Pressure pulsations in sieve-tray columns.

Destructive vibrations occur in industrial sieve-tray columns under
certain flow conditions. The vibrations result from regular pulsations
of the gas flow and pressure in the column. These pulsations have been
studied with a 3 m. high perspex model sieve-tray column having a single
active test-tray using air and water as process fluids.
The effects on the amplitude and frequency of the pressure pulsations
of gas velocity, tray liquid head, liquid crossflow and tray and column
geometry were determined using pressure transducers with real time signal
analysis. Pulsations were produced with fifteen tray geometries, including
five hole diameters between 4.76 mm and 15.87 Mm. Most of the pulsations
were produced at gas velocities below 12 mls and liquid heads below 200 N/m2.
The pulsation fundamental frequency varied between 12 Hz. and 40 Hz., with
an r.m.s. amplitude of up to 60 N/m2•
The gas-liquid behaviour on the sieve-tray was studied using high speed
cine-photography and electrical conductivity probes. This showed conclusively
that, during the occurrence of pulsations, the gas-liquid interaction
was highly regular and synchronised with the pressure pulsations. Detailed
measurements were obtained of the degree of synchronisation and of the
liquid motion on the tray. Several modes of gas-liquid interaction were
identified. During pulsation production, the pulsating jet and the imperfect
bubble were most common. Calculated fluctuations in the gas flow
rate through the test-tray based on measured pressures agreed well with
the results of the film analysis.
A model of the synchronisation process is proposed based upon control
by the pulsating jet. The model explains the measured limits of pulsation
occurrence and permits some prediction of these limits for systems
other than air-water. A simplified system flow analysis accounts for
effects of gas density and column geometry on the pulsation frequency.
The form of the empirical correlation of frequency with experimental
variables is consistent with a physical description of the pulsating jet.