2 replies to this topic

[flash]

I am sure this must have been discussed in the past, but would like to get feedback from the experts on the following points:

(1) Boundary conditions and assumption to use the venting requirements given in the main body of API2000, Annex E, section E2.a reads as “the tank is fully filled with vapour (no liquid is in the tank” – what does it really mean? Tanks are supposed to store liquid. Does it mean if we have liquid in the tank we cannot use the venting method as given in section 4.3.2? Could someone please interpret the correct requirements here?

(2) To determine liquid transfer out-breathing rate, API suggest a method for product having vapour pressure < 5kPa (section 4.3.2.2.1.a) however, for thermal in-breathing it considers hexane (higher vapour pressure liquid, 35kPaa at 40degC) as a base chemical to choose C factors (section 4.3.2.3.3). The basic fundamentals do not appear to match up.

My question is – is there any guideline when to use the venting requirement method given in the main body of API and when to use the one given in Annex A. Could we say - the method in the main body is for higher vapour pressure liquids (solvents) and the one in Annex A is more for less vapour pressure liquids (diesel, gasoline)? I am sure it won’t be that simple but would appreciate if somebody could please share his views.

[Marc-Andre Leblanc]

Hello, 

 

I will try to clarify the question you have : 

 

1) When determining the normal inbreathing and out-breathing requirement, it is required to consider the tank is fully filled with vapour, or liquid level near 0 because it is more conservative and it can happen during normal operation.

 

The reason is because the variation of density of the fluid resulting from the change in temperature are higher for a gas than they are for liquids (therefore the required breathing rate is higher if the tank is full of vapour). The flow requirement consider the tank to be full of vapour because it can be full of vapour or the liquid level can be near 0 during normal operation.

 

If you would consider the tank to be at normal level or full of liquid, the change in density for a liquid for the same change in temperature is a lot smaller than that resulting from the change in density of the gas, resulting in grossely undersized venting equipement.

 

2) a ) This section explain how to convert the filling rate (of liquid) to a flow air being vented. The 5 kpa vapor pressure is to make it clear that the vapour being vented is mostly air. If higher vapour pressure is encountered you can no longuer neglect the presence of vapour given off by the liquid as part of the flow being vented. 

 

This is only for outbreatthing since inbreathing will be air from the atmosphere in general (no process vapour outside tank!)

 

2) b )The hexane assumption is for an entire diffenrent case where the venting does not come from the flow of liquid in and out of the tank, but because of the change in temperature of the atmosphere outside the tank, or other atmospheric condition (like cold rain during hot summer day). The C factor consider the condensation / contraction of the vapour inside the tank and consider that liquid with vapour pressure lower or equal to hexane require less breathing than a liquid with higher vapour pressure.

 

2) c ) Finally the scope of appendix A is very well define in the standard itself, but it is good to know that prior to the sixth eddition, the Appendix A method was the core method of the standard, this is well explained section A.2.

 

In general the venting requirement from the method in the core of the standard (4.3) is more conservative than the venting requirement based on the Annex A, but the scope of the annex A method is smaller.

 

The vapour pressure is only part of the reduced scope of the annex A, but this also include maximum volume consideration, maximum operating temperature, no insulation ...

 

Unless it is clear you will not have condition outside the scope of annex A, even during of process upset, design should consider the core of the standard.  

 

regards

 

Marc-Andre

Edited by Marc-Andre Leblanc, 18 August 2014 - 04:30 PM.

[explosion]

Just my five cents to this: I was involved in the discussions when the old and new revision had been discussed. One reason to establish the annex is that thousands of US tanks are calculated according to it (API 2000 rev. 5 or older). If there would have been a new standard (without this annex) this would have been considered as stat-of-the-art. And a lot of the US companies would have been forced by legal departments to follow the new state-of-the-art by adding more capacity to their venting valves. They opposed against this because of being it a big investment !

Despite the fact that US claimed to have no problems with the sizing according the annex there are two events in Europe where a tank sized acc. to API 2000 old had seen vacuum and been damaged.

So everybody can judge for themselves ...

Regards,

explosion