Effect of Rapid Steam Off-Take on Natural Circulation in Boilers

Abstract

This paper describes investigations into the effect of a sudden change in system pressure, caused by rapid steam off-take, on a natural circulation loop. The work was carried out on a two-tube experimental boiler with a water drum connecting the bottom of the riser and downcomer tubes, which were respectively 2 in o.d. x 9 s.w.g. and 11/4 in o.d. x 6 s.w.g., giving a riser to downcomer flow area ratio of 3·9.

The experimental range covered starting pressures of 1000, 800, 650, and 500 lbf/in² (gauge), with pressure drops of 100 lbf/in² at rates ranging from 1 to 34·5 lbf/in² s. Heat fluxes to the riser tube ranged from 21 000 to 136 000 Btu/h ft² and initial mass velocities from 0·70 to 1·11 million lb/h ft² in the downcomer tube. During the transients, continuous recordings were taken of system pressure, circulation, pressure difference, and fluid density at a point in both the riser and the downcomer tube, using the gamma-ray attenuation method.

The system behaviour was analysed theoretically by setting up the appropriate two-phase conservation equations and solving them using (1) a finite difference method, (2) a stepwise solution on the basis of quasi-steady flow. The first method was eventually discarded in favour of the second. The theoretical flow models considered in the analysis include a vapour core annular type flow in the riser, and a homogeneous type flow in the downcomer, during the transient. The theoretical results are compared with the experimental data.