Flash Point Matters
Flash Point Matters
Why is the flash point of a rejuvenation fluid relevant? The temperature of an electrical arc is 35,000°F. And everything is flammable from the utility standpoint, just as nothing is unbreakable from a lineman’s viewpoint.
That’s a really interesting question. And the observation, “nothing is unbreakable from a lineman’s viewpoint” is apt. The fallibility of equipment, the inevitability of equipment failure, and the certainty that fluid will one day be released into the confined volume of your pad-mounted transformer are precisely why this issue is so important.The table lists closed-cup flash points. The materials with a red background are defined as flammable liquids by OSHA 29 CFR 1910.1200(c) and by DOT 49 CFR 173.115-120.
It’s also important to understand what a closed-cup flash point represents. Referring to ASTM D93-10 (Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester), a sample of fluid is placed in a closed metal cup at a temperature well below its flash point. The fluid and the air/vapor space above the fluid are mixed well as the temperature of the cup and its contents are uniformly increased at a prescribed rate of 5–6°C (9–11°F) per minute. A small shutter is opened at each 1°C increment, and an ignition source is lowered quickly (0.5 seconds) into the vapor space of the test cup. The ignition source lingers in its lowered position for 1 second. It is then quickly raised and the shutter is closed. This process is repeated until the vapor-air mixture flashes.
|Cablecure/SD Fluid||32°F (0°C)|
|Cablecure XL Fluid||55°F (13°C)|
|Jet Fuel A||100°F (38°C)|
|Cablecure iXL [Perficio 011] Fluid||142°F (61°C)|
|Cablecure 732 [Ultrinium 732] Fluids||>142°F (61°C)|
|Cablecure DMDB Fluid||174°F (79°C)|
|Cablecure 733 [Ultrinium 733] Fluids||>248°F (120°C)|
In order for a fire or explosion to occur, there must be three things: a source of ignition, fuel, and oxygen. In medium-voltage and high-voltage environments, sources of ignition are common. Of course, oxygen is also ubiquitous, and hence the only thing that is missing to create a fire or explosion is the fuel. But just having fuel is still not enough! The key to controlling the risk of fire and explosion is avoiding a flammable mixture of fuel and air.[dt_fancy_separator separator_style=”line” separator_color=”default” el_width=”100″]Spills of fluid (fuel) are inevitable, despite Herculean engineering and procedural efforts to prevent such events. For now, let’s assume the spill does occur from a tank failure or a failure of one of the fittings, the tubing, or an injection elbow. What happens next? Check out figure 2. The fluid will flow to the lowest point of the transformer enclosure. There may be a puddle or perhaps just fluid-wetted soil. In figure 2, the temperature at the bottom of the transformer is lower than the flash point (indicated by the red arrow). Because the temperature is well below the flash point, there will not be enough fluid evaporation to reach the lower explosive limit (LEL). There’s no risk of fire or explosion here. [dt_fancy_separator separator_style=”line” separator_color=”default” el_width=”100″]The situation changes in figure 3 as the temperature inside the transformer base just exceeds the flash point. Now in addition to the blue liquid layer, there are three other possible strata, labeled LEL, Goldilocks, and UEL. Let’s start from the bottom. The UEL is the upper explosive limit. All flammable fluids require a threshold amount of oxygen to burn. As a practical matter, this light green stratum is very, very shallow, and since the likely sources of ignition are higher within the enclosure, its presence is largely irrelevant. Now let’s jump to the uppermost stratum, left clear in the illustrations. In this stratum, there may be some molecules of evaporated fluid, but there simply is not enough to ignite in a self-propagating chain reaction. It’s the red stratum, the Goldilocks zone, where there is just enough (not too much and not too little) fuel (vaporized fluid) and just enough (not too much and not too little) oxygen to support combustion.
Because the transformer enclosure is not well ventilated and the flammable vapors are heavier than air, these strata form at the bottom as illustrated. When a fluid vaporizes, its volume increases about a thousand fold, so even a small spill has the potential to create a large Goldilocks stratum. The rate of fluid evaporation is related to the difference between the temperature at the bottom of the enclosure and the flash point. The higher the flash point, the lower the rate of evaporation will be. There are three things that mitigate an increase in depth of the Goldilocks zone. First, gravity will draw the spilled fluid into the soil. Second, enclosures are not airtight, and hence restrained convection will remove some of the vapor. Third, because of entropy and the second law of thermodynamics, the spill disperses. Both liquid and vapor diffuse through air and soil, acting to reduce the concentration of the fluid vapors. The Goldilocks stratum is checked by these three phenomena. For a given spill, the depth of the Goldilocks stratum will be determined by the difference between the temperature at the bottom of the enclosure and the flash point.[dt_fancy_separator separator_style=”line” separator_color=”default” el_width=”100″]In figure 4, the temperature is well above the flash point, and the Goldilocks stratum is much larger. In this illustration, we imagine that the neutral bleed wire to the elbow does not make an adequate electrical connection, and discharges occur at that point. As the Goldilocks stratum enlarges, it eventually reaches this discharge, and the Goldilocks volume ignites. Because the transformer provides mechanical confinement, an explosion occurs, and the lid is blown open violently. [dt_fancy_separator separator_style=”line” separator_color=”default” el_width=”100″]That is precisely what happened to a circuit owner in Ohio (figure 5).
Now go back to the flash-point table above. Does the temperature in your transformer enclosures ever exceed 55°F (13°C)? Unless you are on the North Slope of Alaska, the answer is probably yes. If Alaska Airlines asks if they can store a gallon of jet fuel A in each of your enclosures, would you say yes? The jet fuel would be safer than the low-flash-point injection fluids.
How many fires and explosions would be too many on your system? Although many have never had a fire or explosion with the flammable rejuvenation fluids that they’ve used in the past, others have. As a standards engineer, you should demand that all suppliers provide a complete accounting of all the fires and explosions that their process and fluids have ever experienced. If a vendor is unwilling to comply with this very reasonable request, you can disqualify that supplier.
In that spirit, here’s the list of Novinium fires and explosions, as of October 18, 2012 (boring as it may be).
|Event Date||Event Description|
Thankfully, most of us have never been in a head-on auto accident, but we take comfort in the PPE designed into our cars, namely the seat belts and air bags. We pay more for these features on our cars, not because we have had a serious accident, but because we wish to avoid the terrible consequences of such an event should it occur. Even better than seat belts and air bags would be a system that eliminated the risk of collision altogether.
Such a system is available to circuit owners enjoying the capital efficiency of rejuvenation. The upside-down pyramid in figure 6 illustrates that the most effective way to deal with safety risks is to eliminate them. At Novinium, we embrace this hierarchy and recommend that you embrace it, too. Eliminate known risks, and substitute safer materials and processes for those that are less safe. Apply a concentrated engineering effort to prevent occurrences, and mitigate the impact when unfortunate and inevitable incidents do occur. Implement—but depend the least on—administrative, behavior-based, and PPE controls.