Unit starts

When the number of starts allocated annually for a unit have expired or are about to expire, a tag is added to the compressor record and a warning is issued to the Controller.

The start counts (attempted and successful) are rolled over at the top of each day, month, and year.

Unit starts can be determined either by status feedback from the compressor station or by measuring the speed of the unit (analog points). Both successful and attempted starts are recorded. This data is maintained in the RealTime database (RTDB) and is available for reporting and display purposes using the tools provided with OASyS DNA or other tools such as Microsoft Excel.

If necessary, the Controller can manually override unit start statistics through easy eXtended Operator Station (ezXOS) to reconcile the start counts accumulated. The override does not prevent the starts counts from incrementing further; the existing count is simply replaced with the new count specified by the Controller. Changes to the start counts are logged to the OASyS DNA Event Summary.

Unit Starts - Status Feedback

Where the status of a unit is monitored through a status point, the determination of attempted and successful starts is performed as follows:

• Attempted starts are counted when the operating state of a unit changes from OFF to TRANSIT.

• Successful starts are counted when the operating state of unit changes from TRANSIT to ON.

The table below shows an example of unit status values.

Note: The status names are configurable and may be different on your system.

Unit Starts - Speed Feedback

Where the status of a unit is not available through a status point, it can be determined by engine speed. Two analog points representing the engine speed and compressor speed are used by the application as shown in the table below.

The determination of an attempted start and successful start is performed as follows:

• Attempted Starts are counted when the engine speed exceeds a specified level. Specifically, the engine speed is compared to a specified stopped speed and started speed. Each time the engine speed is detected to have crossed the started speed threshold from below the stopped speed threshold, an attempted start is counted. The stopped and started speed thresholds are specified for each compressor individually.

• Successful Starts are counted when the unit speed (SPEED2) exceeds a specified level. Specifically, the current unit speed is compared to a specified stopped speed and loaded speed. Each time the unit speed is detected to have crossed the loaded speed threshold from below the stopped speed threshold, a successful start is counted. The stopped and loaded speed thresholds are specified for each compressor individually.

Specifying started and stopped speeds for each analog point used to determine compressor status prevents erroneous accumulation of attempted and successful start counts, as shown in the diagram below.

Due to varying telemetry rates and failures of field devices, the transition from one state to another may not always be detected. If Compressor Performance Monitoring (CPM) detects that a unit has changed from OFF to ON, an attempted start is deemed to have occurred and the attempted start count increases along with the successful start count.

Also, in the case of a cold-start of the OASyS DNA system, the CPM application uses second poll processing to ensure that valid data is used to determine unit start counts. Second poll processing ensures that the telemetry point(s) used to determine unit feedback have been scanned at least once by the OASyS DNA system before a unit start is counted.