Class 35 advanced control strategies – ratio control, split range control
8 likes5,014 views
This document discusses ratio control and split range control strategies. It describes ratio control as using one uncontrolled stream to adjust the flow of a second stream to maintain a desired ratio between the two streams. Split range control involves splitting a controller's output between two or more control valves to sequence their operation over different ranges of the controller's output. Examples are given of using split range control to manage pressure in a separator by opening a flare valve or closing a fuel gas valve depending on the pressure level.
Class 35 advanced control strategies – ratio control, split range control
- 1. ICE401: PROCESS INSTRUMENTATION AND CONTROL Class 35 Advanced Control Strategies – Ratio Control, Split Range Control Dr. S. Meenatchisundaram Email: meenasundar@gmail.com Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
- 2. Ratio Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • An important control problem in the chemical industry is the combining of two or more streams to provide a mixture having a desired ratio of components. • Examples of this mixing operation include the blending of reactants entering a chemical reactor or for the injection of a fuel/air mixture into a furnace. • Figure shows an example of a ratio control system. It depicts a control system for blending two liquid streams A and B to produce a mixed stream having the ratio Kr in units of mass B / mass A. • Stream A, which is uncontrolled, is used to adjust the flow of stream B so that the desired ratio is maintained.
- 3. Ratio Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • The measured signal for stream A is multiplied by the desired ratio Kr to provide a signal that is the set point for the flow control loop for stream B. • The parameter Kr can be adjusted to the desired value. • Control hardware is available to perform the multiplication of two control signals.
- 4. Ratio Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
- 5. Ratio Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • In a flow control loop, the dynamic elements consist of the controller, the flow-measuring element, and the control valve. • For incompressible fluids, there is no lag between the change in valve position and the corresponding flow rate. • For this reason, the transfer function between the valve and the measurement of flow rate is simply unity. • The block diagram also shows a transfer function Gp that relates the flow rate of B to the supply pressure of B. • A transfer function Gm1 is also shown that represents the dynamic lag of the flow measuring element for stream A. • From the block diagram, the flow of B may be written as
- 6. Ratio Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • The control action for a flow control system is usually PI. • The integral action is needed to eliminate offset and thereby establish a precise ratio of the mixed streams of A and B. • Derivative action is usually avoided in flow control because the signal from a flow measuring element is inherently noisy. • The presence of derivative action would amplify the noise and give poor control. 1 2 21 1 pm r c v B A B c v m c v m GG K G G Q Q P G G G G G G = + + +
- 7. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • In a split range control loop, output of the controller is split and sent to two or more control valves. • The splitter defines how each valve is sequenced as the controller output changes from 0 to 100%. • In most split range applications, the controller adjusts the opening of one of the valves when its output is in the range of 0 to 50% and the other valve when its output is in the range of 50% to 100%. • The principle of a split range control is illustrated in the following example:
- 8. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
- 9. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • In figure, PIC-01 controls the pressure of the separator for liquid vapour hydrocarbons, by mean of a split range controller with the output signal split and sent to two pressure control valves PV-A and PV-B. • When pressure increases, the fluid shall be discharged to flare. When the pressure decreases, Fuel gas is introduced to compensate the pressure of the separator. • The fuel gas valve (PV-B) needs to close in response to increasing of pressure of the separator, while the flare valve (PV-A) will need to open when the pressure increases beyond setpoint. • When the pressure increases beyond setpoint in range of with 0- 50% controller output, PV-B shall close from fully open to fully close.
- 10. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • When the pressure increases beyond setpoint in range of with 50- 100% controller output, PV-A shall open from fully close to fully open. • In summary, valve actions by PIC-01 is as follow:
- 11. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • In this case, the service of both control valves is different, with respect to use of fuel gas and flaring for pressure control. • Another case of use of split range control loop is when one control valve cannot be suitably designed to cover the complete operating range of the controller. • In that case, valve with a smaller Cv operated between 0-50% range and the other operates for 50-100% range. • For example, a pressure controller for accumulator drum in overhead of a stabilizer column splits range to open 2 control valves.
- 12. Split Range Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015 • In the low range (0-50 % range in response to high pressure of the stabilizer), the off gas is routed to a gas plant downstream, in the high range (50-100% range in response to high high pressure of the stabilizer) the off gas goes to flare by opening of valve B. • The control valve actions is as follow:
- 13. Dual split-range control strategy: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
- 14. References: • http://www.enggcyclopedia.com/2012/06/split-range-control-loop/ • http://www.controleng.com/single-article/a-dual-split-range-control- strategy-for-pressure-and-flow- processes/e02afa4eb60717657598546e8feb895e.html Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015