Guidelines for Pressure Relief Design

7.0   API Storage Tanks

A.     For API Standard 620 Tanks

API 620 regulates the manufacture of low-pressure oil storage tanks that are larger than 300 feet in                                      diameter. By design, the configuration of API 620 dictates that you should have a flat or elevated bottom to                        store your tank.  The tank should be made of a material with a minimum thickness of 3/16 inch and have a                              central, vertical axis of revolution.

B.     For API 620 Tanks

(2.5-15 psig design pressure) the pressure relief devices must prevent the tank pressure from exceeding 105                     % of the design pressure, unless the hazard is due to external heat, such as a fire.  In that case additional                               pressure relief devices must prevent the pressure from exceeding 20% of the design pressure.

C.     For API Standard 650 Tanks

API 650 applies to both aluminum and carbon stainless tanks that are usually used in pipelines and refineries.                    It sets standards for above-ground storage tanks both open and closed top and of different sizes. By design,                      these cylindrical tanks should not weigh more than the roof plates, meaning their pressure should not exceed                    the atmospheric pressure.

D.     API 650 Tanks (atmospheric to 2.5 psig, design pressure)

The pressure relief device must prevent the tank pressure from exceeding the design pressure with sufficient                      pressure differential and device diameter to provide adequate emergency relief.

E.     For API Standard 2000 Tanks (Full vacuum to 15 psig design pressure)

When determining the possible causes of overpressure or vacuum in a tank, sizing is based on either SCFH Air                    or NCMH Air, consider the following :

a.   Inbreathing from liquid movement out of the tank

i.      Vip = 8.02 Vpc , when

1.        Vip = inbreathing rate  (SCFH) and

2.        Vpc = liquid discharge (gpm)

b.   Out-breathing from movement of liquid into the tank

i.      Vop = 8.02*Vpf, where

1.        Vop  out-breathing rate (SCFH Air),

2.        Vfp max fill rate(gpm)

ii.      Vop =16.04*Vpr, if fluid is volatile

c.    Inbreathing from thermal effects

i.      VIT = 3.08*C Vtk0.7*Ri  where

1.        VIT  max thermal flowrate cooling (SCFH Air),

2.        C = depends on storage factors.

3.        Vtk tank volume(ft3),

4.       Ri (insulation factor)

d.    Out-breathing from thermal effects

i.      VOT = 1.51* Y*Vtk0.9* Ri , whre

1.        VoT  is max thermal flowrate heating (SCFH Air),

2.        Y (factor for latitude),

3.        Ri (insulation factor)

e.    Fire case

i.      Q = 3.091 * (Q * F /L)*(T/M) where

1.        q -  required flow capacity for fire SCFH air

2.        Q  - heat input from fire  (use table 4 API 2000 sec. 3.3.3.3 0)

3.        F - Environmental factor  (see API 2000 Sec 3.3.3.3. table 3)

4.        L - Latent Heat of vaporization of stored liquid at relieving conditions

5.        T  - absolute temperature degree Rankine

6.        M - Molecular weight of vapor

f.    Other circumstance resulting from equipment failure or operator error not covered in standard.

F.      High temperature failure – A problem can arise if the anticipated metal temperature during relief is greater                           than the vessel design temperature.    Additional or better insulation or water sprays may reduce the potential                     for the design temperature being exceeded, or reducing the set pressure on the relief device(s) may be                                   necessary for existing vessel.