ZTCTuningGuidelines

1、PAGE 1PAGE 6ZTC Tuning Guidelines January 14, 2009General ZTC is a control algorithm used specifically for Sorbex type Process Units, such as Parex, Molex, and MX Sorbex. It operates the Pumparound and Pusharound circulation valves in the chamber section of these process units during the flow rate s

2、tep changes known as zone transitions, hence its name “ZoneTransition Control”.The basic idea involved with ZTC is that the control system records the position of the circulation valves during a particular zone. The following cycle, when the process is entering the same zone again, the ZTC algorithm

3、 places the valve in that same position. ZTC accomplishes this task by suspending automatic P-I-D control, holding the existing position of the valve for a short time (T0). Next, ZTC uses an “overshoot” calculation to place the valve in an “overshoot” position for a short time (T1), and finally move

4、s the valve to the previous cycles recorded value for a final time (T2). With this approach to controlling the circulation valves position, flow rate changes between zones can be reduced from typical times of 3040 seconds, down to 13 seconds. It has been shown that a relationship exists between the

5、duration of zone transitions and the recovery performance of the process unit. The shorter the zone transition, the better (higher) the recovery.AdjustmentsA basic review of the shape of the zone transition flow rate can be used to determine the best settings for the ZTC parameters. First a discussi

6、on of each of the parameters:T-1T-1 is the amount of time before the Time to Step Timer counts to zero, that the ZTC functionality can start. Starting ZTC before the step timer counts to zero is a way to compensate for a fast RV step and a slow control valve response, by starting ZTC before the Exte

7、nd command is given.T0T0 is the duration that ZTC holds the circulation valve in position before moving it to the overshoot, or final position associated with the new zone flow rate. As a current zone is progressing towards a transition, the last 20 seconds of the valves position are averaged. (This

8、position is placed in the OPc or “Output Current Zone” data register.) ZTC places the circulation valve in manual, at value OPC, for time T0, when a zone transition occurs.T1 T1 is the duration that ZTC holds the circulation valve in an “overshoot” position, before moving it to the final position. T

9、he “overshoot” position, OPx, is a calculated value. It is the difference between the two zone rate valve positions (the next zone position, OPp - the current position, OPc) multiplied by a constant “k” and then added to the value of the current position. Theequation is OPx = k * (OPp OPc) + OPc.T2T

10、2 is the duration that ZTC holds the final valve position in the new zone, before turning the valve back to automatic P-I-D control. The final valve position is called OPp, or “Output Previous Cycle”. When the circulation is at a new zone rate, an average of the last 20 seconds of the first bed is u

11、sed as the OPp for the next cycle. Thus the OPp data is actually the value from the “previous” cycle, but used as a guide to position the valve for the upcoming zone flow rate. Start with Initial Values of T-1=0, T0=0, T1=1, T2=2, and K=1.0k“k” is a constant that multiplies the difference in valve p

12、osition between the two adjacent zones. In the equation, OPx = k * (OPp OPc) + OPc, a value of k greater than 1.0 will produce an overshoot, a value of k less than 1.0 will produce an undershoot. A value of k equal to 1.0 will produce a value of OPx equal to OPp.Regarding the t-1, t0, t1, and t2 tim

13、ers: Make sure that the sum of (t-1) + t0 + t1 + t2 is 0.3 seconds greater than the RV Step (Extend Stroke) Time shown on the RV DATA graphics page. If not, the ZTC control may turn back to Automatic control before the Zone setpoint has changed to the new zone setpoint.This is because the “three out

14、 of four voting logic” needs time to complete after the step and checking the Limit Switch and Bed Position indication changes.Also, when K=1.0 it does not matter how the total t1 plus t2 time is distributed between the two timers. However, when k does not equal 1.0 the distribution of the time betw

15、een the two timers is important. And frequently when k is not one, the t2 timer is set to 0.0seconds.A Recommendation: If t-1 0, then t0 = 0.Meaning, if t minus one, does not equal zero, then t zero, should equal zero.Otherwise they partially cancel each other out. And t-1 appears larger than it rea

16、lly is.It is also acceptable for both t-1 and t0 to equal zero.Typical ZTC DiagramIn the above diagram, ZTC places the controller output in manual and holds the valve at position OPc for duration T0. After T0 is complete, the controller output is changed to the OPx value and held for duration T1. Ne

17、xt, after timer T1 is complete, the controller output is changed to the OPp value and held for timer T2. Finally, after T2 times out, the controller output is put back into automatic mode.Case 1: The circulation flow “overshoots” the targeted flow rate for the next zone setpoint.In the above illustr

18、ation, ZTC causes the flow to overshoot the desired flow rate. It does not stay below the setpoint too long, only that it overshoots. First reduce the “k” value for the Zone IV transition. Next, the duration of timer T1 can be reduced. Both of these adjustments will decrease the magnitude and durati

19、on that the valve output stays below the target value. Remember: Everything is connected in this liquid full system. Meaning that tuning one controller has an impact on many of the others too. This is most obvious with the Pusharound flow. So when tuning the Pumparound ZTC you might see what looks l

20、ike a very good transition, but also look to see if there are any Pusharound flow spikes. If so this could indicate that the t-1 or t0 numbers need adjusting to make the Pumparound flow transition better correspond with the actual flow changes created by the Rotary Valve stepping.This can be better

21、understood by looking at the Parex or Molex or MX Sorbex Wheel. When a stream is taken out of a chamber it reduces the chambers pressure. When a stream is put into a chamber it increases that chambers pressure. So if a stream is no longer going into a chamber, due to an RV step, that chambers pressu

22、re goes down, and the pressure of the chamber it is now going into goes up.When these pressures temporarily change, until the Pumparound flow adjusts, the difference in chamber pressure affects the Pusharound flow rate. And by seeing if the flow spiked high (higher delta P) or spiked lower, this can

23、 help us decide how to adjust the ZTC t-1 and t0 stepping timers.Case 2: The circulation flow “overshoots” the targeted flow rate for the next zone setpoint AND stays below the setpoint too longIn the above illustration, the flow overshoots the desired flow rate and also stays below the setpoint too

24、 long. In this case, reduce either the “k” or the T1, while at the same time reducing the T2 timer. Additionally, increase the Integral action of the controller a small amount to help obtain a better OPp for the next cycle.Pumparound Error Integration: - This is a newer feature that aids in the Pump

25、around ZTC tuning. This functionality integrates the error between the Pumparound Process Value (PV) and Set Point (SP) for an entire Rotary Valve cycle. It is a normalized value reported in units of percent of Pumparound Scale. This number varies from unit to unit, depending on quality of installed

26、 equipment and unit feed rate. Well tuned with good equipment and digital positioners see a Pumparound Error Integration value of around 0.28% at 100% of Design Feed Rate. And will increase as the unit throughput is increased above 100% of design feed rate.Note: The optimum ZTC tuning values will ch

27、ange as unit feed rate changes.So, to minimize the need for time consuming retuning, tune for the feed rate most commonly operated at. And only retune if really necessary when operating far away from the normal point. Two sets of ZTC tuning numbers are usually enough. If commonly operating at over 1

28、00% of design feed rate, three sets of ZTC tuning numbers may be necessary.Case 3 The circulation flow “undershoots” the targeted flow rate for the next zone setpoint .In the above illustration, the flow undershoots the desired flow rate and also stays above the setpoint too long. In this case, increase either the “k” or the T1, while at the same time reducing the T0 timer. This adjustment