Control System Instrumentation - Process Control - Lecture Slides, Slides of Process Control

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Control System Instrumentation I Controller/process | interface ——] ~ oe bo Process M = Measurement C = Controller A = Actuator Docsity.com Standard Instrument Signals • Pneumatic (air pressure): 3 – 15 psig • Electrical: 4 – 20 mA • I/P or E/P transducer Docsity.com fo) i o — oe i] ra oO Table 9.1 On-Line Measurement Options for Process Control Temperature Flow Pressure Level Composition Thermocouple Orifice Liquid column Float-activated Gas-liquid chromatography (GLC) Resistance temperature Venturi Elastic element —chain gauge, lever Mass spectrometry (MS) detector (RTD) Rotameter —bourdon tube —magnetically coupled | Magnetic resonance analysis (MRA) Filled-system thermometer | Turbine —bellows Head devices Infrared (IR) spectroscopy Bimetal thermometer Vortex-shedding —diaphragm —bubble tube Raman spectroscopy Pyrometer Ultrasonic Strain gauges Electrical (conductivity) | Ultraviolet (UV) spectroscopy —total radiation Magnetic Piezoresistive transducers | Radiation Thermal conductivity —photoelectric Thermal mass Piezoelectric transducers | Radar Refractive index (RI) —tatio Coriolis Optical fiber Capacitance probe Laser Target Surface acoustic wave Surface acoustic wave Electrophoresis Semiconductor Electrochemical Paramagnetic Chemi/bioluminescence Tunable diode laser absorption Docsity.com 20>- -~--------~------------ | | 16 | \ for) 12 ; } Tm (mA) I A (ok at i © ! <= O whee ; i <————-Span = 100°——____» Zero = 50°, 0 | | ! 0 50 100 150 T (°C) Figure 9.4. A linear instrument calibration showing its Zero and span. Docsity.com Range and Scale Factor ( ) range 50 to 150 20 4scale factor 0.16 mA / 150 50 0.16m C C G s = − = = − =   Docsity.com Thermocouple leads OG oOo i h Thermowell e. rr] Sy) bem <= Fluid O at temperature T x Tm Ton Lo Dm Figure 9.16. Schematic diagram of a thermowell/thermocoupie. Docsity.com Measurement / Transmission Lags • Temperature sensor make as small as possible (location, materials for thermowell) • Pneumatic transmission lines usually pure time delay, measure experimentally (no time delays for electronic lines); less common today compared to electronic transmissions. ( ) 1 ( ) 1 m s s m m s s T s m C T s s U A τ τ = = + C ha pt er 9 mτ Docsity.com Transmitter/Controller C ha pt er 9 May need additional transducers for Gm if its output is in mA or psi. In the above case, Gc is dimensionless (volts/volts). Docsity.com fo) Ld ® — ror © = oO 20 16 12 Tm (mA) Figure 9.5. | I | t J | I LL ! | I | t I I | | | . I Operating point 2 | | f 1 | | -T-T- TTT /Operating point 1 I I I ! | I | | ! ! | J 0 50 - 100 150 T (°C) eo Gain of a nonlinear transducer as a function of operating point. Docsity.com Most likely True value measured value 14ke Total error (maximum) 12 | Systematic ! | error (bias) ° | 8 10r | oOo e 3 | Random | 2s | — 52 error a <b) 8 5 8 | (repeatability) _— =o x roy Se gL | py a0) oa | | Ea x xX | i on 25 gl | | | O | | x x x | | x Ea x | 2 mon | t Pe Es x x x “ x x x y tp ee Tat te ) 9 O20 0.25 0.30 . 0.35 0.40 ilo q, flow units Figure 9.14. Analysis of types of error for a flow instrument whose range is 0 to 4 flow units. Docsity.com Precision, Resolution, Accuracy and Repeatability • Precision can be interpreted as the number of significant digits in measurement, but more accurately it refers to the least significant digit which contains valid information, e.g., 0.01 in the present case. Therefore, 0.33 is more precise than 0.3. • Resolution is defined as the smallest change in the input that will result in a significant change in the transducer output. • Repeatability is +/- 0.02 in the present case. • Accuracy is 0.39-0.25=0.14, i.e., maximum error. Docsity.com spring Diaphragm Valve position indicator T | ITH Valve stem Actuating Valve body signal Flow __ Valve plug direction V Valve seat —_ Za Docsity.com Control Valve Characteristics (Inherent) ( ) ( ) Design equation for liquids : flow rate, gpm : valve coefficient, valve size : valve lift, 0 1 (fraction open of the valve) : pressure drop across valve : specific gravity v v s v v v s Pq C f g q C C P g φ ∆ = = ≤ ≤ ∆    Docsity.com i Quick opening (square root) fo>) Flow, f ho ® 2 qo] Equal percentage a ea | 0 0 i Lift, @ Figure 9.9. Control valve characteristics. 3 psi < airpressure —> 15 psi to valve Docsity.com or AP P Pump characteristic Flow rate, q Docsity.com Design Guideline Since and for ease of control high for low cost low 1 1 to at design flow rate 3 4 s v v v v d P P P P P P q P = ∆ + ∆ → ∆ →∆ ∆ ⇒  Docsity.com P; = 40 psig Pz = 0 psig (constant) C-) (constant) Heat exchanger Control valve Pump APhe = 30 psi at 200 gal/min AP, = 10 psi at 200 gal/min Figure 9.10. A control valve placed in series with a pump and a heat exchanger. Pump discharge pressure is constant. (o>) ho ® — ok © <= O Docsity.com Rangeability (Turn-Down Ratio) maximum controllable flow level flow at 95% lift minimum controllable flow level flow at 5% lift rangeability=19 for linear valves rangeability=34 for equal-percentage valves (R=50) rangeability=3 for q = uick-opening valves Docsity.com Example 2 If the flow rate is reduced to 25% of the design level, 5030 1.9 (psi) 200 40 1.9 38.1 (psi) 50 0.06 (almost closed) 127 38.1 he v v s P P qf PC g  ∆ = × =    ∆ = − = = = = = ∆  Docsity.com Installed Valve Characteristics • Desired behavior: the flow rate is a linear function of valve lift. • Let us assume that the control valve has linear trim and it is necessary to increase the flow rate. If ∆p through exchanger did not change, then valve would behave linearly (true for low flow rates), since it takes most of ∆p . For higher flow rates, ∆p through exchanger will be important, changing effective valve characteristics (valve must open more than expected ⇒ nonlinear behavior). Docsity.com 240 ! T ] 1 — a = 200+ oo + 160 - “ o Cc a s rT) = i20L 4 = & a wwe Gy <= 80 Yo 1. Linear 125 7 rs) of of 2. Eq% 115 27° 3. Eq.% 138 40 3 ~ 4. —q.% 92 7 0 | ! L I 0 0.2 0.4 0.6 0.8 1.0 £ Figure 9.13. Installed vaive characteristics for Exarnple 9.2. Docsity.com  =~ Actuator & spring Ss Diaphragm (o>) - (3-15 psig) iH Valve position fb) Valve pressure indicator = indicator S. Valve rs] plug = stem O Actuating Valve body , signal di Flow __ Valve plug VW Valve seat — irection UE Xt _ Figure 9.8. A pneumatic control vaive. Docsity.com Control Valve Transfer Function ( ) gpm gpmor p 1 where si %C O v v v v t KG s s qK p q p τ →∞ = + ∆ = ∆ ∆ ∆ = ⋅ ∆       ∆   Docsity.com