Sizing Tank Blanketing Regulators

Steve Attri Global Product Manager

Steve Attri
Global Product Manager

In March of 2014, API Standard 2000 (Venting Atmospheric and Low-Pressure Storage Tanks), 7th edition was released.

In a whitepaper, Sizing Tank Blanketing Regulators Using the Latest API 2000 7th Edition Guidelines, Emerson’s Steve S. Attri describes how the latest changes affect the sizing of tank blanketing regulators. The whitepaper also includes how backpressure regulators are used for vapor recovery systems.

He opens the paper noting the fundamentals that impact tank pressure remain the same—liquid flow (or pump-in and pump-out) and changes to temperature still form the fundamentals to the sizing calculations.


Steve provides the history of the API Standard 2000 over the last several editions:

The API Standard 2000 5th Edition takes into account Tank Volume, Liquid Flow, and Temperature Change. It was written as a basis for the pressure control of hydrocarbons, and considered industrial tanks as well. It is this 5th Edition that is probably in widest use today.

In 2009, this was updated to the API Standard 2000 6th Edition. Note that the 5th Edition remained in the form of Appendix A, so it was not made obsolete per se. The additional factors of Average Storage Temperature, Vapor Pressure, and Latitude were added in the 6th Edition, as an additional focus was placed on alcohols which have higher vapor pressures and can significantly increase the inflow requirements.

This year, minor changes were implemented and the latest guideline is now the 7th Edition. One of the changes has to do with a simplified calculation for volatile liquids. Figure 3 summarizes the similarities and differences between the three latest editions of the standard.

He highlights the role of latitude in two factors, C-Factor and Y-Factor, introduced in version 6. For the C-Factor:

The key latitude categories are “Below 42 Degrees”, “Between 42 and 58 Degrees” and “Above 58 Degrees.” These latitude lines are where weather will patterns shift enough to cause meaningful differences in tank pressure.

The Y-Factor:

…is only dependent on latitude and is used when making out-breathing calculations, which are required to size a vapor recovery regulator.

Steve shares the general steps for determining in-breathing to determine the flow requirements for the tank blanketing regulator:

The general steps are:

  1. determine the volumetric flow rate required to replace the liquid being pumped out;
  2. determine the volumetric flow rate required due to temperature drop;
  3. add the results of (1) and (2) together.

The whitepaper highlights the specific calculations performed. The implications of these new factors are that in-breathing requirements are greater—and may be much greater:

…depending on the C-Factor which takes into account vapor pressure, average storage pressure, and latitude.

The sizing calculations for out-breathing determine the flow requirements for a vapor recovery regulator. The basic calculation steps include:

  1. determine the volumetric flow rate required to compensate for the liquid being pumped in;
  2. determine the volumetric flow rate required due to temperature rise;
  3. add the results of (1) and (2) together.

There are two sets of equations for both non-volatile and volatile liquids. The 7th edition simplifies the calculation by doubling the constant for volatile liquids, which means the out-breathing flow requirements are twice that of non-volatile liquids.

Read the whitepaper to see calculation examples.

Steve concludes:

The latest API Standard 2000 7th Edition represents minor changes to the previous edition. However, it is important to recognize that the changes set forth by API Standard 2000 6th Edition remain intact. Although the 5th Edition continues to be widely used for sizing blanketing and vapor recovery regulators, it is expected that the newer editions will gain more acceptance and use in the years ahead.

These newer editions often result in greater flow requirements, especially for in-breathing. So it is important to recognize that the selection of the appropriate regulator may be impacted. Once the required flow is determined, other factors to consider would be the type of application, pipe size requirements, pressure, the desired set point, and the chemical compatibility of materials.

You can connect and interact with other pressure regulator experts in the Regulators group of the Emerson Exchange 365 community.


  1. Please check this calculation and give answer for ” is it correct”

    Inside Diameter Of Tank = 3800.00 mm
    Capacity = 49.33 cu.m
    Internal Design Pressure = 100.00 mm. wc
    External Design Pressure = -25.00 mm wc
    Max. Volume of Filling Rate = 40.00 Cu.M / hr.
    Max. Volume of Dischare Rate = 1.43 Cu.M / hr.
    Service = Diesel Oil
    Boiling Point = 288.00 Deg.C
    Latent heat or vaporization = 236533 J /kg
    Absolute temp. reliving vapor = 561.00 Kº (corresponding to boiling point point at the reliving pr.)
    Vapour relative molecular mass = 237.00 g/mol
    Temperature media storage = 21.00 Deg.C

    2.0 Outbreathing as per clause A.
    Q = Vot + Vot
    Vot = 4.97 NM3/hr Table A.3
    Vop = 40 M3/hr
    Flash Point = 1.01 Table A.1
    Qout = 45.37 N M3 /hr

    2.1 Inbreathing as per clause A.
    Q = Vit + Vit
    Vit = 8.33 NM3/hr Table A.3
    Vip = 1.43 M3/hr
    Flash Point = 0.94 Table A.1
    Qin = 9.6742 N M3 /hr

    2.3.1 Fire relief requirements as per clause
    For tanks not provided with a weak roof-to-shell attachment, the required vent capacity, q, is given by Equation (11):
    q = 906,6 (Q F / L) (T/M)^0.5
    Where :
    Q = Heat imput by Table 3 = 2454681 W (Awt=68.6m^2)
    F = Enviromental factor from Table 9= 1
    L = Latent heat or vaporization = 236333 J / kg
    T = Absolute temp.relieving vapour = 561.00 Kº
    M = Vapour relative molecular mass = 237 g/mol

    q = 14487.507 NM3/hr

    Table 3. Heat input, Q
    Wetted surface area Design pressure Heat input Heat input
    ATWS (m2) kPa g Q (W) Q (W)
    <18,6 =18,6 and <93 =93 and <260 =260 >7 and =260 <= 7 4129700 Not applicable Where the fluid properties are similar to those of hexane, the required venting capacity can be determined from Table 5

    Table 5. Venting capacity
    Wetted surface area Design pressure Venting Capacity Venting Capacity
    ATWS (m2) kPa g Nm3/h of air Nm3/h of air
    <260 =260 =260 >7 and 219073.91
    Flow area of vent device is safe

Leave a Reply