Test 3 Flashcards
(15 cards)
1
Q
PID
A
- Also known as 3 mode control
- Like reset action, rate action is a time adjustment
- Rate action responds to the rate of change of error
- The rate at which the process is moving away from the SP
- Derivitve or rate action spikes the output of the controller in response to the rate of change of the error and stops when the rate of change of error is 0
- PID responds to magnitude and direction of error, rate of change of error, as well as off set
2
Q
Rate Action
A
- Also known as derivative action
- Is used in large-capacity processes where there may be large lags or sudden changes/upsets in the process
- Used in temperature control applications and closed-loop composition control systems
- Provides a large, fast output response immediately upon a change in the process variable
- On fast processes, this would result in cycling therefore not necessary
3
Q
Derivative Mode
A
- The proportional mode considers only the present state of the process error (responds to input change)
- The integral mode looks at past history (checking if the proportional response fixed the problem)
- The derivative mode anticipates the future state of the process and acts on that prediction
- As the size of processing equipment increases, the mass and thermal inertia increases
- For very large processes, it is not enough to respond to an error that has already evolved because the inertia or momentum of these processes makes it difficult to stop or reverse a trend once it has evolved
- The purpose of derivative control is to predict process errors before they have evolved and take corrective action before the error occurs
- When the rate of error change is zero, the contribution of derivative action to the output is zero
- When the rate of error is rising, the contribution of derivative action is positive and correlates to the slope of the error curve
- The unit of derivative action is derivative time
- This is the length of time by which the derivative mode looks into the future
- If the derivative time is set for 10 seconds, it will generate corrective action equal to what the PB would have generated 10 seconds down the road
- The longer the derivative time setting, the further into the future the derivative mode predicts and the greater its contribution to controller output
4
Q
Methods for controller tuning
A
- Trial and error
- Ultimate sensitivty method
5
Q
Control Valves are…
A
A device for adjusting or throttling passage or flow to regulate something
6
Q
Control valve parts
A
- Actuator
- Body
- Positioner
7
Q
Globe valve
A
Throttling valve where fluid flow enters through horizontally, makes a turn through the plugging seat, then turns to exit valve
8
Q
Globe valve body types
A
- Single, double, angle, 3 way - port
Point of angle: - Mounting
Point of 3 way: - Mixing or splitting flows
- Tempered water systems, mixing hot and cold
Point of Double port:
High pressure
9
Q
Plug cage type valve
A
Equal percentage:
- Opens at a constant rate
Quick opening:
- Flow rate is large initially
- Often used in processes with large lags
10
Q
Valve Trim Definition
A
- Internal components of the valve that contact the process
- Typically when you change the valve trim, you would change the plug, stem, and seat ring
11
Q
Valve plug guiding
A
- Different methods are used to guide the valve plug through its stroke to keep it from moving and allowing proper flow
- Valve guides keep the valve from vibrating, which would damage the valve
- Used for good control characteristics
- Valve trim can be changed
- Good for high pressure / temperature applications
- Many body styles are available
- Not used for slurry applications or where much suspended solids are present
- Not used on corrosive chemicals where the flow is large as the body would be made from stainless steel, thus very expensive
- Not typically used with large flows
- Can be up to 16” in diameter (pipe size on/attached)
12
Q
Valve Guiding Types
A
- Top and bottom guiding
- Cage guiding
- Top guiding
- Port guiding
- Stem guiding
13
Q
Ultimate sensitivty calculations
A
PI:
- %PB = 2.2ultimate proportional band
- Repeats/min = 1.2/ultimate period
PID:
- %PB = 1.6ultimate proportional band
- Repeats/min = 2/ultimate period
- Rate = ultimate period/8
14
Q
When is a controller properly tuned?
A
When the process returns to setpoint as fast as possible with no overshoot
15
Q
Ultimate sensitivty method
A
- Start with controller in manual and process variable equal to setpoint
- Lockout the reset (integral value), turn to maximum if setting is in repeats per minute (minimum if in minutes per repeat)
- Arbitrarily choose a proportional band setting of 75%
- Switch to automatic and induce a small upset by changing the setpoint (10%)
- Observe the process reaction and allow the system to stabilize
- Reduce the PB by 20% and again induce an upset and observe the process reaction
- You must find the point when the process will first start to oscillate at a small but constant amplitude
- Once constant cycle is observed, take note of the proportional band setting and the period of cycles (1/frequency)
- Cycle getting smaller -> PB needs to be lowered
- Cycle getting bigger -> PB needs to be raised
- Dont time cycles per minute -> Time one cycle
- The final adjustments should be made using trial and error method
- If you are unsure which mode to use, do the calculations for 3 mode
- If the rate value is very small (<1), use calculations for 2 mode
