I have a DP level transmitter that is ranged from LRV -21" H2O to URV 0" H@O. This is a wet leg operation via tubing. I did measure form the center of the low side to the center of the high side taps and they are 21" apart. The tubing drops down from the vessel to the transmitter about 13'. When the legs were filled the transmitter went to a -27.5mA. I could use some help if someone could help me understand why the transmitter would go to a negative saturation level instead of -21" H2O.

 

I have a DP level transmitter that is ranged from LRV -21" H2O to URV 0" H@O. This is a wet leg operation via tubing. I did measure form the center of the low side to the center of the high side taps and they are 21" apart. The tubing drops down from the vessel to the transmitter about 13'. When the legs were filled the transmitter went to a -27.5mA. I could use some help if someone could help me understand why the transmitter would go to a negative saturation level instead of -21" H2O.

 

1. The configured zero is not zero, but is -6 in w.c.

2. Mud crap/sludge from the tank got into the impulse tubes. Mud/sludge has a higher SG than water, so a smaller 'level' of mud/sludge will produce more 'pressure' than 6 inches of water.

examples:
high side 0" minus low side +27 inches = -27", where the low side vertical drop tube has mud = 6" w.c.
High side 6" minus low side 21 inches = -27", where the high side has mud = 6" w.c.
or
some combination of mud/sludge accumulations that apply a combined pressure = 6" w.c. higher on the low side than on the high side pressure

Are there drain valves at the bottom of the vertical impulse lines?

 

1. The configured zero is not zero, but is -6 in w.c.

2. Mud crap/sludge from the tank got into the impulse tubes. Mud/sludge has a higher SG than water, so a smaller 'level' of mud/sludge will produce more 'pressure' than 6 inches of water.

examples:
high side 0" minus low side +27 inches = -27", where the low side vertical drop tube has mud = 6" w.c.
High side 6" minus low side 21 inches = -27", where the high side has mud = 6" w.c.
or
some combination of mud/sludge accumulations that apply a combined pressure = 6" w.c. higher on the low side than on the high side pressure

Are there drain valves at the bottom of the vertical impulse lines?

Should have explained it better, first time posting. This is a steam drum, new construction. The tank is empty. Before I got to the job the crew had filled the high and low side tubing with water. After the water was put in the tubing the transmitter then went to a -27.5. The tubing lines have drain valves, I drew them in on the second pic I posted.

 

The density in the steam drum, and in the liquid filled legs are temperature dependent.
That's why you have an off-set.

 

In looking at your tube connections to the transmitter, I must say they are rather unconventional, as they do not facilitate routine filling, venting, or flushing of the sensor lines.