Follow-Up Questions from “Manhole Explosions” Webinar, Part 1
The below questions and answers were generated as a response to the webinar, “Manhole Explosions Part I: Why They Occur”. Some of the questions we answered during the live webinar, but additional answers have been provided for elaboration or clarification. Don’t miss Part II of this webinar on Tuesday, November 6, 2018 to learn more about how to stop manhole explosions.
Causes of Manhole Explosions & Events
1. What are the origins of these gases or rather why would we find them in manholes?
Slides 11 through 30 of the webinar explain the origins of the explosive gases in manholes. You can see the on-demand version of the webinar here.
2. Speaker asserted that arc temperature starts at 30,000 F. It was my understanding that arcs start around 3,000 F and can go up from there. Can the presenter give a source for the “starts at 30,000 F” figure?
Per the Wikipedia definition:
Arc flash temperatures can reach or exceed 35,000 °F (19,400 °C) at the arc terminals. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extraordinary force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion (see calculation below). The result of the violent event can cause destruction of equipment involved, fire, and injury not only to an electrical worker but also to bystanders.
3. It always surprised me how much copper expands when it vaporizes. Is this expansion a significant factor in the event of an arc event in the manhole? Copper expands 67,000 times when it is converted from solid to vapor.
As described in the webinar, a secondary chemical explosion that results from a primary or transmission cable arc flash is a relatively small event, because the total mass of fuel created is relatively small. Atomic copper, however, is part of that fuel and when mixed with oxygen it will contribute to the smallish chemical explosion.
4. Carbon burning—like an explosion in a grain silo? All the particulates burn?
Carbon readily burns. Coal, for example, is primarily carbon. Coal dust and grain dust explosions used to be commonplace before industry learned to effectively prevent their occurrence.
5. Has the transition to poly and rubber cables from lead sheaths increased the frequency of manhole explosions?[from live webinar] I don’t have any data, but lead sheaths fail and then the cable leaks oil. Discharges occur as oil is lost. [appended] There is no historical data available. Even contemporary data is incomplete as incidents are typically underreported.
6. You used a term, “burp”. Our linemen use that term when a joint makes that noise, but does not fail…Is that a result of a hydrogen buildup in the molded rubber joint? Or does the hydrogen dissipate too quickly for that to be a cause?
Hydrogen gas burbs out the cable-component interface when the internal pressure exceeds the component hoop strength.
7. Are networks (as opposed to radial feeds) at considerably more risk for explosions?
Old cables with high loads or nearby steam lines are most likely to develop cracks in the insulation that allow surface tracking to occur. Surface tracking, exasperated by salt water, is the primary cause of explosions.
8. Event 13 looked like it had the most extreme peaks of O2 vs CO2—can you elaborate on that event or is that a statistical outlier?
Event 13 is notable in that it reached the highest peak burn rate. Compared to the 7.5 g/min peak burn rate of Event 6b, Event 13 peaked at about 25 g/min. Event 13 lasted about 2 hours, while event 16 lasted about 24 hours. The aquarium manhole enjoys active ventilation and hence there was never a chance of explosion for any of the identified events. If the aquarium where not actively ventilated there would have been a greater chance of an explosion associated with Event 16, because the total gas produced over its 24 hours is about five times that of Event 13. The two events are reproduced nearby with similar scales. The June 28th event may have been tidally extinguished.
9. How long do these combustible gases accumulate prior to an explosion?
Flammable gases accumulate over the course of minutes or several weeks where active ventilation is not employed. When active ventilation is employed, flammable concentrations can never accumulate. See Part 2 of this webinar to learn more.
10. Did you say HV terminations will eventually fail? If so, would failure be electrical or thermal?
All solid polymeric surfaces in interfacial compression creep away from the interference fit over time. This creep is faster at higher temperatures. Eventually a gap will develop between the two surfaces and electrical tracking will commence. Thermal failure can happen anytime excessive current is applied and this failure mode is largely independent of the tracking, except that higher temperatures accelerate polymer creep.
11. If you have a fully flooded manhole and you pump the water out can you raise the possibility for explosion?
A flooded manhole cannot accumulate explosive gases, so there’s no explosion possibility. When the water is pumped out, the space inside is largely being backfilled by makeup air from the surface. If a cable in a conduit is experiencing or begins to experience arcing/failure, the gases generated would begin to enter the manhole when the water level dropped below the conduit level. If the intention for people to enter the vault, copious fresh air is being supplied from an external blower that would prevent flammable gas accumulation. Smoke from a substantial duct fire might be visible to the crew.
