ch_client = “kb123″; ch_width = 300; ch_height = 250; ch_type = “mpu”; ch_sid = “Chitika Default”; ch_backfill = 1; ch_color_site_link = “#0000CC”; ch_color_title = “#0000CC”; ch_color_border = “#FFFFFF”; ch_color_text = “#000000″; ch_color_bg = “#FFFFFF”; Free Electrons Electrons are in rapid motion around the nucleus. While the electrostatic force is trying to pull the nucleus and [...]
Solenoid Actuated Valves Solenoid actuated valves provide for automatic open-close valve positioning as illustrated in Figure 35. Most solenoid actuated valves also have a manual override that permits manual positioning of the valve for as long as the override is manually positioned. Solenoids position the valve by attracting a magnetic slug attached to the valve [...]
Electric Motor Actuators Electric motors permit manual, semi-automatic, and automatic operation of the valve. Motors are used mostly for open-close functions, although they are adaptable to positioning the valve to any point opening as illustrated in Figure 33. The motor is usually a, reversible, high speed type connected through a gear train to reduce the [...]
Ball Valves A ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow. The ball, shown in Figure 12, performs the same function as the disk in the globe valve. When the valve handle is turned to open the valve, the ball rotates to a point where [...]
All control modes previously described can return a process variable to a steady value following a disturbance. This characteristic is called “stability.” Stability is the ability of a control loop to return a controlled variable to a steady, non-cyclic value, following a disturbance. Control loops can be either stable or unstable. Instability is caused by [...]
In the last example, the control of the lube oil temperature may initially seem easy. Apparently, the operator need only measure the lube oil temperature, compare the actual temperature to the desired (setpoint), compute the amount of error (if any), and adjust the temperature control valve to correct the error accordingly. However, processes have the [...]
Temperature The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon the molecular activity of the material. Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material. Therefore, temperature is a measure of the kinetic energy of the material in [...]
Thermocouple Operation Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit. Figure [...]
Thermocouple Construction A thermocouple is constructed of two dissimilar metal wires joined at one end. When one end of each wire is connected to a measuring instrument, the thermocouple becomes a sensitive and highly accurate measuring device. Thermocouples may be constructed of several different combinations of materials. The performance of a thermocouple material is generally [...]
Functions of Temperature Detectors Although the temperatures that are monitored vary slightly depending on the details of facility design, temperature detectors are used to provide three basic functions: indication, alarm, and control. The temperatures monitored may normally be displayed in a central location, such as a control room, and may have audible and visual alarms [...]
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