Values for mixed-phase density and pressure from the proprietary software flashes are than tabulated in a spreadsheet to enable the calculation of mass flux at each pressure drop point, as shown in TABLE 1. Isentropic flashes modeled in proprietary simulation software a The general energy balance for isentropic nozzle flow forms the basis of the Bernouli equation for the mass flux calculation, as presented in API 520.įIG. The following is a stepwise presentation of calculating the two-phase relief valve sizing technique described in API 520. This article will not discuss the Omega method. However, results have shown wide vairance and, as such, it is not a favored method for two-phase sizing. The Omega method was developed to avoid performing isentropic flashes and instead use a number of interpolations to achive the result. Homogenous means the velocities (momentum flux) and temperature are at phase interface and the phases are also at equillibrium. The major postulate is that the fluid model is homogenous and at phase equillibrium, hence the acronym (HEM). The HEM method assumes the fluid mixture behaves as a “pseudo-single-phase fluid” with a density that is the volume-averaged density of the two phases. Two-phase relief requires 2x–10x more orifice area than single-phase relief would require.ĪPI 520 describes two methods for solving two-phase flow: HEM and Omega. API 520 has detailed scenarios that can be considered two-phase in Annexure C, Table C.1-professionals can consult the table for more reference. 3.1 below illustrates how these objects can be connected to one another to form a network. The nodes represent junctions, tanks, and reservoirs. The links represent pipes, pumps, and control valves. Normally, two-phase relief is only considered for the device where two-phase flow enters the device or that two phase is produced as fluid moves through the relief device. EPANET models a water distribution system as a collection of links connected to nodes. Two-phase relief flow occurs when mass of flow is a combination of a vapor and a liquid. After calculating the mass flux density, the area of the orifice or flowrate can be known if the other is known. Data from the flashes are then used for numerical integration. An important part of the technique in solving the flow equation is numerically integrating the density-pressure term, which is done by performing a series of constant entropy flash calculations from the pressure upstream of the nozzle to some downstream pressure. The point at which mass flux density is at a maximum corresponds to the choking pressure and flowrate. The technique requires solving the Bernouli flow equation for mass flux density (lb/s/ft 2). The procedure is simple but rigorous and generally applicable. This article will present the sizing technique for two-phase flow through relief valves and orifices. The formula for calculating the needed foam concentrate at. AUTHOR: Asutosh Chavda, Sarnia, Ontario, Canada In order to understand the simple formula, it is important to know the components in a foam formula.
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