Moisture in Impulse Line of DP Transmitter

You don't give sufficient technical details, but I do know that, at low pressures a bead of moisture in a tube which is supposed to be sensing the pressure will stop accurate readings. My experience is in measuring pressure in human respiration.

The tube needs to be a large enough bore such that moisture cannot block it.

Del

Go forth and educate thy self!

Best Practice Guide Impulse Lines for Differential-Pressure ...

Depending on what the liquid is and the temperatures and pressures, liquid accumulation on the sensing diaphragm can flash to gas or condense back to liquid. But maintaining flash temperatures in impulse tubing is not common.

Presumably the DP transmitter is mounted above the elevation of the process line so that liquid drains back down and into the process line ?

And the tubing diameter is not small enough to support capillary action?

Pay attention to the statements from the gents at NEL on recommendations for wet gas on page 13 pdf (best practices link above).

Personally, I'm always suspicious of a faulty seal on the equalizing valve on a 3 or 5 valve manifold, although those are usually continuous leaks from high to side to low.

It depends on the range of the instrument and the size of the slug of liquid, neither of which have been presented to the forum.

If the liquid is water, each foot of differential height in the impulse tubing represents an error of about 0.43 psid in the pressure reading. This may or may not be significant in the specific application.

I smell homework.

Enough condensate will block the line(s), if as a liquid will 'slow' the signal down, or if freezing will 'stop' the signal - in either or both lines.

Draw a picture of each possibility and work out for yourself how it affects the differential pressure - which will probably be too high, too low or too late, and even negative.

Then work out the effect the reading will have on the process - and whether it matters.

May be i am wrong but i am thinking a bit differently.

Unless the tube is completely blocked i don't think the reading of the transmitter will change much if the transmitter is mounted such that the tube is horizontally attached to the transmitter and your liquid is water (because it is incompressible ).It starts to affect the transmitter reading when due to condensation it creates a water head (vertical configuration) in the impulse tube.

Maybe in theory, but in a small tube with a low pressure differential a bead of water acts more like rubber than water, surface tension, capillary action etc.

Del