FPC Test 2 Flashcards
When do you use cascade?
When disturbance is on the supply-side.
What does supply-side mean?
What is load-side mean?
Where energy/M is being introduced in to the system.
Where PV is introduced into the system.
When do you use feed-forward?
When disturbance is load-side.
What type of control is used in the slave loop?
It only needs P but often PI is used.
How is the ratio maintained between a slave and a master flow?
Flow is measured in the master flow and this signal is sent to a flow controller in the slave flow which adjusts the valve.
How do you get good flow control?
PI action with high gain.
What are the four things that are measured most in industry?
Flow, Temperature, Level, Flow
What must you determine about control loops if it is impossible to meet all set points?
Which loops control material balance and which loops control product quality
What do you use when material balances are allowed to be kept within a range?
Controllers based on PV as they have an offset. Use low gain P action.
What do you use if it is essential to maintain product specification?
Tightly tuned PID controllers so product stays close to set point.
What do you do it the system seems to show supply-side disturbances?
Apply cascade or feedforward strategies.
What is the disadvantage of cascade?
It is expensive.
How can you ensure product quality if on-line product composition measurement and sampling are both unavailable?
If it is a binary feed, the bubble point temperature can tell the exact composition.
Why does temperature not change at the top and bottom of a distillation column?
Because the reflux and reboiler keep temperature fairly constant at both ends of the column
Where do you place temperature sensors in a distillation column?
At the trays where the temperature STARTS to remain constant. In other words, where the temperature profile begins to break.
What controller action do you use for level control at condenser and reflux drum?
Proportional / P.
What controller action will you use for pressure control in a distillation column and why?
PID, because it is a direct way to control product quality.
What controller action will you use for temperature control and why?
PID, because it is a direct way to control product quality.
What action do you use for reflux flow control?
PI, because it is a secondary way to control product quality.
When do you use material balance schemes?
When reflux ration is high ~above 5.
When you have high product purity.
When there are frequent disturbances to energy balance.
When do you use energy balance schemes?
When the reflux ration is small ~below 1.
When there are frequent disturbances to feed rate and/or composition.
When there is a large reflux drum.
How do you control the residence time in a reactor?
Level control.
The size of the tank.
What must you do if measuring feed parameters is not enough to control product quality?
Have an operator perform sampling and change flow parameters accordingly.
What are common mistakes in process control?
- An over-defined system: too many control valves.
- Not creating a control loop with the valve: one stream is measured but a different one is controlled.
- Bad use of symbols a temperature sensor going to a flow controller. Temperature only manipulates, not controls the flow.
- Misplaced sensors and valves such as a valve directly before a pump or a valve directly after a pump.
What makes distillation columns in series less robust? What effects can this have and how can this be solved?
Noise in one will be transferred to the other. Optimum temperature may not be reached in the second column, solved by adding a trim reboiler.
Why is using a water jacket preferred when cooling a system?
It only needs to be tuned once because the temperature in the water jacket stays constant despite things such as seasonal temperature change.
What are some design methods for better overall control? What are their advantages and disadvantages?
Larger vessels. They are less susceptible to fluctuations but they are more expensive.
Buffer Tanks. This is a vessel between two operational units so disturbance is not felt immediately in the next unit. They add additional cost.
What does the symbol PI stand for?
Pressure Indicator
What does the symbol TR stand for? Why does it have a line through the symbol?
Temperature Recorder.
Because it does not show the reading locally, but instead sends it to the control room.
What do the symbols Fr and Pd mean?
Flow ratio
Pressure differential
What functions do the following symbols represent?
A, C, I, R, S, T
Alarm, Control, Indicate, Record, Sum/integrate, Transmit
What do the following abbreviations represent?
HPA, LLA, HC
High Pressure Alarm, Low Pressure Alarm, Hand (manual) Control
What are the British Standard symbols for pneumatic signals and for electrical signals?
A tight-angled line, a staple-shaped line
On a P&ID, what is the difference between a fail-open and a fail-close valve?
Arrow pointing from transmitter to valve if it’s fail-close, other way round if its fail-open.
What do the step and ramp functions represent?
Step is when the valve is turned from one position to another quickly.
Ramp is when it is turned slowly and continuously.
What is an impulse? What happens after the impulse?
When a process variable is changed quickly without changing the position of the valve. It will revert back to original state after some time.
In a temperature bypass valve, how is heat controlled?
If the temperature is too hot, bypass valve is opened slightly which lets some cold stream into the stream after heating, which cools it down. If temperature is too cold, then valve is closed more.
Why is direct composition analysis rare?
Because it is a lot more expensive that any other form of variable analysis.
What is a “dirty” fluid?
A fluid which has more than two phases.
How do time of flight ultrasonic flow meters work?
What conditions must be met?
They send an ultrasound signal and the flow velocity of the liquid will directly affect the time it takes for the ultrasound to reach the detector as the distance is known and constant.
Fluid must be clear and there must be no fouling on pipes.
How does a Doppler meter work?
What is the advantage of this?
It reads the pitch of the ultrasound which is directly related to the fluid velocity.
It can be used for dirty liquids.
What does turndown ratio mean?
The ratio between the highest value that the instrument can measure and the lowest value.
Give 3 features of orifice plate fluid flowrate sensors.
Low cost Non linear scale (D/P measured not flowrate) Very robust Wide range of process fluids Moderate accuracy
Give 3 features of rotameter flowrate sensors.
Differential pressure method
Linear scale
Float position can be sensed remotely
Should be used with clean fluids
Give 3 features of turbine meters.
