City Gate Station Bypass

[Feedyourhead]

Thought this thread discussing gate station bypasses was interesting.

thread378-465800

So let me ask:

Should natural gas gate stations have a manual or semi-automatic bypass? What sorts of designs would constitute a sound bypass run? A bypass with OPP, no OPP? A manual or automatic throttling valve? What kind of valve?

What are you going to do when the transmission pressure drops to or below the MAOP of your system? Should sound gate station design include some sort of fail-safe mechanism for when the transmission inlet pressure drops too low? What other fail-safe devices could be used except for a bypass? Is on the spot tampering with regulators to reduce the pressure loss across them justified in emergencies? What obligations do engineers have to the public to continue service under such circumstances? Consider that now people are going to freeze if you lose service, which could lead to deaths -- but people also might die if you overpressure the system. Though, downstream district regulator stations are likely to catch any overpressure in the system. Frostbite or hypothermia could become a risk after 12 or 24 hours without heat. Should peakshaving facilities be utilized before attempting to use a bypass run, or should a bypass run be utilized before peakshaving facilities?

What sorts of operational procedures should be written for the safe use of a bypass run?

 

[georgeverghese]

Humans WILL err at some time or other, no matter how elaborate operational procedures may be - you HAVE to make provisions for such eventualities.
There are rare special circumstances when operational procedures alone are used without automatic safeguards. Such arrangements are major operational risk hazards and must have strong economic and operational justifications. Local govt authorities would have a stake in decision making also when major risks to the general public are implied.

 

[1503-44]

Manual. This should be activated only for gate station maintenance purposes only and closely monitored (human eyes on) when doing so. A manual valve, or auto, with another manual shut off valve might work. I'd tend to manual, since this should be a closely supervised operation.

When feeding to local distribution, it is very important to maintain at least the absolute minimum system pressure. Active flow at any lower pressure is a "black start" or an "initial fill" procedure" which must be closely monitored to prevent possible explosive gas-air mixtures from backflow into the system, or during air purging.

When transmission pressure falls below absolute minimum supply, shutoff is the only alternative. That would of course be after at least one, if not several, low pressure alarms sounded at central ops.

When you run out of gas (below abs min pressure), you run out of gas. All other alternatives reduce to the prevention of that event. A connection to an alternate city gate supplied by a competitor is the typical solution, from which feed to the grid is guaranteed, or restored.

Various criticality levels of guaranteed service are typically set with gas purchasers for interruptable, or non-interruptable service. Normally local emergency officials coordinate supply with the gas distributors and will call for shut down of various industries, office complexes first, acvording to their contracted service level, then ladtly to private homes. Hospitals and other emergency service providers will be the last to shut down.

Some customers prefer to build their own storage facilities, or arrange butane tank bullets, trucked in temporarily, or in other cases they might contract gas storage with private storage operators that will store gas underground, typically in a depleted gas field, all depending on what facilities are available in their area. Sometimes these are operated by the gas transmission companies themselves. A number of gas companies have large LNG Spheres they use for peak shaving. Some just operate by increasing their pipeline pack pressures a few days before they expect high demands. All of that depends on what levels they are focused on supplying and what criticality level any customer might need, ranging from schools and hospital, local mfgr plant, electric generators, local distribution company, several of them, or an entire region New York, Denver or Chicago.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Feedyourhead]

At 3 am when shit hits the fan good luck getting ahold of governing agencies for their stake in the decision making. I don't know what makes them competent in this situation because they don't know your system.

There is no need to provide a bypass for maintenance because there should already be more than one regulator run. So you just shut off whichever regulator run you are maintaining and use the other during the summer time.

Connection to an alternate city gate is not an option because there are none. Neither will the government allow any more transmission pipelines to be built. They have a guaranteed delivery pressure to the gate station but that doesn't mean its a 100% certainty. The pressute has dropped below the minimum guatantee before. There are a number of excuses they could make like a failed compressor station.

If we are beginning to shut off customers then that is really our call not local emergency authorities. But yes that could be a viable option. You shut off some customers to save the gate station.

 

[LittleInch]

The job of a gas distribution company is to distribute gas.

It needs to do so for as long as possible given that it can't generate its own gas it needs to use the supply it has been given.
At the same time it needs to operate at or below the MAOP of the downstream system.

There are so many different ways of doing this that it is not possible to answer your questions other than you need to take all reasonable steps to ensure continuity of supply without endangering the system.

