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When tanks are draining, the potential may exist for a swirling vortex to form leading from the liquid surface to any of the bottom-exit or side-exit nozzles connected to downstream piping (Figure 1). One important aspect of the vortex is whether it will entrain air or other gases into the discharge flow. Such vapor entrainment can lead to a host of problems, ranging from vacuum collapse of the supply tank, to over-pressurization of the receiving tank, to a disruption of the vapor seal between the tanks. Meanwhile, if the entrained vapor is allowed to collect into pockets in elevated pipe loops, it can lead to two-phase flow, which can form liquid slugs that could damage downstream equipment. Similarly, if the flow from the tank is to the suction inlet of a pump, these gas pockets may result in surging, stalling (air-locking) or vane erosion. During continuous operations, such as when a tank is being filled and emptied at the same rate, or when a reboiler is being operated on the side of a column, vapor entrainment may cause pulsating or inconsistent flow.
According to publications available in the open literature, a variety of "vortex breaker" designs have been suggested and are reviewed below. When placed over the tank drain, they help to block or prevent the formation of vortexes. However, what is missing from the literature is useful guidance on when to use a vortex breaker. In general, vortex breakers should also be used judiciously to reduce capital and maintenance costs, and because they may be susceptible to fouling or plugging by solids.
Later, this article presents design information and rules-of-thumb for avoiding gas entrainment that have been gathered from the literature. It also provides several expanded design charts to help users both determine when the potential for vapor entrainment could arise, and evaluate various operating conditions or proposed tank and piping design choices.
Vortex breaker designs
Eastman Chemical Co. (the authors' employer) uses the vortex breaker design from Process Industry Practices (PIP) [2], as the company standard. This vortex breaker design relies on a baffle arrangement, either flush with the bottom of the tank, or suspended just off the tank bottom if the nozzle extends above the tank bottom. Figure 2 shows a 4-bladed...