Location of diverter valves and LN vaporizers

[PEDARRIN2]

I have a client that has an existing liquid nitrogen tank and a single vaporizer.

They are wanting to go to 24/7 so are wanting to put in a second vaporizer and a 3 way diverter valve to alternate the vaporizers.

The question we are considering is whether the diverter valve should be on the liquid side, making it a cryogenic valve or on the gas side, making it a gas valve.

All the set ups i have seen has the valve on the liquid side.

Would there be any problems with putting the valve on the gas side?

A side issue is the client wants the valve to be electropolished. The gas side valves can be electropolished, but I cannot find any cryogenic valves that are electropolished. They can be cleaned to Class 100 cleanliness, but not electropolished.

 

[iainuts]

The only issue with having the valve downstream of the vap's would be sizing. The valve will need to be a bit larger on the warm side due to pressure drop, but that generally isn't an issue. I also think you'd have a more reliable valve at that location since it wouldn't be exposed to the cryogenic temperature and possibly thermal cycling.

 

[PEDARRIN2]

A concern I have (not sure if it is valid) with putting the valve on the downstream side is any residual cryogenic liquid in the off line vaporizer.

We are designing the system to have one vaporizer offline for 6-24 hours so, even though the piping is insulated, the cryogenic liequid will likely vaporize in the piping upstream of the offline vaporizer. I am concerned the expanding gas will force its way into the inlet pipe of the on line vaporizer. I don't know if this is a problem, but I would prefer to keep any vaporized gas in the vaporizer or downstream of it.

Am I being overly cautious?

 

[iainuts]

If there's liquid in the vaporizer that you close off with the diverter on the outlet, the gas would simply go back through the inlet of the closed off vap, go to the inlet of the open vap and through. I'm assuming you're not adding check valves or something funny to the inlet of the vaporizers that might prevent gas from reversing direction inside the vaporizer.

 

[Compositepro]

Why not put the vaporizers in parallel and tee the gas lines to the two users?

 

[PEDARRIN2]

No check valves and the vaporizers are in parallel.

So there is on concern with two phase flow in the upstream piping or is that considered acceptable?

 

[iainuts]

No, there's no concern with warm nitrogen gas going into the liquid feed to your opposite vaporizer. Just adds a little heat is all, which is a good thing since you're trying to warm it up. There's no structural concern regarding the piping or vaporizers.

Regarding putting 2 in parallel, the problem with that is that as frost covers the vaporizer fins, it acts as an insulator and eventually can cover a very large portion of the vaporizer. I've seen vaporizers covered more than half way in a gigantic ice ball. Putting 2 in parallel without a diverter can help but it means the vaporizers are only going to defrost and shed all that insulating ice if the entire system goes down. Hence the need for 2 separate vaporizers with a diverter.

 

[Compositepro]

Now you tell us. It would be better to have enough vaporizer area so you never have to worry about defrosting. This does require that the vaporizer fins be designed to be far enough apart so that ice cannot bridge between tubes and block airflow. Ice will simply build to an equilibrium level. So if you operate both in parallel there will be less build-up and you may not need to defrost at all. I suggest that you do not use 3-way valves but block valves. Of course you are also aware that you need pressure reliefs on any section of piping that may get shut-in.

 

[PEDARRIN2]

The cost of a single vaporizer that would never need defrosting was about 2x the cost of two parallel vaporizers and the diverter valve. Plus it takes up about 1.5x the space.

 

[PEDARRIN2]

The equipment supplier (liquid nitrogen tank) is recommending using a Sharpe D88 diverter valve (

http://www.sharpevalves.com/pdfs/series-d88.pdf)

on the upstream (cryogenic) side. I have concerns about this and think the Sharpe Series C diverter valve (

http://www.sharpevalves.com/pdfs/series-c.pdf)

is more appropriate since it has the extended bonnet.

The system will be under 24/7 operation.

Should I be concerned?

 

[iainuts]

Yes, there's a significant safety issue. Although I've seen lots of valves used without extended stems in cryogenic conditions which function relatively well, they're all globe valves which don't have the possibility of trapping liquid inside the valve body cavity. Ball valves can trap liquid which has been known to expand to the point of the valve exploding. I seem to remember a technician a few years ago, killed by a ball valve in CO2 service that didn't have a hole to relieve pressure drilled into the ball. I think it was in Mexico.

The cryogenic valve by Sharpe specifically says they've provided a relief hole in the ball and stem slot to accommodate the potential of trapped liquid. Get the cryogenic valve and I'd suggest warning your equipment supplier about the hazards of ball valves in cryogenic service.

I could also recommend Worcester valves for this application. We've used their valves extensively.

 

[PEDARRIN2]

Is there a danger of trapped liquid if the three way valve is used?

Looking at the cut sheet, it appears the only time there would be trapped liquid is if we used the valve for shut off. We have cryogenic valves on the tank for this so we would not be using the diverter valve for that purpose. We are also specifying the actuator to use 180 degree turns.

I also have looked at the Worcester/Flowseerve valves. The contractor/supplier submitted Sharpe.

 

[iainuts]

That's a question you should ask your supplier. There's still a pocket outside the ball and inside the valve body that could trap liquid.