Release Rate Calculations

Below you will find information to help calculate the release rate for liquids and gasses based on the assumed hole size for the DM. This is needed for the S-RBI People Detail Consequence Assessment (Step 8).

Note: You can also find calculators on the internet to do this.

Release of Liquid through a hole

Liquid release rate of pressurized Equipment is estimated by the Bernoulli equation:

Where is the release rate or mass flow rate [kg/h],the density [kg/m³] and the pressure difference [bar].

The two figures below show the calculated release rate as a function of hole diameter and pressure for a liquid with a density of 650 kg/m³ [41 lbm/ft³].

Release rate as a function of pressure (top) and hole diameter (bottom) for a liquid with density of 650 kg/m3 [41 lbm/ft3].

Release rate as a function of pressure (top) and hole diameter (bottom) for a liquid with density of 650 kg/m3 [41 lbm/ft3].In the case of a refinery, the release rate is not very dependent on the type of liquid, because the density of typical refinery liquids does not vary much. Heavy fractions are usually processed at higher temperatures which reduces the density to a value not very different from lighter fractions. Furthermore, the release rate is not very density dependent (to the square root of density). For instance, a 20% density increase will only lead to a 10% release rate increase.

Release of Gas through a hole

The calculation for a gas release is more complex, but the release of gas remains proportional to the hole size.

Gas release rate through a 6 (0.25” – top) and 12.5 mm (0.5” – bottom) diameter hole. 

Gas release rate through a 6 (0.25” – top) and 12.5 mm (0.5” – bottom) diameter hole. 

The above two figures show the release of light gases through a 6 (¼”) and 12.5 mm (½”) hole. Because of its lower density, the mass flow rate of a gas is much smaller than that of a liquid.

The release of LPG is a special case: it is usually liquid at operating conditions, but gaseous at ambient conditions. The figures below from a general chart to estimate the amount of released LPG. If one conservatively assumes that all liquid vaporises, relatively large flammable clouds can originate from such a release.

Release rate of LPG gases through a 3 mm (0.125”) hole as function of pressure (top) and temperature (bottom). 

Release rate of LPG gases through a 3 mm (0.125”) hole as function of pressure (top) and temperature (bottom). 

Release rate of LPG gases through a 12.5 mm (0.5”) hole as function of pressure (top) and temperature (bottom).

 

Release rate of LPG gases through a 12.5 mm (0.5”) hole as function of pressure (top) and temperature (bottom). 

High Pressure Consequence of Gas Pipes

The high gas pressure part of the explosion Consequence questionnaire uses stored energy (pressure times volume) as a measure of the Consequence of Failure (CoF). For pipes, the effective volume is estimated by taking a section with a length of fifty times the diameter, D, i.e., V=50D*¼D². The figures below are a chart for a quick estimate of the stored energy interval in which a certain pipe is situated.

Pipeline stored energy as a function of pressure and pipe diameter. 

Pipeline stored energy as a function of pressure and pipe diameter.