Negative and Above Range Readings on Flow Transmitter

Have it calibrated, from 0-100"H₂. Might be just offset your zero at 100" H2O and reversed the port at the manifold.

When the technicians came in they did calibrate it. I will double check the connections though and make sure they are not reversed. The transmitter started giving us problems a year ago when we got it checked out by a different group of technicians. Lets call the technicians that came in last week new and the ones that came in a year ago old.

So if the old changed the offset and when the new came in and did not realize. Would the new have caught that during their calibration? If not do you have any other suggestions?

Thanks

Could also be defective diaphragm as commonly experienced on this type of equipment. But have your preliminary check on the LP and HP side ports if not reversed. Have it calibrated on the lab.

If problem persist after calibration, you may replace the transmitter or have the tech's repair the diaphragm if its doable.

Sounds like an obstruction....take it apart and clean it....Is your velocity in spec?

SolarEagle. Take apart what, the manifold and transmitter lines or the orifice plate or both?

I would think the obstruction, if there is one, would be effecting the entrance or flow to the pressure measurement device....I can't see the mechanism, if it's serviceable, or must be replaced....The fact that it works properly while being tested and not in regular use should provide a clue...Possible the flow characteristics of the steam have somehow changed?

Do you have a straight, uniform pipe run upstream and downstream of the orifice plate?

JohnDG

Yes I have a straight uniform pipe and it used to work fine.

Thanks

Have you checked the wiring between the x-mitter and gauge for chaffed wires, loose or corroded terminals? Being a milliamp system it doesn't take much to effect the output readings.

DP is high side minus low side, so a high DP reading can be caused by a low reading on the low side. A lower than expected low side reading makes the DP a higher than expected reading.

Some reasons for lower-than-expected low side readings:

- low side impulse tubing or tapping port in the pipe/orifice flange union is plugged with scale/rust

- low side leak to atmosphere

- low side vent on the back side of the transmitter's process head leaks

- low side impulse tube is not filled to where it should be with condensate. There should be a condensate pot or a tee at an elevation high point on the low side line to provide a fill point where condensate can be added to the low side.

- someone turned on the square root function in the transmitter so the transmitter is now square rooted flow rate values not pressure values. (the square root of a value less than 1.0 is bigger than the value being square rooted. Try it on your calculator.)

- Someone turned on square root extraction in the receiver and in the receiver to produce double square root extraction where the reported values are erroneously higher than expected.

- low side manifold valve is closed

- low side manifold valve is cracked open but the opening is clogged with rust/scale

I presume that to "calibrate" or check the instrument that it was isolated from the main line (by valves or similar) and then re-connected. (Not necessarily removed)

If the upstream port/valve was still closed/isolated, then the pressure from downstream would be relative to the small (fixed) amount in that end of the pipe.

Apparent flow would then really be related to absolute pressure in that downstream port. Thus you could then get negative or positive values and values beyond FSD as nominated.

I will presume that you have a 3 valve or 5 valve manifold attached to your D/P transmitter.

If the transmitter is located a considerable distance from the steam line, feel of the

input tubing.

If both of them are hot to the touch you have a leak in the equalizing(center)

valve,allowing condensate and/or steam to circulate,giving erroneous readings

Be sure to follow the proper sequence of opening and closing the valves or you could

blow all of the condensate out of the sense lines,allowing live steam to contact the

transmitter diaphragms and damage the transmitter.

I have included a link with instructions for manifold valves sequence below.

Good Luck.

https://lh6.googleusercontent.com/-qpdUJ1HWqX8/TXUhs8IJ_wI/AAAAAAAAAIQ/2OFLkMPKPZQ/s1600/3-Valve+manifold+of+transmitter.jpg

I bet you will have to analyze or troubleshoot using this.

Here is a better link for the manifold operation sequence.

http://www.instrumentationtoolbox.com/2011/03/dp-transmitter-valve-manifolds.html#axzz3MBb4NOiN

The previous link was incomplete.

Thanks everyone for the help. I found out that one of our lines were completely blocked and the other partially blocked.

Thank you for the update.