Wet leg for Dp Transmitter


Thread Starter

phil dryburgh

Hello List About 12 months ago I seen a thread about keeping the wet leg for a Dp transmitter open from blockages. The application is for sump levels in a coal preparation plant. Where slurry or magnetite especially after a shutdown have a tendency to block the wet leg of the Dp transmitter. I can't remember whether this is a regulated water or air stream passing across the diaphragm. If anyone can remember or knows of this technique, comments would be appreciated. Regards Phil Dryburgh

Richard King

Hi, If it is a wetleg, then a constant regulated water purge will keep the wet leg full and most the process out. If you are using a bubble tube, a constant air flow will keep the tube full of air and process out. Some applications use water and air for bubble tubes. If the level is for a sump, the transmitter might be above the fluid level. If this is the case, the best solution might be a bubble tube. If a slurry is involved, then a Air/Water combination might assist in keeping the probe clear. I would only use dp Level Tx on clean/non-scaling processes, or where I don't have a choice. Note: Not many Dp Level Txs work well, if the lower Tapping point is buried in slurry. I have seen this cause pressure surges. regards, Richard King
There are a number of ways that this can be done. The first is to purge the wet leg with a material that is innocuous to the process. Air, N2, and water are common materials. I don't know that I would use steam. There is a caveat in that if you are using a small connection, you may get back pressure that can effect the measurement. Special flanges are available with purge connections. Another way is to use an extended diaphragm. If your transmitter is mounted on a flange mount, you use an extended diaphragm that is flush or sticks out into the tank slightly. If the material set up is due to temperature, some people steam or electrically trace the tank side connection. Bill Mostia ======================================================= William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169

Tony R. Gunderman


I'm not familiar with the thread (fairly new to this site), but one technique we have used to avoid build up and eventual blockage of dP level transmitter wet legs is to set up a timed blow or flush of the leg at a higher pressure than the normal "bubble" medium. We usually implement this with a simple solenoid valve to route in the blow/flush medium. When the timers indicate it is time to blow the wet leg, the PV is frozen (to avoid control issues) and then the blow is done. The PV freeze is maintained for some predefined wait after that to allow the signal to stablize on the normal "bubble" medium. The downside is that while this cycle is in progress, the PV is unknown. However, for most of our slow moving level applications, we have found this is much more reliable than waiting for the transmitter to become unpredictable due to a plug in the wet leg.


Tony R. Gunderman