Used with clean, lubricating fluids
Wide operating range
Linear signal
Give 3 features of an electromagnetic flowmeter
Liquid needs to be conductive Voltage produced by liquid is proportional to velocity No restriction to flow More expensive Can handle slurries (non-newtonians)
Give 3 features of vortex meters.
Use the flagpole effect
Linear scale
comparable cost & performance to orifice plate
Give 4 features of positive displacement meters.
Counts volume packets passing through meter Pulse/frequency signal Used to meter quantity Very accurate Very broad operating ranges Very expensive
What is the difference between positive displacement meters and all the other flowmeters?
Positive displacement meters measure flowrates but the others measure velocity
How can you use fluid flowrates to measure the mass flowrate of gases?
Measure the temperature and the pressure and then use volumetric flowrate in the ideal gas equation
What are the advantages of a Coriolis flowmeter?
It can measure mass flowrate directly
It measures fluid density directly
How can temperature be measured?
By measuring level of a fluid with high expansion coefficient
By measuring the resistance of a metal
Thermocouples
Should sensors appear before or after the control valve?
Before
Should valves come before or after a pump?
After
In a two stream system of gas and liquid, ow would you keep the liquid stream at strictly 10% excess?
Use flow ratio control. Measure the gas stream and control the liquid stream using this.
How can you determine whether a control system works?
Change a parameter and see how it affects the rest of the process.
Why is level control not self-regulating?
It does not reach equilibrium again after a step change, meaning it will keep draining.
Why may tuning requirements be different for level control?
Often the flow out of the tank has to be controlled, not the actual level.
How do you measure the volume of a tank which is not a vertical cylinder?
You have to run a plant test. Adjust the flow so there is a small difference (dF) between inlet and outlet flows. Then, measure the time taken for the level to change by a certain percentage (dL%). If the vessel is not cylindrical, then make dL the whole range of the vessel.
What is the formula to calculate the volume of a vessel?
V = (100 * dF * t) / dL dF is in m^3/h t must be in hours dL and 100 are in % V in m^3
Which variables must be known to tune a level controller?
d - maximum tolerated deviation in % from set point. Normally this is the point where an alarm would be.
f - normally expected flow disturbance
F - range of flow controller
ts - controller scan interval
How do you find the range of the flow controller?
If there is a cascade system where the level controller sets the set point of the flow controller, then just read the range of the flow controller.
If no cascade and only level controller controls valve, either make F the maximum value that goes through the pipe with valve fully open. If this is unavailable, make F 1.3 x the designed flow.
What is tight level control?
If the level changes, the controller will respond immediately to try to keep level constant
What is average level control? Why is this useful?
The controller will allow the level to fluctuate before it returns back to the set point.
It smooths the flow out of the vessel so that there are no major disturbances in flow.
What parameter must be high for the controller to work fast?
The process gain.
How do you calculate Kmax?
Kmax = V / (F * ts)
What happens when gain goes above Kmax?
There is overshoot.
How do you calculate the offset at Kmax?
((f * ts) / V) * 100%
Why is integral often not required for tight level control?
Because the offset is often very small so it is not necessary.
How must you set up ts for tight level control?
It must be very fast, less than 5s.
What is surge capacity?
A vessels ability to handle a large variation in inflow.
In level control, what is the equation for dM?
dM = f/F
How do you calculate Kmin in averaging level control?
Kmin = 100f / Fd
How does averaging level control prevent disturbances in outlet?
When there is a sharp disturbance in inlet flow, the controller will allow the level to increase up to the alarm point and just slowly increase the outlet value during this. Overall level will be increased at the end of this control.
Why is proportional averaging level control not practical?
Because it leaves the level at alarm point.
What is the equation for the time constant in averaging level control?
T = Vd / 100f
Which control system is responsible for tight level control and which for averaging?
Direct is for tight
Cascade is for averaging
What do special control algorithms achieve in averaging level control?
They make better use of the surge capacity by not reacting to small disturbances, only big ones.
What is the equation for process gain in level control?
Kp = F/V in hours^-1
ALWAYS LOOK AT FUCKING UNITS OF EVERYTHING HERE
What is used to measure the flow for cascade control and what special condition is required?
Orifice plates with a 30 pipe diameter straight run
Which parameter indicates that cascade control is needed?
Varying pressure
What is the difference between feedback and feedforward?
Feedback changes MV after a disturbance has occured, feedforward changes MV before the disturbance has a chance to affect PV.
When is feedforward control necessary?
When a key variable can change by more than 20%.
How do you combine feedback and feedforward? Give 2 types
By using decouplers.
Ratio and bias.
What is the ratio decoupler equation?
M = I * R
M - output
I - Input
R - Ratio
What is the bias decoupler equation?
M = I + B
Where must three-element control be implemented?
Steam drums
When is dynamic compensation used?
When process lag or deadtime for each of the responses varies by more than 25%
What is DV?
Disturbance Variable
What is load-side and what is supply side disturbance?
Which needs cascade control?
Supply-side is MV, load-side is PV.
Cascade should be used for supply-side.
Revise revision lecture distillation column controllers. Memorise them in correct places.
DO IT
If you want to control the flow of one stream using the flow of another, how would you do this?
Put a temperature transmitter on the first stream and a temperature controller on the second stream.
What are the 4 main categories of level sensors?
Floats
Differential pressure
Ultrasound / energy beam
Put tank on scales
What is the value of K in feedforward ratio?
What is it in bias?
K = 1 K = -Kpd/Kpm
What is the equation for thetas?
theta + theta(m) = theta(d)
What are T1 and T2 equal to?
taff(m) and taff(d) respectively
What do you do if theta is negative?
Set theta to zero and increase T1 by theta(m) - theta(d)
