Local Natural Gas Distribution Systems

by Dawn Pisturino on April 18, 2022

If you’ve ever wondered where your natural gas comes from and how it gets to your house or business, here’s an example of how a local natural gas distribution system works. Regulations and construction requirements may differ from state to state.

Case Study: Local Natural Gas Distribution in Kingman, Arizona

Unisource Energy Services (UES) in Kingman, Arizona receives natural gas from the Transwestern Pipeline, which originates in Texas. The steel transmission pipeline is 30 inches in diameter and operates at pressures from 200 to 1500 psi. The pressure is reduced to 60 psi at UNS regulator stations and then reduced again to 0.25 psi before it enters the service lines that connect to home appliances (Unisource Energy Services, 2019; Energy Transfer, 2020).

In Arizona, there are four compressor stations along the Transwestern Pipeline at Klagetoh, Leupp, Flagstaff, and Seligman. The compressors were built by USA Compression (Energy Transfer, 2020; USA Compression, 2020).

“Natural gas is compressed for transmission to minimize the size and cost of the pipe required to transport it” (Busby, 2017, p. 49). Friction inside the pipe reduces the pressure and flow rate. The gas is re-compressed at compressor stations in order to boost the pressure in the line. Compressor stations are generally found every 50 to 100 miles along a pipeline. They are a crucial part of the transport system that keeps the natural gas flowing through the pipe at the right pressure and flow rate. Air is mixed in with the gas to lower emissions from the compressor stations (Busby, 2017, p. 49, 51).

Reciprocating compressors use high compression ratios but have limited capacities. They are driven by internal combustion natural gas engines (Busby, 2017, p. 49).

Centrifugal compressors use lower compression ratios but have high capacities. They rotate at 4,000 to 7,000 rpm and are driven by natural gas turbines. They are energy efficient and cost less to install and use (Busby, 2017, p. 49-50).

The Transwestern Pipeline has an interconnect point at Kingman, Arizona, at legal description 21 N, 16 W, in Section 19. There is a measuring station there that measures the amount of gas delivered to the Kingman interconnect (Energy Transfer, 2020).

Transwestern utilizes all types of meters in its measuring stations: orifice meters to measure gas received or delivered at an interconnect; turbine meters or ultrasonic meters to measure displaced gas; and coriolus meters, all according to the AGA Gas Measurement Manual (Energy Transfer, 2020).

“Metering of gas flow is an important function of pipeline gas operations” (Busby, 2017, p. 51) because customers along the line pay for the amount of natural gas they receive. Meters must be accurate in order to ensure accurate and reliable billing (Busby, 2017, p. 49).

Many natural gas companies handle seasonal demand by storing natural gas in underground facilities or storing it as liquefied natural gas (LNG). “Peak shaving is one of the most common domestic uses for LNG today” (ADI Analytics, 2015). When seasonal demand (usually in the winter) requires a bigger load of natural gas, the “LNG is regasified and sent to the distribution pipelines” (Maverick Engineering, 2016).

Natural gas supplies can also be augmented with synthetic natural gas (SNG) during seasonal demand. SNG is actually propane-air or liquefied petroleum gas air (LP-air) which “is created by combining vaporized LPG with compressed air” (Transtech Energy, 2020). During seasonal demand, peak shaving facilities inject SNG into the natural gas distribution system to augment the real natural gas in order to meet increased demand (Transtech Energy, 2020).

Another way to increase the natural gas supply during peak demand is line packing. This requires installing oversized pipes in transmission lines, which is incredibly expensive, and not always worth the cost (INGAA Foundation, 1996).

Unisource Energy Services has no storage facilities so it must plan ahead and estimate how much natural gas it will need to meet the winter peak demand. Then it must contract with a reliable natural gas supplier, schedule the deliveries, and submit its Gas Supply Plan to the Arizona Corporation Commission. This arrangement is called an asset management agreement (AMA) (American Gas Association, 2019; Arizona Corporation Commission, 2017).

As local natural gas distribution companies grow, they must install new lines to accommodate new customers. The state of Arizona requires that all excavations be reported to the state at least 2 days before the actual trenching. All utility lines must be discovered and clearly marked. Only manual digging can be used within 24 inches of marked lines (Arizona 811, 2020). These requirements were put in place for safety reasons, to prevent damage to buildings and injuries to humans.

Joint trench requirements can differ from city to city, county to county, and state to state. But a typical safe guideline is as follows: a trench at least 36 inches deep and 18 inches wide; 24 inches between gas and electric lines and between natural gas and water lines; 12 inches between natural gas and communication lines; 24 inches between natural gas and sewer lines (Arizona Public Service Electric, 1995; Lane Electric Cooperative, 2020).

Unisource Energy Services is required by law to follow state and local construction requirements and trenching laws. The company uses plastic pipes 0.5 to 8 inches in diameter and coated steel pipes 0.75 to 16 inches in diameter (Unisource Energy Services, 2019). Like all natural gas companies, company engineers must consider line pressures, the length of the line, and estimated load when deciding which pipes to use.

References

ADI Analytics. (2015). A new role for small-scale and peak shaving lng infrastructure.

Retrieved from https://www.adi-analytics.com/2015/06/03/a-new-role-for-small-scale-and-

peak-shaving-lng-infrastructure/

American Gas Association. (2019). LDC supply portfolio management during the 2018-2019

winter heating season. Retrieved from

Click to access whs-2018-2019-report-final-12-20_2019-.pdf

Arizona 811. (2020). Proper planning. Retrieved from https://www.arizona811.com.

Arizona Corporation Commission. (2017). Winter preparedness. Retrieved from

https://www.azcc.gov/docs/default-source/utilities-files/gas/winter-preparedness-2017/uns-

gas-2017-winter-prepardness.pdf?sfvrsn=daa79b6f_2_.

Arizona Public Service Electric. (1995). Trenching requirements. Retrieved from