LSG Dewar Tank Venting

[soundofsparks]

Here's a group "have you seen this before" question:

I have a LeMaitre LSG with a 350 CO2 Dewar Tank, the tanks we've been getting lately have started slowly venting through the emergency release valve at about 250 PSI (the release should kick in at full at 350 PSI). The tanks also build themselves over night to levels near or at 350. We normally run the effect for 6 minutes/performance. There are 8-9 shows/week with one day off.

Does this seem normal?

Am I losing CO2?

If it is constantly venting will the emergency release ever engage?


My distributor seems to suggest that this problem is not normal, but sounds aggravated when I call for a replacement on what I feel is a faulty tank. Am I wrong? Should I just be letting it vent and not be such a whiner? If it's not normal why have three tanks in a row had the same problem?

I'm particularly interested in responses from anyone who has toured or done a long run with an LSG effect.

 

[Les]

Venting is normal, and yes you are losing Co2. However, there is a large amount of Co2 in the Dewar so it will take a while to lose a significant amount. Since Dewars are self-cooling vessels, bleeding off pressure is how they achieve this. I have read that this process can net a loss of around 3lb/day of Co2, but don't quote me on that.

Actual emergency venting is the part where they're so loud you don't want to be near them. We have had dewars in our warehouse and they would usually emergency vent once every few days or so in the heat of summer. A lot of it is based on the ambient temperature, but if they're in emergency multiple times a day, that might be a maintenance issue with the tanks.

 

[firewater88]

The last 2 shows I ran used a LSG. The first one one of the spare tanks we had in the shop vented the whole time and was found to have a faulty relief valve. The tank in use would vent once it gets to pressure. It does use itself to cool and build pressure with the valve on top.
For the second show, we had 2 on stage running and one would vent more than the other, but not a lot. They had a little bit different style tanks. We would mark the level each day and see the change from day to day. Got through the whole run on one tank each unit. It was close, but the effect worked.

Like Les said, a lot depends on ambient temp and how much it is used. The more its used with the pressure builder valve open, the cooler the tank will be getting. Then as it starts to regulate to the room temp, it might pop the relief to stay below limit pressure.

 

[dmx]

As a Liquid Oxygen technician for the military, I can attest that the venting as you described is a normal function. All cryo products are stored in vessels well above their boiling point. The venting mechanisms are usually mechanical devices that use a spring and pressure plate to regulate dewar pressure. If left unchecked, the dewar pressure will build until the vessel ruptures. Most dewars maintain 3 safety devices. 1- the pressure buildup valve (which is venting for you now), 2- emergency relief valve (which Les described above) and 3- a rupture disc that will burst at 2-3x normal system pressure or about 1.5x the pressure the emergency relief valve is set to.

Loss due to venting should not exceed 10% of tank volume in a 24 hour period. (Or at least, that is the standard we use)

Hope this helps

--Matt

 

[soundofsparks]

All of these responses have been fantastically helpful. Thank you all!