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[Prashant Process]

Hydrocarbon gas can be vented to atmosphere based on considerations outlined by API 521.

 

a.The decision to intermittently discharge flammable and toxic materials to the atmosphere requires careful attention to ensure that disposal can be accomplished without creating a potential hazard or causing other problems, such as the formation of flammable mixtures at grade level or at elevated structures, buildings, process and breathing air intakes; exposure of personnel to toxic materials; ignition of relief streams at the point of emission; excessive noise levels; and air pollution. ISO 23251 or API RP 521 covers in greater detail these associated potential hazards or problems.

 

b.The duration of atmospheric relief discharge should be limited by the use of pressure-limiting instrumentation, reseating relief valves, or operator intervention. However, in addition, the following criteria shall be met for flammable and/or toxic relief streams:

 

1.The discharge velocity shall be sufficient to reduce the concentration of flammable material at a suitable distance downstream of the point of discharge to below the lower flammable limit, but not such a high outlet velocity that a build-up of static electricity could occur.

1. The additional effect of wind-assisted dispersion between the jet and any source of ignition may be taken into consideration.
2. The use of pilot-assisted relief valves may be necessary (in services in which these types of valves can be used) to achieve adequate jet velocities.
3. The distance downstream is set by plant layout and environmental considerations.

2.To maximise dilution in atmospheric discharge, every pressure relief device should have its own discharge line where practical.  The discharge pipe shall be adequately supported and sized for rated capacity of the relief device.

 

3.Dispersion analysis and consequence analyses shall be performed to confirm that the discharge does not represent a significant impact to personnel safety, environment or to equipment/ plant integrity.

 

4.There shall be no significant condensation of flammable or toxic material released from the discharge pipe at the minimum expected ambient temperature.

1. Criteria for assessing condensability require specific calculations for each case.
2. As a preliminary guide, a hydrocarbon vapour of average molecular weight 100 or less should not generally condense under typical discharge conditions unless the minimum expected ambient temperature allows condensation to occur. 
3. For higher molecular weights a more detailed analysis is required; involving the examination of vapour cooling rates and dew point conditions at the specified minimum ambient temperature for the site.

How to determine whether a quench drum is required for the vent stack or not.

Can a P&ID of such a cold venting system be made available.

 

Look forward to your valuable advice.