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Old 19th September 2018, 09:02 PM   #41
cmikes
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Originally Posted by Trebuchet View Post
(OT) I used to design control systems for commercial airliners. Never had anything I did go at all seriously bad, but the thought was always there. I did wind up having to fix some problems that had had serious consequences.

Scott Adams said in one of his books that every engineer's goal (at least the ones he knew) is to get through their career without causing, or being blamed for, a disaster.
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Old 20th September 2018, 10:31 AM   #42
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Apparently there was some over pressure in some lines. Sounds like the pressure control senser was on a line they cut out, so the system jacked up the pressure in the rest of the system.

But still, have you ever seen the way a regulator is designed? Pretty failsafe, excess input pressure pushes the valve shut. So no, no excess pressure into homes. But an underground pipe that leaked due to high pressure could make the sewer system a bomb.

Two facts- 1) A second underground leak was spotted by a police helicopter. 2) all those open manhole covers? Blown out by the explosion.
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Old 20th September 2018, 12:03 PM   #43
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Originally Posted by casebro View Post
Apparently there was some over pressure in some lines. Sounds like the pressure control senser was on a line they cut out, so the system jacked up the pressure in the rest of the system.

But still, have you ever seen the way a regulator is designed? Pretty failsafe, excess input pressure pushes the valve shut. So no, no excess pressure into homes. But an underground pipe that leaked due to high pressure could make the sewer system a bomb.

Two facts- 1) A second underground leak was spotted by a police helicopter. 2) all those open manhole covers? Blown out by the explosion.
"Pretty failsafe" is why there were less than 100 fires. Well over 99% of them functioned well enough, even if they were old. It would not be surprising to find that a few did not in such a large system.
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Old 20th September 2018, 02:28 PM   #44
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Originally Posted by Dr. Keith View Post
"Pretty failsafe" is why there were less than 100 fires. Well over 99% of them functioned well enough, even if they were old. It would not be surprising to find that a few did not in such a large system.
If they were able to fail under a 6 psi* overpressure, they would not have sealed in usual use- the stove would have been a blast furnace. They way they work is excess pressure closes the valve, the more pressure, the more tightly. While a corroded seat might leak, making the blast furnace, an overpressure on the inlet side might even fix the leaky seat.

* Latest news is "a 12 times normal pressure, 6 psi instead of the usual 1/2". But that was not in someones house even, it was in a main line somewhere. IIRC, single digits is under the street, the regulator to 1/2 psi is at the big meter at your house. The big cylindrical thing is the meter, the round disc is the regulator. The large diameter means it is for low pressure.

So, the line pressure caused a leak into the sewer. The triggering explosion was in the sewer, which blew gas into the houses through the sink, tub, toilet drains.

Has anybody heard mention of how old the buried pipes are? A hundred years maybe? They were getting worked on...
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Old 22nd October 2018, 12:22 AM   #45
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Evidently the NTSB released a preliminary report about the cause of the fires. It very much indeed was overpressure in the pipes.

And it was a result of the workers replacing the old local .5 psig service mains with new ones. They successfully switched the gas flow from the old mains to the new ones; however, they neglected to disconnect the old mains' pressure regulation sensors from the system and connect the new mains' sensors in their place - possibly because the work manual for the job from Columbia Gas did not include any mention of the sensors or instructions for replacing or reconfiguring them.

As the old, disconnected mains began to lose pressure, the control system saw that data and began opening the regulator valves between the new .5 psig service mains and the 75 psig system distribution mains to compensate - more and more, as the sensors continued to show falling pressure. According to the NTSB report, at one point the valves were fully open, meaning there could have been up to 75 psig in the local service lines. Needless to say, that was substantially more pressure than the service main or customer connections were designed to deal with.

The older gas system in these particular areas did NOT have pressure regulators at the customer connections or meters; pressure was regulated at the main by the company only. So when the line overpressured, all of that pressure went unchecked directly into peoples' houses. There was no leaking of gas into the sewers. Or perhaps some did leak there due to rupturing lines, but that was not where it ignited and the explosions in buildings did not come from sewer lines, they came from the actual gas appliances.
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Old 22nd October 2018, 10:34 AM   #46
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Originally Posted by Checkmite View Post
Evidently the NTSB released a preliminary report about the cause of the fires. It very much indeed was overpressure in the pipes.

And it was a result of the workers replacing the old local .5 psig service mains with new ones. They successfully switched the gas flow from the old mains to the new ones; however, they neglected to disconnect the old mains' pressure regulation sensors from the system and connect the new mains' sensors in their place - possibly because the work manual for the job from Columbia Gas did not include any mention of the sensors or instructions for replacing or reconfiguring them.

