Double Isolation for High Pressure CO2 Applications

[Sawsan311]

Dear All,

With regards to the hazards associated with high pressure CO2 used in liquid/dense phase process applications, I would like to get your views on the requirement for double isolation or what is conventionally known as DBB for such application, I support that for all CO2 applications double isolation is required due to the following:

1- High CO2 expansion ratio in case of a release or a leak / passing valve i.e. liquid CO2 increases by 700-800 times in volume when expanded to atmospheric conditions.
2- CO2 cryogenic challenges associated with high J-T coefficient in conjunction with isentropic expansion associated with CO2 liquid evaporative cooling hence when released , very low temperature in magnitude as low as -80 to -90 C is envisaged. density exceeds density of air ( at atmospheric conditions) creating additional challenges of dispersion and risks of acute asphyxiation.
3- The risk of CO2 solid formation upon expansion which have an erosive characteristic over any nearby personnel/equipment.

If we need to rationalize the DBB requirement for CO2, what would be the pressure class rating recommended as a minimum criteria for double isolation. Based on the simulations I have done, even a 45 barg gas CO2 -300# (below critical point) is sufficient to have an expansion ratio of 80-90 times which is still very high inducing a secondary layer of isolation irrespective to the pipe size.

Appreciate your views.

Regards,

 

[Latexman]

This FAQ is more general.

Good Luck,
Latexman

 

[LittleInch]

To add to your list I think you should add

4) The risk of CO2 leakage and asphyxiation due to loss of oxygen or injury or death due to CO2 poisoning in any confined space. CO2 is classified as a toxic fluid as it becomes toxic at quite low percent over time which reduces as the percent CO2 increases.

On that basis I would say any pressure of CO2 should require double isolation.

Without being too pedantic, many people think of DBB incorrectly and what they really mean is Double isolation and bleed (DIB)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sawsan311]

Thanks LittleInch for the feedback, the terminology of block in my case is isolation.

We agree that CO2 is highly hazardous specially due to what you have mentioned and its high expansion ratio to atmosphere with significantly high escaping velocity through any passing element i.e. valve, leak, pipe flange etc. I also came across that secondary isolation barrier is needed irrespective to the CO2 pipe size (expressed to be required for hole as low as 8 mm).

Regards,

 

[Compositepro]

I do not really understand the point of this thread. CO2 has some well known hazards having to do with pressure and toxicity or asphyxiation, which applies to the majority of chemical substances. Sure we have to be aware of, and mitigate these, but there is nothing unique to CO2. Every installation must be assessed individually for potential hazards and consequences.

 

[georgeverghese]

In my opinion, the effect of CO2 in the human respiratory cycle is similar to that for N2 - it is merely an asphyxiant. It does not actively interrupt respiration like CO or H2S. At atmospheric total pressure, all it does is to reduce the partial pressure (at high concentrations of CO2) of O2 in the lungs, thus reducing hemoglobin O2 uptake capacity. On the other hand, CO and H2S chemically bind to hemoglobin (irreversibly if I recall correctly).

 

[Sawsan311]

Dears, you are only looking at the general aspects of CO2 being hazardous in terms of acuteness and asphyxiation.

Dense phase CO2 system are quite more complicated than this, there are other thermodynamic characteristics attributed to CO2 being highly cryogenic, cO2 ice formation and the expansion ratio being quite very high. Dedicated literature addressing such unique applications hence YES CO2 processing is Unique.

 

[LittleInch]

George, I don't believe you are correct.

CO2 is toxic. Remember Appollo 13?

It becomes toxic at around 7%

This is a useful guide

https://www.hse.gov.uk/carboncapture/carbondioxide.htm

But yes dense phase co2 is strange stuff but you seem to be aware of that.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Okay, so CO2 dissolves into blood at high concentrations from that last doc in the article (Peter Harper - UK HSE).
Though I wouldnt comment on the requirement for DBB for CO2 containment equipment, we know the risk of solids formation is present with water wet hydrocarbon gas for high pressure letdowns. In fact, the pressures when water ice can form is even lower. In these systems, DBB applies only in pressure classes ANSI 600lb and above.
So, the argument for DBB for pressure classes 600lb and below in CO2 containment systems can only be from a toxicity point of view. CO2 is far less toxic than H2S. In H2S laden streams, DBB applies for all pressure classes.