LNG Tank Venting

This page is intended to provide information to help operators of LNG-fueled vehicles avoid losses of fuel due to LNG tank venting. See the More Resources section below for general safety and best practices information for LNG.

The clock starts when the engine stops

Liquefied natural gas (LNG) must be kept at extremely low temperatures, and can spend only a limited amount of time in transit. LNG tanks are highly insulated, but a small amount of heat inflow is unavoidable. As the tank warms, the pressure in the tank rises as liquefied gas turns to vapor. When the pressure in the tank reaches the design limit, the tank will vent excess vapor to the atmosphere, wasting fuel and making an outsized contribution to greenhouse gas emissions.

Venting should not pose a problem under normal operating conditions. As long as the engine is running, it will draw fuel from the tank and decrease the pressure at a much faster rate than warming can increase it. But as soon as the engine is stopped, pressure again begins to build. To avoid venting, the engine must be restarted, or the tank must reach a service facility, before the pressure reaches the limit.

LNG tanks are rated for "hold time", the length of time that a freshly filled tank can remain below the pressure limit if no fuel is withdrawn. Since hold times for commercially available tanks are typically several days to a week or more, ample time to deal with most disabled engines or weather-related delays, it might seem that venting should occur only under very rare circumstances. But, as explained below, if the tank is not full when the engine stops, the time left before venting may be significantly less than the hold time for a freshly filled tank.

Examples

These examples are scaled for a typical long-haul class 8 truck. The hold times will be somewhat longer for the larger LNG fuel tanks used for rail and water, but the overall conclusions apply to LNG fuel tanks of all sizes.

Let's start with the most optimistic scenario. The tank is filled and the vehicle departs immediately and runs normally until the engine is stopped. If the stop occurs very early in the journey, after only a few percent of the fuel has been withdrawn, the time left until the tank starts to vent will be only slightly shorter than the rated hold time, as expected. But if the stop occurs after more fuel has been used, the hold time decreases substantially. Consider the following:

• A typical truck-sized LNG fuel tank holds about 90 gallons of liquid when filled, and might have a rated hold time of seven days. Suppose the truck has used 10% of the fuel and the engine is stopped at that point. The time until the tank vents will be only a little lower (6.5 days, in this example). But note that the hold time has decreased by half a day, even though the truck has only been on the road a short time (less than an hour).

• Now suppose the truck runs until it has drawn down 90% of the tank's contents, and the engine is shut down at that point. Even though the truck has only been on the road for a matter of hours, the time until venting will be down to 2.5 days.

If that still seems like a safe margin, consider that any delays in departure, or any previous stops along the way, will decrease the time until venting still further.

It should also be noted that even minuscule leaks of air into the vacuum seal between the inner and outer shells of an LNG tank will degrade the insulating properties of the tank substantially. For older or damaged equipment, the times in these examples could be considerably shorter.

Explanation

Why would an emptier tank "overflow" faster than a full one?

Heat flows into an LNG tank at a slow but steady rate, gradually warming the fuel inside. The rate of heat flow into the tank is about the same regardless of how full or empty the tank is (the rate depends primarily on the temperature difference between the inside and outside of the tank, not on how full it is). The less fuel in the tank to absorb that amount of heat, the faster the temperature in the tank increases. A faster temperature rise means a faster pressure rise, and therefore a shorter hold time for an emptier tank.

Running the engine and removing fuel from the tank helps keep the temperature down, since some liquid will evaporate to fill the freed-up space, which has a cooling effect. But once the engine stops, the warming starts.

What you need to know

The time until venting from the point when the engine stops depends both on the amount of fuel in the tank, and on the temperature of the tank at that time. The temperature in turn depends not only on how much time has passed since the tank was filled, but also on such variable factors as the pattern of starts and stops since filling. It's not possible to predict in advance what the remaining hold time will be at any stage of the journey. However, after the engine has stopped, vehicle operators and schedulers can get a reasonably accurate estimate of the time remaining until venting as long as the tank is equipped with both a level indicator and a temperature or pressure gauge. (It's actually sufficient to know any two of fuel level, temperature, and pressure, since the remaining value can be calculated from the known two.) Ideally, manufacturers will provide time-until-venting information in easily accessible form on the driver's display, much as Prius drivers can access fuel economy data built into the display software. At the very least, purchasers should make sure that the display includes, in addition to fuel level, either the in-tank temperature or pressure, and that the information is available whether or not the engine is running.

More Resources

A proprietary standard (SAE J 2343-2008, Recommended Practice for LNG Medium and Heavy-Duty Powered Vehicles) is available for purchase from ANSI. General U.S. regulatory safety standards are collected in 49 CFR 193, Liquefied Natural Gas Facilities: Federal Safety Standards. As of 2013, no best practices resource specifically for LNG vehicle operators appears to be publicly available from any of the relevant U.S. agencies (DOT, OSHA, DOE, or EPA).