12. Would steam leaking into a manhole provide sufficient enough heat to degrade power cables?
Yes, we have recorded temperatures approaching 100°C which is the temperature of steam at atmospheric pressure. XLPE cables generally have a design operating temperature of 90°C, but the manufacturers never intended for these cable to operate at that maximum temperature over their entire lifetimes. Further is the air temperature being 90°C, the conductor and insulation temperature are necessarily at a higher temperature unless their load is zero. EPR cables have a higher design temperature, but whatever the design temperature, the rate of oxidation approximately doubles for each 10°C and hence of all cables will be reduced by high temperature operation.
Solutions for Explosions & Events
13. Can sensors effectively warn of events?[from live webinar] Sensors are a bit of a double-edged sword. Carbon monoxide, carbon dioxide and hydrocarbons are common in the underground vault environments. Sensors alone may result in false positives or what the fire industry calls ‘nuisance alarms.’ For example, the sensors will indicate a change in the carbon dioxide or carbon monoxide levels, resulting in the deployment of fire trucks and other costly and limited resources. There might be action needed, but the fire crew was not necessary. Plus, it might be difficult to pinpoint where the particular problem originated. There are strategies that are required to stop these nuisance alarms or false alarms—one of the most important being continuous active ventilation; as it solves both the detection problem, the false alarm problem and stops the explosion from occurring.
14. My agency uses two vent stacks per vault with equipment installed, typically located near the ceiling at each end of the vault. How would vent stacks impact the probability of vault explosion?
Please attend part II of this webinar series. Please join us on Tuesday, November 6th at 11 a.m. Pacific / 2 p.m. Eastern for the second webinar in the series. Register now: Manhole Explosions Part 2: How to Stop Them.
15. Does duct seal to prevent water intrusion impact explosion probability?[from live webinar] Sealing ducts is a complicated subject. At the surface l, there are pros and cons. To answer the first part of that question; you can seal the water out of a duct for a while, though eventually every seal is going to fail. Thermal expansion with cable makes it very difficult to create a perfect seal. Water is going to fuse through the plastic pipes resulting in water no matter what you do. The other sort of related question to that is how does that affect fires and explosions? As mentioned in the presentation, burning isn’t so much of a problem for explosions, if there is an actual fire in the duct and only water vapor and carbon dioxide are created—there can’t be an explosion. If the oxygen supply is cut off, the levels of carbon monoxide, hydrocarbons and other explosive materials will increase. It’s better to keep a healthy airflow through the ducts. We can sense when a fire starts and take action, rather than ‘bottling it up’ and creating a bigger problem when it explodes down the road. [appended] First of all it is important to recognize that there is no such thing as a duct seal. Gases emanating from a fire will rise to whatever pressure is required to create a path to atmosphere. Therefore, attempts to seal the duct will not adequately prevent the gases being generated from an electrical failure within a conduit from entering the manhole space. The topic of duct sealing will be addressed in more detail by the author in Part III of this webinar series in 2019.
16. Cost effectiveness of sensor and active ventilation combination?
A single fatality cost ConEd $1B. Preventing explosions with active ventilation and monitoring for fires saves human life, reduces outages (improving SAIDI and SAIFI), protects vital infrastructure from damage, and improves the public perception of those that proactively deploy it. If that were not enough, underground infrastructure runs cooler and drier, which extends reliable equipment life.
17. How can we solve this issue? As a utility service provider, how we can mitigate this issue?
Manhole explosions are a nightmare for the public and manhole owners. Unpredictability, life-threatening potential, and the setting in crowded urban areas, make manhole explosions particularity dangerous. Please join us on Tuesday, November 6th at 11 a.m. Pacific / 2 p.m. Eastern for the second webinar in the series: Manhole Explosions Part 2: How to Stop Them.
18. Visual inspection of a fault, can one tell what caused it?
Sometimes you can and sometimes you cannot.
