Posted By: Vivek Gharote

June 18, 2016
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Overview

The primary requisite of a circulation system design is to ensure that nucleate boiling is maintained for all anticipated operating conditions. An usual design criteria is the acceptable percent steam by volume (SBV) or the corresponding percent steam by weight (SBW), throughout the fluid flow path. The inverse of SBW is the circulation number, which is the ratio of the quantity of mixture flowing through the circuit to the quantity of steam produced in the circuit. At low pressures circulation is not a critical problem; but at pressures above 80 atm this attains significant importance. The permissible limit of SBV or SBW is a function of many variables including pressure, heat flux and mass velocity. For each pressure and heat flux, there is a maximum permissible quality which is dependent on mass velocity. At constant heat flux and mass velocity, the DNB occurs at a lower percentage of steam by volume as pressure increases. High pressure designs require a detailed analysis of the probable distribution of heat fluxes, throughout the furnace during operation. These fluxes are a function of heat input burner which affect the quality to ensure that the final circulatory arrangement will have an location, type of fuel fired etc. It therefore becomes a necessity to investigate all the factors percent steam by weight not only on the top of the riser circuit, but also incrementally along its length. Minimum acceptable internal velocity, which is an interdependent variable with quality and heat flux distribution is also an important criterion. Low velocities in sloping tubes may lead to steam blanketing, and eventual failure. Solid deposition may occur at low velocities, leading to subsequent overheating and failure. Low velocities in sloping tubes may lead to steam blanketing, and eventual failure. Solid deposition may occur at low velocities, leading to subsequent overheating and failure. Design of Circulation Piping: A high shock loss at entrance to the circulating piping at boiler drum is to be avoided when water entering the piping is at saturation temperature. Under these conditions, the entrance loss will reduce the pressure below the vapour pressure of water and a definite portion of the water will immediately flash into steam. This steam will condense again, when the increased pressure due to change in elevation equals the sum of pressure losses due to shock at entrance and friction loss. Condensation of steam bubbles at lower elevation may take place with a sudden collapse having all the characteristics of an explosion thus producing water hammer pressure of great intensity in the piping. Tubes should have an entrance velocity of not greater than 3m/sec and should have a vertical drop of not less than 1 m before turning in a horizontal plane or connecting to a header. The piping sizes must be selected so that with the desired rate of flow, the friction and shock losses in the downcomers when subtracted from the hydrostatic head will provide an available head equal to friction and hydrostatic head in the heated tubes and risers. The piping size therefore is fixed by the physical arrangement and the desired circulation ratio.

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