Pakarab fertilizers limited multan
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- 2.7 Pressure switch
- 2.8 Liquid or Steam Pressure measurement
- 2.9 Failures and Abnormalities Over-Pressure
| 2.6 Low pressure measurement Vacuum: pressure below atmospheric pressure is called vacuum or absolute pressure. One atmosphere of pressure is equal to 760 mm Hg. One torr (shorter name of Torricelli, scientist) is equal to 1 mm Hg. 48 Field Instrumentation GEN Rev) Low pressure measurement 2.7 Pressure switch A pressure switch turns an electric circuit on or off at a preset pressure. This pressure is referred to as the set point of the switch. The contacts in a switch maybe normally open or normally close. Dead band is the difference between the value at which a control action occurs set point) and the value at which the control action is cancelled (reset point. 49 Field Instrumentation GEN Rev) 2.8 Liquid or Steam Pressure measurement When a liquid pressure is measured, the piping is arranged to prevent entrapped vapors which may cause measurement error. When steam pressure is measured, the steam should be prevented from entering the Bourdon tube. If the gauge is below the point of measurement, a siphon is provided in the pressure line to the gauge. 50 Field Instrumentation GEN Rev) 2.9 Failures and Abnormalities Over-Pressure: All of the pressure sensors we have analyzed are designed to operate over a rated pressure range. Plant operating systems rely on these pressure sensors to maintain high accuracy over that given range. Instrument readings and control functions derived from these devices could place plant operations in jeopardy if the equipment is subjected to over pressure (over range) and subsequently damaged. If a pressure sensor is over ranged, pressure is applied to the point where it can no longer return to its original shape, thus the indication would return to some value greater than the original. Diaphragms and bellows are usually the most sensitive and fast-acting of all pressure sensors. They are also however, the most prone to fracture on over- pressuring. Even a small fracture will cause them to read low and be less responsive to pressure changes. Also, the linkages and internal movements of the sensors often become distorted and can leave a permanent offset in the measurement. Bourdon tubes are very robust and can handle extremely high pressures although, when exposed to over-pressure, they become slightly distended and will read high. Very high over-pressuring will of course rupture the tube. Download 3.37 Mb. Share with your friends:
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