pressure Control of Air supply is important for accuracy Valve Air Supply 11

12 Pressure Transmitters Remote Seals Reliable, Simple, Easy to Use, Well Understood, Flexible Uses: Diaphragm Seals extend limitations due to process conditions such as: high temperatures corrosion viscous materials suspended solids plugging sanitary needs Differential Pressure Transmitter 12

13 Pressure Transmitters Typical Pressure Applications CPI / HPI Batch reactors digesters fractionators distillation column bottoms and reflux drums separators surge drums reservoirs intermediate storage... Power drum level dearators... Pulp & Paper Headbox Stock Tanks Chemical Storage tanks Evaporators Low concentration liquor tanks... Food and Beverage Fermentors storage tanks aging tanks brew kettles And many more!

14 Hydrostatic Tank Gauging (HTG) It is basically a method for measuring mass. Top transmitter PT only used on nonatmospheric tanks to compensate for ullage pressure differences. Middle transmitter PM located at a specific distance H above PB to calculate liquid density (PB-PM) Bottom transmitter PB located at the base of the tank measures static head. RTD measures tank contents temperature for reference correlations. 14 A local processor handles all the real-time calculations for transmitter algorithms and equations for MASS, DENSITY, LEVEL and VOLUME.

15 Hydrostatic Tank Gauging (HTG) A system approach to tank inventory Effective Level Measurement Options: PT Mass = (P B - P T ) x Area Density = (P B - P M ) / H H HBT PM TT Volume = Mass / Density PB Level = (P B - P T ) /Density + H BT 15

16 Hydrostatic Tank Gauging (HTG) Top Pressure Transmitter RS-485 MODBUS Communications (can take up Network to 31 AIMs) SCADA Package Middle Pressure Transmitter Temperature Sensor Bottom Pressure Transmitter HART Communicator Smart Application Module (SAM) (1 unit per tank) [Handles tank calculaton] Application Interface Module (AIM) (obtain data from SAM & convert to std MODBUS outputs) (can take up to 14 SAMs) All Smart transmitters are in multi-drop mode - Digital signals only 16

17 Hydrostatic Tank Gauging (HTG) Advantages of HTG Multi-Parameter Measurements MASS: Inventory and Billing DENSITY: Quality Control VOLUME: Inventory & Billing LEVEL: Tank Capacity Continuous Density Measurement Improved Accuracy High Reliability Low Maintenance Diagnostic Easy Installation Non-Intrusive No Moving Parts 17

18 Hybrid Inventory System Top Pressure Transmitter Radar Gauge RS-485 MODBUS Communications (can take up Network to 31 AIMs) SCADA Package 18 Middle Pressure Transmitter Temperature Sensor Bottom Pressure Transmitter HART Communicator Smart Application Module (SAM) (1 unit per tank) [Handles tank calculaton] Application Interface Module (AIM) (obtain data from SAM & convert to std MODBUS outputs) (can take up to 14 SAMs) When the Middle transmitter is removed & the Top Pressure Transmitter is replaced by a Radar Gauge, then the whole system will be known as Hybrid Inventory System

19 Hybrid Inventory System H D L Top Pressure Transmitter Radar Gauge Middle Pressure Transmitter Temperature Sensor Bottom Pressure Transmitter Measurement Options: LEVEL is calculated by Radar Gauge L = H - D VOLUME is computed by Radar Gauge Through Strapping Table relationship level to volume DENSITY is computed by the system S.G = Head Pressure / Level Head pressure measured by bottom pressure transmitter MASS is computed by the system Mass = Density X Volume CORRECTING for Density & Volume back to standard values is computed 19

20 Hybrid Inventory Systems Advantages of Hybrid System Highly accurate Multi-Parameter Measurements MASS: Inventory and Billing DENSITY: Quality Control VOLUME: Inventory & Billing LEVEL: Tank Capacity Radar can be installed without removing the tank from service Radar unit can be used with or without a stilling well Process conditions up to 375 F (190 C) and 150 psi (10 bar) Optional average temperature measurement Good for density-stratified products 20

