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
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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)

Save money with roof trusses

With the decreasing availability of large structural lumber work with trusses for your roof construction can save you money and also comply with all structural needs for your project. The length of lumber subject to bending stress are broken into smaller sections, and that’s the main reason you can work with cheaper lumber products. But take care of some extra costs when doing trusses for your roof.

Image result for trusses types
Photo credit: click HERE

You have a lot of types of truss designs, and during the design of your project you have to adapt all your load needs. The maximum allowable span depends on the type of wood you are using in your construction (Southern Pine, Douglas-Fir and Spruce Pine-Fir – also known as SPF – are the most common) and also on the pitch of the top cord.

You have some places in the internet to help you on calculating the allowable span for trusses in your project. Click HERE to redirect you to one of these websites.

A final message: remember that in the majority of the houses you will be building with roof trusses you’ll need a crane to help one the truss installation. The costs will vary from region to region, and for a house around 2,500 sf, ranch style, you will need at least 8 hours of crane work if you have a trained team to installed them. Don’t forget to include this cost in your project.

Keep in mind these numbers in your design

Image result for small kitchens
Photo from the website

Sometimes we see dwellings with old designs that looks really strange. Kitchens that are very difficult to use, boots that do not fit in the closet, and windows that are to high or too low. Here are some important numbers to remember:

  • Windows – 38 / 42

Window heights are important on both inside in outside. From an outside view the ideal is to align the top heights of the windows and exterior door, mainly in the main entrance (ideally for the entire building). Sill heights of windows adjacent to furniture or counters should be at least 42″. View windows sill should not exceed 38″

  • Closets – 24 / 36

Plan to allow at least 36″ of closet pole per occupant, with a hanger depth of at least 24″. Remember that an ambient is only consider a bedroom if you have a closet there – so, provide at least one closet for each room. You can add closets near front and rear entrances (for coats and shoes also, in case you have ancillary entrances for a mud room), a linen closet an at least one generous walk-in closet in the main bedroom.

  • Passageways: 36 / 40

Width are dictate by the needs to move large furniture. For stairs, landings, main and minor hall, interior and exterior doors the minimum is 36″ but we recommend at least 40″ to have good room for movement. Basement doors need a minimum of 36″ but ideally use 48″

  • Kitchen work aisle: 42 / 48

The width of a work aisle should be at least 42″ for one cook and at least 48″ for multiple cooks. You need to measure it between the counter frontage, tall cabinet and the appliances. Also have in mind that the passageway close to the working aisle should be at least 36″

  • Shower size: 30 / 36

The recommended size is 36″ x 36″ with a minimum of 30″ x 30″. If a person with disabilities will live in the house you need to revise the ADA of 1990 (Americans with Disabilities Act) – Appendix A of page 36 – that specifies all the needs for this population. We will do a post on this specific topic soon.

Keep the quality on your roof edges

APA – The Engineered Wood Association has a lot of useful information for builders in their website, and here we will share a very important tip: how to choose your panels for soffit applications.

Quality APA panels are a great alternative to other materials used in soffit applications. There is a variety of APA face grades from which to choose. Selecting the appropriate panel depends primarily on whether the soffit is open or closed.

For appearance purposes in open soffit construction, you have to provide adequate blocking, tongue-and-groove edges, or other edge support such as panel clips. Minimum capacities are at least 30 psf live load, plus 10 psf dead load.

For open soffit construction (figure 1), panels designated Exposure 1 may be used.

Figure 1 OpenSoffit

Exterior panels should be used for closed soffits (Figure 2).

Figure 1 ClosedSoffit Wise Home Building

In open and closed soffit construction where Exposure 1 sheathing is used for roof decking, you have to protect panel edges against direct exposure to the weather with fascia trim.

Fascia Subfascia plan drawing Wise Home Building
Plan drawings showing a fascia, sub-fascia and trim to protect the edge of a roof (red arrow) in an open soffit design

Finishing. Although unsanded and touch-sanded grades of plywood are often used for soffits, optimum appearance and finish performance is achieved by using panels with Medium Density Overlay (MDO), or textured (such as APA 303 Siding) or sanded A-grade faces. Top-quality acrylic latex house paint systems perform best and are the only systems recommended for A-grade faces.

You can find more information in the APA website:

Engineered Wood Products: the new normal

Engineered wood products includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibers, or veneers together with adhesives or other fixation methods.

They can be divided in a lot of categories:

  1. Plywood: one of the most recognized and trusted wood building products for decades. Manufactured from thin sheets of cross-laminated veneer and bonded under heat and pressure with strong adhesives, plywood panels have superior dimensional stability and an excellent strength-to-weight ratio and are highly resistant to impacts, chemicals, and changes in environmental temperature and humidity. Suitable for a variety of end uses including subflooring, single-layer flooring, wall and roof sheathing, sheathing ceiling/deck, structural insulated panels, marine applications, siding, webs of wood I-joists, concrete forming, pallets, industrial containers, mezzanine decks, and furniture.
  2. Oriented Strand Board (OSB): widely used, versatile structural wood panel. Manufactured from waterproof heat-cured adhesives and rectangularly shaped wood strands that are arranged in cross-oriented layers, OSB is an engineered wood panel that shares many of the strength and performance characteristics of plywood. OSB’s combination of wood and adhesives creates a strong, dimensionally stable panel that resists deflection, delamination, and warping; likewise, panels resist racking and shape distortion when subjected to demanding wind and seismic conditions. Relative to their strength, OSB panels are light in weight and easy to handle and install. Also suitable for a variety of end uses (similar to plywood)
  3. Glulam: Glued laminated timber, or glulam, is a highly innovative construction material. Pound for pound, glulam is stronger than steel and has greater strength and stiffness than comparably sized dimensional lumber. Increased design values, improved product performance, and cost competitiveness make glulam the superior choice for projects from simple beams and headers in residential construction to soaring arches for domed roofs spanning more than 500 feet. Glulam has a reputation for being used in striking, exposed applications such as vaulted ceilings and other designs with soaring open spaces. In homes, churches, public buildings, and other light commercial structures, glulam is often specified for its beauty as well as its strength
  4. I-Joists: I-joists are strong, lightweight, “I” shaped engineered wood structural members that meet demanding performance standards. I-joists are comprised of top and bottom flanges, which resist bending, united with webs, which provide outstanding shear resistance. The flange material is typically laminated veneer lumber (LVL) or solid sawn lumber, and the web is made with plywood or OSB. The robust combination of structural characteristics results in a versatile, economical framing member that is easy to install in residential and light commercial projects. I-joists are used extensively in residential floor and roof framing. They are ideal for long spans, including continuous spans over intermediate supports. Because I-joists are straight and true, it’s easier for builders to avoid crowning and maintain a level framing surface.
  5. Structural composite lumber (SCL):  includes laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL) and oriented strand lumber (OSL). They represent a family of engineered wood products created by layering dried and graded wood veneers, strands or flakes with moisture resistant adhesive into blocks of material known as billets, which are subsequently resawn into specified sizes. Typical uses for SCL include rafters, headers, beams, joists, studs, columns, and I-joist flange material. Two or three sections of SCL can be joined together to form 3-1/2-inch or 5-1/4-inch members. These thicker sections readily nest into 2×4 or 2×6 framed walls as headers or columns

Source: APA website