Most systems I see have two dual 100% regulator streams with main and standby regulators in each stream. These regulators tend to be self contained gas powered. This avoids failure in the event of loss of power for a long period.

I have seen some manual bypasses in the form of several small tubes with orifices and then someone watches the pressure and turns them on and off as required as the flow varies. Even then I think there was a gas powered slam shut to prevent over pressure and I would be very wary of any manual system which didn't include this. Overpressure would probably be higher risk than someone dying of cold. Having your house explode is different from having to huddle around a fire or electric heater. Or just driving somewhere where there is some heat.

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

 

[GBTorpenhow]

The one station bypass I designed before mostly escaping the gas transmission and distribution world was explicitly intended for use if the main gate station, which fed two large metropolitan distribution systems at different pressures, was ever completely knocked out by a fire, terrorist attack, meteorite impact, or other catastrophic incident.

The bypass station was located in a separate fenced enclosure a prudent distance away from the station proper and consisted of one run for each distribution system with a manual globe valve with locked closed upstream and downstream ball valves. For OPP, the downstream distribution systems were each provided with a full capacity pilot operated relief valve battery.

ETA: Inlet pressures were something like 750-800 psig MAOP with an actual range of 450-700 or so, the distributions systems were 60 and 35 psig MAOP.

 

[1503-44]

Yes, there are often two feed regulators, but not always. If you have one, then a bypass can be handy.

Some areas have more than one transmission line in the area and they might have feeds from both sides of the city, others 3 or 4. Depends where uou are located and everybody's gas demand. All areas are somewhat different. Some areas have no gas, except bottled or bullet LPG tanks. Trailer parks often have their system feed off of bullets.

Service levels are established in the gas sales contracts. Order of supply is as I said. If that doesnt work for you, get bottled gas backup.

They can't go below minimum pressure. That is a dangerous situation. Restart of pilot lights are supervised operations and involve house to house. They will shut you down at minimum, rather than go to lower pressures. Nothing else can be done in that regard.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[davefitz]

Hmmm. I seem to recall someone opened such a bypass during modification of the distribution lines in the Boston area a few years ago, and by bypassing the pressure regulating station they blew out all the low pressure consumer regulators and filled a couple of houses with natural gas, and killed a few people.Supplying and using such a bypass has consequences. While the designer of the system may assume that the bypass is only opened by a competent operator who would manually prevent overpressure,, there really is no guarantee that the assumption is correct.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[LittleInch]

I think the Boston one what happened was that they isolated the section of line that had the pressure tapping on it which fed back to the regulator a false downstream signal which the regulator then kept opening further and further trying to increase pressure base don this false reading?

But yes, high pressure will kill people probably more easily than getting a bit cold.

Loosing gas supply to a distribution system is just terrible news for the company involved so extreme measures may be justified, but do as much as possible to avoid manual bypasses or pressure control and even then protect against over pressure.

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

 

[1503-44]

Correct. 1 person died and they did a billion of property damage in the town. Plus Scared hell out of my 84 year old auntie.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Feedyourhead]

Yes the Merrimack Valley incident was due to improperly placed sense lines not a bypass. But there are also incidents on record where people have died in the cold.

"It’s been a year since millions of Texans woke up to find their power cut during the freezing cold. What followed were days without heat, electricity or water. Hundreds did not survive. But, on the anniversary of what became the worst blackout in Texas history, some say many of those who died have yet to be officially recognized.

On Dec. 31, the state’s Department of Health and Human Services released its final report placing the number of dead at 246. The study aimed to include deaths caused directly by the storm (such as hypothermia), as well as “indirect deaths” — people who died later of injuries related to the blackout and freezing temperatures."

There is danger to human life no matter which way you slice it.

"They can't go below minimum pressure. That is a dangerous situation. Restart of pilot lights are supervised operations and involve house to house. They will shut you down at minimum, rather than go to lower pressures. Nothing else can be done in that regard."

well they didn't discontinue service. I'm not sure if they would. I don't know if the company is doing a bad job with litigation or if the transmission company just doesn't care. but litigation can't save you in the moment.

it seems like the issue of underpressure at gate stations is mostly overlooked and yet it has serious consequences to public safety. the natural gas industry is being squeezed by politicians and it is going to start happening more and more as transmission pipelines are booked to capacity and protestors block new sources of supply.

