What is wet leg calibration?
What is wet leg calibration?
Wet leg calibration is a way to calibrate differential pressure (DP) level transmitters that are installed on closed tanks or pressured vessels. In this procedure, the low-pressure (LP) impulse line is kept full with liquid on purpose. The wet leg is the name for this liquid-filled impulse line.
There is gas or vapor pressure above the liquid in closed tanks. If you don’t make up for this pressure correctly, the level measurement will be wrong. A condensate pot or seal pot fills the LP side impulse line completely with a known liquid, which is usually condensate or the same process fluid. This keeps the hydrostatic pressure on the LP side of the DP transmitter at a steady level.
When calibrating the wet leg, the constant pressure is taken into account when figuring out the transmitter’s Lower Range Value (LRV) and Upper Range Value (URV).
The transmitter doesn’t read zero DP when the tank is empty. It reads a negative DP instead, which is caused by the height of the wet leg fluids.
The DP goes up when the tank is full because the process liquid height is higher than the wet leg pressure.
The main benefit of wet leg calibration is that it automatically cancels vessel pressure. This is because gas pressure affects both the high-pressure (HP) and low-pressure (LP) sides of the transmitter equally. The only thing left is the influence on the liquid level.
People often utilize wet leg calibration in boiler drums, separators, reactors, and pressured storage tanks, where condensation is likely to happen. To calibrate correctly, you need to know the height of the wet leg and the specific gravity of the liquid. This is because any change in these numbers will directly affect the accuracy of the level.
In short, wet leg calibration makes sure that level measurements are consistent, dependable, and precise when employing DP transmitters in closed tank applications.
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