Electrical: Valve monitoring and control circuits

Transcription

1 Electrical: Valve monitoring and control circuits Valve monitoring and communication

2 Electrical: valve monitoring and control circuits Overview This section covers valve monitoring circuits for interfacing to PLC or DCS control systems. Mechanical and proximity sensors are described for use in conventional intrinsically safe (IS) and non-is applications. 2

3 Electrical: valve monitoring and control circuits Process control inputs and outputs (I/O) 1. PLC/DCS System 2. Logic control 3. Power supply (Typically will need auxiliary power supply for output power or bus network.) 4. Input cards (~8 discrete) 5. Output cards (~4 discrete) 6. Bus interface card 7. Analog input card 8. Analog output card 1 (3-8) (1-8) 3

4 Electrical: valve monitoring and control circuits Conventional automated valve installation 4

5 Electrical: valve monitoring and control circuits Monitoring and control circuits Power Source I/O system Closed input Open input Valve monitor Closed switch Open switch Power Source Switching output Solenoid valve 5

6 Electrical: valve monitoring and control circuits Valve monitoring circuit Power source Computer input Switch 24 to 240 volts Estimate over 95% provide feedback into solid state (computer) inputs Market split between 120 VAC and 24 VDC in North America and 240VAC and 24VDC in many other parts of the world Current levels typically 20 ma or less 6

7 Electrical: valve monitoring and control circuits Intrinsically safe switch circuit Power source Computer input Switch 24 VDC or less IS barrier Many monitoring and control circuits are intrinsically safe Barrier typically drops 2 to 3 volts Current and voltage are typically less than 20 VDC and 10 ma at switch 7

8 Electrical: valve monitoring and control circuits Intrinsically safe NAMUR sensor circuit Power source Computer input Repeater barrier l = 0.5 ma to 5 ma 8 VDC NAMUR sensor 24 VDC or less IS barrier Barrier off state (target off): Current in NAMUR sensor circuit > 2.1 ma Barrier on State (target on): Current in NAMUR sensor circuit < 1 ma Best circuit to use for IS on/off valve monitoring Defined by EN

9 Electrical: valve monitoring and control circuits Proximity two-wire sensors Power source Computer input Switch 24 to 240 volts Used as a more reliable direct replacement for mechanical and reed switches May be used in 24 VDC, 120VAC, or 240VAC VAC circuits with computer inputs or moderate power switching applications Leakage current and voltage drop must be considered 9

10 Electrical: valve monitoring and control circuits Proximity two-wire sensor, off state Power source Computer input Off state current (Leakage) Switch 24 to 240 volts Leakage current is required to keep sensor functioning properly in off state Computer inputs are in off state with current < 0.5 ma StoneL leakage current < 0.5 ma 10

11 Electrical: valve monitoring and control circuits Proximity two-wire sensor, on state Power source Computer input On state current Voltage drop (7V) Switch 24 to 240 volts Voltage drop is amount of voltage required across the sensor for proper operation in the on state Computer inputs typically require up to 180VAC in 240VAC; 90 VAC in 120 VAC; up to 14 VDC in 24 VDC Compatible with most computer I/O but need to check Voltage at computer input = PS voltage switch drop 11

12 Electrical: valve monitoring and control circuits Analog input circuit Analog input 24 VDC Position transmitter 4 to 20 ma circuit Current changes with changing valve position Feedback present throughout entire valve stroke 12

13 Electrical: valve monitoring and control circuits Valve solenoid circuit Computer output Power source 24 to 240 volts Solenoid valve Computer output energizes or de-energizes solenoid valve Voltages are typically 24 VDC, 120 VAC or 240 VAC AC coils have inrush when first energized; DC coils do not. Typically have minimum current of 10mA for AC applications Universal input valves that work on both 24 VDC and 120 VAC are available (Axiom C-module). 13

14 Electrical: valve monitoring and control circuits Intrinsically safe valve solenoid circuit Computer output Power source 24 VDC Solenoid valve IS barrier Voltage at solenoid valve = PS voltage IS barrier drop Barriers typically drop 12 V so on a 24 VDC system, a 12 VDC IS solenoid is required To meet IS requirements, the valve must be IS approved and typically ½ W or less and its entity parameters match those of the barrier 14

15 Voltage Valve Monitoring Switching Applications INTERNAL 220V A Computer Inputs & Intrinsically Safe 65% of applications 120V B C B Computer Inputs 30% of applications C Work Circuits 5% of applications 30V A 0V 0mA 20mA 50mA 100mA Current

16 Date Author Title Valve Monitoring INTERNAL Limit switch specifications 2 SPDT, 10 amp 240VAC, DCS input Hermetically sealed 3 24VDC, PLC input 2 SPDT, 1 amp 24VDC, intrinsically safe 2 DPDT, redundant contacts 24VDC, PLC input 1 SPDT for Open & Closed limit switch indication 16

17 Electrical: Switches and sensors Valve monitoring and communication

18 Electrical: switches and sensors Overview This section discusses the various types of switches and sensors used by StoneL, along with their advantages and disadvantages. 18

19 Electrical: switches and sensors Switch/Sensor types Mechanical Proximity; Reed type Proximity; Solid state 19

20 Electrical: switches and sensors Mechanical switch types Single pole double throw (SPDT) silver contacts Use for 125/250 VAC only Operating Life of 400,000 cycles Single pole double throw (SPDT) Common gold contacts Use for 125 VAC and 30 VDC computer inputs Operating life of 100,000 cycles Normally closed Normally open Double pole double throw (DPDT) silver contacts Use for 125/250 VAC only Operating life of 250,000 cycles Common Normally closed Normally open Normally closed Common Normally open 20

