Tech Tip: BLow off Fitting

You hear several people talking about blow off fittings, or pop off fitting, or even pipe away fitting. All are the same thing, and very important to a safe nitrous system. It is the small hex fitting opposite to the outlet of your NX nitrous valve. (pn 11707 for standard blow-off, 11709 NRHA blow-off, and 11712 for replacement discs.)
The Blow off fitting is a pressure release device. It is designed to rupture if the bottle is over pressurized. Over pressurization can be cause by several things. Mostly caused by over heating a bottle. This is cause by installation error with a bottle heater, sitting in direct sunlight, in the back on the car on very hot day. OR overfilling a bottle, putting 11lbs in a 10lb bottle.
The reason for the blow off is to protect any persons in the area. Once a bottle reaches 3000psi a disc will rupture, releasing all the nitrous and pressure that was in the bottle to the atmosphere. This will also be a very loud noise and definitely get your attention if you are anywhere in the area. Also freezer burn can happen if nitrous comes into contact with your skin, so do not try to stop the bottle from emptying.
Once the bottle has emptied itself, you must replace either the entire fitting, or the burst disc. (Depending on your application) NX recommends 15ft of torque when tightening the standard blow off or the NHRA Blow off fitting. Over tightening this fitting will damage the disc and may rupture at a lower PSI. Also no thread sealer is needed with this type of fitting.
Ricky

Thats some good info. :D

Do you know what the regulations are for the tube? I need to go with a flexible line since each of my bottles has the blow off port on different sides of the bottle. One is NOS and the other is a NX bottle.. Last I knew, the blow off was required, but they never stated that it had to be a hard line. Is this correct? Currently I've got it plumbed with some 6-AN Parker push lok hose, but could easily convert it over to a 6,000 psi hydraulic hose.

thanks!

-Tom

All companies blow off fittings are -8 an. You can use hardline or steelbraided hose either works fine. I like the steel braided hose give a bit more flexabiltiy.
Ricky

Ok, great... 8-AN is what I meant to say... It's big stuff.. I know that NHRA and IHRA now accept Russell and Aeroquip rubber hoses for use in transmission and fuel lines. Do you think this would apply to N20 also, or does it HAVE to be braided?

Thanks for the info.

-Tom

I would do braided no need in risking a hose blowning problem if that ever happened. Go steel braided.
Ricky

A word of caution to those using braided steel line. You need to use a threaded STEEL bulkhead fitting when you route the line out of the passenger compartment. You don't want a hose whipping around uncontrollably as it trys to discharge 3000 psi. Worst case you kill or injure someone, best case the hose whips back through the hole in your floor and fogs the inside of the car in which case you have'nt gained anything over a regular pop off fitting. The rules are in place to prevent a driver from losing control due to a white discharge cloud obstructing his vision but the reality is that most overpressures occur from heat when the car is just sitting. This increases the likelyhood that yourself or others will be near the bottle so just be safe and spring for a $10 fitting.
End of rant.

James

I already have the bulkhead fitting in place, with 8-AN fittings on each end of the current hose... It sounds like all I need to do it change out the hose. When you think about it though, almost ANY hose should work. The Parker hose I currently have on there is rated to 300psi. There will also be virtually no pressure buildup inside the hose since the other end is open to the astmophere. Also, the hose is about 10-15x larger in dia. than the hole in the blow off disc would be.

Is the parker hose in the trunk of your caprice? If it is, then you don't have to have it at all to pass tech. I've used that same hose before and think that the hose would blow long before the fittings came loose(I think that braided line is actually easier to build). I say leave it, but thats just my opinion.

Yes, it is in the trunk.. Every time I goto the track though, they ALWAYS check to make sure I have a blow down tube. At the first part of the season I'll be dialing the car in with the new 383, so won't be using the N20 till later. I'll have plenty of time to feel out the techs at the local tracks and get their opinions on things.

In the end, maybe I'll have to find someone that will sell me just 2 ft of the braided hose.