DP Xmtrs on Towers

[ProjEngr]

Our process group would like DP's on the Atmospheric Crude Tower around every section. All DP's are in the vapor space of the tray. All the xmtrs are mounted above the taps with the sensor lines sloping toward the taps. I have steam traced the sensor lines, but we are still having problems with (we think) condensables building up in the tubing, causing erroneous measurements. Are there any alternatives to purging the sensor lines? I would like to keep the DP xmtrs I have purchased (in lieu of buying sealed leg diaghrams.)

 

[chop]

You could try to put the dp below the taps to keep the tubing full of condensable. If the condensable stays in liquid phase up to the taps you should be fine. Be sure the tracing temperature keep the tubing above the freesing point and below the flashing point. You will have to ofset your zero. Make a try on one to see if it is stable.

Good luck.

 

[MarkTX]

I have run into this problem many times. Usually it works when first installed then after a few hours or days the vapors start condensing. If you hold your finger over the end of straw and pull it out of a glass of water, the water stays in the straw. This could be what is happening in your case. One solution is to install 1" pipe or tubing. The diameter of the tubing is too large for the condensing liquid to collect so it drains back down the walls of the tubing.
The majority of places I work use 1/2" tubing for impulse lines so this problem surfaces.
The past few years I have run into more installations where two transmitters are installed, one at the top and one at the bottom. A subtracting block is used in the DCS to calculate the difference in pressure. You need very accurate transmitters for this. I wouldn't do it with two pneumatic transmitters.
Mark

 

[ProjEngr]

Thanks for the response. Ultimately we went for the purge. We installed a natural gas purge to each tap on the tower. At each tower tap, I installed a Brooks rotometer (vareameter) sized for 1/2-5 scfm. The DP's have now become a very reliable reading. You can only do this on towers that have an overhead drum with off-gas or some other vapor in it. For towers without O/H drums or with liquid filled O/H drums, I recommend the much larger sensor tubing, a sealed leg diaphragm differential pressure transmitter, or dual pressure transmitters.