dp transmitter

This transmitter used to measure dp between orfice and there is a problem that the reading is always 20.4 ma but when we isolate it by five way manifold it reads zero we clean the manifold and change transmitter with new one and found that the same problem exists,we connect 24v direct with external power supply and gives the same reading ,
URV:1050 inwater,LRV:0 in water and this ranges isn't changed with hart what i can do ?

It sounds like your differential pressure transmitter has a measurement range that is too low for the pressure difference. In other words, when the full pressure difference is applied to the transmitter, it immediately is driven to its full output of 20 miliamperes, because the pressure difference is larger than its range.

Probably you need a different model of transmitter with a higher pressure range.

But the transmitter reading is correct in past but changed to overrange itself

What could have made the pressure change? Is it a vacuum (negative pressure range below atmospehric)? If so, you could have a leak in your supply tubing to the transmitter.

1) What brand and model number DP transmitter?

2) How are you reading the mA values? On an HMI or with a calibrator or milliammeter in the loop circuit?

3) >we connect 24v direct with external power supply and gives the same reading.

I don't know what you mean by the 'same reading'. Please use numbers in your replies.

4) When the equalizing valve is opened on the valve manifold does the transmitter read zero?

5) When you isolate the transmitter from the process and apply a pressure on the vent connections of the 5 valve manifold, how does the DP transmitter's output relate to the applied pressures?

Please provide numbers, like
50"w.c. = 0mA
200" w.c. = 20.4 mA

When you apply a pressure, what is the reading?

Rosemount transmitter 3051CD and i read values at hart communicator and Hmi is the same reading gives 1130 in water (20.4 mA) ,when equalizing open the reading is zero

...I read values at HART Communicator, and HMI is the same reading, gives 1130 in water (20.4 mA), and when equalizing open the reading is zero.

Click to expand...

"1130 inches of water (20.4 mA)" is probably the maximum reading for a transmitter with a "Range 3" (see attached picture from Rosemount manual). If your dP is greater than 1000 inches, you need a different range sensor (Range 4 or Range 5) for your transmitter.

If your maxium pressure difference is a lot less than 1000 inches, then you need to do the Rerange Procedure using your HART communicator.

"when equalizing open, the reading is zero" I think should be normal when no pressure difference is being read.

If you need to reset the range limits, here is one procedure that does not require the HART Field Communicator Device.

2 different transmitters have shown a DP higher than 1130 inches w.c.

Why shouldn't 20.4mA indicate the real process differential Pressure?

You failed to answer the question about what the transmitter reads when you apply a known pressure from a calibrator. Have you done so? What are the results?

Without knowing what the transmitter outputs (mA) at a known, applied pressure (500"w.c.) within its configured range (which proves the transmitter "sees" the DP correctly), it's all speculation.

speculation #1 leaks in impulse lines.

Speculation #2 check all values at the transmitter and the orifice fitting, make sure they are in the proper position.

I agree with danw, do a calibration to see what it looks like.

2 different transmitters have shown a DP higher than 1130 inches w.c.

Click to expand...

Either they are showing the actual pressure DIFFERENCE, or they were maxed out at their URL (Upper Range Limit).

It is not clear to me whether Shkoko2000 thinks the sensed pressure difference of 1050 inches is too low, or too high. Because 1050 is approximately the URL of the Range 3 sensor, I am thinking the Rosemount transmitter is seeing a pressure that is higher than its URL.