Introduction
Temperature Measurement
Process technicians are required to closely monitor the temperatures of process streams. When heat energy is applied to an area, molecular activity increases, and energy is transferred from molecule to molecule. As this process occurs, pressure and temperature increase in an enclosed environment, materials expand, and density changes. Temperature is defined as the degree of hotness or coldness of an object or environment. Two commonly used scales are Fahrenheit and Celsius. Fahrenheit scales operate by using 32°F as the freezing point for water and 212°F as the boiling point of water. Celsius uses 0°C as the freezing point of water and 100°C as the boiling point of water. Process operators use Fahrenheit and Celsius thermometers to measure temperature. Local temperature indicators usually contain a bimetallic strip that differentially expands with increasing temperature, causing a deflection that is correlated with temperature. Bimetallic thermometers (thermocouples) can range from –300° to 800°F. Another familiar type of thermometer has a primary element or sensor that consists of a filled thermal bulb and capillary tubing, resistance bulb, or thermocouple. In industry, mercury is not used in thermal bulbs or capillary tubing. The most common temperature-measuring devices used in the chemical processing industry are thermoelectric.
Pressure Measurement
Pressure is an important variable that must be carefully monitored and controlled in an industrial environment. Pressure is often defined as force per unit area—that is, the amount of force exerted by fluid on the equipment in which it is contained. In physics, the term pressure usually is applied to a fluid, which, in this context, can mean either a gas or liquid. Pressure is measured in pounds per square inch (psi) in the English system, kilograms per square meter (kg/m2) in the metric system, and newtons (N) per square meter, or pascals (Pa) in the System of International Units. (A newton is 1 kg-m/sec2.) Two of the most common types of pressure are atmospheric and hydrostatic. Atmospheric pressure is the force exerted on the earth by the weight of the gases that surround it. At sea level, atmospheric pressure is about 14.7 psi (1.013 Pa). This pressure decreases with altitude because of the reduced height (and therefore weight) of gas. Hydrostatic pressure is the pressure exerted on a contained liquid and is determined by the depth of the liquid.
Fluid Flow Measurement
Fluids flow through a series of pipes, valves, pumps, and vessels. Knowing and controlling the flow rate of a particular process stream are critical to the operation of the unit. Continuous chemical reactions require precise measurements to ensure that all of the reactants (raw materials) are combined in the proper proportions to form the final products. Feed rates and product rates must be accurately controlled for economic reasons. Process flow measurements can be taken by any kind of flow meter, but flow control most often requires a flow transmitter.
Level Measurement
Process technicians use fixed reference points, typically vessel taps, on which to base level measurements. The lowermost tap represents zero level, and the uppermost tap is 100%. Correct level readings and control help make modern processing possible and profitable. Level measurements can be continuous (levels monitored continuously) or single point. In single-point measurements, readings are taken from a s ingle point or from multiple points on a vessel. Single-point measurements are used to turn equipment (valves, pumps, compressors, motors, alarms) on or off and to detect high and low process levels. Level-measurement devices can also be classified as direct or indirect.
References
1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas