Pakarab fertilizers limited multan
DIFFERENTIAL PRESSURE METHOD
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- Wet Leg System
- 4.9 Level Compensation
| 4.8 DIFFERENTIAL PRESSURE METHOD Dry Leg System A full dry leg installation with three-valve manifold is shown in Figure 4 below. If the gas phase is condensable, say steam, condensate will form in the low pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter. A technique to solve this problem is to add a knockout pot below the transmitter in the low pressure side as shown in Figure 4. Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid. In practice, a dry leg is seldom used because frequent maintenance is required. One example of a dry leg application is the measurement of liquid poison level in 87 Field Instrumentation GEN Rev) the poison injection tank, where the gas phase is non-condensable helium. Inmost closed tank applications, a wet leg level measurement system is used. Wet Leg System In a wet leg system, the low-pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg. A level transmitter, with the associated three-valve manifold, is used in an identical manner to the dry leg system. At the top of the low pressure impulse line is a small catch tank. The gas phase or vapour will condense in the wet leg and the catch tank. The catch tank, with the inclined interconnecting line, maintains a constant hydrostatic pressure on the low-pressure side of the level transmitter. This pressure, being a constant, can easily be compensated for by calibration. If the tank is located outdoors, trace heating of the wet leg might be necessary to prevent it from freezing. Steam lines or an electric heating element can be wound around the wet leg to keep the temperature of the condensate above its freezing point. 4.9 Level Compensation It would be idealistic to say that the DP cell can always be located at the exact the bottom of the vessel we are measuring fluid level in. Hence, the measuring system has to consider the hydrostatic pressure of the fluid in the sensing lines themselves. This leads to two compensations required. Download 3.37 Mb. Share with your friends:
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