Vapor relief Low-Pressure storage tank API-2000 1

[Luuk21]

Hi,

I have a question regarding the calculation of the required capacity for a Low-Pressure storage tank, using API-2000.

I'm using the 5th edition of API-2000, we don't have the newer ones available.

The case is that 100 m3/h liquid ethanol is pumped into an ethanol storage tank, displacing 100 m3 of ethanol vapor. The set pressure of the valve is 1 barg / 15psig (still within limits API-2000), temperature 25 degrees, vapor density 3 kg/m3.

If I understand correctly, the required relief capacity for this particular case is:
202 Nm3/h - based on Table 1B - Normal Venting Requirements, API-2000 5t edition.

However, I'm getting told that you should convert the relieved gas to Normal Cubic Meter Air, which is 1m3 of Air at 0 degrees (273.15K). This because the vendor spec of the valve is in this unit.

According to our tool, this would lead to:
Relieving flow @normal cond. FV,N = FV,R · (T2 · P1 · Z2) / (T1 · P2 · Z1)
FV,N = 200 * (273.15 * 2 * 1) / (298.15 * 1.013 * 1) = 200 * 1.83 = 362 [Nm3/h] Ethanol

Density @ normal cond. rhoN = Mw*1.013/273.15/0.083144
rhoN = 46.068 * 1.013 / 273.15 / 0.083144 = 2.05 [kg/m3] Ethanol density at zero deg C & Patm

Equivalent diagram flow for vapor relief FV,dia = FV,N · sqrt[(rhoN·T1·1.013) / (rhoAir·273.15·P1)]
FV,dia = 362 * sqrt[(2.05 * 298.15 * 1.013) / (1.2 * 273.15 * 2)] = 352 [m3/h] equivalent flow Normal Cubic Meter Air

My question is;
Is this correct? Do you have to compensate for the Normal conditions or can you just apply Table 1B? My feeling is that you can simply apply Table 1B, since the distinction is made between low boiling / flashpoint liquids and high boiling / flashpoint liquids.

Then, liquid relief. Yes, I know that breather valves are not designed for liquid relief, but our tool does indicate there is a relation to determine the capacity:

FV,dia,L = FV,L / sqrt(rhoair / rhoL)

In case of a 30 m3/h overfilling this leads to;

FV,dia,L = 30 / SQRT(1.2/875) = 810 Nm3/h

I could not find any reference on this conversion. It makes sense since the correction in conversion to Normal Cubic Meters are is done Square-root. However, liquid behaves different than gas. Is this approach conservative?

See attachment for clarification

 

[TiCl4]

Luuke21,

My copy of API 2000 states in the header subtext of Table 1B that the relief values given are "NM3/hr of Air per Cubic Meter per Hour of Liquid Flow".

The table already gives you required flow in NM3/hr of air. There is no conversion necessary to correlate this to vendor-specified equipment, as most usually spec relief in NM3 or SCFH air at various pressure differentials.

 

[Luuk21]

The table already gives you required flow in NM3/hr of air. There is no conversion necessary to correlate this to vendor-specified equipment, as most usually spec relief in NM3 or SCFH air at various pressure differentials.

Thanks! I had the same interpretation. However, someone showed me the new API-2000 7th edition, and there Table 1B is removed. Instead they use a calculation method that indeed does convert to Nm3/h. This only applies if the temperature is above 49 degrees though.

So I guess the new standard requires you to convert the relief gas to Nm3/h - Table 1B 5th edition is not longer valid.

Kind regards,

Luuk

 

[TiCl4]

I'm a bit confused - is your tool referencing the older standard with the Table 1B or the newer, formulaic standard? You shouldn't mix and match versions. If the tool references the older standard, then conversion still isn't necessary. If it references the newer standard, then I can't comment on all your work as I don't have the standard.

You have calculated the Vop at relieving conditions (200 m3/hr) and translated that to NM3/hr correctly, as far as I can see. Just check to ensure that you use the correct value (actual vapor flow or normalized vapor flow (200 vs 362)) in the correlation in Annex D.9 (which I can't see).

 

[Luuk21]

If it references the newer standard, then I can't comment on all your work as I don't have the standard.

Basically the new standard (7th edition) takes the calculated relief amount (f.e. 200 m3/h of Ethanol vapor at 40 degrees and 2 barg) and converts it to Nm3/h air.

However, in Annex A of the 7th edition, the alternative method of Table 1B is also displayed (the old 5th edition way).

So you can use both I guess.

My tool is a big mess, I'm trying to clean it up and use the correct standards.

 

[pierreick]

Hi,
You may want to take a look at this document.
Pierre