Figure 117

Schematic diagram of displacement-type level transmitter functions as a first-order lag. As shown by Figure 11.8, this chamber has two key features: (1) a hood or cap to keep out liquid from the last downcomer or the reboiler return nozzle, and (2) a 1-inch-diameter hole in line with the lower level nozzle to facilitate rod out. The combination of orifice and chamber gives about 10:1 damping (attenuation) at about 0.5-0.6 cps. If there is a concern about solids collecting in the chamber, a second 1-inch hole may be cut in the lower section of the chamber. The damping will then be 10:1 at 1.0-1.2 cps.

It is probably apparent that almost any desired damping may be obtained by the proper choice of cross-sectional area in the chamber and of orifice-flow cross-sectional area.

External Damping for A P Transmitters

When a AP transmitter is used for level measurement and is installed at an elevation above the upper level tap and equipped with purges, one may provide damping as shown in Figure 11.9. Here a restriction and volume pot are installed in each impulse line to ensure that the inputs to the transmitter are dynamically equalized. Many combinations of volume pots and restrictors are possible; as an example, a pot with 12 in3 and a Taylor snubber (58S104) will give 10:1 attenuation at about 2 cps.

Useful for level measurements, this design is almost mandatory for specific gravity measurements via AP for slurries. It may also be used with gas or steam flow-meter installations with self-draining impulse lines.

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