Gas Distribution Data Model Overview
The gas distribution system is composed of the connected features that convey natural gas from a source such as a regulator or town border station to the customer. Principle components of the gas system are pipes (mains and services), devices which control and regulate flow in those pipes, fittings that join pipes, and metering equipment that measures the flow of gas within pipes.
Mains are pipes that carry gas from a source, such as a regulator or town border station, to a point adjacent to a customer premise. Service pipes transport gas from mains to meter locations. At the town border station (also called a city gate), gas transmission is converted to a distribution system. These features may have associated regulators, regulating meters, overpressure devices, and odorizers. Regulating stations define the location of one or more pressure regulators.
Several types of devices control the flow of gas through a set of pipes as well as the pressure at which gas is delivered. A regulator is a mechanical device used for the controlled reduction of pressure in a gas distribution system. Monitor and backup regulators are included in this feature type. A valve operates in a pipe to permit flow in only one direction or regulate the flow by means of a flat, lid, plug, or other mechanism to open or block the pipe. Valves designated as ‘key’ are critical to modeling and analysis. Flow control devices include any fitting that is not a regulator or a valve that can control the flow of gas and is machine operated.
Steel pipes buried in corrosive soils will corrode. Coatings of epoxy, polyethylene, or other materials are common methods for inhibiting corrosion. Cathodic protection is another method of protecting underground metallic structures, such as steel pipes, fittings, and valves, from corrosion.
Metal structures deteriorate as stray electric current normally present in the ground flows from the relatively ‘anodic’ structure into the relatively ‘cathodic’ soil. By inducing a small electric current on metal structures to make them ‘cathodic,’ stray current flows from the soil to the structure and, as a result, the structure is protected.
Protected portions of the distribution system must be electrically separated from nonprotected portions. This is often accomplished by insulated fittings such as insulated flanges or insulated compression couplings.
The components of the gas distribution system are grouped into three general, logical categories:
Devices and Facilities Overview
Pipes and Maintenance Overview
Cathodic Protection Overview
These categories contain features classes that share common properties and/or behavior. For example, devices can be grouped together because they detect and/or control the flow of gas through pipes. Some devices measure the flow (e.g., meters), and some regulate the flow of gas (e.g., regulators). After establishing a basic grouping of objects, you can identify more specific similarities between objects. During this grouping process, you can define new classes (called subclassing) and merge some classes (subtyping). The final result is a set of root abstract classes, intermediate abstract classes, leaf classes, and relationships.
When you begin to define the properties of each leaf class, common properties emerge. For instance, both meters and regulators have manufacturers and model numbers. Rather than duplicate each property in both objects, you create a higher-order class (Gas Device), which is an abstract class, to contain these properties. This class contains properties common to all objects subclassed from it and will never be a standalone object. This process of generalizing properties results in a set of intermediate classes that represent, or model, the gas utility system.