Teaming two solutions can be counterproductive

Sometimes things just don’t add up as expected. Andrew Sloley discusses how sometimes two solutions aren't always better than one solution.

By Andrew W. Sloley, contributing editor

Consider what happened at a plant that had a problem with a level controller on a distillation tower. The tower has multiple side-stream draws from collector trays and each collector has its own liquid level controller (Figure 1). During operation the liquid level controller LC1 on the heavy product suffered from plugging as the process fluid would set up to form a solid.

Two solutions were proposed to make the existing level indicator work:

  1. Adding insulation to prevent heat loss around the level instrument — to keep the heavy material hot enough so that it doesn’t solidify.
  2. Flushing with a lighter material that would not set up — in this case, the medium product from the column. (The small amount of flush wouldn’t create product purity problems.)

In an attempt to make sure the problem got fixed, the plant put both solutions in place at the same time. After the fix the level instrument remained in service. Plugging from solidification stopped.

However, later during HAZOP review, unit operators complained about upsets due to intermittent spikes in level readings, with seal failure of the heavy product pump their major concern. Field verification by checking a sight glass on the vessel showed that the level spikes in the instrument occurred even when the collector tray level didn’t change. Evidently, some problem with the level instrument caused false readings.

Careful review of Figure 1 reveals the reason.

Figure 1. Adding insulation as well as flushing the unit hurt rather than helped performance.

Figure 1. Adding insulation as well as flushing the unit hurt rather than helped performance.

The medium product leaves the tower in vapor/liquid equilibrium at 430°F. It cools as it goes through several units before a slip-stream returns to flush the heavy product level tap. The medium product will vaporize at the tower pressure if heated to above 430°F. The heavy product draw tray temperature is 613°F.

If the heat loss from the level instrument keeps the purge liquid below 430°F until it enters the tower, the level instrument reads correctly. However, if the insulation works too well then heat conduction can raise the temperature of the fluid to above 430°F and the instrument flush vaporizes in the instrument, causing bad level readings.

The solution was to remove all insulation from the level instrument except that necessary for personnel protection. After this no more false readings occurred.

So, remember, teaming two separate solutions may make matters worse instead of better.

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