Should a bypass valve be provided in a City Gate Regulator Station? 2

[TexasPE]

In a natural gas city gas regulator station with two parallel regulators, one primary and one secondary both with backup monitor regulators for overpressure protection, should a bypass be allowed? The station is taking gas from an 800 psig MAOP pipeline, and after regulation down to 60 psig and then after odorization is delivering it to two systems. One has a set of meters for an industrial customer and the second to a stub out for a low pressure polyethylene pipe LP distribution system. There is no relief or high pressure shutdown being proposed on the system so it is strictly relying on the dual regulation to provide overpressure protection. It is my belief that the addition of the bypass should mandate either a front end high pressure shutdown if the back end gets above the MAOP of 60 psig. While it appears the DOT regulations and ASME B31.8 code does not mandate this, it would seem that sound engineering would. Operating with a bypass open would make any overpressure protection by the regulators effectively non-existent. Can I get some discussion on this?

 

[1503-44]

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Gas Main Explosion in Massachusetts- Update
thread815-466119

 

[alchemon]

CFR 192.199 and CFR 192.201 do not say that the device needs to be “automatic”. All a regulator is, is an “automatic” valve that opens and shuts. Instead of a pilot doing it, a human does it.

Please go back and read my original post which said to double valve with threadolets for venting. This meets the intent of the code and is functionally the same as a worker-monitor set-up with internal reliefs. I also said in original post to consider an additional OPD.

The emphasis on the last post for an additional OPD was that you, as the design professional, need to meet your duty to the public and your client. So if they want this, you should try to make it work - which may mean adding it. The extra OPD can be a waste of money though.

If you don’t like this, don’t design it that way. But I have had multiple stations installed across the US with this arrangement and the PUCs all agreed with this interpretation.

 

[LittleInch]

Alchemon,

I've gone back and reread our posts.

Now maybe when ou say "double valve" in your head you may mean double control valves, but then also say " well ball valves aren't very good at pressure control", so maybe not.

You do seem rather obsessed with instances where the inlet pressure into the regulators becomes less than its set outlet pressure, but I'm still not sure why a full open regulator is that much worse than full open bypass.

Anyway whether the regulator is human powered or some other actuation method, the wording and intent of 192.195 seems pretty clear to me.

Part b 1 and 2 makes it clear that you need to design to prevent over pressure and that the device "could be activated in the event of failure of some portion of the system". Now to me that means the OPD or slam shut isn't some sort of optional thing you imply, but is in fact a mandatory requirement. Or a full flow relief valve.

I'm quite sure there are many such systems which hopefully never get used in anger, but the fact they are there creates a risk that they will be used and not in the manner you thought of. And that's the issue here - as designers we can't mandate against operators doing things that you didn't allow for. If the bypass wasn't there they can't open it.

I've lost count of the number of times in a design I've removed bypasses around control valves, had the argument " but we always do it this way", discussed it in a HAZOP or a technical meeting and always had them removed or had significant additional safety features added (e.g. ROs, over pressure trips and slam shut valves or relief valves).

So Texas PE - I think the discussion has run it's course - what do you think?

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

 

[alchemon]

it is a manual bypass my friend - closed 99.999999% of the time. It is activated and controlled manually by a human. The human uses the device to prevent over pressure protection. I fully agree that you must protect the downstream, but the code does not state that the device/method for so doing must be “automatic”.

Now whereas if a vandal were to operate it - that is a different question. They do make locks for such purposes. Lock the manual bypass closed.

I appreciate the otherwise kind words, and I agree too we circled this one as many times as we can.

 

[1503-44]

Part 197 requires a "service regulator". That is so obviously not a human being operating a bypass that I hope we don't have to go down that rabbit hole any further.

Alchemon, you have left many questions outstanding about your previous post. I'm not sure of the value of going back to those. As far as your PUC reasoning goes, the PUC's, if they do any design review at all, often approve poor designs that do not meet CFRs, or even basic good engineering practice, as do a number of operating companies. Most PUC members are not engineers at all. Many "pipeline companies", especially in the gas gathering sector, do not have an engineer of any kind on board. I once arrived to work for the largest gas producer in south Texas, bigger than EXXON (in terms of maximum sustainable daily gas production rate), to find out I was the only degreed engineer working there at all. 850 wells, 2000 miles of gathering pipelines and a 300 mile, solely intrastate gas transmission pipeline with 3 compressor stations. They hadn't had an engineer working there for more than 10 years. All of which are causes of what got us here where we are today and is precisely why the NTSB's made their new recommendation to require PE reviews. They recognize that those old practices must change.

Ya, I'm finished.

 

[TexasPE]

I want to thank everyone for the candid discussion. It looks like for now the plans are on hold. The OPS has an interpretation that is about 30 yes old and a bit outdated so I have asked for another. I tend to agree with the requirement for a relief or a HP shutdown if the bypass is being operated. I think that 192.195 mandates it. Maybe the answer is to require double block valves with the Overpressure protection Device inside those block valves for a bypass. Then if both are open and the bypass is being used the upstream valve would be used to throttle for a relief. Or if it is a hp shutdown, monitor the downstream and use the downstream valve to throttle. just a thought.

 

[TexasPE]

I would add I believe the code(ASME B31.8) and dot regulations (Part 192)do require all above ground valves to be locked.

 

[1503-44]

Thought I was outta' here, but OK...

I haven't looked at that section lately, however such valves used to require "protection". I belive that is the key word to look for there. It used to be anyway. If the valve location is not fenced (protected) for some very good reason, a lock on the valve wheel, lever, etc. (chain-padlock wrapped around such as to immobilize a handwheel or the lever) would suffice. Protected can also mean just having a fenced site with lockable gates, typically a minimum of 2, on opposite sides of the valve, and perferably the plot too. In that case the valves would e protected by the fence and not need locks themselves. All lockable gates must posess emergency opening apparatus (panic bar, etc.) operable from within the plot's interior to eliminate the need for fumbling around for a keys, should an operator (human being type) be inside the fence and wish to make a quick get-a-way.

 

[1503-44]

PS, I'll look forward to seeing a diagram of the final arrangement, if you would be so kind.