DP Transmitter Dry Leg System
A full dry leg installation with three-valve manifold is 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 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 the poison injection tank, where the gas phase is non- condensable helium. In most closed tank applications, a wet leg level measurement system is used.
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