Remote steam pressure sensors

[merganzer82]

Hi,

This is my first post here, so bare with me; hopefully you can understand what I'm trying to explain.

I have 2 remote steam pressure sensors from which I need to get the 4-20mA signal back to a PLC in the central steam plant. The measurement is used as feedback for the PLC to adjust the VFDs for the pumps.
One sensor is 500m (1640') away and the other is 800m (2625') away, so its obvious that some sort of long distance data transfer is required.

I have researched several transmitters that have ethernet capability, but cannot seem to be able to find a PLC that you can get an expansion module to simply plug the ethernet cable into and get the required data. And obviously, I need to know what protocol that is I need to use.

For example, for one manufacturer, the transmitter converts the 4-20mA signal to RS-485/Modbus which then connects to an Ethernet/IP module that converts from RS-485 to ethernet. The wiring diagram shows that the Ethernet/IP module can connect directly to a PLC via CAT6. But, will this data over the CAT6 be something that is usable by the PLC? Should there be something at the PLC converting the data back to 4-20mA?
I can't seem to any PLC with modules that would accept this kind of data (plenty of PLCs with ethernet capability, but its for connecting multiple PLCs together or to a LAN).

Also, the end-user wants a direct connection of the sensors to the PLC; does not want to go through a LAN, so that option is out.

Any suggestions would great. I apologize for my lack of knowledge in the instrumentation/process engineering; my expertise is in electrical power/controls.

Thanks.

 

[xnuke]

4-20 mA current loops can run a long distance, much farther than an Ethernet signal can be run without a repeating device. What makes you think you can't run those signals that far over even 16 AWG STP? A quick calculation shows 20 mA only drops 0.42 V on 16 AWG at 2625 ft. A 24 V DC supply should have plenty of capability to drive the sensors, and though you didn't specify a make/model, most pressure sensors for industrial process control are compatible with that supply voltage.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[FIG Journey]

merganzer82,

I agree with xnuke. I have been to many oil field construction jobs installing and commissioning. The distances run from transmitters to DCS/PLC systems are massive and have no ill affect on 4 to 20 mA signal transfer. You should be fine. Just remember not to run your 4 to 20 mA communication cables beside power cables.