A gas day does not have to start at midnight, and in North American pipeline practice it usually does not. The contract hour is the operator-configured hour of the day at which one gas day ends and the next begins, and it is the boundary that closes each daily quantity record on a flow computer. This guide explains what the contract hour is, why a pipeline and its shippers have to agree on it, and how a contract hour set differently in the flow computer than in the accounting system pushes volume into the wrong gas day.
Contract Hour in one line: The contract hour is the operator-set hour of the day that marks the start of a gas day and the boundary for daily quantity accumulation in a flow computer. In much of North American pipeline practice the gas day begins at 9:00 a.m. rather than midnight, so all parties measuring and accounting for the same gas must configure the same contract hour. If the flow computer and the accounting system disagree on it, volume gets allocated to the wrong gas day even though the total over a long period is unchanged.
A gas day is the 24-hour accounting period over which gas nominations, deliveries, and imbalances are tracked, and its start is a contractual choice, not a physical fact. The contract hour is that chosen start: at the contract hour each day, a flow computer rolls its daily accumulators, closes the record for the day that just ended, and opens a fresh one. Every daily quantity record is therefore bounded by the contract hour on each end rather than by calendar midnight.
In North American pipeline practice the widely used convention places the start of the gas day in the morning, commonly at 9:00 a.m. in the operator's applicable time zone, which is why gas-day totals do not line up with calendar days. The choice traces back to how nominations and scheduling are coordinated across the day, and once adopted it becomes the fixed reference for how volumes are grouped. The specific hour is less important than the fact that everyone touching the same gas uses the identical one.
Because the contract hour also defines the hourly boundaries within the day, it shapes the hourly quantity records too. Each hourly record spans from one clock hour to the next, but the daily record that aggregates them begins and ends at the contract hour, so the first and last hours of a gas day belong to different calendar days. Getting this boundary right is what makes a flow computer's daily total mean the same thing as the pipeline's gas-day total.
Gas measurement is a shared accounting: a producer, a gatherer, a pipeline, and a purchaser may all measure or account for the same molecules, and imbalances between them are settled per gas day. If they do not all define the gas day the same way, they are effectively counting different 24-hour windows, and their daily totals will not reconcile even when every meter is accurate. Agreement on the contract hour is therefore a prerequisite for the numbers to match, independent of measurement quality.
The agreement has to hold in two places: in the field, where the flow computer decides which gas day a given hour of flow belongs to, and in the office, where the accounting or allocation system assigns volumes to gas days for settlement. Both must use the same contract hour, or the same physical flow will land in different gas days depending on which system you ask. Because the contract hour is a configuration parameter on the flow computer, it is also something an audit can and does check.
The stakes are highest around the boundary itself. Gas that flows in the hours just before and after the contract hour is exactly the gas that can slip into the adjacent day if the definitions differ. On a steady, unchanging flow this may only shuffle volume between two days with little net effect, but where flow is changing rapidly near the boundary, a mismatch concentrates the discrepancy precisely where the volumes differ most.
Consider a flow computer configured with a 9:00 a.m. contract hour feeding an accounting system that assumes a midnight gas day. The gas that flows between midnight and 9:00 a.m. is attributed by the flow computer to the previous gas day but by the accounting system to the new calendar day. Every daily total is then built from a different 24-hour slice, so the two never agree, and each day's imbalance carries an error equal to the flow in that nine-hour offset. The long-run total is unaffected because the same gas is eventually counted, but the day-by-day allocation, which is what settlements are based on, is wrong.
This class of error is insidious because nothing is broken. Both systems are internally consistent and every meter may be perfectly calibrated; the totals simply refer to different windows. It typically surfaces as a persistent, roughly constant daily imbalance that will not close no matter how the meters are checked, and the fix is not a calibration but a configuration correction to align the contract hour, applied as a logged change so the record reflects when the alignment happened.
A cloud SCADA such as Merobix reduces the chance this hides by exposing the flow computer's daily rollover and its configured contract hour alongside the accumulated volumes. When an operator can see across a field that every flow computer rolls its day at the same hour, and can compare that hour to what the accounting system expects, a misaligned contract hour is caught as a configuration discrepancy rather than as a mysterious imbalance weeks later. Surfacing the boundary in the same view as the volumes turns an invisible allocation error into something a measurement team can spot and correct.
It is a contractual convention in North American pipeline practice rather than a physical requirement, rooted in how nominations and scheduling are coordinated through the day. The important point is not the specific hour but that everyone measuring and accounting for the same gas uses the identical contract hour, so their daily totals cover the same 24-hour window and can reconcile.
The same physical flow gets assigned to different gas days, so daily totals never agree even if every meter is accurate. The error equals the flow in the offset window between the two definitions and shows up as a persistent daily imbalance that will not close. The fix is a configuration correction to align the contract hour, not a meter calibration.
No. Over a long enough period the same gas is eventually counted, so the cumulative total is unaffected. What changes is the day-by-day allocation, because volume is pushed into the wrong gas day. Since settlements and imbalances are computed per gas day, that misallocation still matters financially even though the long-run sum is correct.
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