DP flow transmitter problems

Field Instrumentation
1

I have 3051 Rosemount DP type flow transmitter installed in main steam line to calculate flow. But, when steam valve is closed from boiler side the flow meter shows half tonn per hour flow rate. Full scale flow is 17500 kg/ hr and Max Diff Pressure is 2500 mm h20. How can I fix it?

2

It could be an old installation and now giving problem ? or if it is a new installation, check your DP reading it should be zero, if it’s not zero check DP calibration, your valve may be leaking, also check for line leakage after DPT. if any.

3

  1. Why do you think that the valve is not leaking and the DP flow transmitter is actually telling you what is going on? Are you the king, killing the messenger with the bad news?

  2. If the installation is typical, the transmitter is mounted below the steam line and the high and low impulse lines are filled with condensate to isolate the transmitter from the steam heat. There should be condensate pots at the top of the vertical runs of impulse tubing, but many times there aren’t and the head of condensate is not ‘full’ in each impulse line. If the high side impulse line has more liquid condensate in it than the low side impulse line, then there will be a constant, positive offset error, most noticeable when the steam is turned off, because the error appears as a positive flow when there is not flow. The solution is to fix the impulse lines with correctly installed condensate pots (impulse line from steam pipe slopes DOWN to the steam pipe).

  3. When a steam line is shut down, the remaining steam condenses and creates a vacuum that can suck condensate from the condensate pots or the impulse tubing. Another cause of unbalanced impulse wet legs.

  4. A true experience: an accumulation of pipe scale (rust and dirt) in the space between the sensing diaphragm and the process head on the high side was loading (putting pressure) on the diaphragm so that the diaphragm could not return to its zero, unloaded, zero position. The constant offset appeared as a pressure/flow when there was no flow. Cleaning out the scale and re-zeroing the transmitter fixed it.

  5. Blockage of the low side impulse line that creates a constant low side ‘low’ pressure lower than what the process is applying, resulting in a higher than expected reported DP because ‘high side minus low’ side when the low side is lower than it should results in a subtraction that is higher than it should be.