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Gas Leakage and Effect on Oxygen Concentration:

Example Calculations

Introduction

It is important to consider the consequences of a gas leakage from a pressurised gas cylinder as part of the risk assessment exercise.   Leakage could reduce the concentration of oxygen to levels capable of causing asphyxiation (e.g. release of carbon dioxide or nitrogen).  Equally, a release of oxygen could create an oxygen enriched atmosphere allowing fires to start more easily.

Effects and Symptoms of Oxygen Depletion / Carbon Dioxide Enrichment

 

Example Calculations

It is difficult to determine an accurate rate of gas discharge from a cylinder in the event of a tube blow-off.  Figures supplied by Gas Safety UK Ltd estimate a discharge from between 1.2 to 84.0 m3 per hour could be anticipated.  A rate of 1.2 m3 / hr could represent a slow release scenario whereby the contents of a cylinder would be discharged over the course of several hours (i.e. a working day or overnight), whilst 84.0 m3 / hr could represent  a sudden release, whereby the cylinder contents are completely discharged within the space of a few minutes (the worst case scenario).  Gas suppliers (e.g. BOC) will be able to supply information on the quantities of gas contained in different sized cylinders- it can therefore be estimated how long a cylinder would take to empty at a given release rate.

Since the process of risk assessment examines the likelihood of an event occurring, it is perhaps more appropriate to look at slow release scenarios as examples.  These are more likely than a sudden or fast release, and there is also a greater chance that a slow release would go unnoticed (a sudden failure or rapid release would give rise to e.g. hissing, popping or other audible / visual warnings that would alert anyone in the vicinity to the fact that something was wrong). 

Oxygen concentration resulting from a leakage of gas from a pressurised gas cylinder may be calculated as follows:

Resulting oxygen concentration (%) =

C = L
      Vn

Where:
C = gas concentration in room
L = gas release from cylinder (m3)
V = room volume (m3)
N = air changes per hour

 

Example Calculation 1:
Effect of slow CO2 gas release from cylinder in a laboratory

  • A laboratory with dimensions 10m x 8m x 3m contains one size K vapour withdrawal CO2 cylinder connected by a flexible tube to an incubator.  The tube accidentally becomes detached and goes unnoticed.  The CO2 is released at a rate of 1.2 m3 / hr. The laboratory is mechanically ventilated giving 5 air changes per hour.
  • C= 1.2 / 240 x 5 (x 100) = 0.1% CO2 (O2 level reduced from 21 to 20.979%)
  • The O2 level remains above 20% and the CO2 level within the laboratory reaches about 0.1%- below both the short term (15,000 ppm or 1.5%) and long term (5,000 or 0.5%) exposure limits
  • Note: if the room was smaller and had poor ventilation (e.g. 0.4 air changes per hour) the same scenario could give rise to a CO2 concentration above the exposure limits and reduce oxygen below 19%.  Additional safety precautions would need to be considered if release of gas could reduce oxygen concentration of air below 19%.  Measures to be considered include:
    • improved ventilation and / or
    • fixed point oxygen monitoring

 

Example Calculation 2:
Effect of slow O2 gas release from cylinder in a laboratory

  • A laboratory with dimensions 10m x 8m x 3m contains one size K O2 cylinder connected by a flexible tube to a gas chromatograph machine.  The tube accidentally becomes detached and goes unnoticed.  The CO2 is released at a rate of 1.2 m3 / hr. The laboratory is mechanically ventilated giving 5 air changes per hour.
  • C= 1.2 / 240 x 5 (x 100) = 0.1% O2 (O2 level increases from 21 to 21.021%)
  • In this case the O2 levels do not rise significantly and do not create an oxygen enriched atmosphere.
  • Note: if the room was smaller and had poor ventilation (e.g. 0.4 air changes per hour) the same scenario could give rise to O2 enriched atmosphere presenting an increased fire hazard.  Additional safety precautions would need to be considered if release of gas was likely to give an oxygen enriched atmosphere.  Measures to be considered include:
    • improved ventilation and / or
    • fixed point oxygen monitoring

 

 

 

 

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