Tank Overflow Nozzle Requirement

[Sawsan311]

Dear All,

I have seen tanks which are fixed roof mainly utility water/diesel are provided with overflow nozzle. Do you think for other hydrocarbon tanks where flammable/hazardous contents are stored, no overflow nozzle is mandated since there will be greater risk imposed with such a nozzle and since it would be mandatory to ensure available overfill safety time of at least 20 minutes between the maximum high high liquid level and critical high level as per API 2350.

I would like to know when do we have to always consider an overflow nozzle in our designs

thanks

regards,

 

[JStephen]

The point of the overflow is to protect the tank from damage in the event of overfilling. Overflows are almost universal on potable water tanks.
With a petroleum tank, damaging the tank due to overfilling is a financial/safety/environmental disaster. But then again, overflowing the tank into the containment is just a different financial/safety/environmental disaster. So the general trend is that overflows have gone out of style in favor of increased monitoring, alarms, and controls. See H.5.3 in API-650.

 

[LittleInch]

A lot of hydrocarbon tanks are either external or internal floating roof to reduce emissions.

Internal floating roof tanks often have vents in the roof which act as overflow nozzles.

Some EFR have cut outs in the top of the shell to do the same thing to stop the roof collapsing.

But JStephen has nailed it. Both are bad so put more effort into stopping it happening.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

The overflow issue should be an accident scenario done as part of the HAZOP review of the process system.

Unfortunately, commonly this has not been yet done at the time that the tank is being specified so a specified overflow sometimes becomes a "spare nozzle" by the time the tank is constructed. As stated above, augmented monitoring, alarms, and controls are more common

The actual overflow is only important when expensive tank/environmental damage will occur ..

This is kind of a fun report to read regarding the importance of tank overflows and people being mindful of their assignments ....

https://www.icheme.org/media/13923/buncefield-miib-final-report-volume-2a.pdf

MJCronin
Sr. Process Engineer

 

[IFRs]

There is also the notion that petrochemical storatge tanks are often filled at high rates - 5,000 to 30,000 barrels per hour and a gravity pipe would have to be huge to flow that much. Plus, an overflow pipe is a continuous source of dangerous vapors that are better exhausted up high where they can disperse.

 

[Sawsan311]

thank you so much all