From Working Guide to Process Equipment, Third Edition
In this age of advanced computer control, problems are still caused by the field instruments. The field instruments we discuss in this chapter are
This chapter is particularly important when we consider that the data displayed in the control room are for operator control of the process. Data for engineering purposes should be obtained locally, at the instrument itself. Further, a large percent of control problems are actually process malfunction problems.
9.1.1 Level Indication
What is the difference between a gauge glass and a level glass? Simple! There is no such thing as a level glass. The liquid level shown in a gauge glass does not correspond to the level in a process vessel. Figure 9.1 is a good example. This is the bottom of an amine fuel-gas absorber. This tower is used to remove hydrogen sulfide from the fuel gas. At the bottom of the tower, there are three phases:
Fuel gas: 0.01 specific gravity
Hydrocarbon liquid: 0.60 specific gravity
Rich amine: 0.98 specific gravity
Figure 9.1: A gauge glass functions as a manometer.
Because of the location of the level taps of the gauge glass, only the amine is in the glass. The gauge glass simply measures the pressure difference between two points of the tower (points A and B in Fig. 9.1). That is, the gauge glass functions as a manometer, which measures the pressure difference in terms of the specific gravity of the...
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