Hey there,

I am having a hard time understanding how this differential pressure transmitter is working for a certain application.

Okay so the Low side of the transmitter has a capillary liquid filled sensing line with a flange connection. It is about 20 feet lower then the actual transmitter unit. This flange connection is just upstream of a heat exchanger.

The High side of the transmitter also has a capillary liquid filled sensing line with a flange connection. This connection is about 2 feet higher than the transmitter unit. The flange is downstream of the heat exchanger. I believe upstream of this flange is a turbo expander which creates a vacuum pressure on this point.

I am mainly confused because when i plug my hart into the transmitter the URV is -197 Kpa and the LRV is 57 Kpa. When I blow down both points and open each side (high and low) to atmosphere I read 60 Kpa for the PV on my Hart. Why is that? Also, in our control room the span for the transmitter is 0-250 Kpa and when I blew it down in the control room they saw about 30 Kpa.

Any help understanding this would be much appreciated.
Thanks

 

I don't know about the range of the transmitter or the control room range because those are specific to the application, but the HART PV of 60kPa is sort of in the ball park for an LRV.

The PV 'zero' value for a DP with dual remote seals depends on the elevation differences between the tap points, the specific gravity (SG) of the capillary fill fluid and the temperature effect on closed filled systems.

To illustrate, I'll use the round value of 1kPa = 4.0"wc, to match your round numbers of guesstimated 20 feet and 2 feet elevations.

The high side has 2 feet of positive head applied by the weight of capillary fill fluid on the transmitter's high side diaphragm.
2 feet = 24 inches (water column) = 6kPa

The low side has 20 feet of negative head applied by the weight of the capillary fill fluid pulling away from the transmitter's low side diaphragm.
-20 feet = -240 inches = -60kPa

A DP measurement is the applied pressure on the high side minus the applied pressure on the low side.

6kPa minus negative 60kPa = positive 66kPa

66kPa is the rough calculated DP measurement with no external pressure applied to either side, ie, open to the atmosphere, give or take for elevation uncertainties, unknown fill fluid SG and the temperature effect on a closed filled system.

The 'calculated' 66kPa value is in the ball park of the 60kPa you read as the PV with HART when the transmitter is open to atmosphere and the original 57kPa LRV, given the uncertainties of elevation, fill fluid SG and temperature.

What the URV value is depends on what the range of pressure the sensors 'see' under operation.

As to what the control room representation of the 4mA 'zero' and the 20mA 'span' is, it's something else again. It's just scaling.