Understand the type of limit

The various types of limits are as follows:

• Volume — Limits are defined with regards to deviation of volume balance.

• Percent flow — Limits are defined with regards to deviation of volume balance as a percentage of flow. Balance as a percentage of flow is obtained by dividing volume balance over the interval by the total metered volume into the region over the interval, and expressing this ratio as a percentage.

• Switching — Limits are defined using both Volume and Percent Flow. If the meter increment for the Pipeline Monitor (PLM) cycle is greater than the value configured in the Switch Limit field, then the Percent Flow limits will be used. If the meter increment for the PLM cycle is less than the value configured in the Switch Limit field, then the Volume limits will be used.

• Voting — Limits are defined in regards to the amount of instantaneous deviations that occur within a particular integration window.

  Note: Voting is not available when the Extended Alarming Method field is set to Disabled.

Voting is intended to help to reduce the number of leak alarms by known, operationally-induced events, such as the startup or shutdown of pumps. The balance condition and resulting alarm depends on number of one minute intervals having a balance deviation rather than the entire interval.

Normally, PLM sums the instantaneous deviations over each integration window. If the deviation for a window exceeds configured thresholds an alarm is generated. However, if you select PLM Voting in place of normal PLM alarming, the system no longer sums the instantaneous deviations. Instead, it classifies each deviation as either "good" or "bad" on the basis of configured thresholds. The system allows a maximum number of bad deviations for each integration period. When the integration period reaches or exceeds this number, a PLM alarm is generated.

Voting can be used with the following term intervals:

• Short Term 1

• Short Term 2

• Short Term 3

• Long Term 1