Figure 119

External damping for AP level measurement

Many AP transmitters on the market have adjustable damping in the feedback. This leaves adjustment up to the instrument mechanic, who seldom has a rational basis for setting it. But an excessive amount of damping can have an adverse effect on control; changing the damping while the process is in operation may have a serious effect on control-loop stability. This feature therefore is not recommended. Where such instruments are bought on a project, the variable damping feature should be left wide open and external properly designed, fixed damping provided where required.

Damping for Externally Mounted Displacer Transmitters

As mentioned earlier, most displacer-type transmitters have a sharp resonance in the range 1—3 cps. For typical displacer housings ("boots"), we have calculated that 10:1 attenuation at 2 cps will be obtained by installing a 1-inch-diameter orifice in the lower line connecting the displacer housing to the process vessel. This provides a fiddleproof design; the guard valves should be left wide open in normal operation and need be closed only for maintenance work. For extremely noisy installations, both internal and external damping may be used, provided the time constants are selected with some care.

It is pertinent at this point to mention certain potential problems with displacer-type transmitters:

1. If excessively long connecting lines are provided between the vessel and boot, the natural frequency of the "manometer" may be low enough to interfere with level control. This is discussed in a very interesting paper by Sanders.4

2. If a restricting orifice is installed as suggested in the lower line connecting the level measuring chamber to the column base (or if an internal damping chamber is used), vapor condensations may occur in the upper line. This may take place fast enough to cause a significandy higher level in the chamber than in the column base. The chamber and upper line should be insulated and heated; in difficult cases a small gas purge to the upper line is helpful.

3. As discussed later, if it is desired to have a liquid purge and damping, the purge rate must be fixed and the liquid level elevation in the chamber known. One could deliberately use the upper connecting line as a condenser, send a fixed amount of condensate via a metering orifice or capillary to the measurement chamber, and overflow the remainder back to the column.

Level Measurement Errors Due to Velocity Effect

As shown in Figure 11.10, level is sometimes measured with the nozzles located on vessels or pipelines of different diameters. Two cases are of interest: downflow and upflow.

Down flow

By far the most common case, downflow is occasionally encountered on distillation column bases or vaporizers with the upper level tap on the column base and the lower tap on the bottom-product line. If the liquid velocity V in the bottom-product line is in feet per second, then the error he in inches of process liquid is:

When V = 1.9 ft/sec, he = — 1 inch. In other words, the transmitter output signal will indicate a level that is 1 inch lower than the true level.

Upflow

In this case there is no velocity error.

Specific Gravity Compensation for Level Measurements

Occasionally we find enough difference between column-feed specific gravity and normal bottom-level specific gravity that there is a serious base-level measurement error at startup. Also, at times, a given column may have a different bottom specific gravity as a result of changes in bottom-product specifications.

1. D0WNFL0WCASE

He = -028U",2 = IN.PROCESS FLUID ERROR IN MEASURING H

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