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Consequence of Thermophysical Approximations on Flare and Hydraulic Networks. A Case Study

Srinivasa Rao Karanam, Asuman N. Yilmaz, and Sabyasachi Sen. Invensys SimSci-Esscor, 26561 Rancho Parkway South, Suite 100, Lake Forest, CA 92630

Fluid-flow network analysis simulators used for designing flare and hydraulics systems are comprised of several inter-dependent and layered components. They are:

* The thermophysical properties calculation sub-system,

* The suite of correlations for pressure-drop calculations, and

* The network solver

Of these three layers, the thermophysical properties calculation component represents the core of a simulator and, therefore, plays a critical role in all simulation calculations. It follows, therefore, that the fidelity of this thermodynamics core plays a major role in the accuracy of the predicted relief load that a flare network is designed for. This is an important first step in the avoidance of over-sizing of relief valves which handle relief loads, on account of the significant costs involved. The case for good design is further warranted by the fact that, for safety reasons, uncertainties in thermophysical properties are typically countered by over-designing the system.

Commercially available fluid flow simulators offer a variety of options for calculating thermophysical properties, ranging from non-rigorous methods based on rules-of-thumb and simple bundled correlations, to rigorous equation of state models also involving three-phase flash calculations. Non-rigorous “simplified thermodynamics” approaches yield only rough estimates, and do not, typically, include calculations involving discrete substances. Furthermore, the simplicity often results in inability to describe reasonable vapor-liquid-equilibrium which, in turn, mandates calculations based on single-phase flow assumptions, even if the system experiences multi-phase flow conditions. On the other hand, the solution of flow networks utilizing “simple thermo” can often be an order-of-magnitude less CPU-intensive than those incorporating rigorous component-based thermodynamics and, consequently, a popular choice for analyzing and designing flare networks, even though this approach may lead to over-design.

This case study focuses on several scenarios, comparing and contrasting flare / hydraulic network calculations based on simple versus rigorous thermodynamics. In addition, this investigation seeks to address the over-design penalty, and how savings may be realized with “rigorous thermo” approaches. Several key quantities are compared, including state variables such as density (“microscopic quantities”) and design parameters such as relief load (“macroscopic quantities”). Some rules-of-thumb (guidelines) are proposed for flare / hydraulic network calculations based on single-phase flow when, in reality, two-phase flow occurs.