Products Line Imbalance During a Batch Interface
A 16-inch refined-products line, 85 miles long, is moving 3,200 bbl/h of diesel with a gasoline batch interface near milepost 41. Over 45 minutes, SCADA's volume balance shows 3,200 bbl/h in and 2,980 bbl/h out. Line pressure at the origin is steady, but a downstream pressure transmitter at milepost 60 has dropped 8 psi. The prior night's ambient temperature fell 12°F. The receiving tank gauge was proved yesterday, but the transmitter has a known 0.3% bias. A contractor is doing permitted road work near milepost 44, and the one-call ticket is current. The controller also sees a 0.5% change in gravity at the receiving station, consistent with the batch interface arriving. Company procedure requires a leak check before shutting down for a suspected release, but a confirmed release requires isolation and notification within 15 minutes.
1. Does the SCADA volume imbalance confirm a leak? Show the calculation and reasoning.
Read the worked answer
Not by itself. The imbalance is 3,200 minus 2,980 equals 220 bbl/h; over 45 minutes, 220 times 0.75 equals 165 bbl. The 85-mile, 16-inch line holds roughly 100,000 bbl, so a 12°F overnight cooldown can change line pack by hundreds of barrels through thermal contraction. Receiving-tank transmitter bias of 0.3% at 2,980 bbl/h is about 9 bbl/h, small but not zero. The 8 psi pressure drop and 0.5% gravity change are more concerning, but the gravity shift is consistent with the gasoline interface arriving. Therefore, the data support a possible leak but do not confirm one. The correct action is to verify tank gauges, correct for temperature, check meter proving, and look for corroborating pressure-gradient changes before declaring a release.
2. What immediate response and notifications are appropriate before confirmation?
Read the worked answer
Start a formal leak check: independent tank gauging, meter proving, line-pack calculation, and pressure-trend review. Notify the supervisor and control center, and alert the nearby permitted contractor to stop work and clear the area until the line is verified. Do not shut down solely on the imbalance if procedure requires a leak check first, but set a clear trigger: continuing loss, growing pressure divergence, odor report, or visible release means isolate immediately. If a release is confirmed, isolate the section and notify emergency responders, the pipeline operator's emergency line, and affected stakeholders within the 15-minute requirement. Document times and data. The key judgment is to keep the conservative trigger while avoiding an unnecessary shutdown that could create its own hazard.
3. How would you localize the suspected release if the imbalance persists?
Read the worked answer
Use the hydraulic gradient. Compare calculated and measured pressures at milepost 60 and other transmitters; a leak produces a break in the gradient between the upstream and downstream sides. Close remote block valves in stages to bracket the section, but avoid creating a surge or isolating a large volume unnecessarily. Check the permitted excavation at milepost 44 first because it is the highest-consequence known activity, but do not assume it is the cause. Dispatch field personnel to drive or fly the right-of-way, look for staining, odor, dead vegetation, or soil disturbance, and use leak-detection equipment. Coordinate with the contractor and emergency responders. Keep the control center informed and preserve SCADA trends for later analysis.