Do Liquid Injection Right

A variety of factors influence performance.

By Andrew Sloley, Contributing Editor

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For our situation, the cSB was 2.12 with a vapor velocity of 56.6 ft/s. Two-phase flow calculations showed operation comfortably in a "distributed" regime. For comparison, this system at 10 ft/s flow velocity would have a cSB of 0.38 and a predicted segregated flow pattern (Beggs-Brill). At 13.0 ft/s the cSB reaches 0.5, but a segregated flow pattern still is predicted. Vapor velocity has to rise to 50 ft/s with a cSB of 1.9 to reach a predicted distributed flow regime. For our case, the 56.6 ft/s velocity required no modifications regardless of the criteria. In other cases, low velocities may necessitate a smaller piping size for good mixing.

When injecting a liquid, consider erosion, corrosion and thermal stress. Erosion can result from direct impact of liquid. The interface zone, where the pipe may be wetted some times and dry other times, can pose a particular corrosion problem. Finally, if quenching occurs, thermal stresses in the pipe can cause major mechanical issues. For the case here, mechanical issues weren't difficult.

The situation is relatively simple if the piping network consists of a simple line. Problems occur with downstream branches. If you must distribute liquid through multiple branches, keep the piping as symmetrical as possible. Maintain the symmetry in both the horizontal and vertical planes and at every split. Figure 1 shows several examples of symmetrical and non-symmetrical piping. In non-symmetrical systems the liquid will concentrate toward the last part of the pipe that takes a turn. To avoid symmetry problems, the plant used separate injection points downstream of the last major branch in the line.

Your requirements may include other factors and pose much different tradeoffs than those here. So, delve into the particulars.


ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at Asloley@putman.net

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