As the old, disconnected mains began to lose pressure, the control system saw that data and began opening the regulator valves between the new .5 psig service mains and the 75 psig system distribution mains to compensate - more and more, as the sensors continued to show falling pressure. According to the NTSB report, at one point the valves were fully open, meaning there could have been up to 75 psig in the local service lines. Needless to say, that was substantially more pressure than the service main or customer connections were designed to deal with.

The older gas system in these particular areas did NOT have pressure regulators at the customer connections or meters; pressure was regulated at the main by the company only. So when the line overpressured, all of that pressure went unchecked directly into peoples' houses. There was no leaking of gas into the sewers. Or perhaps some did leak there due to rupturing lines, but that was not where it ignited and the explosions in buildings did not come from sewer lines, they came from the actual gas appliances.
That's what I call a major screwup. That gas company is probably going to get sued into bankruptcy. I've never seen a gas hookup without a regulator in series with the meter. Usually individual appliances have regulators too, but I don't think they're designed to handle that much overpressure, and even if they are, it would only take a small leak to deliver way too much gas.

ETA: I also wonder about the electronic pressure control. Mechanical regulators are pretty reliable, though a leaky valve or diaphragm can cause them to fail. I would think some redundancy (maybe both mechanical and electronic systems) would be desirable.

Last edited by CORed; 22nd October 2018 at 10:39 AM.
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Old 22nd October 2018, 11:06 AM   #47
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Originally Posted by CORed View Post
That's what I call a major screwup. That gas company is probably going to get sued into bankruptcy. I've never seen a gas hookup without a regulator in series with the meter.
Indeed. I guess it turns out most modern systems have relatively high service pressure all the way to the customer's connection, and it's the customer's meter that provides all of the pressure regulation. This older system must not be very common anymore.

Originally Posted by CORed View Post
ETA: I also wonder about the electronic pressure control. Mechanical regulators are pretty reliable, though a leaky valve or diaphragm can cause them to fail. I would think some redundancy (maybe both mechanical and electronic systems) would be desirable.
You would certainly think so. This regulation system seems to have been quite poorly designed. Again, from the NTSB report:

Quote:
Minutes before the fires and explosions occurred, the Columbia Gas monitoring center in Columbus, Ohio, received two high-pressure alarms for the South Lawrence gas pressure system: one at 4:04 p.m. and the other at 4:05 p.m. The monitoring center had no control capability to close or open valves; its only capability was to monitor pressures on the distribution system and advise field technicians accordingly. Following company protocol, at 4:06 p.m., the Columbia Gas controller reported the high-pressure event to the Meters and Regulations group in Lawrence. A local resident made the first 9-1-1 call to Lawrence emergency services at 4:11 p.m.
So somewhere on this system, there WAS a sensor that did accurately detect an overpressure in the service lines...and triggered an alarm in a monitoring center in Ohio, with the people monitoring that information having to make a phone call to the techs in Massachusetts to inform them about it. For some reason, the automatic pressure regulators in the system itself were not tied in to that sensor's data or alarm conditions. If they had been, perhaps a safety program could have directed a shutdown or override of the bad underpressure readings from the disconnected main.

I'm not a gas system engineer; but I can think of reasons why a very simple and unintelligent "pressure low, open valve/pressure high, close valve" pressure regulation system might not be ideal for something like flammable gas service when there are alternatives available. I don't think I would ever have imagined a scenario like this (where the perfectly-working pressure sensors somehow find themselves not measuring system pressure anymore); but it's a general rule of thumb in pressurized systems that ANY falling-pressure condition indicates a leak somewhere; and in the case of, say, a ruptured gas main, why would you want to start pushing more gas into that line?
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Old 22nd October 2018, 11:27 AM   #48
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Originally Posted by Checkmite View Post
I'm not a gas system engineer; but I can think of reasons why a very simple and unintelligent "pressure low, open valve/pressure high, close valve" pressure regulation system might not be ideal for something like flammable gas service when there are alternatives available. I don't think I would ever have imagined a scenario like this (where the perfectly-working pressure sensors somehow find themselves not measuring system pressure anymore); but it's a general rule of thumb in pressurized systems that ANY falling-pressure condition indicates a leak somewhere; and in the case of, say, a ruptured gas main, why would you want to start pushing more gas into that line?
I can understand that such a system would be good for dealing with variable demand that may spike at certain times of the day, but yeah, there needed to be a failsafe system if the pressure sensors were not seeing the increased pressure.
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Old 22nd October 2018, 11:51 AM   #49
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Maybe someone in the local union accidentally caused this. Accidents happen. Lawrence is a nice town. It would be a shame if there were to be another accident.
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Old 22nd October 2018, 12:06 PM   #50
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Scraped a thin layer of ice off my car window this morning, just south of Boston. People without gas heat must be very unhappy right about now.
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