Ripple Effects
These two videos are screen captures I did to convey a problem to the commissioning team instead of writing an annoyingly long e-mail with pictures that explained the problem (which, as some may know, I am very capable of doing).
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Both of these videos are from the same project. The first video illustrates how an upset in one control process can ripple out and cause upsets in other control processes.
Specifically, int this example, an upset in the fan speed control loop, caused by an operating mode change that needed a larger differential, was rippling out and causing a significant disturbance in the zone temperature control processes; about a 2°F change in temperature in an hour or so. |
The diagnostic technique involved locking the fan speed down and observing the result. When we did that, the zone temperature stabilized, indicating that the upset we were seeing in the zone was being driven by the upset we were seeing in the fan speed control process (vs. the zone temperature upset driving the fan speed upset).
That observation focused our troubleshooting on the fan speed control process, which led us to the realization that there was an operating mode change that was being enabled and disabled too quickly. When we spread out the differential on that process, the fan speed control process stabilized, as did the zone temperature.
This video also features cameo appearance by Riley Sellers as the dog barking in the background.
That observation focused our troubleshooting on the fan speed control process, which led us to the realization that there was an operating mode change that was being enabled and disabled too quickly. When we spread out the differential on that process, the fan speed control process stabilized, as did the zone temperature.
This video also features cameo appearance by Riley Sellers as the dog barking in the background.
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This second video is alluded to in the first video. It is another example of how an upset in one system can ripple out and cause issues in a different system.
In this particular instance, a very minor (3-4%) hunt in the control process for a chilled water valve serving one of three air handling systems in an archival storage facility was causing the chillers in the central plant serving the facility to trip out on low evaporator temperature and pressure. |
The diagnostic approach we suggest is identical to the approach suggested in the first video; i.e. lock down the suspected culprit control process and observe what happens.
When we did that the problem went away, which focused our attention on the control processes associated with the errant chilled water valve. That revealed another mode change issue with a differential that was set too tight. In this case, the system was jumping from a temperature based control process to a humidity based control process due to very minor variations in the temperature sensed by the sensor triggering the change; like a hummingbird fluttering by the sensor would likely trigger a change-over event. Spreading out the differential solved the problem.
When we did that the problem went away, which focused our attention on the control processes associated with the errant chilled water valve. That revealed another mode change issue with a differential that was set too tight. In this case, the system was jumping from a temperature based control process to a humidity based control process due to very minor variations in the temperature sensed by the sensor triggering the change; like a hummingbird fluttering by the sensor would likely trigger a change-over event. Spreading out the differential solved the problem.
