Wet leg in differential pressure measurement

  • Thread starter Chronis Theodos
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Thread Starter

Chronis Theodos

When wet leg design is implemented, which side of the differential pressure transmitter (DPT) is connected to the wet leg? (+) or (-)? Does the density of the filling fluid have to be taken into consideration, and if yes, how (heavier or lighter than process fluid)? If the filling fluid is condensate (steam level measurement), then which side of DPT is connected to the wet leg? If it is connected vice versa, is it wrong?

Wet leg is used normally in level measure, because if the gas or steam (water, HC) inside the drum condensate to ambient temperature, this condensate will begin to fill the empty tubing connected to the top of the drum, then this produces a small column of liquid that will go rising through the time and your LRV (zero) will not be the same that you calculated. It is the same if you connect the high side on the top or vice versa, the only consideration if you do that is that you must take care with the calculus range. Usually the rule is to put the high side(+) in the bottom and the low side (-) in the top of the drum. The density of the filling fluid is taken in consideration because this density is used to calculate both ranges the upper and the lower range. It is normally heavier than process fluid. We use glycerin, glycol, water... all depends on the temperature, compatibility with the process fluid. One point with respect to the density is that the span (when you calculate the range) has to be more high than minimum span than is capable to measure with the transmitter.

Instruments Technician

Bryce Gillan

The DP cell is connected to the process with the (+) and (-) connected as they would on a system without wet legs. The (+) is connected to the process with the higher pressure e.g. on level measurement the bottom of the vessel and the (-) to the top of the vessel. The density of the material in the legs needs to be taken into account in the overall calculation of the DP cell.

On a DP system you can have a wet leg implemented onto either (+) or (-) or both. If you are measuring condensate level then you should have both (+) and (-) sides of the DP cell configured with wet legs otherwise if you are trying to keep a dry leg, condensate will build up where you don't want it and affect the measurement.

To do the calculation, you just have to add up the pressures on each side of the DP cell at 0% level measurement and 100% level measurement and you get your range. To get the pressures you have to multiply the heights of the liquids multiplied the sg and you get pressure values in units of mmH20 if the distances were in mm.

0% = (+) pressure - (-) pressure (vessel at 0% level) to 100% = (+) pressure - (-) pressure (here you will have additional pressure on the (+) of the head of condensate with the vessel full).

Chronis Theodos

Thank you very much guys.

Your advice has proved to be extremely valuable!!

Chronis Theodos