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Chapter 9: How Instruments Work Levels, Pressures, Flows, and Temperatures

By Norman P. Lieberman
From Working Guide to Process Equipment, Third Edition

Overview

In this age of advanced computer control, problems are still caused by the field instruments. The field instruments we discuss in this chapter are

  • Level indicators

  • Pressure indicators

  • Flow indicators

  • Temperature indicators

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 Level

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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Liquid Level Floats
Liquid level floats, sometimes known as float balls, are spherical, cylindrical and similarly-shaped objects - made from either rigid or non-rigid material - that are engineered for buoyancy in water and other liquids. They are used for surface demarcation, level measurement, flow control, cable support, and to block UV light. They may also be designed for integration with valve, switch or fluid-instrument mechanical assemblies, to control or monitor liquid levels and tank volumes.
Mechanical Pressure Gauges
Analog pressure gauges are mechanical devices that include bellows, Bourdon tubes, capsule elements and diaphragm element gauges.
Level Sensors
Level sensors are used to detect liquid or powder levels, or interfaces between liquids. There are two basic level-measurement types: continuous and point or multi-point.
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