EXPERIMENT 2 LEVEL CONTROL SYSTEM 1.0 OBJECTIVE To study the response of level control process in open tank using the PID controller. 2.0 INTRODUCTION TO THE APPARATUS (MODEL WLF922). a) The process consists of a level tank T31 and a collection tank T32 connected with the appropriate pumps and piping system. b) T31 as an Open or Closed Tank To operate T31 as an Open tank, the top vent (V) and overflow drain (D) manual valves are opened, so that the top space of T31 is at atmospheric pressure. CONTROL SYSTEM AND INSTRUMENTATION Field-Mount Instruments Transmitters, Control Valve, Gauges, etc LT31 FE31 FT31 Differential Pressure (DP) Level Transmitter Measures level of tank T31, Open or Closed. Calibrated Range = -800 to 0mm Water Gauge (WG) Impulse line from the top space of T31 to the low pressure chamber of LT31 must always be completely filled with water. Transmitted to LIC31. Orifice plate connected to FT31 Differential Pressure (DP) Flow Transmitter with square root function Calibrated for 0-3.0 m 3 /Hr Transmitted to FIC31 LCV31 PP 1 Control Valve, Air-to-Open (ATO) Pneumatic Positioner (PP) for LCV31, with By-pass. LCY31 Current-to-Air Converter Converts 4-20 ma to 3-15 psig proportionally for PP/LCV31 LS32 Conductivity Level Switch with three conductivity probes 1
PG31, PG32 SV31 Pressure Gauges Solenoid Valve, normally open (NO) PRV31, PRV32, PRV33 Pressure Relief Valves Panel-Mount Instruments and Annunciators LIC31/FIC31 One unit panel Controller configured for- LIC31-Loop 1, PID1, Level PID Controller FIC31-Loop 2, PID2, Flow PID Controller In Cascade mode, LIC31 is the primary, FIC31 is the secondary. Three control strategies can be operated as described earlier LIC31, Level Single Loop, PID1, Loop 1 LT31-LIC31-LCY31/PP/LCV31 Selector switch in Position 1 LIC31. LIC31 in Auto (A) or Manual (M) mode. FIC31, Flow Single Loop, PID2, Loop 2 FE31/FT31-FIC31-LCY31/PP/LCV31 Selector switch in Position 2 CASCADE LIC31-FIC31. FIC31 in Auto (A) or Manual (M) mode. LFR31 2-Pen Recorder with pen/bar graph and selective display in engineering units. Red pen (Channel 1) Level, Calibrated Range 0-800 mm WG Green pen (Channel 2) Flow, Calibrated Range 0-3 m 3 /Hr Analog display in % x Maximum (calibrated range), engineering units = Actual reading, engineering units. The chart drive is set for fast speed (500 mm/hr). The recorder chart drive is started by pressing ON the RCD button with the front swing cover opened. The Annunciators are- LAH31 FAL31 LAH32 Tank level at T31 measured by LT31 exceeds the preset High level alarm limit. The High alarm limit is set at PH1 at the PID1 page Inflow to T31 measured by FE31/FT31 is below the preset Low alarm limit. The Low alarm limit is set at PL2 at the PID2 page Tank level at T32 exceeds the High level limit (shortest probe) of the Level Switch LS32. 2
3.0 EXPERIMENTAL PROCEDURE 1. Tank T32 should be filled with water up to and just below the level of the shortest level probe (not visible) of the Level Switch LS32 which is slightly below the tank (T32) overflow pipe outlet. Top up the water later whenever necessary. 2. Operate Model WLF922 independently. Open the by-pass manual valve B33 of pump P33 to divert any flow back into T32. Shut the interconnecting pipeline from Model WLF922 to Model WT922 at the appropriate manual valve MV-T. Also shut the manual valve MV-D at the discharge of pump P33, leading to the drain. 3. Quickly check the various manual valves as follows- All pumps (P31, P32, P33) suction, discharge and by-pass valves are fully opened. Operate T31 as an Open tank with the top vent (V) and overflow drain valves fully opened. The pressurizing air inlet to T31 is isolated at its inlet manual valve (with the valve handle at 90 to the air supply inlet tubing), located next to the preset Air Regulator AR31 near the top of tank T31. Operate T31 as a Self Regulating process. Open only the manual GATE valve at the gravity discharge pipe at the tank bottom. The second manual globe valve at the second gravity discharge pipe must remain shut, thus operating only one gravity discharge pipeline at the bottom of tank T31. Shut the two manual discharge valves of pump P31, one of which is at the strainer inlet. For a Self Regulating process, pump P31 must be OFF. The manual by-pass valve around the control valve LCV31 should be always shut but its two adjacent manual valves should be always opened. The bottom manual drain valve of T32 is always shut. 4. Compressed air is required to operate the control valve system LCY31/PP/LCV31 and to pressurize tank T31. Check that the pressure is in accordance to the pressure indicated at the air pressure regulator (IAS. Do not pressurize T31 yet. Shut the pressurizing air inlet manual isolation valve. 5. Note the following switches and pushbuttons but do not switch ON any pump yet. Pump P32 Pump P31 Pumping inflow from T32 to T31. To be switched ON during operation. Do not use unless T31 is to be operated as a Non-Self Regulating process, in which case the bottom gravity discharge pipes of T31 must be all shut. Check that the two manual discharge valves of pump P31 are shut. 3