19. Are periodic infrared exams (for overheating cable and connections) effective at preempting an explosion?[from live webinar] In fact, they’ve been shown to be completely ineffective. Pepco, (this is a public case) Potomac Electric Power Company, was ordered by the Maryland State Utility Commission to do scheduled manhole inspections. They studied the published results and found that their aggressive inspection program had no impact on manhole events. On the other hand, occasionally they did find things that were broken and needed to be fixed. I would suggest that a more logical way to deal with inspections is to have continuous online monitoring with active ventilation. This approach would allow for a more proactive, efficient response. For example, if there is an electrical discharge at a splice that generates nitrous oxide, nitric oxide and ozone, not only can the gases be detected one can determine that manhole XYZ has a discharge occurring inside of it…..resources can then be dispatched to go find it. So rather than searching every single manhole, search through and do repair work on the ones that are having actual problems. [appended] Periodic IR inspections of cables in manholes will identify cables and connections that are overheating at that exact moment in time, under those exact cable loading conditions. As cables continue to age, and loading conditions fluctuate, cable temperatures can change suddenly. Having real-time visibility into the conditions within a manhole is a much more effective method of preempting a manhole event. PEPCO was forced by the Maryland PUC to do such inspections over a multi-year period. The results were more than disappointing. For all the money and resources expended the effort had zero impact.
20. Are you a proponent of tethered manhole covers?[from live webinar] Number one, we are proponents of stopping the explosion from occurring in the first place. In the vast majority of cases, it is completely possible to do so. Any technological approach has some vulnerability; for example, if active ventilation is deployed and power is lost. Because the ventilation depends on power, one can imagine a case where gases would build up and it would only take a lit cigarette to ignite it. The explosion could occur even with active ventilation to the extent the active ventilation failed. Therefore, I’m all for tethering it as a ‘belt and suspenders’ approach to keep it from going out in the worst-case scenario. Fortunately, there are simple and inexpensive ways to restrain manhole covers. [appended] Tethering manhole covers to prevent them from becoming a projectile during a manhole event is an option. Preventing the event from happening is even better. To learn more, please join us for the webinar Manhole Explosions Part 2: How to Stop Them.
21. From a designer perspective, what things would you want the engineer to consider when designing manhole ducts and cable training to help avoid things like this from happening?
Active ventilation and monitoring should be in all new installations or refurbishment efforts. Learn more at Part 2.
22. Would fire wrapping 15KV cable improve or make worse the chance for explosion inside the manhole?
I can’t imagine how fire wrapping 15kV cable would impact vault explosions one way or the other.
23. What does active ventilation entail?[from live webinar] Passive ventilation is dependent on the wind blowing up on the street in order to change the air down below in the manhole cover. At a minimum, gasses are accumulating 10 or 15 feet below the surface and in some cases much deeper. Unless it’s a hurricane blowing up above, the odds are there are gases accumulating near the bottom. Active ventilation is recognition of that reality. The application of a vented manhole cover, along with a duct that goes all the way to the to the bottom of the manhole, sucks air from the bottom and throughout the air column so that air is turned over in the vault every few minutes. The higher the frequency of air circulation, the whole system will run cooler and drier; preventing an explosion. [appended] Active ventilation specifies fans, always running in a normal mode, and accelerating as needed when specific gasses are detected above acceptable levels. Passive ventilation speaks to manholes or grates over the underground space. We know passive ventilation is an issue as many gases of concern are too heavy to vent quickly out on their own without active intervention.
24. In the Aquarium example, was there no explosion or big fire due to the active ventilation?
Because there was active ventilation deployed at the Aquarium an explosion was not possible. See Part 2 to learn why.
25. So the best solution to prevent this is the ventilation?[from live webinar] Our recommendation is active ventilation. A utility in the Northeast conducted a study on how passive ventilation impacted their networks. The findings indicated that the total incident rate increased because they introduced more salt water into the system. You can see an excerpt of the study here. That last problem can be solved by using an appropriately designed manhole cover that doesn’t let the water in, but also it doesn’t really address the similar-to-air density gases and HTA’s. Active ventilation is the only thing that gets you everything. [appended] To lean more please join us November 6th at 11 a.m. Pacific, 2 p.m. Eastern for the 2nd portion of this series. You can register by visiting: Manhole Explosions Part 2: How to Stop Them.
26. When is the preventative presentation going to come out?
Please join us November 6th at 11 a.m. Pacific / 2 p.m. Eastern for the second portion of this series. You can register by visiting: Manhole Explosions Part 2: How to Stop Them.
27. Can we see that PEPCO study? That is a great resource!!
The Pepco study can be seen here: https://edocket.dcpsc.org/apis/pdf_files/7bcef30b-0edc-4b17-8838-dacdc9159275.pdf. On page 9, you can see where the inspections are showing less reportable conditions, then the report goes on to say: we note that the number of manhole events recorded by Pepco in DC during Year 201Q has increased by 29 (35.4%). There is a table provided on page 9 showing findings for years 2007 to 2010. Page 35, section 2.8 shows the type and severity of historical events.
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