Environmental and Safety Considerations of Double Block and Bleed 1

[sshep]

My Friends,

A few days ago I was suprised to discover that most of my project's environmental technical staff did not realize that the use of automated double block and bleed (to atmospheric "safe location") systems was normal on fired heater fuel gas trip and interlock logic. This configuration is so common to me that I cannot remember ever having seen it any other way.

The issue came up because the project wants to use a similar configuration on fired heaters some other safety and isolation systems. Naturally the environmental concern is the impact of leaking valves as fugitive emissions, and potential safety concern if the "safe location" is used liberally (as it often is) to be a local vent. When the line is flowing the emissions concern will be through the single closed bleed valve, and when the system is blocked in through either block valve to atmosphere via the open bleed.

Now I am struggling to explain how something so common in my experience as a safety feature is not an environmental concern. Do you guys have any references which can help me on either the safety requirement (NFPA, API) or environmental side of the issue?

best wishes,
sshep

 

[bimr]

Your comment identifies a lack of experience for your environmental staff.

In general, the NSPS and other air pollution regulations does not apply emission limits during startup and shutdown. The operating conditions exempted are listed in the environmental operating permit. Any safety requirements will also be exempted.

Suggest you obtain a copy of the permit and review it.

 

[MJCronin]

The safety importance and state requirements for fuel gas trains covers boilers, fired heaters and thermal oxidizers.

The fuel gas devices and trains ARE NOT DESIGNED BY THE PROCESS ENGINEERING STAFF

They must be of a certified design, accepted by the state authorities and:

There are several organizations such as UL (Underwriters Laboratory), FM (Factory Mutual), IRI (Industrial Risk Insurers), etc, that publish requirements for the various components which make up a fuel train for specific burner output. Also, several codes and standards such as NFPA and ASME publish requirements for the entire assemblies.


Boilers, furnaces and heaters BLOWUP and PEOPLE DIE when the rules are not understood.

The national board has a good (but dated) discussion of the organizations and the rules:

http://www.nationalboard.org/index.aspx?pageID=164&ID=204

MJCronin
Sr. Process Engineer

 

[sshep]

Thanks,

This was useful but i will gladly take further comments and reference.

To clarify, I see that the first sentence of my second paragraph has a typo. It was supposed to say "... use a similar configuration to fired heaters on other systems ...". Basically the project envisions an interlocked block and bleed to positively cut out a process gas, and cut in purge gas in a trip; and then reverse in the restart sequence. The application would not be fired equipment- that was only a common example of what is envisioned.

Incidentally, we never design any fuel gas detail in house. We always specify fired equipment as a package from qualified vendors.

My subsequent research has found several documented cases where these style vents have caught fire due to valves leaking through in fuel gas service, so leaking valves is a real possibility.

Best wishes,
Sshep

 

[davefitz]

Interesting topic. The same "double block and bleed" rules should likely be applied to some relatively new technologies that have flown in under the radar in the last 10 -20 yrs. In particular, the combined use of fuel gas performance heaters on combined cycle plants and the use of air cooled condensers at the same plants has introduced a risk for LeL events ( lower explosive limit events , AKA fuel-air explosions), mainly during outages. One such event occurred in March 2012 in Mexico, one death, and about $100.E6 USD damage.

"Whom the gods would destroy, they first make mad "