For measuring Boiler Steam drum water level by DP transmitter, I noticed always using Low side of DP transmitter is connected to the filled wet leg. while the high side of the DP transmitter connected to the bottom side of the drum so the reading is always negative. Is there any reason for this installation way? why not reversing it so that reading be in positive?

 

Actually, the reading should not always be negative. The high pressure side of the transmitter should be connected to the low side of the drum - this pressure will be higher than the upper side of the drum. What you are doing with this type of level measurement is measuring the weight of the water and steam above the lower tap on the drum. Then, by adjusting for the density of the water and steam (it is at saturation conditions so all you need to know to get the density is the drum pressure) you can calculate the actual level. The transmitters are usually calibrated to read zero level at normal water level.

Note that this arrangement will give an off-scale high level indication in the event of a broken connection to the transmitter which is a trip condition. Note also, high level trip condition is somewhat more hazardous to equipment (the steam turbine) than extremely low level, so this arrangement is the default "fail safe" method.

In the cold, pre-start condition, the level usually will be below normal water level and thus will be negative. As the unit warms up and the water expands, the level will rise and it should eventually reach the normal level, and will likely approach the first high alarm level during the startup.

 

After checking with your system engineer, you could switch the hi and low ports to get a positive value. But accepting the negative value and just reversing in the controller or hmi/scada is easier.

You may know this, but others may not. A negative signal is generated when the hi port has a lower pressure than the low port, or vise versa.

Similar to what happens to when you reverse the hi and low for a LIM instruction in a PLC. (Exactly like what happens when you reverse an opamp circuit.)

The pressure differential and electrical differential are outputing differences not + or - values. I say check with your systems engineer first, because the rest of the system may be (and most likely is) dependent on the original set up. Which in your case appears to be a negative value. So it is not just about swapping ports to get positive value as a differential measurement, but insuring all aspects of the system can handle the change you made.

 

The reading is positive, as the level goes up the output goes higher.

Zero Level = 4 mA, 100% level = 20 mA

It's just offset by the wet leg height that's all.