Tanker Discharge Rate vs. Vapor Recovery Capacity
You are the barge supervisor on a 10,000-barrel inland tank barge discharging gasoline at a shoreside terminal. Shore vapor recovery is connected and the terminal reports the VRU is operating at its design capacity of 400 scfm. Your cargo pump is rated 1,200 gpm at discharge pressure. After 45 minutes, the barge's high-pressure/vacuum relief on the vapor header begins venting intermittently, and the terminal's vapor line pressure alarm sounds. The terminal operator asks you to keep pumping and 'just crack the vapor valve to atmosphere when it builds.' Wind is light and variable; the barge is moored alongside a loading rack next to a residential-adjacent riverbank.
1. Should you continue pumping at the current rate? Walk through the vapor balance and state what you will do.
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No. At 1,200 gpm of liquid out, the barge must take in roughly 1,200 gpm of vapor to avoid drawing a vacuum on the cargo tanks; the VRU can only handle 400 scfm, which is far below the vapor displacement rate of the pump. That mismatch is exactly what produces the intermittent PV-relief venting and the high-pressure alarm.
Actions:
- Stop or throttle the pump immediately so vapor generation matches the VRU capacity - a safe target is well under the VRU's rated scfm once you account for line losses and any simultaneous vapor space changes.
- Notify the terminal that the vapor line is over-pressured and that you will not vent to atmosphere.
- Do not 'crack the vapor valve' - that releases gasoline vapor to atmosphere next to a residential riverbank and defeats the pollution-prevention intent of the VRU connection.
- Log the event, notify the PIC, and confirm the PV relief is functioning correctly before resuming at a reduced rate.
The judgment point: vapor balance is a capacity constraint, not an optional convenience. The pump rate must be set by the weakest link in the vapor path, not by the pump's rating.
2. The terminal insists the VRU is fine and asks you to raise the pump to full rate 'to get the ship out.' How do you handle the conflict?
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You hold the rate where the vapor system can absorb it and document why. Your responsibility for the cargo transfer and pollution prevention does not transfer to the terminal operator.
Steps:
- State the numbers plainly: pump at full rate displaces vapor far in excess of the 400 scfm VRU capacity, and the high-pressure alarm and PV-relief venting are objective evidence of a mismatch.
- Offer the compliant alternative: continue at a reduced rate that keeps the vapor header within limits, or stop and have the terminal verify the VRU.
- If the terminal still insists, escalate to the PIC/company and, if necessary, stop the transfer. A stopped transfer is recoverable; an atmospheric gasoline release near a residential bank is not.
- Record the disagreement, the alarm times, and the rates in the deck log and the transfer record.
The exam judgment: 'the customer wants it' is not a basis for exceeding a safety or environmental limit. You are the supervisor of the transfer, and the vapor path is part of the transfer.
3. What would you check on the barge side before resuming at any rate?
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Before resuming, verify the barge side of the vapor system is sound:
- PV relief/vent valves: confirm they are in the correct operating position and not stuck or fouled; the intermittent venting may indicate a valve that is lifting early or a blocked line.
- Vapor header and hose: check for liquid carryover (a slug of liquid in the vapor line will choke the VRU), condensate, or a kinked/partially closed valve.
- Tank vapor spaces: confirm no tank is being overfilled or drawn down in a way that changes the vapor load unexpectedly.
- Gauging and alarms: confirm high-level and high-pressure alarms function and that the transfer is within the approved plan.
Only after confirming the barge vapor path is clear and the terminal confirms VRU capacity do you resume, and then at a rate the vapor system can absorb. Document the checks.