Flame Arrestors

[StoneCold]

I have an interesting application where flame arrestors were reccomended by a third party. I have looked at the possiblility of installing the flame arrestors, but I can not come up with a good reason to do it.

The situation is batch reactors venting direclty to atmosphere. The batch reactors are always nitrogen blanketed so when you vent the reactor down it releases solvent vapors and N2. The second venting scenario with these reactors is a reaction releases butane or hydrogen and it is directly vented to the air.

So at all times the vapor in the vent line should be above the UEL and the flame front should not ever be able to travel into the piping and back to the reactor.

The one exception to that, would be the first few seconds that you begin venting, the vent pipe would have air in it that had to be displaced.

So I don't see much benefit in adding the flame arrestors to the ends of the vents.
Maybe chokes would be more important to help disperse the vented material and prevent a vapor cloud explosion.


Am I wrong?
If you remove hydrogen from the scenario does that affect your decision?

Thank you for your input.

Regards
StoneCold

 

[tickle]

In the second scenario, the vapour space is filled with butane and hydrogen. Is the vessel drained drawing in air. I am guessing that the N2 blanketing is applied. But what if the N2 fails?

It sounds as though a batch HAZOP may be required.

In determining the hazardous area, the insides of the tank would be considered a zone 0 - flammable material is "always" present, regardless of whether is is above its UEL.

I am not sure what you mean by chokes. I prefer the in line flame arrestors that can be serviced at the tank roof level, they also tend to direct the vapour directly upwards.

 

[JimCasey]

If you just have a conservation vent, the vessel will vent saturated (conc>UFL) vapors when it is being filled. If that vapor column gets ignited, the conservation vent does a pretty good job of slamming shut and stopping propogation of the flame front, which can't go any farther than the conservation vent anyway. When the vessel's contents are being consumed, the blanketing system should respond and replace any lost volume with nitrogen.
Potential danger occurs if (1) N2 supply fails and air is drawn into the tank(2) while the tank is draining (3)While there is an ignition source inside the tank. This would indicate he need for a small flame arrestor on the VENT line, to keep external flame from being drawn in, if there is a fire and the drain lines are left open /Fail.

Other note: Butane and H2 go up, so they're not going to hang around and become concentrated. Solvent vapors should go to a flare stack. There should be a Flame Arrestor in the vent line near the flare stack.

 

[786392]

Or Further to what has been rightly identified by 'Casey'&'tickle'.

A pressure/vacuum type conservation vent of appropriately calculated size for the system conditions along with

a 'Flame Arrestor each loop on top(i.e.downstream side) of the PV vent

1)towards atmosphere venting pipeline end and

2)towards the Flare end

Must be employed;just to safeguard/ arrest the Flame(s) sucking-in possibilities

for this butane& Hydrogen rich environment's Fire and/or Explosion


Hope you realize!

Best Regards
Qalander(Chem)

 

[StoneCold]

Ok
A couple of mis conceptions about the facility in question.
We don't have a flare. If we had a vent header that fed the flare all of this would be mute. I could install one detonation arrestor by the flare, and feed nitrogen into the vent header and I would be covered. Not that expensive for one, abiet large detonation arrestor.

The real issue is so many small vents.
I really do not see how you are going to get burn-back into a narrow pipe with solvents only in the piping.

The Choke comment would be like a venturi to speed up the exit velocity of the vapor coming out and cause it to disperse more. I.E. tring to reduce the possibility of a vapor cloud explosion. Which I still don't think would burn back into the vent piping. It might burn continuously near the exit of the piping but not burn back, probably not even when the vent was closed off.

Regards

StoneCold

 

[786392]

Stone I don't know how small are the vents in quetion.

However if this is some fool proof NRV(Non Return Valve)or Choke system

I can not comment since none recalled to be witneesed by me in past 30 plus years.

Other Knowledgeable peers may come up with better/useful thoughts.

Best Regards
Qalander(Chem)

 

[StoneCold]

Qalander
Thanks for you response.
The vents are 2" and 1-1/2" sch 40 pipe.
Maybe Choke is not the best term. The item I am thinking of is more like a nozzle at the inlet of a venturi system. Possibly so crude as to just be a butt weld 2"x 1" concentric reducer welded to the end of the pipe to help increase the exit velocity.
Not an NRV or anything that fancy.

Regard
StoneCold

 

[786392]

I feel still you should find very small threaded flame arrestors as the system contains Butane and Hydrogen.

Since the concept you make me envisage;is a greatly reduced safety system.

Best Regards
Qalander(Chem)

 

[StoneCold]

Qalander
I think we all feel better with some layers of protection in place. I just wonder what the real risk is. Not that I want to find out on a large scale. If I remain real curious I might set up my own little experiment. But even then I would put a flame arrestor before my vapor generator. Maybe I don't believe that the flame would propagate back but I am not willing to take an exploding metal tank in the face to convince me I was wrong. Dead wrong!

Thanks

StoneCold

 

[786392]

Thanks,
Sorry for myself being overcautious in process safety approach.

Best Regards
Qalander(Chem)

 

[StoneCold]

Qalander
I don't disagree with you. I just wonder if I am pouring money into 30+ flame arrestors when I should be building a vent header and a flare. You know there is always only about half the money you need to go around.

Regards
StoneCold

 

[786392]

If this is the scenario;You are most probably

better off having a larger dia header(Lower pressure rating)

connected to some flare system via knock out(small)drum.

Floating With slight purge gas +ve pressure(might be Nitrogen) or

even a combustible/flammable gas mixture burning in the flare tip

Thus avoiding air ingress from atmosphere.

Best Regards
Qalander(Chem)

 

[djack77494]

Stone,
Based on your descriptions, it appears unlikely an explosive atmosphere could ever develop within your tank. If an adequate hazard analysis considers your situation, it should be possible to identify whether or not this is true. If the atmosphere inside the tank can never burn, then the flame arrestor would serve no purpose.

 

[rmw]

Blanketing systems fail, tank and vessels breath in and out with ambient temperature changes as well as pump in/out rates, etc, so air could get back into the tank and dilute the mixture conceivably.

Things catch fire occasionally, ranging from nearby processes to grass on the ground near by.

So conceivably, some day some time a flame could be present and drawn into the process.

Flame arrestors are about preventing flames present outside the tank/process vessel from entering the vessel and not about preventing what goes on in the atmosphere once whatever is in the process is vented to its surroundings.

If it would make everyone feel warm and fuzzy, put the frazzling flame arrestors on. It doesn't sound like they would be very large. The value of the time spent discussing this in this thread is probably more than the cost of the vents. I think you could buy a lot of 1-1/2" and 2" flame arrestors for what it would cost you to engineer and build a vent header system.

rmw

 

[786392]

Dear rmw Thanks;
You have summarised the ingress and cost issues in a focused way.

Best Regards
Qalander(Chem)