Calculate the sum of all products VBN(i) * X(i) where X(i) is the mass fraction of each component: VBN = ∑ (Xi * VBN(i)) VBNblend is then: VBN / ∑(Xi) 3. Calculate the Viscosity Blend Number (or Index) for each component using the following formula: VBN(i) = 14.534 * ln + 10.975 2. The first method that will be discussed here is the calculation using the Refutas equation, since this is a very easy method to implement in an excel spreadsheet, or in a smartphone app, and it requires a minimum of known parameters.ĭownload dropbox for free REFUTAS ALGORITHM As already mentioned the Refutas algorithm involves three steps: 1. For each component the kinematic viscosity at two temperatures must be known. ASTM Blending Method (D7152), which uses D-341 temperature charts for obtaining blend viscosity graphically. It calculates the Viscosity Blend Index for each component, then sums up the product of each VBI and its weight fraction, and finally establishes the blend viscosity by using the formula for the Viscosity Blend Index in reverse. ![]() The Refutas equation, as proposed by British Petroleum is a three step algorithm. In order to calculate the resulting viscosity when blending two or more components together the following methods are popular ones in use: 1. This article focuses on the calculation of a blend viscosity of a fuel or lubrication oil mixture. A variety of methods and formulas exists to assists blending engineers in performing the necessary calculations for composing a specific blend, and sometimes the required specification of the end product calls not only for a specific viscosity but also for (a limit to) a certain flash point and / or pour point. Where VBN is the viscosity blending number of the blend and e is the transcendental number 2.71828, also known as Euler's number.One of the much researched topics in the petroleum industry is the topic of blending Fuel oils and lubrication oils are often blended from a variety of components to achieve a certain viscosity in the final product. Once the viscosity blending number of a blend has been calculated using equation (2), the third and final step is to determine the viscosity of the blend by using the invert of equation (1): Where w is the weight fraction (i.e., weight % ÷ 100) of each component of the blend. The next step is to calculate the VBN of the blend, using this equation: It is important that the viscosity of each component of the blend be obtained at the same temperature. Where v is the viscosity in square millimeters per second (mm²/s) or centistokes (cSt) and ln is the natural logarithm (log e). The first step is to calculate the Viscosity Blending Index (VBI) of each component of the liquid blend using the following equation: ![]() Liquid viscosity blending calculations to determine the viscosity of a blend of two or more liquids having different viscosities are best performed by using what is known as the Refutas equation in a three-step procedure.
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