Gas safety relief pipe size 3

[stanfea]

In the process of designing a gas supply line to a multi fuel boiler. I am providing a safety relief system vented to the atmosphere. The supply line is 8". The relief line however I am not exactly clear on what size it should be. It has been suggested that the vent line will be about 2". I am looking for a code requirement that sizes this vent as I don't believe that it must be designed for the full flow of the line...

All help welcomed
Stano

 

[CRG]

What is the design pressure for the 8" line?
What is the upstream pressure for the line?
What might cause this line to exceed the design pressure?
Is this an industrial facility, and if so, is this a B31.3 design?
If industrial and not B31.3 then what pipe code have you designed to?
Is this a commercial building, and if so, does this fall under IFGC or UPC?
I do not know how to answer your question without additional information.

 

[TD2K]

What is the purpose of the safety relief system? Is it to vent the gas between a pair of block valves as a double block and bleed (this is pretty common on fired equipment to ensure fuel can't enter the firebox accidentally)?

Do you have a spec change on the fuel gas piping coming to the boiler and must protect the downstream piping from poential overpressure?

Something else?

As CRG said, we need some more information what you are trying to accomplish here.

 

[stanfea]

Yes it is a block and bleed. It is in an industrial plant. The pressure is about 3psig, @ 200,000 SCFH. The relief is near to the boiler (25'). The inlet pressure to the new piping system is historically very stable and does not fluctuate, which is irrelavant I realize.

Thanks again
Stano

 

[Guidoo]

Following text is copied from an article by Walter Driedger called "Controlling Fired Heaters". It was published in Hydrocarbon Processing in April 1997. Complete text is available from his website:

http://www.driedger.ca/ce5_fh/CE5_FH.html

According to the table you need at least a 3 1/2 inch vent line. By the way, I would not call this item a "safety relief pipe" but a bleed or vent line. I must add that I checked the latest (2003) edition of NFPA 86 but could not find such a table. Here they only show a manual leak test valve between the main shutoff valves that is used to test whether main valves are leaking, not to vent blocked volume each time main valves are closed...

The vent valve makes certain that there is never any gas pressure on the second shutoff valve despite any leakage through the first one. Since its only purpose is to vent leakage, the vent line has a smaller bore than the supply line. It must be fail-open. The Canadian Gas Association code CAN/CGA-3.9-M879 repeats the following table from NFPA 86A7 for determining vent line sizes:

Gas Supply
Line Size Gas Vent Line Size
NPS mm NPS mm
1½ ( 40 ) ¾ ( 20 )
2 ( 50 ) 1 ( 25 )
2½ ( 65 ) 1¼ ( 32 )
3 ( 80 ) 1¼ ( 32 )
3½ ( 90 ) 1½ ( 40 )
4 (100) 2 ( 50 )
5 (120) 2 ( 50 )
6 (150) 2½ ( 40 )
8 (200) 3½ ( 90 )
>8 (>200) >15 % line cross-sectional area

For low molecular weight fuels such as hydrogen (Mol. Wt. = 2) or methane (Mol. Wt. = 16) it is sufficient to vent the valve to atmosphere outside the building. For fuels heavier than air, such as ethane, propane and butane (Mol. Wt. = 30, 44, 58 respectively), the vent should be piped to a flare header.

 

[stanfea]

Guidoo

Thanks for the reference. I was hoping the line would be smaller but had a feeling it would be a pretty good size. My line is 8" so it looks like I am putting in a 3.5".
I have done a fair bit of reading and many people seem to have the opinion that a DB&B is not an acceptable safety isolation. It seems like an excellent system imo, what are the drawbacks to the DB&B? If the first valve doesn't seal well then the vent is there and the second valve also, seems pretty fool proof? I do agree however that complete blanking or disassembly is always the best by its nature, but the DB&B seems good too. Comments?

Thanks again.

Stano

 

[Guidoo]

Stano,

We are moving away from the original subject, but OK.

DB&B is normally not considered safe (positive) isolation in case you have to enter a vessel or a tower. Some operating companies accept it only if other ways of isolating (e.g. spectacle blind, spade, removable spool) is not practical and if the DB&B is clearly tagged and locked. Other operating companies do not accept it. Reasoning is that chances are too high that someone may open the valves without knowing that someone is inside the vessel.

In your case (fuel gas lines to heater), the reason for the DB&B is to prevent leakage of gas into the heater box that could result in an explosive atmosphere inside the heater box. Here I have always seen the DB&B as described in the article from Driedger. In case the heater is stopped, the two main valves close and the valve in the vent line is automatically opened. Only in case of small pilot gas lines I have sometimes seen that only the double block valves are used, without the vent.

Now there is a more general discussion about how to use the bleed in the DB&B: Is the bleed used to remove pressure on the second block valve (in which case it should be open when the block valves are open and hooked up to safe location), or is the bleed used to frequently test whether one or both block valves are leaking (in which case the bleed is normally closed and only opened to do the leak testing of the block valves). Reasoning of the second possibility is that if you frequently test the valves, the likelihood that the two valves in series will fail is very small, in other words: a tolerable risk.

It seems like NFPA 86 now recommends to use the bleed is a leak testing device instead of a way to remove pressure from the second block valve.

A few weeks ago we did some research to see what the generally accepted method is to use of DB&B and found that there is no such general acceptance (e.g. between operating companies or international standards).