21 Electrical: switches and sensors Mechanical switches Advantages High power (Up to VAC ) Disadvantages Wear out and corrode Oxidize and fail in 24 VDC computer inputs Require seal off in hazardous areas PLC and DCS inputs are low power so generally not suited for mechanical switches. Competitor differentiation None; Competitor switches are similar 21

22 Electrical: switches and sensors Proximity reed type switches Single pole single throw (SPST) Ruthenium contacts Use for 30 VDC and 125 VAC computer inputs Operating life of 5 million cycles Magnet Common Normally open Single pole double throw (SPDT) Tungsten contacts Use for 125/250 VAC only Operating Life of 5 million cycles Single pole double throw (SPDT) Rhodium contacts Use for 24 VDC and 120 VAC computer inputs Operating life of 5 million cycles Normally closed Magnet Common Normally open 22

23 Electrical: switches and sensors Proximity reed type switches Advantages Moderate power switching (up to 3 30 VDC up to VAC) No seal offs required Long life Disadvantages May lose hermetic seal over time Contact sticking at 120 VAC (non Tungsten) Need special application knowledge StoneL competitive advantages Protection circuitry guards against contacts sticking LED 23

24 Electrical: switches and sensors Proximity solid state switches NAMUR sensor (EN ) Three-wire solid state Two-wire solid state 24

25 Electrical: switches and sensors Solid state sensing technologies Capacitive Operation: Triggers off any object Analysis: Reliable technology but may indiscriminately trigger from undesirable targets Hall effect Operation: Magnetic field generates voltage Analysis: Magnetic field strength changes with temperature and vibrational movement which could affect accuracy Magnetic resistive (Axiom, Prism PI, and Hawkeye HX ) Operation: Resistance changes with changing magnetic flux direction Analysis: Reliable operation and cost effective (accuracy of 0.5 degrees) Inductive (Quartz, Eclipse, Prism PM and Hawkeye HK) Operation: Triggers off conductive metal object Analysis: Reliable, proven, minimal power, low cost and very stable with temperature extremes Target Sensing head Sensing circuits Switching circuits 25

26 Electrical: switches and sensors Proximity NAMUR sensor EN Unoperated Operated Sensing circuit Current output Sensing circuit Current output Current greater than 2.1mA Metal target Current less than 1mA Used extensively with IS systems (majority of hazardous applications in Europe) Various sensing technologies may be used with NAMUR output 26

27 Electrical: switches and sensors Proximity NAMUR sensors (IS only!) Advantages Less costly than switching solid state sensors Compatible with IS repeater barriers Optimal for point-to-point IS monitoring applications Can determine open circuit or shorted circuit with the right barrier Disadvantages Not usable in standard monitoring circuits Most customers not familiar with sensors Need specialized application training Competitor differentiation Must outsource from other manufacturers; standard with StoneL Part of dual module/c-module system with StoneL Less costly 27

28 Electrical: switches and sensors Proximity three-wire solid state sensors Sinking sensor Sourcing sensor + 24VDC VDC - Used to eliminate leakage current and voltage drop problems with solid state switches Very common in factory automation applications 28

29 Electrical: switches and sensors Proximity three-wire solid state sensors Advantages Eliminate leakage current and voltage drop problems Non-arcing, no seal offs required Disadvantages Confusing to apply Often miss-wired causing problems Competitor differentiation StoneL only uses on special applications Used by Turck, Effector and Pepperl+Fuchs, but confusing and expensive 29

30 Electrical: switches and sensors Proximity solid state two-wire sensors Advantages No moving parts with unlimited life 5 year warranty Suitable for both AC and DC computer inputs Easy to apply; direct replacement for mechanical switches Triggers off any metal including nonferrous Disadvantages May not be optimal in intrinsically safe circuits (voltage drop) Competitor differentiation StoneL makes all proximity sensors; low cost, high reliability StoneL integrates into Eclipse and Hawkeye. Others use reed or mechanical proximity switches 30

31 Valve Monitoring: Dual Module + wires + terminal strip 31

32 Electrical: switches and sensors StoneL s C-module system 32 Features Continuous (C) proximity sensing High intensity LEDs Solid state position sensing Integrated sensing and communication SST short circuit protection SST universal AC/DC solenoid input Sealed terminal strip and electronics Push button settings Benefits Eliminates thru-shaft and enables diagnostics Clearly show position and solenoid status Ultimate in reliability Minimize costs Prevents switch burnout with short circuit Use same unit for entire plant Prevent contamination and impervious to shock and vibration Quick and precise position setting

33 Valve Monitoring Technology evolution Mechanical switches Moving parts Mechanical proximity switches Mechanical seal Solid state inductive proximity switches Long life Magnetic integrated circuit sensors Long life Corrodes easily Wide temp High power Gold for low power Limited life Limited temp High power Special contacts for low power High cost Limited temp Low current rating Ideal for computer Medium cost Wide temp Low current rating Ideal for computer Low cost Analog versions Low cost 33

34 Comparative Summary Basic Types Mechanical Switches Reed Switches Inductive Proximity Magnetic Sensors Reliability Good Better Best Best Cost Best Good Better Best Flexibility Good Good Better Best Size Good Better Better Best 34

35 Valve monitoring and control wiring Questions What are the most reliable switches to use in computer input applications? Are SPDT switches required for computer input applications? What % of monitoring goes into computer discrete inputs? What type of switches provide us with the best competitive advantage? Why do you think the C-module was created? 35