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Nitrogen for a pneumatic positioner

Nitrogen for a pneumatic positioner

Nitrogen for a pneumatic positioner

(OP)
Hello,
If possible, can I get some thoughts on the use of Nitrogen as a motive fluid for the pneumatic positioner on the control valve? The valve is located inside a building that has some air movement. It also has a O2 analyzer for oxygen deficient atmospheres.

Is it really better utilizing instrument air?
Thank you,
Sr

RE: Nitrogen for a pneumatic positioner

SrChemE,

What is the reasoning behind using N2 for the motive force for actuation? It's more expensive and creates a hazard for use in enclosed environments (even if O2 sensors are located nearby). Is there a compatibility concern with the process fluid? Is this related to your earlier chlorosilane post?

We had a few of instrument "huts" in the middle of our processing area. The instruments required an N2 purge due to compatibility concerns (handling Cl2, TiCl4). These huts had O2 sensors with warning lights/sirens if the O2 dropped low. Even with the hundreds of tons of Cl2 and other nasties being processed around, nothing scared me more than stepping into one of those huts, even if the light was green and the O2 sensor was showing good through the window. Note - at this site we used instrument air for all valves. N2 was only required for certain instruments.

Maybe it's me, but I'd avoid N2-purged anything indoors if at all possible.

RE: Nitrogen for a pneumatic positioner

Yes it is. Even if you need a small air receiver and compressor.

Nitrogen is just too dangerous in an enclosed space even with "some" air movement.

You can get electric positioners
https://www.pumpeng.co.uk/products/wear-free-no-po...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Nitrogen for a pneumatic positioner

In open areas (plant with high ceilings) we used it all of the time.
Valves and cylinders lasted much longer and required much less service.
We had 'excess flow' valves in the lines so if one failed they would trip and close the line.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Nitrogen for a pneumatic positioner

(OP)
Good morning,
Thanks for all your feedback. This installation is not for the chlorosilane project (it is still in development stages).
This is a control valve located on a nitrogen purge system to a star valve packing - sometimes chlorine entrapped in the system. The building has nitrogen lines for purges for clearing pluggages, therefore the O2 meters inside the building.
Thank you,

RE: Nitrogen for a pneumatic positioner

SrChemE,

Based on your posting history, am I correct in assuming you are working in a TiO2 plant? You reference TiCl4 leaks, working with toluene, N2-purged star valves, and liquid Cl2. All of which are found in the front end of the TiO2 process, specifically the oxidation area. If I had to hazard a guess (but keep the details still general enough for discussion here), you are asking about a control valve that controls an N2 purge on a salt system in the oxidation area. Since N2 is nearby, you want to use that N2 to supply the actuator?

I would not do this. If I am correct in the above assumptions, this purge is located in a rather...sensitive...part of the process. I speak from experience when I say that introducing N2 purge into this area (which is commonly done), ALWAYS has the potential for backflow of the process into the N2 lines. I've seen this happen with a double failure, and with check valves not holding. In our case, the N2 system either was tied directly to process or was used for pump/valve packing purge. The valve packings/pumps started leaking out fumes, but that was limited in quantity due to the tightness of the packing.

If you put this N2 feed on the control valve, you have a direct exhaust path that could evacuate your process directly to atmosphere. Depending on the failure mechanism and the actuator design, you could have as much as an 1/8" exhaust hole for venting, which at process pressure will get the entire area into IDLH VERY quickly for CL2 or TiCl4 due to the valve constantly modulating and releasing material.

If this "control" valve is instead an on/off valve and not a positioning valve, then the risk is lower due to the valve not exhausting except when opening or closing.

As always in this area, even a minor change like this deserves proper scrutiny. Based on the specifics of your valve/actuator/process tie-ins, you may find this is a low risk or high risk. Either way, I think you should gather a PHA team to review this with exact details of the design. My personal feeling is this: If this is a positioning valve, do not do this. If this is an on/off valve that only vents when moving, then this may be okay with proper safety review.

RE: Nitrogen for a pneumatic positioner

(OP)
TiCl4,
Yes, I work in a TiO2 plant.
We had issues with moisture in our air lines before and that triggered the question of using Nitrogen as a motive fluid for the pneumatic positioner on the control valve.
We conducted a “What If”, as was recommended. There is atmospheric monitoring in the room, but the option of porting these exhausts outdoors through tubing would not eliminate the hazard when the valve is changed out, and the tubing for directing the exhaust outside is not re-installed.
We will be using air to prevent displacing the oxygen in air inside the building.
Many thanks for your comments and advice.

RE: Nitrogen for a pneumatic positioner

A better air dryer might be better....

I think the risk of not installing the exhaust to outside is very low myself. This would / should be part of the design or work order / method statement. You can always add a notice to the actual actuators saying vents to be external.

Or put it this way, doing it now reduces the lowish risk. Chances of needing to replace valves would normally be quite low / infrequent and then risk of not re attaching the exhausts very low.

So adding exhausts to outside would be very advisable in my opinion.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Nitrogen for a pneumatic positioner

Moisture in air lines ? You must be using oil lubricated air compressors for generating plant / instrument air, and the PSA driers are bunged up with lube oil / thermally degraded lube oil light vapor carryover.
Switch out the lubricated machines with non lubricated air compressors, and replace the poisoned PSA driers.

RE: Nitrogen for a pneumatic positioner

Hi,
If you want to stick to N2 , you can use a PSA system with proper filter for oil and performing dryer.
This worked for me in an API production .
Good luck
Pierre

RE: Nitrogen for a pneumatic positioner

I don't see a problem with using N2, even in a closed area. The consumption volume is typically very low. You should be able to determine whether that is enough to effect the environmental conditions enough to cause a hazard. As others stated, motive fluid problems are usually the result of liquid in the gas system. N2 should eliminate that possibility.

Brad Waybright

The more you know, the more you know you don't know.

RE: Nitrogen for a pneumatic positioner

I live in a cold region and here it is a widespread practice to use N2 or CH4 instead of instrument air. Mostly because of dew point of those. This is unsafe and is prohibited by many codes and practices and anyway instrument air is being replaced with readily accecible gases like CH4 in remote locations where control is weakened.

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