21 Radar Gauge Frequency Cycles/second Electromagnetic Spectrum gamma rays x-rays Wavelength, Meters Radar is an Electromagnetic Wave Radio Detection And Ranging ultra violet visible light Radar, 3-30 GHz super high freq ultra high freq TV broadcasting FM Radio Microwave oven, 2-10 GHz low frequency Cellular, pager, MHz

22 Radar Gauge Radar Techniques Pulse» Measures range ( distance )» Transmits a pulse and measure time until echo is received» Accuracy depends on ability to measure time Radar signals travel at the speed of light. Must measure in picoseconds ( x10-12 )! Cost-effective electronics do not exist to do this accurately! 22

23 Radar Gauge Radar Techniques FMCW: Frequency Modulated Continuous Wave» Does NOT calculate time-of-flight» Evaluates the phase difference between the transmitted and return signal» Plotting these phase differences against the transmitted signal yields a result proportional to distance 23

Radar Gauge Advantages of Radar Gauge Non Contact, Non Intrusive Tolerates Wide Range of Process Conditions» Corrosive Processes» High Temperatures» Changes in Vapor

24 Radar Gauge Advantages of Radar Gauge Non Contact, Non Intrusive Tolerates Wide Range of Process Conditions» Corrosive Processes» High Temperatures» Changes in Vapor Space» Variable Density» Variable Dielectric» Viscous or Sticky Products Low Maintenance No Special Licenses Required Can measure long distances Liquids, pastes, solids 24

25 Radar Gauge Radar Application Considerations Sensors can be completely removed from process by use of a window made out of a nonmetallic material, such as Teflon, Ryton, Ceramic Sensors can be removed from the process without opening the vessel Radar Limitations Cost May not work with processes with low dielectric constant May not work in applications with large amounts of turbulence Process connections tend to be large (>4 flanges) 25

26 Radar Gauge Possible Applications Pulp & Paper»High Density Storage»Color tanks»bleach tanks»hydropulpers»retention tanks»black liquor tanks Pharmaceutical»Batch reactor»chemical storage Power»Slurries Chemical»Polymers»Latex»High temp»lpg tanks»butane sphere»batch reactors»two-phase sludge»cyclohexane Minerals»Steel Scale Holding Tanks 26 And More!

27 Radar Gauge Radar vs. Ultrasonic Gauge Similarities: Both technologies Top down, non contact Easy to install Good for abrasive materials, slurries Not affected by changing fluid properties: density, conductivity, dielectric 27 Differences: Radar Full vacuum to several hundred psi Wide temperature limits Can handle steam, fog, vapors Can handle some foams and agitation Can be used with windows Ultrasonic Very slight vacuum to about 100 psi Narrow temp band (<200 F) Is greatly affected by changes in vapor space Signal is lost in foam and agitation

28 Radar Gauge Radar vs. Ultrasonic Gauge Gas Temp ( o C) RADAR ULTRASOUND million m/s m/s Dry Air Water vapor Carbon Dioxide Ammonia Acetone Source: Instrument Engineer s Handbook, Liptak

Float Other technologies Float Mechanism Internal Still pipe to guide the float External Still pipe to guide the float Indicator Float Tank Isolating Valve Drain Valves 29 Indicator Float-operated

29 Float Other technologies Float Mechanism Internal Still pipe to guide the float External Still pipe to guide the float Indicator Float Tank Isolating Valve Drain Valves 29 Indicator Float-operated gauge level-indicator, indicates liquid level in cone or flat roof unpressurised tanks. Recommended for use on tanks storing water, fuel, oil, chemicals or other liquid products where operations do not require extreme accuracy.

30 Other technologies Capacitance Probe A capacitance instrument measures amount of capacitance between two plates of a capacitor. The capacitance of a capacitor increases if a dielectric is placed between the plates Circuit applies high frequency signal to probe C = KE o Ad where K = dielectric constant of material E o = permitivity of vacuum A = Area of plates (probe) C = capacitance (pf) d = distance between plates k d 30

31 Other technologies Capacitance Probe d How Capacitance varies with process fluid? Nonconductive Coating Level is proportional to dielectric change Nonconductive Fluid Process fluid is the dielectric barrier Tank Wall forms second plate The variation of dielectric is the measurement Conductive Fluid Level is proportional to plate area change Process fluid is the second plate Insulation on probe is dielectric The variation of the plate size is the measurement 31

