NFPA 85 and low BTU fuel gas

[fvincent]

Dear colleagues

One of my clients intends to have their existing boilers complying with NFPA 85.

The boilers operate firing blast furnace gas, coke oven gas, steel furnace gas and some sort of tar occasionally.

I suppose NFPA does not apply to such fuels. Am I correct?

Anyway, as a safety criterion, NFPA 85 would help the client to achieve a high safety level.

However, some requirements seem infeasible, as the installation of a double shut-down valve plus bleed arrangement for each of the BFG burner (there are 8 BFG burners per boiler, three boilers)

Does it sound acceptable for you to using a U-water seal for each burner instead of the double valve plus bleed?
How do you think such solution compare to the NFPA requirements?

Thanks a lot


fvincent

 

[pmover]

why not nfpa 85?

"This code shall apply to single burner boilers, multiple burner boilers, stokers, and atmospheric fluidized bed boilers with a fuel input rating of 12.500 MBtu/hr (3.663 MW) or greater, to pulverized fuel systems, and to fired or unfired steam generators used to recover heat from combustion turbines [heat recovery steam generators (HRSGs)]."

the code is not fuel selective.

Chapter 3 pertains to multiple burner boilers.

not quite clear what is meant by BFG burner and the U-water seal. pls elaborate . . .

hope this helps some.
-pmover

 

[fvincent]

Well, as I mentioned the boilers fire blast furnace gas (so, the acronym BFG). It is a very low BTU gas (its LHV is ~ 1/10 of Natural Gas LHV).

NFPA 85 paragraph 3.3.71 lists the kind of fuels which are referenced in the other parts of the text. For gaseous fuels there are only LPG (and similar)and NG (and similar). Both are high LHV fuel gases.

A very poor fuel gas as BFG which is fed at low pressures (8000 Pa above local pressure) require a very very large ducting (instead of piping).

It seems very unfeasible to have double valve + bleed valve arrangement for each burner.

That is the point.

Thanks

fvincent

 

[fvincent]

As to the U-water seal, I meant a U shaped piping (or duct) which is fulfilled with water and operates as a seal, preventing gas from flowing from the main gas system to the burners. Water is drained when flow is authorized, by opening a drain valve. A water inlet valve is opened and the drain valve is closed when sealing is to be established again.
regards

fvincent

 

[Combustor1]

fvincent,

Who designed the water U tube arrangement - sounds ancient and in sorry need of replacement.

Without knowing alot about the installation and the details, I need more info - do you have a combustion fan or 8 combustion fans and what about an exhaust fan? Also how many Btu/hr are we talking about?

If they are using a water seal for gas flow then what kind of safety switches do they have, Low presure air or flow or draft, low or high fuel pressure, what happens if a burner or several burners go out?

On the cheap maybe you can get away with one main gas rack with double block & bleed valves and a single shutoff valve for each burner (including a low and high pressure switch).

Therory - once system purge is complete and all fans and switches (proper interlocks) are operating then one or all burners should be able to come on (I would need to see a detailed schematic/drawings to be sure) burners could go on and off. If a shut off valve were to leak then hopefully there would not be a buildup of gases in the furnace. Again drawings would be helpful.

It sounds like this system is not been handled by a combustion specialist and really needs to be! If you are a vendor with little experience in this area I suggest you contract out or leave this one alone for liability reasons.

I handle combustion systems for a varity of applications in many various plants in our corrporation and have been in the biz for long enough.

Let me know if you are in need of more assistance.

 

[fvincent]

Thanks for your answer, combuster.

The U-tube arrangement was installed at the time of the boilers erection (1977).

The interlock philosophy is going to be adapted according to NFPA 85, but as to the pressure switches they are already available.

There is only one FDF and one IDF per boiler.

Boiler capacity is 150 t/h (330,000 pph) at 65 bar (925 psig) and at 485oC (905oF), corner fired furnace with four levels.

The risks of not using a double shutoff valve plus bleeding will have to be addressed by a specific study, probably. As to the NFPA 85, it seems mandatory to have such double valve arrangement per burner if any burner can be off while the remaining are on.

My question still remains: is there other accepted arrangement to provide proven leakfree closing of valve other than double valve arrangement?

I must confess that I am not such an enthusiastic of U-tube arrangement, either..

Anyway it seems to be reasonable to consider that solution specially for a burner which is fed with up to 19,000 Nm3/h of BFG at 8000 Pa (or 11,800 Scfm at 32 inches WG)through a duct with diameter 50 inches.

Thanks again

Regards

fvincent