12 replies to this topic

[wenchop]

How do you know when a hydrocarbon storage tank is to be vented to atmosphere, and when the vapours have to be recovered, e.g. with a vapour recovery unit ?  

 

I am thinking of the hazard of ignitable vapour that may form outside the tanks. 

 

[proinwv]

You should start with your local and national laws.

 

Also, as you correctly state, there may be a fire hazard. And too, the atmospheric air may contaminate your product.

 

You should search this forum for many comments on this subject.

[wenchop]

There are measures e.g. blanketing, PVRV to prevent air ingress into the tank, so no ignition can occur inside the tanks, but I am wondering about the venting to atmosphere or to vapour recovery.

 

I agree that environmental laws and regulations would state the minimum standard.

 

But from a standpoint of fire hazard, e.g. a flammable vapour cloud, what is the rule?

 

thx,

[wenchop]

I am looking in API 2000 but cannot seem to find a statement on the issue..

[proinwv]

API 2000 is a venting standard, not an environmental rule. You will not find in it what media to blanket, etc.

 

If I believed my tank could produce "a flammable vapour cloud" as you state, I would take the necessary actions before hand to eliminate that hazard.

[wenchop]

Found something:  

 

Alberta, Canada Directive 60 Upstream Petroleum Industry Flaring, Incinerating, and Venting: 

 

"Hydrocarbon products stored in atmospheric storage tanks at gas plants, compression 

stations, and gas batteries must not have a true vapour pressure of more than 

83 kilopascals (kPa) at 21.1°C if the tanks are vented to the atmosphere."

[wenchop]

API 2000:

 

"When selecting vent devices for areas with strict fugitive emissions regulations, the maximum leakage

requirements during periods of normal tank operation shall be taken into consideration."

 

No talk about ignition hazard here..

[fallah]

magonz,

 

Your exact query isn't so clear...

 

Anyway, the VRU (vapor recovery unit) mostly is applied for the cases in which a liquid is stored well blow the ambient temperature and flaring would cause wasting a lot of valuable inventory...Obviously other than such cases the vent of flammable liquid storage would be connected to flare...

[wenchop]

fallah:

 

let me rephrase:  a liquid hydrocarbon storage tank that vents, sometimes they vent to the atmosphere, sometimes to a vapour recovery unit, the question is, from a standpoint of Fire Hazard OUTSIDE of the tank due to a vapor cloud, what is the rule?  When can we vent to the atmosphere and when to a VRU?    for example, a PVRV can be connected to a VRU suction or vented to atmosphere.  

 

I do not ask about environmental standards which are another topic.  Those are non negotiable legal requirements that are not up to interpretation.  

 

thx,

 

ps.   when you have for example hydrogen sulphide in the vapour space, in that case for many reasons you cannot vent to atmosphere. This case is very obvious, toxic stuff cannot be vented to the atmosphere.

Edited by magonz, 16 May 2014 - 12:42 PM.

[fallah]

magonz,

 

Basically a VRU has nothing to do with tank's venting system and is to be applied for recovery of the valuable vapor would mostly be created in low temperature storage facilities due to heat receiving from the ambient. Then a VRU is normally connected to a storage system by a pressure control valve but a PVRV is to be terminated to the atmosphere or a safe location and if would relieve a flammable vapor will be connected to flare.

[wenchop]

I have seen VRUs installed whenever a hydrocarbon liquid sent to storage drops (same temperature) in pressure as it reaches storage (boot and tank). That alone will can cause flashing of significant amounts of light hydrocarbons.  

 

I have seen many PVRV with flammable vapor not connected to flare, are they all doing something so wrong?  Is dispersion modelling required to demonstrate that LEL will not be reached?

[fallah]

magonz,

 

Yes, the VRU can also be applied in recovery of vapors due to flashing a liquid to low pressure condition.

 

PVRVs handling flammable vapors might not to be connected to flare due to one of the following reasons:

 

1) Basically there wouldn't be a flare network near the storage facilities,

 

2) Amount of the vapor to be vented is so low,

 

3) There would be a safe location based on which the PVRV outlet can be piped away in order to reach to that location.

 

I think a dispersion modelling might not to be applied for PVRV outlet due to low velocity of the relieving vapor.

Edited by fallah, 16 May 2014 - 02:41 PM.

[wenchop]

Also, a tank would be connected to a closed system (VRU, flare) if : 

-the release of flammable vapors would result in excess of 50% LFL at grade, any platform or equipment.

-in case of ignition, would result in inacceptable radiation levels to surroundings, people, equipment, etc. 

 

Today there is software available for vapor cloud releases and many other scenarios ( leaks, pipe ruptures, relief devices, vessel ruptures, venting and more).