hazardous area classification for diesel oil storage tankfarm 2

[giraffetall]

As the flash point of diesel oil (or even fuel oil) is above 65 deg.C most of the people consider the diesel oil storage tank farm as non-hazardous area when preparing the Hazardous Area Classification drawing. Another argument also is: as the auto ignition temperature is very high, 257 deg.C, most of the time the storage tank farms are classified as non-hazardous area. Recently there are some arguments on this subject.
Any one who could contribute on this subject?
Would appreciate very much for your valuable comments urgently.

 

[dpc]

It has been some time since I dealt with this, but AFAIK, NFPA does not required explosion-proof treatment of #2 diesel storage. Diesel generators are routinely supplied with day tank level switches that are not explosion-proof.

This may have changed in recent years, since there have been cases where aviation jet fuel vapors in wing tanks is believed to have ignited in the presence of a spark source.

 

[itsmoked]

Yeah but jet fuel is kerosene...

I cannot speak directly to diesel storage.

Also the level sensors in a billion automobiles are not explosion proof and they are in gasoline.

 

[dpc]

OSHA considers kerosene and #1 and #2 oil to be Class II combustible/flammable materials. Kerosene is essentially the same as #1 fuel oil AFAIK.

Gasoline vapor is definitely much more explosive than either and is well covered in the NEC and NFPA.

Don't know much about automobile gasoline level sensors, but I doubt that there are arcing contacts inside the gas tank.

Our standard spec used to require explosion-proof treatment of diesel generator fuel storage tanks, but the suppliers routinely took exception, based on the fact that the NFPA did not require this.

That's basically all I know about it.

 

[itsmoked]

dpc; interesting, thanks.

Car sensors are generally wiper rheostats. Sounds like arcing to me.

The rheostat then runs a 12V heating element in the dash gauge. This scheme was to thermally average the reading of the sloshing fuel via a bi-metal pointer.

I guess the attitude was that gasoline while highly flammable is flammable only right around 13.7% air/fuel which should (never) occur in a fuel tank.[bomb

 

[zeusfaber]

I think it's possible to read too much into the jet fuel comparison. In the one case that's really been given a lot of attention (TWA 800), there were, at least as far as I can remember at several years' remove, two extenuating circumstances:

1. The Aircraft had been running its APU on the ground for an extended period, dumping heat into the fuel in the tank. This increased the vapour pressure of the fuel.

2. The aircraft flew, reducing the ambient pressure in the vapour space above the tank. This increased the Vapour Pressure to Ambient Pressure ratio (= %age vapour concentration at equilibrium).

The two effects together were just enough to get the vapour concentration above the lower explosive limit.

Nobody knows what the source of ignition was.

Kerosene jet fuels have a specified minimum flash point of 38C, and even the special stuff (JP-5/AVCAT) used for naval aviation is more volatile (minimum flash 61C) than your diesel - though 65C for diesel sounds a little high to me: I would have expected something more in the range 56 to 61C.

A.

 

[zeitghost]

Have you ever seen a gas tank sensor?

It's a naked wirewound pot...

Presumably the fuel/air ratio in the tank makes explosion unlikely?

 

[itsmoked]

Triple cross post yack!

 

[dpc]

No, I haven't seen a gas tank sensor - it may be power limited to such an extent that the vapor cannot be ignited, I dunno. Gasoline vapors are heavier than air, as well.

But gasoline vapor can certainly explode quite easily, I think the concentration range is like 1.5 to 8% as I recall.

zeusfaber,

I understand the extreme conditions related to TWA 800. I thought investigation centered on a solenoid valve inside the tank but maybe I dreamed that. Also understand that Boeing has never accepted the findings.

But knowing NFPA, I thought they might have revisited requirements for #2 based on their normal abundance of caution.

 

[zeitghost]

<Triple cross post yack!>

Yay! Great minds think alike or something... or what was that saying about fools again? )

 

[zeusfaber]

Interestingly, given the way some of the rest of this thread is going, in August 2000 the NTSB posted this opinion on the most likely source of ignition:

[ul]
[li]Short circuit outside center wing tank[/li]
[li]Excess voltage transferred to fuel quantity indication system wires[/li]
[/ul]

- they thought this more likely than (among many other things) a meteorite strike.

