(41av) Summation of Causes: Inappropriate Use of Layer of Protection Analysis

Pumping, piping, and storing liquids are ubiquitous operations in the oil, gas, petrochemical, and chemical industries. Engineers are taught that liquids are incompressible fluids, and this assumption often proves useful in engineering design and safe operation. Conceptually, many engineers can recognize the overpressure hazard posed by boiling liquids trapped in a fixed volume, such as a reactor vessel. Comparatively, guidance documents and engineering judgement may overlook or discount the hazards posed by trapped liquids that do not undergo a phase transition and experience much smaller volume changes. This seemingly logical assumption is further bolstered by industry experience that demonstrates many systems and components are capable of handling the additional stresses posed by expansion or contraction of a liquid in a fixed volume fluid system. However, there are circumstances where the finite volume expansion (or contraction) of liquids may pose significant hazards in a fluid system. This often-overlooked, underappreciated, or misdiagnosed overpressure hazard from thermal expansion of a trapped liquid has the potential to damage equipment and result in consequences ranging from leaks to catastrophic failures. This paper will introduce the underlying thermodynamics of thermal expansion and provide recommendations on evaluating thermal expansion hazards for liquid-full process components.

Hazards associated with thermal expansion or contraction of liquids in fixed volume systems without an associated phase change will be examined. Specific scenarios will be presented to illustrate the phenomena and underlying thermodynamic considerations. These scenarios will then be further developed to demonstrate key factors that can influence the risk of a loss of containment event. Guidance in industry standards, as well as existing gaps, will also be discussed. Finally, tools and techniques will be presented for risk-based analysis to evaluate systems and identify areas or components that are potentially vulnerable to this often-overlooked hazard.

Specific scenarios will be analyzed for thermal expansion mitigation, including liquid-full piping between two block valves, liquid-full piping between a block valve and check valve, and liquid-full vessels. To understand and address the potential risks of each scenario, the following questions will be addressed:

• How does a volume of liquid become trapped?
• What is the fluid and its potential trapped volume?
• What factors influence the thermal expansion hazard?
• How do the design and operation of the system correlate with the risk?

Potential engineering and administrative controls for managing thermal expansion hazards will be discussed that depend on the answers to the above questions. Case studies from the literature and the authors’ experience will be introduced to support the concepts.