Length of Vessel
The minimum dimensions in Figure 300-5 are based on the experience and laboratory studies mentioned above. Use these dimensions or increase Dimension “C” if required for liquid residence time. If surge volume is unspecified, a minimum of 3 minutes residence time should be provided within the controllable range. For compressor protection, a minimum of 5 minutes residence time, based on maximum expected liquid rate, should be provided between the high level alarm (LAH) and automatic shutdown (LSH) levels.
If liquid residence time requirements result in an excessively long vessel, increase the diameter. In vertical vessels at moderate pressure, length/diameter ratios up to 3.0 or 4.0 are common.
The LSH (maximum liquid) level is just below the “donut” baffle ring. The minimum liquid level in a vessel with ellipsoidal heads is just above the tangent line; with hemispherical heads, the level may be down within the bottom head.
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