Reverse-Acting D/P Transmitters and Steam Drums

Your last thread had a few good answers, but I'll try to make it a little simpler:

A NORMAL d/p transmitter has an increasing output with an increasing pressure on H (L being constant). This is an undeniable fact.

In order to get the transmitter to increase output on increasing level (drum head), the H side MUST be connected to the bottom of the drum. Period.

The L side is connected to the top of the drum to establish your point of reference, and allowed to fill with condensate in a wet-leg system.

Imagine an empty drum. If the L leg is allowed to fill with water, that will drive the transmitter downscale, right? So re-zero the transmitter to raise the output up to zero indication (in your imagination).

Now imagine the drum filling. This puts pressure on the H side, driving the transmitter upscale. The transmitter output is now FOLLOWING drum level, and now your operator is happy.

Then someone came along later and tried to complicate things, and invented a reverse acting D/P transmitter, so that the H side of the transmitter could be piped to the High side of the drum, and vice versa.

This is what's called a marketing gimmick, and I'm sure the transmitter manufacturer charged handsomely when this feature was installed.

Reverse meaning that the output follows the LOW side now.

History: I suspect that this happened when an un-trained engineer designed a level system, got snakebit, and came up with a way to implement a dp transmitter in a level system HIS way. My 2 cents.

Thnx mkruger21 .... u really nailed it ... i couldn't have asked for a better explanation .... once again lots of thnx ....

Keep in mind too that if you have a DCS system it's really easy to set up the system to characterize the input from a regular transmitter that is set up with the high side to the low side lead and low side to the high side to read in reverse without doing a think to the transmitter.

DonC, you did make me think of something that will cause me to retract my sarcasm-

If you have performed an FMEA on your system, and have analyzed certain piping/instrument failures, then having the flexibility of a reverse acting transmitter may help you better address your FMEA.

I have never actually seen this implemented, but is certainly possible. I know moving from analog to DCS has certainly improved flexibility in failure action/direction from a casualty standpoint.