Instrument Tech.


Thread Starter

Mike Rofkahr

We are updating instrumentation on a steam turbine condenser which originally used a displacement level transmitter. We have purchased an electronic DP transmitter to replace the displacement type. At first glance the changeover looks easy, simply by connecting the'H' input to the measured variable and the 'L' input to the condenser shell, thereby providing a diff. measurment. But, I am afraid the 'L' input will eventually fill with condensate because of the tremendously humid environment in the condenser.
Second thought is to fill one leg with condensate on purpose as a reference and setting the TX up as reverse acting, but the reference leg would evaporate out during down times....

Any suggestions would be appreciated.

Mike Rofkahr

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Brian T. Smith


Just to back up a step: why was the displacer replaced? Was it because of changing liquid specific gravity as well as to update the
Instrument? A dP transmitter will produce a measurement error if the fluid SG varies.
The error could be worse than a Level displacer.

You could seal the low pressure leg line with a fluid that does not evaporate at your operating
temperature and does not mix with Water. If freezing is an issue then the fluid must not freeze or it needs to be heat traced. The Specific Gravity of the seal liquid must be higher than the vessel fluid.

A remote diaphragm sealed capillary would be a better choice than the liquid sealed leg line.
Usually both sides of the transmitter use identical seals and capillaries.
For best accuracy use a large diaphragm, short capillary.

You may want to consider just replacing the Displacer head with an electronic Transmitter head.

Brian Smith

Sastry P.S."

The DP type may not work for this application because H will eventually be vacuum which will be varying and hence the DP measurement varies
which may not give correct level measurement.

Stockler Paulo MSM AD0432 BR

The solution is classic. Use horizontal condensing chamber in the L tap slightly upper of the center line of the tap.(may be 12" of 2" pipe with caps). Don't invert the taps, it's preferable to use elevation zero adjustment at the instrument.
Good luck!

Johan Bengtsson

Well, the simple answer is of course:
Either you have to fill it, and continue to fill it, or empty it and continue to empty it...

Of course this is easier said than done....

Case 1: (keep it filled)
Is it possible by some way to put a small continous flow that overflows at the top? Small and steady enoug to not affect the measurement?

Case 2: (keep it empty)
Let the pipe continues down (below the DP cell) where the condensate is collected. A simple level switch could tell when it is about to become full and needs to be emptied, this could perhaps even be done automatically by a small valve or pump small enough to not affect measurement and not open/running when there isn't any condensate.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
You have received adequate responses. Now, if the change in specific gravity is of concern, e-mail me that request. I will give you the correcting function for SG as function of temperature up to the critical point. If you e-mail for that, I will also put it on this forum so that others may use. Personally I would use the traditional wet leg.