Gas explosions: understanding the complex system behind natural gas delivery

In New England we were very worry about gas explosions that happened last week, and it is quite natural to become curious about WHY it happened. To better understand some possible source for the problem, we have to understand how the gas system works.

A house on fire
Photo Credit: theatlantic.com

The gas flowing from higher to lower pressure is the fundamental principle of the natural gas delivery systems. From the well, the natural gas goes into “gathering” lines, which are like branches on a tree, getting larger as they get closer to the central collection point. A gathering system may need one or more field compressors to move the gas to the pipeline or the processing plant.

From the gathering system, the natural gas moves into the transmission system, which is generally composed of about 272,000 miles of high-strength steel piper. These large transmission lines for natural gas can be compared to the interstate highway system for cars. They move large amounts of natural gas thousands of miles from the producing regions to local distribution companies (LDCs). The pressure of gas in each section of line typically ranges from 200 pounds to 1,500 pounds per square inch (psi), depending on the type of area in which the pipeline is operating. As a safety measure, pipelines are designed and constructed to handle much more pressure than is ever actually reached in the system. For example, pipelines in more populated areas operate at less than one-half of their design pressure level. Many major interstate pipelines are “looped” — there are two or more lines running parallel to each other in the same right of way. This is important to provides maximum capacity during periods of peak demand.

Another important part of the system are the compressor stations located approximately every 50 to 60 miles along each pipeline to boost the pressure that is lost through the friction of the natural gas moving through the steel pipe. The majority of the compressor stations are completely automated, so the equipment can be started or stopped from a pipeline’s central control room. The control room can also remotely operate shut-off valves along the transmission system. The operators of the system keep detailed operating data on each compressor station, and continuously adjust the mix of engines that are running to maximize efficiency and safety. Natural gas moves through the transmission system at up to 30 miles per hour, so it takes several days for gas from Texas to arrive at a utility receipt point in the Northeast. Along the way, there are many interconnections with other pipelines and other utility systems.

When the natural gas in a transmission pipeline reaches a local gas utility, it normally passes through a gate station. Utilities frequently have gate stations receiving gas at many different locations and from several different pipelines. Gate stations serve three purposes. First, they reduce the pressure in the line from transmission levels (200 to 1,500 pounds) to distribution levels, which range from ¼ pound to 200 pounds. Then an odorant, the distinctive sour scent associated with natural gas, is added, so that consumers can smell even small quantities of gas. Finally, the gate station measures the flow rate of the gas to determine the amount being received by the utility.

From the gate station, natural gas moves into distribution lines or “mains” that range from 2 inches to more than 24 inches in diameter. Within each distribution system, there are sections that operate at different pressures, with regulators controlling the pressure. Some regulators are remotely controlled by the utility to change pressures in parts of the system to optimize efficiency. Generally speaking, the closer natural gas gets to a customer, the smaller the pipe diameter is and the lower the pressure is. Distribution lines typically operate at less than one-fifth of their design pressure. Sophisticated computer programs are used to evaluate the delivery capacity of the network and to ensure that all customers receive adequate supplies of gas at or above the minimum pressure level required by their gas appliances. Distribution mains are interconnected in multiple grid patterns with strategically located shut-off valves. These valves minimize the need for customer disruption to service during maintenance operations and emergencies.

Natural gas runs from the main into a home or business in what’s called a service line. Typically, the natural gas utility is responsible for maintaining and operating gas pipeline and facilities up to the residential gas meter. All equipment and gas supply lines downstream of the residential meter are the responsibility of the customer. When the gas reaches a customer’s meter, it passes through another pressure regulator to reduce its pressure to under ¼ pound, if necessary. Some services lines carry gas that is already at very low pressure. This is the normal pressure for natural gas within a household piping system, and is less than the pressure created by a child blowing bubbles through a straw in a glass of milk. When a gas furnace or stove is turned on, the gas pressure is slightly higher than the air pressure, so the gas flows out of the burner and ignites in its familiar clean blue flame.

As you can see, the system is complex, and depends a lot of computers and physical connections, working at different pressure. We don’t know what causes the problem here in Massachusetts, but probably it was related with a high pressure entering a connection system within some pipe that was not prepare to absorb the pressure safely.

(Source: American Gas Association website)

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