Block Valve Fails to Prove Closure Near a School
A 12-inch NGL line is running at 1,100 bbl/h. At 02:10, the controller commands remote closure of block valve MV-33 at milepost 33 after a downstream pressure alarm. The valve reports 'closed' but flow does not drop; upstream pressure rises 40 psi in 10 minutes and downstream pressure falls 12 psi. The valve has a gas-over-oil actuator with a manual handwheel; local operation takes about 45 turns. The nearest operator is 25 minutes away. A school is 0.5 mile from the valve, and a subdivision is 1 mile downstream. Company policy says a valve that fails to prove closure is treated as a potential release. The upstream station can isolate in 4 minutes via remote valves, but that would strand 8,000 bbl between stations. The downstream station can accept a reduced rate, but only if the line remains pressurized.
1. What are the controller's immediate priorities?
Read the worked answer
Treat the valve as failed and the situation as a possible release. Stop trying to prove closure remotely; command upstream isolation first to stop supply, then downstream isolation to limit the release volume. Notify the supervisor and emergency responders, and tell them the valve status is unconfirmed. Dispatch the nearest operator to operate the handwheel manually, but do not wait for that operator before isolating remotely. Check pressures and flow for signs of a release, and consider shutting down or reducing pump rate. Protect the school and subdivision by notifying emergency response and following the emergency plan. Do not open a bypass or increase flow to clear the valve. Document times and commands.
2. How would you decide between upstream isolation and waiting for manual operation?
Read the worked answer
Use consequence and time. The valve is at milepost 33, with a school 0.5 mile away and a subdivision 1 mile downstream; a potential NGL release is a high-consequence event. Manual operation is 25 minutes away plus 45 turns, so waiting leaves the line supplying a possible release. Upstream isolation in 4 minutes stops the source; downstream isolation limits the inventory that can escape. Stranding 8,000 bbl is an operational and commercial cost, not a safety reason to delay. Isolate upstream first, then downstream, and coordinate with the operator. If pressures indicate no release and the valve is only failing to indicate, the supervisor may adjust, but the default is conservative because the valve did not prove closure.
3. What information should be given to responders and the operator?
Read the worked answer
Give the product (NGL), line size, operating pressure, flow rate, valve location and milepost, and the fact that closure is unconfirmed. State the isolation status and the expected time to isolate. Identify the school and subdivision, access routes, and wind direction if available. Tell the operator which valve to operate manually and warn about the actuator, stored energy, and the need for PPE. Tell responders where the product could travel and that NGL is flammable and may travel as a vapor. Provide a control-center contact and update times. Do not speculate beyond confirmed data; label estimates as estimates. After isolation, confirm zero flow and pressure behavior before allowing responders to approach.