32 Other technologies Capacitance Probe Limitation Change in Dielectric creates error Coating on probe by product creates errors With non metallic tanks or tanks without vertical walls, addition of reference probe is required Calibration can be difficult especially since one cannot bench calibrate Changing vapor space can affect output 32

33 Other technologies Capacitance Probe Potential Applications Pulp & Paper Sewage level Liquor tanks Bulk solids Chemical Interface: fatty acid/water, oil/water Carbon black Separators Food & Beverage Storage silos Oil & Gas Water bottom Water cut 33

34 Other technologies Displacers Based on Buoyance Force The displacer is buoyed up by a force proportional to the weight of the liquid it displaces Vertical movement of the displacer is converted to angular movement by mechanical linkages 0 pounds 0 pounds 34 Angular movement is then converted to electrical or pneumatic output. Buoyant force increases as level rise

35 Other technologies Displacers Good for short span measurement Liquid Level Measurement Interface Measurement Density Measurement 35

36 Other technologies Displacers Benefits: Simple, Reliable. Good for Interface measurements. Good for Density measurements. Unaffected by Agitation. Tolerates High Temperatures and pressures. Point or Continuous. Limitations: Does not tolerate viscous, dirty, or sticky fluids Variable density causes errors in level measurement Typically used for smaller spans (cost effective) Must be installed carefully Intrusive & Contact 36

37 Other technologies Displacers Typical Applications: Oil and Water interface Oil and Gas Separators Stripper Reflux Drum Level Dehydration Units Effluent Separators Absorption Towers Condensate Discharge Accumulators Density and Interface Measurements 37

38 Other technologies Servo Gauging Uses a combination of a displacer and a spring balance The servo motor strives to obtain an equilibrium between the displacer and the balance. Any change in level will cause a change in equilibrium. Advantages: Very precise (1 mm accuracy) Can measure level, interface relatively low cost Limitations: Intrusive Mechanical linkages Storage Drum Cable Displacer Balance Detector Servo Motor 38

39 Other technologies Nucleonic Gauging Single Point System Gamma rays are emitted from the source. The presence or absence of the gamma rays is measured by the detector. Nucleonic level switches use radioisotope sources sized to provide measurable radiation at the detector when no product material is present between source and detector. Gamma Source Detector 39

40 Other technologies Nucleonic Gauging Continuous System Nucleonic level transmitters use the same radioisotope sources, but respond to the total absorption of gamma rays as they pass from the source to detector. Source The amount of radiation reaching the detector is inversely proportional to the amount of material in the vessel. Detector 40

41 Other technologies Nucleonic Gauging Advantages: Unaffected by: High temperatures High Pressures Corrosive Materials Abrasive Materials Viscous Materials Agitation Clogging/Plugging Point and Continuous Liquids and Solids Interface (based on H 2 density) Limitations: Large density changes can create errors Layer of coating on vessel walls create errors Licensing Required Leak Checks required Cost 41

42 Other technologies Nucleonic Gauging Typical Applications Chemical Distillation Tower Batch Reactor Storage Tanks Resin Bed level Hydrocracker reactor Pulp & Paper Digester Level Wood Chip Bins Bleach Tower Consistency Effluent Waste Slurries Liquor concentrates Refining Fractionator Tower Surge Tanks Coke Drum Interface Desalter Food and Beverage Hopper Level Blending Vats Mining Crusher Level Storage silos Slurries Utilities SO 2 / Lime scrubber Fly ash Slurries 42

43 Other technologies Laser 43 Function: Uses infrared light to send a focused beam towards surface. Time of travel and reflection is measured. Narrow, focused beam: good for applications with space restrictions. Non contacting: uses a window Accuracy: +/- 1 cm Works best in cloudy, shiny liquids or solids May pass through surfaces of clear, still fluids Cannot tolerate dust, fog, steam or vapors High cost Alignment is critical glass window laser device

44 Other technologies Ultrasonic Gauging A sound pulse(9 to 160 khz) is transmitted and reflects off the surface back to the transceiver. The true reflected echo pulse is extracted and the time interval between transmission and reception is evaluated electronically. Advantages: Non Contact No element contamination Can be used for liquids and solids Tolerates Many Process Conditions: Sound Waves 44 Varying Density Corrosive Processes Viscous Product Varying Dielectric Sludge Buildup The higher the level the faster echo reflected

