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controls test 4
Terms in this set (25)
5 most common measurement sensors used in CPI (easiest to hardest)
what is a valve positioner? why is it used?
type of feedback controller that sense the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly.
used to eliminate valve deadbands and hysteresis, flow rate loading, and other undesirable characteristics due to friction forces in the valve.
Typical dynamic model used for a control valve
control dynamics tend to be fast compared to the process.
the dynamic behavior of the control valve can be approximated by a 1st order transfer function between the manipulated variable and the signal to the control valve
explain why there's a general guideline that a control valve takes 20 to 33% of the available pressure drop in a line at the design flow rate
the valve would be sized to make the most of the pressure drop in the line. this would give the valve max influence over process changes that disturb the flow rate & would give the smallest/cheapest valve.
the most economical operating conditions require the valve to introduce as little pressure drop as possible in order to minimize pump costs.
the guideline is a compromise.
explain why not all control valves are linear valves
a valve with linear behavior would appear to be the most desirable choice, the designers objective is to obtain an installed valve characteristic that is as linear as possible. because the pressure drop across the valve usually varies with flow rate, a nonlinear valve will often yield a more linear flow after installation than will a valve with a linear characteristic.
typical dynamic model for a thermowell? explain why the time constant in this model requires the thermal capacitance of the thermowell be made as small as possible and the thermowell be placed in an area of turbulent fluid flow.
first order- assumptions: well and thermocouple are always at the same temp, the heat is transferred only between the fluid and the well.
tau = mC/UA : if capacitance is small and heat transfer coefficient and area is maximized then the lag is minimized.
placing in turbulent fluid flow allows more heat transfer so a higher heat transfer coefficient which minimizes the time constant and the liar. U is strongly dependent on fluid velocity so the thermowell should be placed in an area of max fluid velocity, near the center line of the pipe or in the vicinity of a mixing impeller will make UA large.
5 categories of alarms
equipment status, abnormal measurement, alarm switch without its own sensor, alarm + sensor, automatic shutdown or startup system
what is SIS? what is ESD? why should they operate independently from the basic process control system (BPCS)? why would SIS and ESD have redundant sensors and actuators separate form the BPCS?
Safety Instrumentation System- type 5 alarm, last resort. designed to provide automatic responses after alarms indicate potentially hazardous situations/have process reach a safe condition. (last resort to bring conditions back to normal/keep it safe)
Emergency Shutdown System- automatically shuts down/starts up equipment. comes into play if conditions reach a critical point where they will be dangerous.
Basic Process Control System- consists of feedback and feedfoward control loops that regulate process variables such as pressure, temp, flow rates, liquid levels. typically provides satisfactory control during routine process operation.
Operate independently because otherwise emergency protection will be unavailable during periods when the BPCS is not operating (malfunction or power failure)
Redundant sensor and actuators are utilized for critical measurements. prevents a single sensor failure from crippling the SIS operation. the SIS also has a separate set of alarms so that the operator is notified when SIS initiates an action, even if BPCS is not operational.
example of interlock not considered part of the ESD
process interlocks aren't part of ESD: the liquid system- the liquid level must stay above a certain value to avoid cavitation in a pump
what is a PLC
programmable logic controller- simple digital device that is widely used to control sequential and batch processes
what is an alarm flood
too many alarms going off at the same time which makes it very hard to find the real problem because of the overwhelming amount of info the operators are receiving- contributing factor in plant failures.
what is state-based alarming
automatic adjustment of alarm limits to accommodate different process conditions such as start up, shut down, changes in feedstock, or changes in production rates
how reliability, probability of failure, and MTBF are related
reliability (R) is the probability that the component doesn't fail in the interval (0,t) and can be determined from the constant failure rate (u) of the component. R=exp(-ut)
failure probability (P) is related to R by P=1-R
Mean time between failures (MTBF) is the reciprocal of the constant failure rate MTBF = 1/u
five rules of thumb for selecting control objectives
1. all variables that are not self regulating
2. must be kept within equipment and operating constraints (temp, P, comps)
3. are a direct measure of the product quality
4. seriously interact with other controlled variables
5. have favorable dynamic and static characteristics.
4 guidelines for selecting manipulated variables
1. inputs that have LARGE effects on the controlled variables
2. inputs that RAPIDLY affect the controlled variables
3. the manipulated variables should affect the controlled variables DIRECTLY, rather than indirectly
4. avoid recycling disturbances ??
3 guidelines for selecting measured variables
1. reliable, accurate measurements
2. have an adequate degree of sensitivity
3. minimize time delays and time constants
motivation for feedforward control? typical configuration? why is proportional control often chosen for feedforward?
to measure important disturbances and take action before they upset the process. offers large improvements over feedback control for processes that have large time delays.
motivation is to reduce the effect of measurable disturbances.
measures disturbance variable instead of the output.
proportional control can be used because feedforward control is often used for averaging control instead of a tight controls, where a slight offset in a new steady state is fine (ex: liquid level in tank)
what is cascade control? which loop should be the inner (secondary/slave) loop? which loop should be tuned first? explain objective of a cascade control scheme.
alt approach to feedforward control that employs a secondary measurement point and a secondary feedback controller. secondary measurement point is located to recognize the upset condition sooner than the CV but the disturbance isn't measured.
main objective is to maintain 2 process variables at specified values.
inner loop should respond faster than the other. usually P or PI control depending on amount of offset that would occur with P only control.
(primary loop is generally PI or PID)
secondary controller should be tuned first after installation with the primary controller in the manual mode
what is ratio control? how is it similar to cascade?
special type of feedforward control. objective is to maintain the ratio of 2 process variables at a specified value. 2 variables are usually flow rates, a manipulated variable U and a disturbance variable D. the ratio is R=U/D.
similar to cascade because both control 2 process variables at same ime.
what is selector control? objectives?
practical solution for choosing appropriate signal from among a number of available measurements. used to improve the control system performance as well as to protect equipment from unsafe operating conditions.
median selectors, overrides, split range control, constraint control
when do you need nonlinear control? 2 common techniques used in non-linear control? gain scheduling?
needed when a highly nonlinear process operates over a very large range of conditions.
nonlinear modifications fo a standard PID control algorithms- making the controller gain a function of the control error, nonlinear transformations of input or output variables- make the closed loop operation as linear as possible. allows process to be controlled over a wider range of operating conditions and in a more predictable manner. controller parameter scheduling- one or more of the controller settings are adjusted automatically based on the measured value of a scheduling variable.
gain scheduling- piecewise constant controller gain that varies with a single scheduling variable, the error signal.
important characteristics of real signals related to process control
real signals determine the actual control for a process and its accuracy.
what is a zero-order hold?
part of the digital to analog converter which allows a pulse to be turned into a continuous signal until the next pulse occurs
what is split range control
several manipulated variables are used to control a single controlled variable
ex: used to maintain temp in a vessel that both heats and cools
explain why negative elements in the RGA are to be avoided
if pairing of inputs and outputs results in a negative relative gain, then the closed loop system will exhibit instability either in the overall closed loop system or in the loop with negative gain. when lambda is negative then the open loop and closed loop gains have different signs. thus, opening loop 2 would have an adverse effect on loop 1 such as oscillation. so these should not be paired together. for lambda<0 the control loops interact by trying to "fight each other" and the closed loop system may become unstable.
Recommended textbook explanations
Adel S. Sedra, Kenneth C. Smith
Introduction to Electric Circuits
James A. Svoboda, Richard C. Dorf
Engineering Mechanics: Dynamics
J.L. Meriam, L.G. Kraige
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