 

[1503-44]

It is important that people depending on gas for survival have their own backup arrangements in place, whatever those may be. I would not depend on anybody for survival, especially a public utility. LPG cans and batteries. As many as you can afford. Or get the town to arrange emergency facilities and transport for any potential extended power outages. Stay safe.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

I meant local Govt authority involvement during the design phase development, especially at design phase HAZOP and Operational Risk Assessment. That way, the gas distributor company wont find themselves high and dry in case things go wrong if the the root cause is some design deficiency.

 

[1503-44]

Locally that's been limited to obtaining zoning and building permits. Technical details are per CFR/PHMSA requirements, administered through the various states oil and gas or public utility commissions.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

@1503,
At some of the larger OpCos, it is standard procedure to get the relevant Govt authorities involved during the concept design phase development whenever the design concept implies a safety or health risk to the public. These risks should be clearly described in the terms of reference for the Hazard Review.

 

[1503-44]

That's good. Glad to hear somebody is doing that.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Feedyourhead]

Ya that could be helpful but certainly regulators are not infallible. They still make mistakes too. And unless you already know the regulator that would be involved in your design then they could just come in with a bunch of bad ideas and then you have to do it because they are the regulator.

I don't know if you have heard the saying too many cooks spoil the broth, but you know what I mean. Also: better the devil you know.

There is always a chance that the regulator could be really brilliant and then they could be helpful. Its a good suggestion, I am just pointing out the cons.

But at the end of the day regulators have the same level of competency as everyone else.

If all engineers do is blindly rely on regulations and codes and standards, or government officials, then they could find themselves putting the public at risk if the regulation or standard they are following is flawed. Regulations are always evolving because they aren't perfect.

In fact I can think of several instances where regulators have forced my company to put people at risk because they are more concerned about following every regulation to a T and implementing narrow-minded safety regulations to have enough oversight to see how that will affect the entire system. For example a regulator could require you to repair a leak in the middle of winter because they want it done in x months, but there may be no way to get the construction permit in that amount of time depending on the location, so the only way is to retire the pipe, but in doing so you endanger the integrity of your system.

So rather add this scenario into your HAZOP: the regulator is narrow-minded, isn't a good engineer, doesn't correctly interpret regulations, and is too politically influenced. Surely there are at least some regulators like that somewhere in the country.

 

[1503-44]

All in all IMO usually the regulating agencies have a sufficiently high level of competence, are receptive to logical arguement and design that follow codes. I've never had a problem. It is the inexperienced client engineers that give me the most grief, but they always fall in line and agree once they understand why I want to do something "my way". I usually also understand why I have to do the occasional thing or two their way.

You normally don't need construction permits to make repairs, especially emergency repairs, the thing is already built. You may need approval of your repair method from time to time, which is normally forthcoming without significant delay.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Feedyourhead]

Ya I'll have to get to know my regulators better.

It might be difficult or impossible to get the permits you need in time if the pipe goes under a highway, railroad, over a bridge especially if it is condemned, or if you need to obtain easements for an alternate route.

 

[1503-44]

Permits can be troublesome for pipelines crossing aquifers, Applicants Trails, Sacred burial grounds, sacred trees (big in Africa and Asia)), rivers, navigable rivers, recreation areas, scenic views, wildlife refuges and national parks, historic sites and monuments, wilderness areas, national and state parks, reservoirs, native tribe reservations, environmentally sensitive sites, densely populated areas and anthropological sites as they should be. Absolutely void moving people and further endangering any endangered species. However if you do a good job routing the pipeline, these areas can usually be avoided. Adequate time has to be included for routing work. Just do not ever thing about having a a straight pipeline to begin with and you won't be too disappointed. I've found I can get most of what I need to do done with a relatively small amount of additional pipe and adding that pipe is worth it in the end, as you don't have to battle out the route in court for 5 years while the project gets balled up in court and withers away or flat out killed or by with year on year budget increases and political wax balls that scare off all the financeers. The practice of some pipeline's CEOs just drawing straight lines and crossing whatever lays ahead at any cost attitude is in most cases totally ridiculous. The best pipeline companies understand the landowner-pipeline relationship as a long lasting partnership. If you've seen what bad pipeline companies do to landowners, you won't be surprised by the resistance that is often raised.

Good route planning is most of the up front work for a long project cannot be rushed without severe consequences. For short pipelines, a county road crossing or two, or even a railroad, if you get their specs and do what they want in advance, isn't going to be too much of a problem, but that does tend to rise exponentially with length of pipeline.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."