Pakarab fertilizers limited multan
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- HEAD PRESSURE
- 4.5 Open Tank Measurement
| 4.4 Magnetic Reed Switches If the liquid is hazardous chemical or at high temperature or pressure, magnetic reed switches are used. It is normally in open state. When the floating magnet outside the tube comes near the switch, it attracts the magnetic pole piece in the switch. This action closes the switch until the floating magnet moves away. These switches are not sealed in the tube, and they never come into contact with the liquid in the tank. 82 Field Instrumentation GEN Rev) HEAD PRESSURE Pressure of a liquid in an open tank depends on two factors. The elevation or height of the liquid above the point at which the measurement is to betaken The relative density (specific gravity) of the liquid. • Head Pressure measurement for any liquid Pressure Head (ft) × Specific gravity × 0.433 (psi/ft) Or, P = PCF × H × RD Where P = pressure (psi, PCF = pressure conversion factor (0.433 psi/ft), a constant, H = height of liquid above the measurement point (ft, RD = relative density of liquid. Head pressure of mercury with 10 ft depth is, Pressure = 10 ft × 13.60 × 0.433 (psi/ft) = 58.9 psi Ina closed tank, pressure is proportional to the liquid elevation above the measurement point plus any additional pressure applied to the liquid. For head pressure measurement of corrosive liquids, liquid seal is used to separate process fluid from transmitter. The liquid seal should meet three conditions. It should be non-compressible. It should have a higher density than the process liquid. It should not react with process liquid. 83 Field Instrumentation GEN Rev) 4.5 Open Tank Measurement The simplest application is the fluid level in an open tank. Figure 2 shows atypical open tank level measurement installation using a pressure capsule level transmitter. If the tank is open to atmosphere, the high-pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere. In this manner, the level transmitter acts as a simple pressure transmitter. We have Phigh = Patm + SH Plow = Patm Differential pressure P = Phigh - Plow = SH The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level. Download 3.37 Mb. Share with your friends:
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