For utilities, as with all businesses, the decision to invest in increased safety provisions is a careful measure of risk versus reality. Utilities want what’s best for their customers and employees, but are beholden to all the traditional project factors—cost, budget, scope and competing priorities—when deciding whether to allocate funds for improvements.
In the case of manholes, most utilities are aware they face aging infrastructure and some are aware of the specific dangers of manhole events—smoke, fires or explosions that emanate from utility manhole vaults, damaging the vault’s contents and often causing disruption in the world above.
Manhole events are a growing problem, more than 2,000 events occur in the U.S. every year. In the worst cases, a flying manhole cover from an explosion can cause injury or even death. This tragic human cost is accompanied by mitigation and repair costs estimated at $100,000 and higher per manhole. Additionally, there are uncapped expenses for litigation, unwanted media attention and public relations.
So how does a responsible utility project manager face this reality and determine a solution that works best for the company and its community? Like other business problems, the key is to first understand what’s causing the problem, and then look at the smartest ways to get in front of it.
Manholes are harsh environments. As underground equipment vaults, they are exposed to outside elements including severe pollution from debris, vegetation, trash, car fumes, acid rain, and high concentrations of salt and fresh water. These pollutants create decomposition that further affect weakened electrical, gas, and water infrastructure that has been damaged by traffic vibrations, rodent infestation and the ravages of age.
The drawing below portrays the development of a typical manhole event. As shown, a variety of electrical and environmental conditions combine to create an explosion or fire. In particular, certain combustible compounds in the air are ignited by faults in aging secondary electrical cabling.
Most manhole events are caused by the decomposed secondary cable initiated by an electrical fault.
To combat this development, it’s necessary to reduce the volatility of the environment in manhole vaults. Utilities can take action to discover, predict and prevent manhole events in a three-pronged approach:
1. DIRECT toxic gases away from manholes using active, ongoing powered ventilation.
2. DIVERT storm water from entering manholes using specially-designed covers.
3. DETECT the conditions in each manhole so you can take action before an event occurs.
Most manholes are ventilated using passive technology: natural convection pushes gases up and out of the vault through the grates in the manhole cover. The reason manhole events occur anyway is because passive venting is insufficient. To make sure fresh air is brought into the vault and gases are circulated out, you need to install an active ventilation system.
The PreVent system uses a duct-and-fan system to provide active ventilation to individual manholes.
Novinium, a Seattle-based utility product and services company, makes a manhole event prevention solution called PreVent™ that includes active ventilation. As shown above, the ductwork attaches to the underside of the manhole cover and uses a fan with an exhaust volume exceeding 100 CFM. The unit is designed for continuous operation using 24V DC and a single splice into a secondary network. From above, you can shut off the power using an accessible, waterproof disconnect.
So, active ventilation is important. But what if we take one step back and look at how to prevent the wrong kinds of elements from entering manhole vaults and causing explosive situations in the first place?
Besides addressing the volatility of aging secondary cables themselves, the main strategy for preventing buildup of gases is to address water ingress. When storm water carries salt and other corrosive elements from the street into the vault, the resulting deterioration and electrical failure conditions create the combustible atmosphere you’re trying to avoid.
What’s needed, ultimately, is a better mousetrap in the form of a redesigned manhole cover for today’s environmental conditions. The PreVent system, for example, includes a replacement manhole cover that fits standard openings but features a domed design that uses built-in dams and draining grooves to redirect water around the vented openings and away from the manhole interior.
The PreVent manhole cover includes features to help reduce water intrusion into manhole vaults.
Flow table testing in the Novinium labs show that its PreVent manhole cover design slows water ingress considerably—by more than 90% when the water flow over the surface is 0.11 inches deep (the equivalent of 5.5 inches of rainfall on a 2% grade) and more than 99.5% when the flow is 0.06 inches deep, or a rainfall rate of 2.5 inches per hour.
With ventilation and covers addressing prevention, now shift your focus to thinking about how to best address manhole events themselves. Due to limited resources, most utilities rarely inspect manhole vaults for problematic conditions. While they want to have a proactive approach, most utilities don’t think they have a way to do it…but this is wrong!
Ideally, a utility manager could sit in one place, know the status of any combustible conditions in your manholes, and proceed with day-to-day business, confident if conditions reach a certain point of risk, an alert will be delivered and the manager can take action accordingly.
All the technology for doing this exists; it’s just a matter of applying it. For example, PreVent uses a wide range of sensors to gather and log critical environmental data, including air flow, temperature, relative humidity, water level, and gas levels for such compounds as CO, CO2, and CH4. This data appears in a customizable dashboard at a utility headquarters or substation, and can push alerts to workers’ devices. From a utility management perspective, this kind of system easily supports communication and action planning for the utility to address risky conditions before they become a problem.
Sample dashboards of a typical PreVent™ active monitoring system showing statuses, analysis, and alerts.
In this way, technology and data provide the accurate and timely information needed to take preventative action with manhole events, while counteracting the high cost and potential human error of ongoing manual inspections.
When you combine these three approaches into a single strategy, the benefits are clear: By applying common-sense prevention measures and using data, not just people, to track the environmental conditions across an entire network of manhole vaults—and by predicting failures rather than waiting for them to happen—utilities can gain efficiency in their operations and potentially save lives.