3. DESARROLLO
3.4. DIFICULTADES EN LA TRADUCCIÓN DE PANZA DE BURRO
NCSM tuning is comprised of two steps. They are the baseline and first step. During the baseline, the pump output % is held at a low value for a chosen duration. During the first step, the pump output % is held at a higher value for a chosen duration. The NCSM data is collected and a set of PID values is computed. They are uploaded to the controller to begin PID control.
1. Select Edit AT ORP (NCSM) in the Configurator
2. Select Tune PID in the control page and select Auto tune in the PID tuning screen.
3. Scavenger feed pump must be set to the optimum stroke setting such that it can account for all changes in load expected in the system. This setting can typically be determined by analyzing data from the monitoring mode.
OM0211_________________________________________________________________________ 4. On the Auto Tune PID Loop screen enter the Auto Tune Settings before starting Auto Tune. The
Configurator Help Screens provide detailed information on the steps and parameters.
• Confirm the Setpoint is correct.
• The Baseline Output % is chosen such that the ORP values during the baseline duration are higher (more positive) than the setpoint.
• The Baseline Duration is set to 15 minutes by default and can be changed by the user. The baseline duration should be greater than the system lag time, although this time should be
less than 1 hour.
• In intermittent systems, the baseline duration should follow the above recommendations as well as exceed the off-time (down-time) by 4 times. For example, if the off-time (down-time) is on average of 5 minutes, the baseline durations should be at least 20 minutes.
• The Step Output % is chosen such that the ORP reading will decrease (more negative) by at least 30 mV to 50 mV during the tuning process.
• The Step Duration is chosen such that the pump output is held long enough at the chosen value to observe the minimum change in ORP values (greater than the system lag time).
• For intermittent systems, the step durations should follow the above recommendations as well as exceed the off-time (down-time) by at least 4 times.
• The total tune duration of baseline and step duration is not recommended to exceed 90 minutes.
Note: The pump speed is set to 0% after Auto Tune. So, the pump speed and setpoint can be
changed after Auto Tune.
5. After all the tune settings are entered, click on the Start Auto Tune button to begin the tuning process. The analog pump output will automatically change as shown in the plot
Graphing of the tuning data will be displayed (example)
6. After the two stages of tuning are complete, the Configurator will analyze the data and calculate a set of PID values. They can be uploaded to the controller to begin PID control. For AT ORP control, clicking on the “Upload” button only uploads the PID parameters and does not start PID control.
NO REBOOT IS NEEDED.
• At the end of NCSM Auto Tune, the scavenger pump will be turned off (manual 0%)
• PID control will resume only when you switch the output from “manual” to “auto” via the Configurator or the controller keypad (analog output controls).
OM0211
Note: NCSM PID Auto Tune always sets “D” = 0. This means the controller is truly running PI control.
Note: The “I” value will typically be a little greater than the system lag time (in seconds). This should
have been measured before tuning (see Section 5.3)
7. If the Configurator is unable to calculate a satisfactory set of values it will display error message(s) to indicate why the calculation failed.
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ORP tuning will typically fail if the ORP readings do not decrease (more negative) when the scavenger feed is increased. Various factors can cause this behavior and looking at the tuning log data and knowledge of the condition of the boiler during tuning will help troubleshooting. Check for problems in controlling the pump. Check for air-locked pump.•
Steam load variation was too high during the tuning process. Repeat the tuning when conditions in the boiler are more stable.•
If the variation in the steam load seen during tuning is typical, increase the first step output by 5% and the first step duration by 5 minutes. Then repeat the Auto Tune procedure.If the tuning process does not satisfy certain quality criteria “Warning” messages will be displayed but the user will be permitted to upload these PID values.
•
Choose the baseline output % such that while the scavenger is being fed the ORP reads close to the desired setpoint (more positive than the setpoint). During the tuning process, the ORP values should change greater than if the pump was held at a constant output.•
It is recommended that the baseline output be some value higher than zero. The first step output is automatically calculated for the user but can be changed if the user is concerned about “fogging” the system with scavenger.•
Even if the setpoint is not reached during the tuning process, the PB and I terms can still be used as long as there is a 30 to 50 mV drop during the tuning process and there aren’t any other error messages.Note: See Section 14.5 for additional PID Troubleshooting.
8. Steps to complete after putting the system in ORP PID control.
a. Monitor the system for the first week after control is implemented to determine whether the system is controlling at the setpoint. A PID autotune is meant to be a starting point and will likely require some adjustment to recover good control. What is perceived as “good control” will vary from site to site and is largely limited by the operational characteristics of the boiler. The overall goal is to minimize excursions away from setpoint.