Boiler Drum - Level Measurement by Differential Pressure Transmitter

Hi Andy,

It's OK, but not too much more. Too much more will decrease your accuracy/sensitivity. It's unacceptable to have a span less than the reference column.

Mike

I am afraid I would have to disagree. If you have the DP transmitter range greater than the column height (assuming constant Temp & press) the instrument will never read empty 0% or full 100%, a very scary situation with a boiler drum. It is quite acceptable to have a DP range less than the column height, which allows 0% and 100% to be possible and a more magnified resolution for control purposes. I have witnessed a plant using the before mentioned situation of DP range>column and all they ever achieved was consistently overflowing the tank or running it dry and damaging the pumps on the extraction lines.

ROK

If you are mounting the dp between two columns (boiler drum and reference column) at the bottom, then you want a dp transmitter with a full differential pressure range no greater than the column height above where the dp transmitter is mounted. Assuming the vapour/gas pressure at the top of each tank is equal the differential pressure will never exceed the column height.

If you use a DP transmitter with a higher range you can normally decrease the span of the output (ratio turn-down) so it matches the full scale height difference you are looking for, thus eliminating the overflow risk. If you go too high with the base differential range you will degrade the accuracy of the DP being measured when you turn-down the output span. e.g. a 2:1 turndown is going to be more accurate than a 10:1 turn-down for the same liquid level span.

If there is no span adjustment in the DP transmitter then you have to make sure the instrumentation measuring the DP is scaled to reflect the correct span.

The best way to test whether the DP transmitter works correctly is to calibrate it prior to use by simulating the pressure with a calibration hand pump and test gauge and comparing the reading to what the instrumentation is measuring. If the DP transmitter is rangeable it could be set to any span and not necessarily what is written on the label, which might only be the base pressure range, so always best to test the setup and check the DP transmitter is calibrated correctly with the instrumentation. This should also be done it regular intervals because sensors drift over time.

Pressure calibration equipment