Lab02 review
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This document provides information about cascade control systems. It begins with equations for calculating vessel level using differential pressure. It then describes a basic level control loop. It explains a cascade level/flow control loop, where the level controller output sets the setpoint for a flow controller in a secondary loop. A block diagram shows the basic configuration of a cascade control system with a master controller for the primary variable (level) and a slave controller for the secondary variable (flow). It includes a diagram of an example system with tanks, pumps, and a chiller using cascade control to maintain both tank level and chiller flow.
Lab02 review
- 1. Basics of Cascade Control EMEC125 – Lab 02 - Review
- 2. Open Vessel Level Measurement 𝑃 𝐻𝑖𝑔ℎ = 𝑃 𝑎𝑡𝑚 + 𝑆. 𝐺.∗ 𝐻 𝑃 𝐿𝑜𝑤 = 𝑃 𝑎𝑡𝑚 Δ𝑃 = 𝑃 𝑎𝑡𝑚 + 𝑆. 𝐺.∗ 𝐻 − 𝑃 𝑎𝑡𝑚 𝐻 = 𝑃 𝐻𝑖𝑔ℎ − 𝑃 𝑎𝑡𝑚 𝑆. 𝐺.
- 3. Level Loop LIT 1 LC 1 I P LCV 1 Tank Fill Level set point 30.0" H L
- 4. Cascade Level/Flow Loop LIT 1 H L LC 1 I P FCV 2 Tank Fill Level set point FIT 2 FC 2 Flow set point FE 2 30.0" LH
- 5. Cascade Control – Block Diagram Master Controller (Primary) Final Control Element Secondary Variable Measurement Process Primary Variable Measurement Slave Controller (Seconary) Controller Output Controller OutputSlave Set Point Master Set Point Level Flow
- 6. Tank A Warm Water 16 Gal. (60L) P4 P1 C1 HV8B C2 HV1B HV3A HV4A HV5A HV2A Tank B Cool Water 16 Gal. (60L) P3 P2 E2 HV8A E1 HV1A HV3B HV4B HV5B HV2B Chiller HV7 Column SV PITL 1 Open to Atmosphere FI 1 L H PITH 2 Three Valve Manifold I P IA HV6A HV6A VFD VFD VFD VFD FE 2 FV 1 YIC UR 24Vdc Notes: 1. UR – Paperless Chart Recorder 2. YIC – Loop Controller 3. PITL – Differential pressure indicator, transmitter measuring level; LIT 4. PITH – Differential pressure indicator, transmitter measuring flow; FIT 4. Tank A and B are open to atmosphere. 5. VFD – Variable Frequency Drive
Editor's Notes
- #3: Open Tank Level Measurement Level measurement in open tanks is the simplest. The differential pressure transmitter for level measurement is normally mounted at or below the tank connection in order to measure the pressure of the liquid above. The pressure is sensed by the process flange and transmitted to the high side of the sensing element of the transmitter. The low pressure side is vented to the atmosphere. The level transmitter can be calibrated to output 4mA when the tank is at 0% level and 20mA when the tank is at 100% level. Level is normally expressed in inches of water column (inH2O). Notes about S.G. A hydrometer is a tool scientists use to measure the specific gravity of liquids. Specific gravity is the ratio of the mass of a liquid to the mass of an equal volume of pure water. Because the density of a liquid changes with temperature, hydrometers are calibrated for different reference and sample temperatures. Pennsylvania Barometric Pressure State Low: 30.08 Hg State High: 30.17 Hg State Average: 30.12 Hg State range: 0.09 Hg
- #4: The typical single-loop control scheme is illustrated on this slide. In this case, the level of water in the tank is monitored directly by a single controller which activates the control valve to steer the measured level towards the set-point value. A sudden change in the output flow due to a change in the downstream load would result in a level change. The controller however, cannot respond to this new situation until the new flow rate has had a measurable effect on the level in the tank.
- #5: This P&ID shows how the cascade scheme works around this shortcoming by using a second loop with a slave controller which monitors and controls the flow at the output of the system. The master controller still monitors the level in the tank but adjusts the set point of the slave controller instead of the control valve. The flow control loop regulates load changes before they have an important impact on the level. Thus, a cascade control system does not have to wait for the primary controlled variable to change before initiating corrective action. A change in the secondary controlled variable is sufficient to do so. Although a variation of the primary controlled variable can occur, this variation is typically not as important when it is under cascade control.
- #6: The master set point comes from the Foxboro loop controller. The master controller is the Foxboro loop controller. The pressure transmitter (LOW) is measuring the differential pressure in the vessel (Primary Variable) and providing a set point to the secondary controller (Slave), also the Foxboro. The secondary controller is reading the pressure from the pressure transmitter (HIGH) which is measuring the differential pressure across the venture (Flow).