Planning of a GC lab gas supply

[HbJ]

After much (almost unholy) delays and strange issues I can finally start moving into my new lab.

While mounting the first tables I stumbled over a small but crucial issue: How do I bring the gases to my GCs? I have around a dozen of GCs (Varian 3700 and some 3400 as well as DANI boxes I rebuild almost from scratch).

I have a wide area of applications where the focus is mostly on trace analysis and reliability. My employer is not very forgiving.

I'll use Nitrogen, Hydrogen and Helium with the option of Argon/Methane for an ECD in the future.

As I can setup my lab only once (for practical and financial reasons) I have several questions to you:

1.) I'd use chopper gas lines (purified for GC purposes of course) as they are cheaper than stainless steel, easier to plump, immune to hydrogen induced corrosion (HIC) and the material is compatible with the inlets of my GCs (stainless steel is harder than chopper and could damage the seat of the receiving fitting in the GC).
So is there any compelling reason to use stainless steel gas pipes?

2.) Should the gas lines be hard soldered (my dad could do that for me) or are Swageok/Gyrolok/... fittings the better choice? My lab should stay (more or less) untouched for the next 20-30 years

3.) I plan to place the gas bottles about 20 meters away from my GCs so what internal diameter should my gas lines have and is it good practice to install a loop of gas piping to damp pressure pulses?

4.) Is it beneficial to install small loops of gas plumping directly at the GC as another means to damp pressure drops?

5.) Regarding the gas quality I chose 5.0 purity all over. When using Helium that purity gets expensive. According to Supelco one can purify 4.5 helium to around 6.5 with the 3800-U heated gas purifier. The data sheet looks interesting and the saving could be substantial. Do you have any experience feeding an Ion trap mass spectrometer with gas purified that way?

6.) I heard that many GC labs use purified air as make-up gas. The savings may be (somewhat) tempting (less plumping, less cylinders etc.) but I see safety problems here (hydrogen flame could under some circumstances backfire into the air line). So what experiences do you have?

7.) Is there a cheap and reliable way to prevent contamination when changing gas cylinders?

8.) What measures are sensible to protected the lab from havoc caused by the GCs? I already installed fire detectors and a hydrogen detector which sniffes gas on the top of the ceiling. In case hydrogen is detected the hydrogen line is to be switched over to nitrogen, in case of a fire alarm all GCs (except the MS) should just be switched off.
Nonetheless there are massive objections to let the GCs run unattended. Which additional measures are sensible to minimize the risk?

Thanks for reading. I hope you can help me a bit

[Bruce Hamilton]

Sorry, I haven't got much time, but a couple of quick points.

Arrange for the gas supplier company to perform a safety audit or HAZOP assessment of your proposal. Their opinion should help reduce internal strife. Not much point in investing in automated instruments if the company has to also employ babysitters. Also ask the instrument manufacturers to comment.

That number of GCs is going to have considerable heat release, so ensure air conditioning can cope, and the room has suitable air changes / hour rates.

In general brazed/silversoldered lines are best, just ensure all internal greases have been removed from the copper tubing before installation.
GC grade isn't always clean. I prefer to rewash, vacuum flush, and oven dry. Use Swakelok fitting where valves/purifiers will be fitted.

Ensure all lines are leak and pressure tested with a small molecule gas ( helium for most, and hydrogen for its own lines ).

The line diameter and volumes should be sufficient to reduce pressure pulses without loops ( especially if using good quality SS diapharm 2-stage regulators ), and remember that the larger the diameter, the thicker the wall needs to be. My guess would be 6-8 mm OD to manifolds, and 3 mm OD to instruments, but the tubing supplier can advise. Avoid dead legs in the system, they can allow tramp moisture to accumulate, keep systems simple.

Try to ensure that lines always will be at pressure to reduce air ingress - may need to add simple toggle shutoff valves at GC if GC controls also are used for pressure setting.

Put a simple restrictor on the hydrogen lines to each instrument so that no more than 200 mls/min ( or whatever your maximum use is ) can be released if the lines to the instrument rupture and the gas is only pressure controlled, not flow controlled.

Ensure flashback arrestors ( or similar ) are used near the source of hydrogen, and that there are pressure relief valves on the external sections feed lines of all gases into the lab, I set them at about 150% of nominal pressure ( usually about 100 psi ), but get the type that don't weep with age - wastes gas.

I used purified factory air, with bottled air backup. There are also plenty of relatively cheap air compresser sources for filling diving cylinders that could be used with suitable purifiers. It's important to add suitable purifiers, but the advantage is that cylinder swapping is greatly reduced.

I used a twin cylinder system going to a single regulator for each gas. That way the high pressure lines to each cylinder can be bleed to remove air while the orher cylinder is supplying. It was designed by BOC, so talk to your gas supplier about design options and their recommendations.

Please keep having fun,

Bruce Hamilton

[Paulhurleyuk]

One thing I would recomend, is a dual cylinder manifold (similar to what Bruce described). I remeber using one in a job several years back where two cylinders were connected to a clever manifold. The Active cylinder (decided on with a big handle on the front) would be used and the inactive cylinder is shut off. When the active cylinder got down to a certain cut-off the manifold would automatically switch to the inactive cylinder (assuming it was at a higher pressure). #This meant you could change cylinders and purge lines while the system was in use, and it was very unlikely to run out of gas overnight. We used this for Helium, Oxygen, Nitrogen, Zero Air and possibly acetylen (it was a while ago). Can't remember who supplied it....

Paul.

[Schmitty]

One thing I would recomend, is a dual cylinder manifold (similar to what Bruce described). I remeber using one in a job several years back where two cylinders were connected to a clever manifold. The Active cylinder (decided on with a big handle on the front) would be used and the inactive cylinder is shut off. When the active cylinder got down to a certain cut-off the manifold would automatically switch to the inactive cylinder (assuming it was at a higher pressure). #This meant you could change cylinders and purge lines while the system was in use, and it was very unlikely to run out of gas overnight. We used this for Helium, Oxygen, Nitrogen, Zero Air and possibly acetylen (it was a while ago). Can't remember who supplied it....

Paul.

We have multiple set-ups like this for all "in-house" gases from Linde/AGA, in the US.

[MikeD]

You are fortunate indeed if you can specify your installation down to the last Swagelok nut and ferrule. Pity the person who must work through building owners who are not the employer, through sub-contractors of sub-contractors, through consultants who say "just tell me what gas and how much -stay out of the design - you are not the budget holder".

sigh

By veteran of two lab relocations.

[JGK]

Have you considered the option of using copressor/gas generator systems as oppsed to cylinders for your nitrogen/air requirements

[HbJ]

Thanks for all the valuable input! I had a busy week


@Bruce Hamilton: Thanks for your suggestions!
- Regarding the safety audit I'll document regular checks of the gas tightness of the gas supply.
- An air conditioning is to be installed (simple air change).
- I'll use specially purified copper lines. I'm unsure if I can clean them on-site.
- Regarding the pressure regulators I'll install the first stage at the pressure cylinder and the second stage near the GC (down from 10 bar to around 4 bar).
- I'll use 1,5 mm restrictors near the GC.
- Flashback arrestors will be part of my setup (thanks to your suggestion!)
- For air I already use an oil-free compressor with a huge charcoal and molsieve trap.
- A twin cylinder system will be in place (again thanks to your suggestion).

@JGK: - For air I use an on-site generator but nitrogen and hydrogen generator seem far too expensive and unreliable to me.