I'd heard gasoline vapour in an enclosed tank was actually relatively safe for the opposite reason - it's so volatile that, unless it's a cold day, the vapour concentration in the headspace is comfortably above the upper explosive limit (if you can ever use a word like "comfortably" in this context). Having said that, I've never done the sums to see if it's true.

A.

 

[giraffetall]

Hi dpc, you mentioned that "NFPA does not required explosion-proof treatment of #2 diesel storage".
But I was told by someone that there is also one sentence in NEC chapter 5 that makes heating oil (about the same as #2 diesel) a Class D fluid that results in the area classification problem for tank farms. I do not have a copy here at my house. Have you heard of that?

The applicable Codes in this case are "NFPA 70 (i.e NEC Chapter 5)" and "IP model Code Of Safety Part 15 (latest edition)". It is an "Oil Storage Terminal Project".
The Owner demanded to be classified the TankFarm as hazardous area whereby Contractor put up the area classification as "Safe" area. Thus there is a dispute on the subject.
Once the "NFPA 70" is specificed, it also refers back to "NFPA 30". Even API RP 500 is also more or less the same as "NFPA 70".
The argument is that under the normal atm. pressure and temperature, Diesel Oil does not produce flammable gas (NOTE: Diesel Storage is under atm. Pressure). Remember in Diesel Engine, in order to achieve combustion you need a compression stroke. In some Diesel Engine they also provide heater to preheat before engine is started. Thus there is no flammable gas around at atm temp. and pressure, thus no hazardous area (safe area).
Another argument also is the auto ignition temperature is 257 deg.C. Thus as long as there is no naked flame (keep the nake flame away)and heat producing equipment away from the TankFarm, it can be a "Safe Area". It all depends on the Plant Layout Design.
Thus in conclusion the TankFarm can be classified as "safe Area" provided the design met the above arguments.
Anybody who disagree on these arguments??
Welcome all further comments!

 

[waross]

Codes notwithstanding, if the contract documents specify,
"The Owner demanded to be classified the TankFarm as hazardous area" Than that is the answer.
If an owner wants and specifies more than the minimum code requirement, and says so in the bid documents, then that is it.
The owner may be keeping his options open for the future storage of other more flamable fluids.
This is possibly not a code dispute, but a legal dispute concerning the wording of the contract documents.

 

[giraffetall]

Hi waross,
The contract document did not specify clear cut that the TankFarm shall be defined as hazardous area. It only simply mentioned that if necessary the Area Classification Drawing for the TankFarm shall be prepared as per "NFPA 70 (i.e NEC Chapter 5)" and "IP model Code Of Safety Part 15 (latest edition)". That's all.
Also note that in this Project there are other TankFarms which handle other products which will create Hazardous Aeras.
That is the reason there is a dispute now.
The Contractor said strictly according to Codes and Standards the Diesel TankFarm shall be non-hazardous Area. Owner argued that it shall be Hazardous Area.
Who is right and who is wrong?

 

[waross]

My codes are badly out of date, but I can find no reference to diesel fuel in either the 1998 Canadian Code or the 1996 NEC. The Canadian code and the NEC have lists of liquids and vapors that are classified and diesel fuel is not included.
I would suggest asking the owner to cite the specific code articles on which he is basing his argument.
In NEC 1996 Article 514-1. Definition.
Where the authority having jurisdiction can satisfactorily determine that flammable liquids having a flash point below 38 Deg. C (100 Deg. F) , such as gasoline will not be handled , such location shall not be required to be classified.
If you can locate this rule in the current code it may be what you need.
While 1996 NEC . Article 500-1 Scope – Articles 500 Through 505. Mentions “flammable liquids”, 1996 NEC Article 500-5. (b) Class I Division 2. uses the wording “volatile flammable liquids”
I don’t believe that Diesel fuel is considered volatile.
Again you will have to check the current codes for the current wording and Article numbers.
Hope this helps
Please let me know if there are serious changes in the codes.
yours