[Dsg-rtpc] Backpressure

[Bob Miller]

Hi Sebastian,

First question; yes you can modify the 1" nylon line as you like.

The other requests will be looked at after all the currently scheduled work for the changeover is complete.

Please check your inventor of gas and if you would like some ordered, send me a final number and I will get it ordered.

Regards,
Bob

________________________________
From: Sebastian Kuhn <kuhn at jlab.org>
Sent: Friday, January 17, 2020 3:48 PM
To: Bob Miller <rmiller at jlab.org>
Cc: JIWAN POUDEL <jpoud001 at odu.edu>; Mohammad Hattawy <mohammad.hattawy at gmail.com>; dsg-rtpc at jlab.org <dsg-rtpc at jlab.org>; Nathan Baltzell <nbaltzel at odu.edu>; Narbe Kalantarians <narbe at jlab.org>; Eric Christy <meric.christy at gmail.com>
Subject: Re: Backpressure

Sorry, forgot to attach the photo…



On Jan 17, 2020, at 3:42 PM, Sebastian Kuhn <kuhn at jlab.org<mailto:kuhn at jlab.org>> wrote:

Dear Bob et al.,

thank you and the DSG people for quickly setting up the “chimney” I requested. One question: Would we be allowed to cut that (very long) exhaust line? I understand that right now much of it coils up on the ground, where it doesn’t do us much good but creates a lot of volume (many liters, if my math is correct) that have to be filled with HeCO2 before the “chimney effect” can even start to work. Ideally, we would like to shorten it to not much more than needed to reach the present exhaust height.

Secondly, I’d like to come back to my earlier request to slightly change the interconnections on the gas panel for the differential pressure meter which presently has one side connected to the 4He buffer volume (highlighted in the attached photo). I would like to connect the other side of this meter to the RTPC pressure line (with a T) as indicated in green. That way, this meter will measure the differential pressure between the 4He buffer and the RTPC, which is our most sensitive pressure differential we have to keep an eye on.

Right now, I believe the port on the slow controls readout unit meant for this differential pressure meter is being used alternatively to monitor the ambient pressure in the EEL via a 2000 Torr probe that we happened to have. Ideally, we would like to rather add another dedicated port of that purpose (and another “virtual” meter in the slow control readout panel) so we can monitor both. But in any case the pressure difference between RTPC and 4He buffer is more crucial, and in the Hall we will have an “ambient pressure” readout anyway. So if we are short on readout channels (or programming time), we should just revert that port to its originally intended purpose.

Finally, we briefly discussed the changeover schedule. If I understand correctly, you will want to take the gas panel down into Hall B rather early on in the 1/30-2/12 time period? In that case, we will definitely either need it back as soon as possible or an alternative way to supply drift gas to the RTPC. In ANY case, once the first RTPC is installed in the Hall, we will want to continue testing our backup RTPC(s) in the EEL, so hopefully we can set up a simple gas system for that purpose. At the minimum, I believe we just need a manually controlled flow controller/meter, a couple mechanical pressure meters, and a bubbler, plus whatever HeCO2 gas might still be available. We have something like that at ODU (we used it with the RTPC prototype before moving that to the EEL), and we could easily modify it for our purpose. One question would be what kind of documentation and permits we would need to bring it into the EEL and use it there. Alternatively, how much work would be involved in replicating just those parts of the present system that provide gas to and from the RTPC, and measure the pressure inside the RTPC?

Finally, speaking of HeCO2 gas bottles: I wanted to make sure that we have enough for the actual run - I believe, presently we have 6 in the gas shed but we may potentially need 8, and perhaps even 10 to be extra sure we have enough headroom. So it might be good to order 4 more of those bottles if possible.

Thanks, and have a nice weekend - Sebastian

On Jan 15, 2020, at 9:37 AM, Sebastian Kuhn <kuhn at jlab.org<mailto:kuhn at jlab.org>> wrote:

Hi Bob,

yes, it would be nice to test this out with a 2.5 m higher exhaust in the EEL. Of course, if we need another meter or 2, it should be easy to modify the gas shed exhaust line exit with an “extra chimney”, right?

Thanks - Sebastian

On Jan 15, 2020, at 9:32 AM, Bob Miller <rmiller at jlab.org<mailto:rmiller at jlab.org>> wrote:

Hi Sebastian,

Based on our Hall model, the exhaust line exit on the gas pad is 2.5 m above the beam line.

If you want pressure lower than 0.1 IWC, just remove the oil from the bubbler.

The exhaust line is 1" ID and about 40 m long.  With a flow of 170 ccm (mixed gas and buffer gas flow for 1 volume exchange per hour), the pressure drop due to flow is 0.001 IWC.

I can see if we have some 1" nylon tube to set up in the EEL and have the exhaust 2.5m higher than the detector if you like.

Regards,
Bob




________________________________
From: Sebastian Kuhn <kuhn at jlab.org<mailto:kuhn at jlab.org>>
Sent: Tuesday, January 14, 2020 5:49 PM
To: Bob Miller <rmiller at jlab.org<mailto:rmiller at jlab.org>>
Cc: Jiwan Poudel <jpoudel at jlab.org<mailto:jpoudel at jlab.org>>; Mohammad Hattawy <hattawy at jlab.org<mailto:hattawy at jlab.org>>; Cyril Wiggins <cwiggins at jlab.org<mailto:cwiggins at jlab.org>>; dsg-rtpc at jlab.org<mailto:dsg-rtpc at jlab.org> <dsg-rtpc at jlab.org<mailto:dsg-rtpc at jlab.org>>; Silviu Covrig Dusa <covrig at jlab.org<mailto:covrig at jlab.org>>; Stephen Bueltmann <bueltman at jlab.org<mailto:bueltman at jlab.org>>; Eric Christy <meric.christy at gmail.com<mailto:meric.christy at gmail.com>>
Subject: Backpressure

Dear Bob,

we have been pondering how we can reduce the backpressure from the bubblers for the drift gas exhaust in the BONuS12 RTPC gas panel. We find that the pressure in the gas return line has to reach at least 0.1 IWC (about 25 Pa or 0.00025 atm) before a bubble can form and leave the exhaust pipe. The overpressure inside the RTPC is undoubtedly even larger, taking flow through the chamber and the pipe connecting it to the DMS (and the DMS itself) into account.

On the other hand, we will be operating the RTPC in Hall B, where the exhaust port will be connected to an exhaust gas line that leads out of the Hall and up to ground level, which I believe is several meter higher (does someone have that number?) This actually HELPS, in the sense that the drift chamber gas (80% He / 20% CO2) is less dense than air, so that the hydrostatic pressure differential should be somewhere around 7 Pa per meter, or about 0.1 IWC for a 3.5 m height difference between the RTPC location and the final exhaust exit port. This is akin to a chimney effect which would significantly help with the gas flow as it more or less could mitigate the extra 0.1 IWC backpressure we are battling right now. (It’s right at the limit of what the RTPC can handle, and furthermore increases its leak rate.)

If my reasoning is correct (I am cc’ing Silviu in case he wants to weigh in), we could simulate the final behavior in the Hall by attaching a (at least) 4 m long exhaust tube to the gas panel in the EEL that we somehow rig vertically so that its end would be > 3.5 m above the RTPC. My question is
a) is this reasonable,
b) do we have suitable piping available
c) and if the answer is “yes” to both, could we either ask the DSG for help with this or rig it ourselves?

Thank you, and best greetings - Sebastian



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