Diesel does not require a hazardous area classification.  Thus storage tank areas may not require explosion-proof fittings etc.  Blanketing diesel with fuel gas would require a hazardous area classification.

bchoate (chemical)

"Why do we need blanketing?" (when).  Blanketing is done to prevent the vapor space of a tank from becoming ignitable and potentially ignite and rupture the tank (loss of primary containment).  A vapor space is ignitable when (1)the material stored is a flammable or combustible material, (2) the vapor concentration (at ambient temperature)of the stored material is between the LEL and UEL (% v/v)of the material, and (3) the oxygen concentration in the tank is above the minimum oxygen concentration for the stored material.  The last requirement for an explosive ignition is an ignition source.  Tank feeds are often setup for submerged fill to minimize one potential static electricity ignition source.  Nitrogen blanketing eliminates the oxygen needed for an ignitable atmosphere.  Design of the blanketing system should consider failure scenario's for any of the elements of the blanketing system.  e.g.  nitrogen supply is closed by operator error or by regulator failure.  Pumping out of the tank causes air ingress from activation of a conservation vent (vacuum).      

@Pons19

You may consider reading the following article:
Kinsley G.R., Properly Purge and Inert Storage Vessels, CEP magazine, Feb. 2001
http://www.aiche.org/uploadedFiles/CEP/Issues/020157.pdf

"We don't believe things because they are true, things are true because we believe them."

Another reason for blanketing pertains to fluid stability.  This too does not apply to Diesel.

My understanding is that diesel is classified as a combustible material and therefore does require an area classification.  It would just be a smaller area than a flammable material like gasoline.  It's true that if you blanket the tank with fuel gas, that will add a Class 1 Div 1 area where diesel would only require a smaller Class 1 Div 2 area.

See API 500 for more details about Area Classification.

Andy
New Orleans, LA
Petronyx Consulting Engineers, LLC
http://www.petronyx.com

See NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas; Table 4.4.2 for number 2 fuel oil.  Also see ASTM D 396 and ASTM D 975 for fuel oil specifications.

The area classification will depend on flash point.
As the F.P. of diesel is something around 65ºC (145ºF) and you store it around 30ºC or a low temperature, there is no problem of hazardous area classification.

I don't see API 500 yet, but I already work with IEC 60079-10. You should see both standards and choose the best for your application, or the stricter one.

(As good manner all vessels with flammable liquids should be classified as Zone 2 around it)

The need for blanketing storage tanks having flammable fluids cannot be overemphasized. Basically combustion occurs with the combination of the following;
1. Flamamable gas source
2. oxygen-(sufficient concentration)
3. and source of heat.
The exclusion of any one of the three conditions prevents the occurence of a fire.
Blanketing simply reduces the oxygen/flammble gas ratio, the use of an inert gas is common but a combustible gas source such as fuel gas can be used provided a closed system is adequately maintained and oxygen analyszers are installed inline.
from experience, "blanketing" is often being ignored unfortunately the resultant losses can be catastrophic.
 I recently concluded the design of blanket gas system for all the existing storage tanks in a particular tank farm( simultaneous blanketing and vapour recovery from in/outbreathing storage tanks).
The project came into being, due to a "freak" fire accident resulting in the loss of a 400,000bbl crude oil storage tank from  lightning.
There is no such thing as a "freak" accident the rules are plain to read, ignoring recommended practices might only work for the time being.
P.S
The Tank farm has being in operation for over 20 years before the accident.