Sizing Relief Systems for High Viscosity Two-Phase Flow

Qualify your design methods for high-viscosity two-phase relief systems against these simple benchmarks. High viscosity two-phase (HVTP) flow occurs in many industrial scale reactors, particularly when runaway reactions (e.g., during polymerizations) are vented through an emergency relief system. The design of a relief system for two-phase discharge can be complicated, as it involves a fluid with a liquid-like density and a gas-like compressibility. Moreover, the fluid may flash as it loses pressure, achieving choked flow at the valve's exit and thus limiting the flow capacity of the relief system. The literature indicates that 30 - 40% of the relief devices that are in existence violate industry guidelines for inlet pressure and drop and backpressure. These studies followed the Occupational Safety and Health Administration (OSHA); Washington, DC; Process Safety Management (PSM) rule, which requires that the relief device design basis be documented and verified. Many of these installed relief systems were designed using best industry practices, such as the American Petroleum Institute's API-520, which details how to design emergency relief devices for low viscosity ingle-phase (gas, steam, liquid) systems.

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