Pump P33 Pump P33 can be selected to either the AUTO or MAN position. MAN position is the OVER-RIDE P33 position. 6. Check the Positioner (PP) manual By-pass switch at LCV31/PP. Make sure the Positioner (PP) is connected to LCV31 i.e. do not by-pass it, but use the Positioner (PP). Set the 1-2 Selector switch to Position 1 to operate LIC31 and display LIC31, Loop 1 accordingly. With LIC31 in Manual (M) mode, adjust MV = 25%. Check that LCV31 is about 25% OPENED (see the indicator scale at the valve stem). Also note that the control valve LCV31 is Air-to-Open (ATO). 7. Start the recorder, LFR31 by pressing the RCD button ON, with the front swing cover opened. Check the recorder chart speed (500mm/Hr) which is regularly printed out on the chart. The recorder chart drive should be stopped (press RCD) when its chart recording is not required. 8. With the controller Selector switch in Position 1, display LIC31 faceplate at the controller. Note that Position 1, PV1, SV1 and MV1 are displayed as PV, SV and MV in the main display page (faceplate). Set the setpoint (SV1) of LIC31 to 400 mm. Note that SV1 should be operated from 200 (25%) to 640mm (80%) only. Set the first (I) trial PID values at the PID1 page or panel as follows- LIC31 (PID1, Loop1) PB1 = 30%, TI1 = 25secs, TD1 = 0 9. When the level is near to the setpoint SV1, switch LIC31 to Auto (A) mode.quickly change to the FIC31(PID2) page and make sure the Cascade(C) mode indicator for FIC31 is lit up. Else press C once. Switch back to LIC31(PID1) page. Throughout the following test and observation, LIC31 is to remain in Auto (A) mode and the control valve Positioner (PP) must be used and connected unless instructed otherwise. 10. The recorder LFR31 chart drive should be at fast speed (500 mm/hr) with RCD ON. Check the chart speed which is regularly printed out on the chart. Observe the level response at the red pen until the response is almost steady to with ±1.0 mm or it displays oscillatory response even after 3 cycles. Write down the setpoint (SV2) and PID values on the recorder chart paper beside its response. These chart recordings constitute the results of your experiment and should be kept for your reports. 11. Repeat the flow (green pen) observation by using the following TEST DISTURBANCE, one test at a time. Start and end each test when the flow response is reasonably steady or remains oscillating even after 3 cycles. (i) Switch LIC31 to Manual (M) mode and quickly step change MV (inflow) by about 10% to 20%. Then quickly switch LIC31 back to Auto (A) mode. This simulates a load step test at the inflow. Observe the level (red pen) response similarly. 4
(ii) Open 1½-turn, hold for 30 seconds and shut fully the pump by-pass manual valve B32. This simulates a load pulse disturbance test at the inflow. Observe similarly. Repeat the test with any of the above disturbances such as above, using the second (II) trial PID values as follows- LIC31 (PID1, Loop1) PB1 = 10%, TI1 = 15 secs, TD1 = 0 secs *LIC31 must be in Auto (A) mode. 12. Level PID control with only Proportional (P) control mode Repeat the test with only Proportional control mode. Integral control can be removed by setting TI1 to its maximum (9999 secs). Proceed as follows Set LIC31 (PID1, Loop1) PB1 = 10%, TI1 = 9999 secs (Max), TD1 = 0 sec. Apply disturbance test. Repeat with PB = 5%. 13. Level PID control with Derivative (D) control mode Repeat the test with Proportional (P), Integral (I) and Deviation (D) control modes as follows, using the same test disturbance. PB1 = 10%, TI1 = 25secs, TD1 = 5secs Double the PB1 to say 20% and observe the damping at both the inflow (Green pen) and level (Red pen) response and at the control valve LCV31. When the level is almost steady at its setpoint again, apply test disturbance. STOP ALL THE PUMPS. MAKE SURE BOTH THE MANUAL DISCHARGE VALVES OF PUMP P31 ARE FULLY SHUT. 4.0 RESULTS/DISCUSSION The recorder LFR31 records the results of the experiments. Relevant portions of the recorder chart responses should be submitted as results of the experiment, with the following pertinent data marked on the corresponding chart recording- Setpoints and PID values SV1, PB1, TI1, TD1. NOTE The Recorder chart speed is occasionally printed on the chart, for example at 500 mm or 0.139 mm or 7.2 secs Hr sec mm 5
Provide your results and discussion by answering the following questions a) Study and compare the level responses at different PID. Use the following terms to explain the dynamics of process i.e. Rise time, Settling time, Overshoot and Period of oscillation. b) Note that from the recorder chart response that even though the inflow (green pen) is fluctuating, the level is held almost steady. Explain why. c) Discuss the response of P-control only. d) Discuss the response when adding derivative (D) control mode.observe the fluctuation of the flow(green pen) and level(red pen). Is derivative control suitable?eplain why. 6