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 Retrofit without recertifying for ASME code
Retrofitting an existing vessel for any of the foregoing suggested remedies has one drawback: it often requires welding new support rings, beams, clips, and other structures to the vessel wall. Most vessels are ASME code certified. Thus, after welding to the vessel wall, the welded area must be heat-treated, and the vessel must be recertified. It is generally desirable to avoid this cumbersome procedure.
Amistco offers expansion rings that are made in sections that can be passed through a manway. (See Figure 9.) The rings are then bolted together and wedged against the vessel wall without welding. The unique double expansion design ensures that the installed ring does not move during operation. Once the rings are installed—either in vertical or horizontal vessels—beams can be bolted to the rings, and complete housings can be built up inside the vessel.
 Using inlet diffusers to relieve carryover
Inlet design is one of the most commonly neglected aspects of a compressor knockout drum design, thus often the cause of poor performance. In the example shown in Figure 10, a poorly selected half-pipe inlet deflector projecting into the vessel produces the following results when the collected liquid is at a high level:
1. Gas jets to the back wall of the vessel. Without enough space to diffuse the jet, gas utilizes only part of the mist eliminator. Due to uneven velocity profile, liquid carryover occurs as in Figure 7.
2. The gas jet agitates the accumulated liquid below, creating a higher mist load.
3. Turbulence spoils normal gravity settling of larger liquid droplets below the mist eliminator. The resulting additional liquid load increases the likelihood of flooding the mist eliminator.
A properly selected inlet diffuser added to an existing knockout drum (Figure 11) provides more effective separation of liquid coming in with the gas and distributes the gas evenly throughout the vessel diameter.
Damage by sudden pressure changes
Carefully review the transient pressures that occur at the knockout drum and mist eliminator during all phases of operations. The suction drum could see a sudden surge of flow in either direction due to compressor recycle or opening an anti-surge valve. Thus, a mesh-type mist eliminator can be subjected to forces not seen in normal operation. This can dislodge the pad sections, leading to compressor damage from liquid carryover or even from fragments of the pad.
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