I have few queries on steam drum level measurement. Basically the boiler utilizes 'wet reference leg' and also utilizing 3 dP transmitters for control.

1) What should be the reading of the transmitter if the drum is empty?

2) Should the low side of the transmitter connected to the reference leg or the high side of the transmitter connected to the high side?

 

1) Transmitter output will be maximum (ie)20 ma when drum is empty and at drum full condition transmitter output will be min (ie)4ma

2) Reference leg to be connected in high pr. side of transmitter

 

Thanks Bala for your input, however I still need some confirmations.

1)What about the reading in mm? The scale of the reading is -450mm to +450mm. Meaning if there is no water, it should go to -450mm which is 20mA?
If the drum is empty, but the reference leg is constant(meaning there is water inside), what should be the reading? Any literature on how the dP measurement works?

2) In my station, the high side is connected to the tank.The low side is connected to the steam. That is where I am confused. We are using Rosemount dP transmitter. Please view this technical note http://www.emersonprocess.com/rosemount/document/notes/00840-0100-4360.pdf
It shows that the low side is connected to the condensing pot.

 

-450mm is No water and +450mm is full water.

+450 mm level tapping will always be in steam side and this end is connected to reference leg with condensing pot. Even in normal operation of boiler, the condensed water will be available and this pressure (due reference leg height) is connected to Dp Transmitter HP side.

When No water inside drum, the differential pressure between LP and HP will be high and TX output will be 20ma

You can see calibration details in Rosemount O&M manual

 

i think if you put low pressure side at steam side, you should set negective pressure. i mean at minimum water level, your reading will be (-) and at maximum water level your reading will be zero.

For example, range is 900.then set at TX from -900 to 0. so i think we can use low side either at stem side or water side. it is just my opinion. If i'm wrong, please correct to me. thanks