The Valve They Tried to Fix Made It Worse

What happened?

At the Valero Meraux refinery, an emergency pressure relief valve on a hydrocracker unit failed in the open position, producing a large and sustained flaring load. Operations staff, seeking to reseat the passing relief valve, closed a downstream block valve — presumably believing that stopping the flow would allow the relief valve to reseat under lower differential pressure.

The upstream side of the passing relief valve had a design pressure of 170 bar. The downstream block valve was rated to the flare system design pressure of 19 bar. The closed downstream valve was consequently subjected to the full relieving pressure of 145 bar — approximately eight times its design rating. It failed catastrophically, resulting in a loss of containment and an explosion of a hydrogen/methane mixture.

“Pressure moves according to physics, not operational hopes and assumptions.”

What caused it?

The incident was caused by a fundamental misunderstanding of how pressure behaves across a high-pressure/low-pressure (HP/LP) interface — specifically, what happens when a valve downstream of a passing relief device is closed.

• When the downstream block valve was closed, it became part of the high-pressure envelope whether the operators intended this or not. The relieving pressure was imposed across the valve.
• The downstream valve was rated to flare system pressure (19 bar) — not to the upstream hydrocracker pressure (170 bar). The pressure differential was lethal to the valve.
• The operators’ mental model of the system was incorrect: they believed that closing a downstream valve would reduce pressure on the relief valve. In fact, it trapped and concentrated it.
• Operating procedures did not adequately address what actions were permissible (and impermissible) when a pressure relief valve was found passing.
• The pressure rating boundary between the high-pressure hydrocracker system and the lower-rated flare header was not adequately communicated to operating personnel.

Lessons learnt

This incident is a vivid illustration of one of the most persistent hazards in the process industries: misunderstood HP/LP interfaces. The consequences were not the result of equipment failure or an unforeseeable event — they were the direct result of a well-intentioned operator action based on incorrect assumptions about how the system behaved.

• HAZOP studies must address abnormal operation scenarios: the scenario of a passing PRV and operator response actions must be included in hazard identification studies, not only normal operating conditions.
• HP/LP interfaces must be clearly communicated to all operators: pressure ratings at system boundaries must be visually marked on P&IDs and physically identified on plant, and operators must understand the consequences of isolating downstream of a passing relief device.
• Procedures for abnormal conditions must be explicit: what actions are permitted when a pressure relief valve is found in a passing condition must be documented, risk-assessed, and communicated.
• Never close a downstream valve under a passing relief device without understanding the pressure implications: this must be a cardinal rule in any facility with HP/LP interfaces.

“Closing a downstream valve under a passing relief device makes that valve part of the high-pressure system — whether the operator intends it or not.”