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 Breaking inlet foam
If liquid in the gas approaching the compressor knockout drum is subject to foaming, it can readily flood a mesh pad and in severe cases even a vane unit. The end result is massive liquid carryover from the vessel and damage to the compressor. A vortex-tube cyclone device (Figure 5) can break up foam in the incoming feed.
Dealing with high liquid viscosity
There are applications in which high viscosity impedes liquid drainage so severely that a mesh pad would flood at prohibitively low velocities and liquid loading. In these applications, a vane unit is the better choice. For high efficiencies, consider Amistco’s double-pocket vanes, which handle high-viscosity liquids with an efficiency of 99.9% for 8-micron and larger droplets.
Mist eliminator spacing in the vessel
An often overlooked but very important aspect of suction drum design that can lead to liquid carryover is proper spacing in the vessel. Figure 6 illustrates guidelines for sufficient distance between the mist eliminator and the gas inlet and outlet. If spacing is too close, the gas will pass though only part of the mist eliminator. This causes localized high velocities with liquid re-entrainment and low velocities with poor efficiency as shown in Figure 7.
In an existing conventional drum where the mist eliminator is too close to the gas outlet, there may not be enough room to lower the mist eliminator. The solution then might be a properly designed flow distribution device located above the mist eliminator to create a more uniform velocity profile.
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