Process Safety Beacon: Thermal Expansion

Some process safety insights

We have developed an index of the Beacons, along with a summary of our process safety analyses, at our Process Safety Beacon Index.

The August 2025 Process Safety Beacon, Thermal Expansion runs hot and cold!, describes an incident in which a section of liquid-filled piping ruptured after being isolated between two closed valves. As ambient temperature rose, the trapped liquid expanded, dramatically increasing pressure until the line failed. The release injured 31 people, including one fatality.

In this post we consider some of the process safety lessons to be learned from this incident.

Beacon Analysis

This Beacon report emphasizes that any liquid trapped between closed valves can build enormous pressure as it warms. Even small temperature changes can raise internal pressure to dangerous levels because liquids are nearly incompressible. Issues to consider include the following:

• Contributing factors included inadequate operating procedures, insufficient hazard awareness, and lack of protective equipment.

• Install relief valves or expansion chambers where isolation is unavoidable.

• Train operators to anticipate thermal-expansion pressures during shutdown, maintenance, or startup.

• Recognize that both heating and cooling can be dangerous; freezing liquids can also rupture equipment.

• A thermal relief path was not provided.

Sutton Technical Books Analysis

Below are some thoughts as to how the 20 CCPS process safety elements apply to this incident. Asset Integrity/Reliability, Hazard Identification and Risk Management, and Operating Procedures are particularly important.

Process Safety Culture

The event reflects a concern sometimes found regarding well-known physical principles. When production pressures override attention to detail, routine isolation steps may seem harmless. A questioning culture would have prompted someone to ask, “Where will the expanding liquid go?”

Compliance with Standards

API 521 and OSHA 1910.119(d)(3)(i)(F) require over-pressure protection for blocked-in liquid systems. Either these standards were not understood or compliance checks were inadequate.

Competence

Operators and maintenance staff did not fully grasp the consequences of temperature change in a sealed system.

Workforce Involvement

If front-line workers had authority to challenge isolation plans, they might have questioned the situation. Empowering the workforce to stop and review unusual configurations is critical.

Stakeholder Outreach

Although not central to this case, the event underscores the importance of communicating physical hazard awareness to contractors and other site visitors who may not appreciate over-pressure risks.

Knowledge Management

Incidents of this type have been documented for decades. Failure to capture and share lessons from previous cases shows weak institutional memory.

Hazard Identification and Risk Management

The hazard — thermal expansion of a trapped liquid — should appear in P&ID reviews, HAZOPs, and pre-startup risk assessments. Either the hazard was missed or dismissed as low-likelihood.

Operating Procedures

Procedures likely directed operators to ‘close both valves’ without specifying a vent or relief step. Procedures must explicitly require pressure-relief verification whenever sections are isolated.

Safe Work Practices

Lockout/tagout should include verification that isolated lines are safely depressurized or relieved. This check was apparently omitted.

Asset Integrity / Reliability

The piping and valves were mechanically sound, yet the design lacked a thermal relief device: an integrity failure at the system level rather than at the component level.

Different grades of steel may have helped avoid this incident.

Contractor Management

There is no information regarding contractor involvement.

Training and Performance Assurance

Refresher sessions should include demonstrations showing how small temperature changes create extreme pressure in blocked-in lines.

Management of Change

MOC was probably not a factor in this incident.

Operational Readiness

Operational Readiness was probably not a factor in this incident.

Conduct of Operations

Routine work can become mechanical. Consistent shift-handovers and checklists could have caught the isolation before temperature increased. The Beacon comment about potential over-pressure in temporary lines and hoses is a particular concern.

Emergency Management

Emergency drills should consider sudden line ruptures and flying debris.

Incident Investigation

The CCPS Process Safety Beacons, that have been published on a monthly basis for many years, have made an important contribution to process safety. The manner in which posts such as this analyze these events in terms of the elements of process safety can enhance the value of these Beacons.

Measurement and Metrics

Indicators such as ‘number of lines isolated without relief’ could serve as leading metrics for maintenance and operations teams.

Auditing

Periodic P&ID and field walk-through audits should confirm that every potentially trapped volume has adequate pressure relief.

Management Review

Senior management must reinforce the principle that engineering fundamentals, such as thermal expansion of trapped liquids are safety issues.

Other Similar Incidents

This incident was not unique. Other similar events are:

• TPC Group, Port Neches, TX (2019)
  Referenced in the Beacon, a blocked-in section of isobutene piping ruptured as temperature rose, bursting a cast-iron strainer. The ensuing explosion injured 31 people and caused massive damage.

• Refinery Propane Line (2007)
  A long-idle propane section fractured because trapped water froze. When thawed, propane leaked and ignited. Several workers were injured.

• Olefins Unit Drain Line (U.S. Gulf Coast, 2013)
  A short, blinded drain spool filled with naphtha warmed under sunlight. The blind flange blew off, releasing vapor near hot equipment. Investigation revealed the isolation had been installed during maintenance without a relief path.

• Cooling-Water Loop at a Power Plant (2016)
  Operators isolated a water-cooled heat exchanger for cleaning and left it full. Overnight freezing split the exchanger shell, flooding the pit on restart.

• Ammonia System Service Line (2020, food plant)
  A liquid ammonia transfer hose, disconnected and capped for storage, warmed in the sun and violently ruptured. The resulting release led to several cases of chemical burns and environmental release reporting.

Conclusion

This incident illustrates that simple physics can defeat complex systems. Even in well-maintained facilities, neglecting the behavior of trapped liquids can cause sudden and violent failures. Each isolation step, however routine, deserves a moment’s thought: Has pressure somewhere got a way out? Even with digital control and smart valves, thermal expansion remains an analog hazard governed by physics. Small-bore lines, pressure gauges, and sample ports inevitably create confined volumes. Good design identifies them, but only vigilant operations ensure they remain protected.

The deeper lesson is that process safety depends as much on understanding physical science as on following rules.