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[US_ChemE]

Dear Forum Members,

This query is related to API Standard 2000 (Sixth edition, November 2009).

Clarification is sought on what formula / methodology to be followed for determining the inert gas flow rate for blanketing valve (PCVs) of blanketed tank.

As per API Standard 2000, there are two different formula for determining the thermal inbreathing rate for non – blanketed & blanketed tanks which are as follows:

• Thermal inbreathing for non – blanketed (un-insulated) tanks

As per section 4.3.2.3.3 of API 2000 Sixth edition, Nov 2009 in breathing rate of the un-insulated tank:

Vit = C x Vtk^0.7 + Vpe .………….Equation 1

Where
Vi = flow rate of inert gas in Nm3/h
Vtk = Tank volume in m3
Vpe = maximum rate of liquid discharge in m3/hr
C = factor that depends on vapour pressure, average temperature and latitude.

• Inert gas for blanketed tanks

please note the below quote from section 4.5.3 of inert gas – blanketed tanks

Quote

“ for tanks that use an inert-gas supply system, the likelihood of potential explosive atmosphere is reduced and there can be benefits related to less severe hazardous area classification. See Annex F for discussion of other benefits & for informative guidance for inert gas blanketing of tanks for flash back protection”

Un-quote

As per Annex F, the inert gas flow rate for blanketed tanks of level 3, is estimated by using following equation -

Vi = 0.5 C x Vtk^0.7 + Vpe ……………Equation 2


As an be observed from equation 2 (annex F), the required inert gas flow rate for blanketed tank is almost half the estimated rate from equation 1 (as per section 4.3.2.3.3).

Considering above mentioned criteria, may I request advice on following points:

• Why in case of inert gas blanketed tanks inbreathing flow rate is less as compared to non-blanketed tank?

• Does it assume that air ingress in the tank through breather valve may happen in case of short fall of inert gas; however this will not form explosive mixture?

• Can we consider the flow rate of inert gas worked out using equation 2 for sizing the inert gas line and the pressure control valve on the inert gas line?

Thanks & Regards,
US_ChemE

[proinwv]

I would hope that someone who sat on the committee would reply to these questions for you. It is often difficult to determine the logic that went into writing a standard.

I can point out that Annex F is not mandatory, but informative. Also Annex F requires that certain additional alarms and controls to be included with the system.