Commissioning an internal floating roof tank and prepping for Maintenance

[DAWN555]

We are putting our first internal floating roof tank into service. Does anyone have any guidance for whether we need to take special precautions for LEL in the upper section of the tank during initial fill. My understanding as as we fill it in order to float it off the legs the vapor will be pushed out a breather vent until the roof begins to fully float on the liquid. Once it it floating the double seals and large vent on the top will be sufficient to prevent LEL build up in the upper section. But I am concerned during the initial fill. The material is a hexane like material.

And in 3 years when we have to take it our of service for an internal inspection and set the roof on its legs by emptying it slowly, air will be drawn through a vacuum break into the section below the floating roof as we fully empty. Again I am concerned about LEL below the roof at that point.

The tank is grounded but is that really enough. The amount of static to light off a hydrocarbon is very tiny (one source says hexane is 0.011 mj) . Trying think of commissioning and prepare for maintenance procedures

thanks

 

[LittleInch]

I repeat what I said in your other post.

You need to assume that the interior of the tank is at or above LEL at ALL times. An IFR reduces considerably the amount of vapour which is otherwise vented out through the roof vents, but it doesn't eliminate it.

IMHO this - "Once it it floating the double seals and large vent on the top will be sufficient to prevent LEL build up in the upper section" is NOT CORRECT.

When first filling the amount of vapour being released into the tank space is large and you need to consider how this excess quantity of vapour is released from the tank vents and if it causes any further risk due to a vapour cloud from the main tank vents.

When you empty for maintenance you might need to think about flooding the interior of the tank with Nitrogen as you lower the liquid level below the roof level to reduce the risk of an explosion.

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

 

[stewbaby]

The key is to limit this transitional time between the 'lean' and 'rich' atmospheres so you don't stay in the explosive range. Standard floating roof storage tank RAGAGEP though does not typically take any special precautions into account (when designed and operated to API650, API 2510, NFPA 30, NFPA 499, NFPA 780, etc. and meeting and operated per 40 CFR applicable CC or Kb subarts). The downside to trying to do things like nitrogen purge for the vapor space above the roof is that you still have a point where you go from rich to lean or back again as the nitrogen is introduced or removed...there is no way to avoid 'paying that bill' without just a continuous blanket or other vapor handling system.

 

[ChalMel13]

The initial fill has to be very slow (flowrate <1m/s), until the rood is fully afloat. We usually start by filling another tank and then slow "cracking" the valve on the tank that will be put back in service. However, there are risk involved as the space in the tank between the product and the roof will be within the explosive range .

From my point of view, the safest way is to float the roof using inert gas such as CO[sub]2[/sub] or N[sub]2[/sub]. We usually fill through the sample point pipes for couple of hours and then starting the filling process. The risk here is that the inert gas might lead to O[sub]2[/sub] deficiency and should be considered when entering into the roof (i.e. leg repositioning).