4 replies to this topic

[AndyChemEng]

Hi, I'm carrying out a review of our vent header arrangement and I was wondering if anyone could provide their thoughts/guidance on the following query:

 

Vent header

The vent header itself serves a chemical facility and consists of 4 individual headers that combine and exit via tall stack. Each header has a collection of vessels breathing into it, blanketed/unblanketed storage tanks and process vessels etc. These vessels emit a range of flammable VOCs (toluene and similar compounds)

 

Basis of safety

The basis of safety for the header is 'absence' of flammable atmosphere allied with absence of ignition source.

This is achieved by inerting the each header with nitrogen at a given flow-rate and avoiding any source of ignition (vents are just pipes, so static from abnormal liquid or propagation of ignition from a vessel).

 

Query

My query is regarding what should this given flow-rate be and is there any guidance available as I've not been able to find any?

 

My thoughts

Keeping the oxygen level below 5% for the 'majority' of the time is my starting point. 

 

Look at all the operations and the corresponding out-breathing rates across the multiple units and assume they are undergoing initially purging (air venting) - if I have eight vessels, do I assume eight simultaneously or do I pick a more realistic number say 1-2 that would cover the majority of realistic cases? 

We also carry out vacuum operations that will inherently flood the vent periodically for a short duration with high levels of oxygen and to accommodate this with higher nitrogen flows seems very uneconomical and impractical.

 

In essence, I'm hoping to hear what approaches/methodologies others have used in their experience along with their justification and if they referenced any particular available guidance?

 

Regards

Andrew

Edited by AndyChemEng, 21 August 2021 - 08:15 AM.

[Pilesar]

My first thought is that you should not have a common stack for atmospheric tank breathing and pressure vessels and each header should be considered separately. I am surprised you can get environmental permits for routinely venting VOCs without a destructive flare. Review API 2000: Venting Atmospheric and Low-Pressure Storage Tanks for some guidance. The amount of gas for vent purging is based on keeping a high enough velocity in the line so that air cannot reverse flow to form flammable conditions inside piping and equipment. Purposely adding oxygen to a hydrocarbon vent header does not seem a safe practice.

[breizh]

Hi ,

Consider the link  attached with examples :

https://onlinelibrar...0470038071.app5.

 

Get a copy of :Safe design and operation of process vents and Emission control system published by CCPS .

 

Note :

*You should contact your local safety bureau about standards and norms applicable to vents in UK and should be in touch with HSE department within your organization. 

 

 

Good luck 

Breizh 

[AndyChemEng]

Thanks for the replies and advice.

 

I'll give that book a read over.

 

Ta

Andrew

[AndyChemEng]

Found this document useful if anyone has to go through a similar exercise in the future:

 

https://www.icheme.o...iv-paper-26.pdf

 

Andrew