Other technologies Ultrasonic Gauging Application

of signal travel and thus, the measurement Changes could be

of the vapor Some units have temperature compensation Gas

45 Other technologies Ultrasonic Gauging Application Considerations Conditions of the vapor space impact speed of signal travel and thus, the measurement Changes could be due to: temperature dust vapor composition stratification of the vapor Some units have temperature compensation Gas blankets can be used to provide uniform vapor space condition Sound Waves 45

Heavy agitation and foam may cause signal to be

open, outdoor installations, wind can blow signal

46 Other technologies Ultrasonic Gauging Application Considerations Process surface conditions can affect signal return. Surface must have ability to reflect signal. Heavy agitation and foam may cause signal to be absorbed Vortex in fluid can misdirect signal In open, outdoor installations, wind can blow signal off coarse Stilling wells can be used to isolate the surface and contain signal. 46

47 Other technologies Ultrasonic Gauging Limitations not suitable for vacuum service Cannot tolerate high temperatures (>200 F) Foam interferes with signal Agitation may distort signal Internal obstacles can create false echoes Nearby equipment could generate frequencies that will cause errors Vapor pressure limited to 50 psi 47

48 Other technologies Ultrasonic Gauging Typical Applications Chemical Distillation chamber Corrosives Slurries Latex PVC Waxes Food and Beverage Dearating vessel alcohol fermenter baking batter chocolate dairy products grain storage Cryogenic systems (point level) Waste water Clarifier Settling tanks Reservoirs Flood control Sludge levels Pulp & Paper Black liquor w/ solids Pharmaceutical Emulsions Lotions Marine Fuel or ballast water indication Bilge alarm 48

49 Exercise 1. Which has the best accuracy on 6 meter High Water tank? A. Hollow or glass fill float with mechanical gauge (1 inch) B. Servo Gauge (1 mm) C. Pressure Transmitter ±0.1% of F.S [ ] 2. An inground reservoir is 5 meters deep. Which of the following method(s) will be suitable to measure and transmit the level without having to dig a hole to reach the bottom of the reservoir or the scour main. (Answer Yes [Y] or No [N]) A. Differential Pressure [ ] B. Servo Level Gauge [ ] C. Capacitive Probe [ ] D. Nucleonic Gauging [ ] E. Ultrasonic Gauging [ ] F. Radar Gauging [ ] G. Bubbler System [ ] 49

50 Exercise 3. For HTG, why is a 2nd Pressure Transmitter added to tank in the middle? 4. Which one of the following tank gauging system is based on Mass? (A) Radar (B) Nucleonic (C) Servo Balance (D) HTG [ ] 5. Which of the following statement about Radar Gauge is NOT True? (A) Top-down mounting (B) Can handle agitated & sticky process fluid (C) Can be used on a tank with non-metallic internal surface. (D) Can handle process with deep vacuum [ ] 50

51 Exercise 6. 5 Assuming SG is = 1.1 What is the volume? ft 3 What is the density of this fluid? #/ft 3 10 What is the mass? pounds What is the pressure level reading? in H 2 O water = 62.4 # / ft 3 51

52 Exercise 7. 5 Now, suppose the SG changes to 1.05 and the level does not change What is the volume? ft 3 What is the density of this fluid? #/ft 3 10 What is the mass? pounds What is the pressure level reading? in H 2 O water = 62.4 # / ft 3 52

53 Exercise P measured = 150 inh 2 O s.g. = 1.5 P in (flow=const) What is the fluid level in the tank?

54 54 Reference A Smith-Corripio [1997]. Principles and Practice of Automatic Process Control 2 nd Edition, John Willey & Sons Ogata [2010]. Modern Control Engineering 5 th Edition, Prentice Hall Rosemont [2002]. Fundamental Control Training

Level MEASUREMENT 1/2016

Level MEASUREMENT 1/2016 AGENDA 2 A. Introduction B. Float method C. Displacer method D. Hydrostatic pressure method E. Capacitance method G. Ultrasonic method H. Radar method I. Laser method J. Level