Safety rating of transducers needed for non-enclosed natural gas measurements

ryangriggs

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Hi, I have been tasked with adding basic telemetry to an outdoor, non-enclosed natural gas substation. The substation is not enclosed in a building, and is basically just a loop of pipe above ground, having two sets of manual valves and regulators. (AC power is available nearby.)



They want to see pressure and temperature readings as the gas leaves the substation. It's already fitted with 1/4" FNPT ports welded directly into the 6" steel pipe.



Gas pressure is specified as 0-60psig.



Do these need to be intrinsically safe or explosion-proof sensors? I'm not finding specific codes/regulations for this application, but may not be looking in the right place. Any other recommendations for selecting the right sensors? Ceramic vs stainless sensing membrane, etc?


Also, should the temperature be sampled from the middle of the flow (i.e a probe), or pipe wall (clamp-on)?





Thanks!
 
Hi, you may have already looked in the NEC, there are sections for electrical construction, wiring methods and what is considered a classified (explosive materials, vapors, dusts, etc.) location and the "barriers" associated with such a location. It will also reference intrinsically safe voltages, equipment and uses permitted.

Most of the temp transmitters I have had experience with in piping (water utility) were inserted in a well in the pipe. Sounds like you may not have a well, and a direct insertion into the medium (natural gas) may or may not be permitted. The manufacturer of the device would have information about what mediums the device can come in contact with. There could be a corrosive issue, I don't know much about gas. The code book has a great index. Hope this helps.
 
Ryan,
At a minimum the area around the equipment will be considered Class 1 Division 2 (area where explosive gas or vapor can exist in abnormal or upset conditions). If there are any sample ports, Loading/Unloading hoses or any ports that are routinely opened under normal operation then I would consider it a Class 1 Division 1 area (area where explosive gas or vapor exist under normal conditions).

If you go with transmitters made for use in Petroleum and Natural Gas they should be Class 1 Div 1 rated. Compaines like Rosemount, Honeywell, E+H and Autrol all make transmitters that will work for you. Autrol is a brand I have been using lately with good results. They are much cheaper than Rosemount.

For the pressure the 1/4" FNPT will work. For temperature you may need to install at least a 1/2" FNPT port to install a Thermowell. Then the Transmitter with a RTD installs into that Thermowell.

Another thing that might work is to install a Mult-Variable Transmitter (MVT). It will bring in the temp and pressure and make the data available via Modbus.

All depends on what you are trying to do.
Hope this helps,
 
Thanks @bullzi, that's exactly the type of info I was looking for... I will check out those brands. I'm currently perusing the NEC Class I requirements.




Ryan,
At a minimum the area around the equipment will be considered Class 1 Division 2 (area where explosive gas or vapor can exist in abnormal or upset conditions). If there are any sample ports, Loading/Unloading hoses or any ports that are routinely opened under normal operation then I would consider it a Class 1 Division 1 area (area where explosive gas or vapor exist under normal conditions).

If you go with transmitters made for use in Petroleum and Natural Gas they should be Class 1 Div 1 rated. Compaines like Rosemount, Honeywell, E+H and Autrol all make transmitters that will work for you. Autrol is a brand I have been using lately with good results. They are much cheaper than Rosemount.

For the pressure the 1/4" FNPT will work. For temperature you may need to install at least a 1/2" FNPT port to install a Thermowell. Then the Transmitter with a RTD installs into that Thermowell.

Another thing that might work is to install a Mult-Variable Transmitter (MVT). It will bring in the temp and pressure and make the data available via Modbus.

All depends on what you are trying to do.
Hope this helps,
 
If you use 4-20ma or 0-10v - wire them through an Isolation Barrier such as MTL7787+ that is exactly what they are for - to make your circuit a Intrinsically Safe Circuit.
 
@damica1 perfect. That's what I was wondering - how to ensure the 4-20mA circuit was non-incendive and/or intrinsically safe. At that point according to NEC it can be wired as if it was a non-classified location, right?


I assume that means I can use a standard transducer too, instead of a Class I rated device?


Then, placing the control box far enough away would eliminate the need to have it explosion-proof classified also, right? How would I determine the distance required to no longer be class I location, since this is outdoors?



If you use 4-20ma or 0-10v - wire them through an Isolation Barrier such as MTL7787+ that is exactly what they are for - to make your circuit a Intrinsically Safe Circuit.
 
Yes, you can use standard sensor, but you must follow wiring outline to the T. (it does have to be wired correctly) but the isolation barrier will provide that information.

I don't know what you are going to be monitoring, there should be some type of guidelines that will tell you how far away will be considered a safe environment.
 
Thanks, it's an outdoor substation handling 0-60psig natural gas before it goes to distribution.


I'll have to get in touch with the engineers to figure out safe distances.


Thanks again.





Yes, you can use standard sensor, but you must follow wiring outline to the T. (it does have to be wired correctly) but the isolation barrier will provide that information.

I don't know what you are going to be monitoring, there should be some type of guidelines that will tell you how far away will be considered a safe environment.
 
1. Natural gas pipe is not inherently 'hazardous'.

The gas trains installed on gas piping (up to 4" pipe) for furnaces or process heaters do not typically require hazardous area rated instrumentation, unless the process area in which the gas train instrumentation is located mandates hazardous compliance. Heat treat shops run dozens of gas trains on dozens of furnaces that are general purpose rated, not hazardous rated.

An instrumented gas train location in a grain milling area might require Class 2 Div 1 or Class 2, Div 2 classification its location falls within the 'circle' of combustibles area.

Same concept for a process heater in a chemical plant - it's the area in which the gas train is located that determines whether hazardous ratings are needed, not the fact that there's gas fuel in the pipe that the instruments are connected to.

It is not uncommon in the design phase to move a gas train 20 feet in one direction or another in order to avoid the gas train being located in a hazardous rated area and thereby save a couple thousand bucks on the gas train instrumentation.

I'm not sure that a gas pipe line in an open field needs to be hazardous rated.

I'd ask the project specifier whether they consider the area a classified area and to what classification.

2. Intrinsically safe instrumentation is sold with nameplates identifying the instrumentation as tested (by a specific approval agency, like FM) and rated for I/S. A specific model number designates that as well as the verbiage on the nameplate. Intrinsic safe instruments have a specific "control drawing" for their installation. All the components in an I/S loop have to be I/S (unless it is characterized as a "simple device", like a switch).

3. Temperature measurement.

I suspect the desired value is gas temperature, not the exterior pipe temperature, but only the entity paying for the data (the owner/operator presumably) can clarify that.

A thermowell for a temperature sensor should be analyzed for its ability to withstand the flow's "wake frequency", which can cause resonance and cause the thermowell to break. The better thermowell vendors will run a wake frequency calc for you given the flow conditions in the pipe. I'm not aware of thermowells that fit into a 1/4 NPT access port though.
 

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