Generator Exhaust Stack

[Dumbo2929]

What is the group's experience with the following? A generator (450W) diesel is being installed to back up a building on a college campus. The generator exhaust is being extended about 4 stories up to get above the building roof line. The exhaust pipe manufacturer is recommending a pressure relief valve, just in case of generator backfire. I've done a lot of generator projects, but don't usually see a pressure relief valve in the exhaust stack. This may be a scam to get another $3k out of the client.

 

[cammax]

In most commercial installations of exhaust piping, we normally use sch 80 welded steel pipe sized by the mech engineer, with this no "safety" is needed, However it sounds like your using a commercial product of a light weight material that may not stand up to a back fire or many years of weather for that matter.

 

[willard3]

What material/thickness has been proposed for the stack?

Is sounds as if either the diameter is too small if a backfire builds that kind of pressure or they are making the pipe from gum-wrappers.

 

[Dumbo2929]

The proposed material is single wall, 304 Stainless, 0.035" thick, 8" diameter.

 

[GMcD]

Do a cost comparison- I agree with Cammax, I've designed a lot of emergency gen-sets for buildings and we always use a minimum of schedule 40 steel pipe, or schedule 80 pipe, if it routes through a noise sensitive area. The lightwall pipe will radiate a lot of noise rather than directing it up to the outlet. For long runs, I normally have the gen-set manufacturer also advise on the exhaust pipe size depending if the engine is turbo-charged or not. Long runs generally require a "one pipe size larger" than the factory size, and is especially critical for low back pressure turbo-charged engines.

 

[BronYrAur]

I have also always used sch 80 black pipe, but regardless we are talking about a line that is open to atmosphere, correct? How will a relief valve help protect a line that is already open to atmosphere?

 

[Dumbo2929]

Yes it is open. I'm not sure how the relief valve helps. I'm guessing if the gen backfires, it saves blowing the pipe to bits, since the relief is near the gen exhaust output.

 

[BronYrAur]

Yea, but it seems to me that by the time you would build up enough pressure to pop the relief, you would either burst your pipe or safely vent it out to atmosphere. this is just speculation on my part; I have no experience with reliefs on a generator exhaust.

 

[MikeHalloran]

1. Doesn't a backfire overpressurize the engine _inlet_?

2. It's actually hard to make a tube that thin at that diameter.

3. Four stories of sch80 at 8" is fairly heavy. I'd be inclined more toward 316L stainless pipe, sch5S or sch10S.





Mike Halloran
Pembroke Pines, FL, USA

 

[imok2]

A generator (450W)! are you sure?

 

[berkshire]

Correct me if I am wrong,
I thought a back fire spit into the intake manifold,
And an after fire made a big bang in the muffler or exhaust pipe. I also do not think you would need an 8" pipe for a 450 watt generator.
B.E.

 

[Dumbo2929]

450kW

 

[berkshire]

That's better.

 

[MikeHalloran]

If there's any kind of silencer, 8" seems a little small for 450kW.



Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

I have only ever heard of gasoline engines backfiring when the exhaust system fills with raw gasoline vapor due to interruption of the ignition system.

Diesels cannot really do that. Excess fuel under really heavy load just causes black smoke. The injector pump simply cannot run unless the engine is started. So what could possibly fill the whole exhaust system with flammable fuel and cause a diesel to backfire ??

The thin stainless pipe should be o/k after the muffler, and 304 is the material of choice for exhaust tube. It is used on piston aircraft engines, it has very good hot strength and resists cracking. Eight inches sounds reasonable too, as a compromise between noise and back pressure. But black pipe would be my choice from engine to muffler.

I would contact the manufacturer of the diesel engine, and ask one of their engineers for their recommendations. As you suggest, the pipe vendor may be a bit too keen.

 

[berkshire]

Warpspeed (Automotive)
I have seen the same thing on a hard to start cold diesel.(system filled with raw vapour.)
The engine cranks without firing, filling the stack with white smoke/vapour, then when the engine does fire, the whole lot goes off with a whoomp, blowing a yellow flame a couple of feet out of the stack.
B.E.

 

[Dumbo2929]

The exhaust stack extension is after the muffler, which is black pipe. The proposed stack (4 stories is 8" 304 stainless). I only question the proposed extra pressure relief just before it goes horizontal, after the muffler. I have not seen it before, nor do I want to pay $3k for it.

 

[Warpspeed]

Contact the engine manufacturers and ask their opinion ?

 

[cammax]

I was not concerned with the terminology the poster used, But I can assure you that diesel engines can have very violent back-fire/ after fires of superheated fuel in both sides of the combustion chamber.
It is usually caused by another mechanical failure of some sort.

 

[Dumbo2929]

I did speak to the manufacturer, and they have not seen it; hence my question to the group.