PTV/cryo inlet trouble

[cjm]

Hi all,

I've just been given the task of operating a brand new 7890A with 5975C MSD with a Gerstel PTV inlet installed and was hoping someone here is familiar with the proper way to set up the CO2 tank used for cryo cooling the inlet. It's been only a couple months and our cryo valve has failed twice. I have the CO2 tank sitting next to the GC, and plumbed directly into the PTV cryo valve using just 1/8" copper. I didn't like the idea of using just narrow copper due to the nearly 850 psi liquid CO2 running through the lines. I'd like to change this to 1/4" stainless, it seems like it would be a better setup, and perhaps safer. Currently we have no in-line traps installed, but have one ordered and on the way. Our go-to-guy for setting our system up initially has suggested there might need to be some dampener installed to "slow" or regulate the flow as the valve goes through it's normal open/close/open/close routine and that perhaps this valve doesn't handle unregulated CO2 flow. I know that we can't lower the pressure because it would no longer remain a liquid.

Also it was suggested by a coworker's friend that "manually" shutting this valve off (while in the software instrument control) to conserve CO2 will damage the valve. This seems odd to me that this would cause damage. In fact, it seems the simplest and safest way to shut off the CO2 while instrument control is open; could this really damage the valve?

Hopefully someone here has a similar setup, is familiar with the gerstel PTV and could make some suggestions..I would really like to protect this seemingly delicate cryo valve and any suggestions would be greatly appreciated. Thanks.

[Peter Apps]

As a general point; narrower pipes handle pressure better than wider ones, so if you go from 1/8 inch to 1/4 inch you lose some of the benefits of going form copper to stainless. 1/8 stainless would make more sense if you want to change, but make sure that it has a decent wall thickness.

To get efficient cooling you must have the minimum of restriction between the tank and where the cooling is supposed to happen.

Doesn't Gerstel have some guidelines on how to set up the CO2 ?, and what qualltiy CO2 to use ?

I have had two Varian 1079 inlets with CO2 cryo running for nearly three years now, and the cryo valves are still fine (touch wood !!).

Peter

[cjm]

Thanks for the info.

I will leave the tubing the way it is. we just installed a new cryo valve (the third now...) and we now have an inline co2 filter. I hear the CO2 can be extremely dirty, there is no purity rating for it! Hopefully contamination is the issue and we've solved it! That would be great.

[Peter Apps]

Crud in the CO2 could certainly be the problem. We use "food grade". The Varian cryosystem has two particle filters in the CO2 line.

Let's hope that it is third valve lucky.

Peter

[cjm]

More than two years later and our cryo valve problem has not gone away. When the temperature in the lab is a cool 65-70 degrees F everything works perfectly. If it's super hot, more like 78-85, the CO2 valves just don't work. They stick closed for whatever reason. Over time they just give up all together. In total we've now changed this valve 10 times in 2.5 years. I am very disappointed with these valves and wish there were an alternative. Anyone know of a good replacement from a different manufacturer?

[chemviren]

h

[Peter Apps]

Opposite to what chemviren says, when temperature goes up, gas pressure rises (universal gas law). Most high pressure valves are designed to use the upstream pressure to help them seal as a fail-safe, and the solenoids have to pull harder when the pressure is higher, so this might be the source of your problem. Insulating the lines is not going to help much, unless you insulate the cylinder as well.

The high temperatures in your lab are probably causing other problems as well; ovens taking too long to cool, solvents evaporating out of samples for example, you can probably install air conditioning for less than the cost of all the valves that you have burned out.

Don't Agilent or Gerstel have anything to say about this problem ?

Peter

[cjm]

I have spoken with Gerstel a while back and explained it to them. They said the ambient temperature should have nothing to do with it. Sure. I got in to the lab this morning and it was a cool 68 degrees. Everything worked great. Now it's pushing 82 (with air conditioners cranking) and the valve is stuck. (A light tap tap tap with a crescent wrench frees it up, but I have to keep tapping until the inlet ramps from 0 C to 310). Interesting what you both say about the line pressure. I didn't consider that the valve may not operate due to the elevated pressure, but it makes sense. I have installed a pressure gauge, it's reading exactly 1000 psi right now. On normal (72 degree) days it reads just under 900, exactly where I would expect it. I will do a bit more research and get in touch with Gerstel again and see if there are alternatives. Thanks for the feedback, i appreciate it.

Also: We had the valve on one instrument mounted outside the instrument itself. It seemed to work better, not sucking up the heat lost from the GC during cool-down. I'm wondering if there's a better location for it.....

[cleh]

What is the pressure on your CO2 cylinder and what is the pressure rating of your valve?

[cjm]

Well it's liquid CO2, so the tank is 850 psi at normal room temp. Now with it so hot in here it's around 1000 psi. I am trying to determine if there's a rating for the valve.

[cleh]

The pressure rating would be on the side of the valve. I ask because one of my customers had a similar problem with LN2. They had a valve for low pressure cylinders. The valve started failing on a monthly basis and needed replacement. They didn't realize that their purchasing guy decided to go to high pressure cylinders because they cost less.

[chemviren]

h

[cjm]

Yep, sure enough, it's only rated to 1000 psi. I'm surprised they'd install a valve only good to 1000 if the normal operating pressure is 900. Thank you.

[cleh]

Let me know if Agilent or Gerstel can't provide you with a valve that will work for you. I have a good vendor and the price is usually better than the OEM.

[Peter Apps]

Peter Apps, I'm fully aware of the universal gas law. However the opposite happens at our facility for some reason, the hotter it gets the more our CO2 pressure drops.

Since you know the gas law you will recognise that this is an anomaly that should be flagged as such when offering advise to other people who are probably not going to see similar anomalous behaviour.

Peter