Figure 5-1: Tying into a Transmission Line...............................................................................................2
Figure 5-2: Main Installation.....................................................................................................................3
Figure 5-3: Installing New Mains..............................................................................................................3
Figure 5-4: Fully Looped Distribution Systems.........................................................................................4
Figure 5-5: Radial Distribution System.....................................................................................................5
Figure 5-6: Mixed System.........................................................................................................................6
Figure 5-7: Regulator Station....................................................................................................................7
Figure 5-8: Plastic Pipe Inserted in a Steel Line.......................................................................................8
Figure 5-9: System Design Capacity vs. Inlet Pressure with a Constant Percent Overload Capacity......9
Figure 5-10: Equipment Requiring High Pressure...................................................................................12
Figure 5-11: Required Space for Maintenance........................................................................................13
Figure 5-12: Regulator Station.................................................................................................................13
Figure 5-13: Ideal Piping Layout Developed............................................................................................14
Figure 5-14: Multiple Shutdown Valves...................................................................................................15
Figure 5-15: Overpressure Protection Equipment...................................................................................15
Figure 5-16: Computer Graphic of Network Simulation...........................................................................16
Figure 5-17: Schematic Diagram of Feeder Systems..............................................................................18
CHAPTER 5: CAPACITY DESIGN OF GAS DISTRIBUTION SYSTEMS
Introduction
Distribution companies must provide safe and reliable gas service. This is done by maintaining adequate pressure for the proper operation of customer appliances and equipment. This safe, reliable gas service must also be offered at a reasonable cost.
Extending or reinforcing a gas distribution system is the primary activity in the capacity design process. Computer or manual simulation of various plans at design conditions is a process called network analysis that is used to refine and optimize these plans. In choosing a plan, the designer’s objective is to balance the possible performance penalties of a closely sized system against the small, economic penalty of designing an oversized system.
Pressure and Capacity Needs
System Planning is responsible for determining the effects of changing load patterns on the piping network and proposing methods to assure adequate system capacity. Network analysis, load calculations, and pressure surveys are among methods used to identify areas with capacity and pressure problems.
The “control point” in a distribution system is often considered the location where minimum delivery pressure first occurs on a peak day. Additional system capacity and higher pressures may be required to address the following conditions:
• Poor system pressures during periods of high demand, such as cold weather
• Additional loads from an increase in the number or size of customers
• Needed expansion of the system to supply a new service territory.
Methods to Address Pressure and Capacity Needs
There are various methods used to improve system capacity and pressure. If system pressures are increased, system capacity will normally increase as well. The optimal method to expand a system or reinforce pressures in an area will depend on the layout and condition of the existing piping. The most common methods are:
• Acquiring additional gas supply sources
• Installing larger diameter pipes to replace small-diameter lines
• Installing additional pipelines
• Interconnecting and looping lines
• Adding new regulator stations to a system
• Inserting new medium-pressure pipe to convert low-pressure lines
• Uprating.
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Acquiring Additional Sources of Gas Supply
A gas company may acquire additional gas from suppliers or transmission companies to meet a growing system demand. This method is best used when an extremely large area or an entire distribution system has inadequate capacity or pressure.
Figure 5-1: Tying into a Transmission Line
Acquiring additional gas supply is unfortunately limited by the location, availability, and cost of the supply. For example, a transmission company may not have additional gas to sell, or a distribution company may incur high costs running a pipeline to connect with an additional source of supply. In addition, a new gate station and equipment modifications are normally required when connecting with a new supply source.
Purchasing additional gas from an existing supplier may be the lowest cost option for acquiring gas supply. However, the downstream piping configuration may be unable to efficiently distribute additional gas throughout the existing distribution system network.
Installing Large-Diameter Mains to Replace Smaller Diameter Lines
Within an established distribution system, a company may choose to replace small-diameter mains with larger diameter pipelines. This method can provide additional gas flow to a limited area or can supply an additional load or new customers. Other customers may also be transferred to a new main to remove further load from an existing system.
CHAPTER 5 — CAPACITY DESIGN OF GAS DISTRIBUTION SYSTEMS 5-3
Figure 5-2: Main Installation
This method is not ideal for extremely large areas because of the flow limitations with individual lines. For example, the size of other pipes in an area can create a bottleneck in the delivery of additional gas. In addition, a company must obtain right-of-way permits for new lines and overcome other construction obstacles involved in the installation of larger diameter pipe.
Installing Additional Mains
A distribution company may decide to install additional mains to supply areas without gas or to improve flow patterns in a limited area of the system. This method can be used to supply new residential or commercial developments, or individual customers. For example, if a new industrial area is built next to an existing residential area, a number of additional mains may be required to supply gas and maintain adequate pressures.
This method may have a limited effect on system pressures and capacity because of the size and layout of pipes in an area. Moreover, the installation of multiple lines in an area is costly and time-consuming.
Figure 5-3: Installing New Mains
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Interconnecting and Looping Pipelines
Interconnecting and looping pipelines can improve pipe pressure and minimize customer outages by providing alternative paths for gas to travel or flow. The connections must often be made in an intersection and require stopping equipment. As a result, this method can be expensive and the amount of pressure improvement may be minimal. However, in Figure 5-4 this method is less expensive than the installation of new mains or regulator stations.
In a fully looped or gridded distribution system, as shown below, mains are installed on all or nearly all streets and connected together at most intersections. The other extreme is a radial or tree system that has no closed loops.
Most low-pressure systems approximate fully looped systems because main blockages were common at the time they were installed. The most common cause of blockage was a build p of water from a high water table or nearby leaking water main. Groundwater entered through leaks and then collected at low points in the gas main system.
Fully looped systems minimized blockages by providing many alternative paths by which gas could reach each customer. Radial distribution systems (Figure 5-5) are less costly than looped systems; however, the radial layout is analogous to a tree. If the trunk is severed, the whole organism dies. Consideration of looped versus radial systems thus involves a trade-off between economy and reliability of service.
Figure 5-4: Fully Looped Distribution Systems
Source: Richwine, T. E., and D. W. Schroeder, "Large-Scale Gas Network Design by Automated Means." AGA Operating Section Proceedings 1986: 314-19.
CHAPTER 5 — CAPACITY DESIGN OF GAS DISTRIBUTION SYSTEMS 5-5
Figure 5-5: Radial Distribution System
Source: Richwine, T. E., and D. W. Schroeder, "Large-Scale Gas Network Design by Automated Means." AGA Operating Section Proceedings 1986: 314-19.
A practical limitation of radial systems is the number of customers that the work force can get back in service within a reasonable period of time. Most distribution system operators would consider 24 hours acceptable except for heating customers in severely cold weather. Estimates of maximum permissible outage size range from 50 customers for small, isolated distribution systems to 30,000 customers for a large, highly developed area located in a warm climate.
The current trend in the design of new distribution systems is to use a mix of looped and radial designs. (See Figure 5-6.) Considerable, though not complete, looping is used for header mains and piping near sources of gas supply. A radial layout is used for extreme ends of a system and fringe areas of a regulator feed area. Large looped systems fed by a large number of regulator stations are seldom built.
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Figure 5-6: Mixed System
Adding Regulator Stations
A new regulator station can provide additional capacity and pressure to a large area within a distribution system. However, the difficulties of adding a regulator station include availability of a high-pressure gas supply, obtaining land in a proper location, and the configuration of downstream piping in the area to handle increased flow.
The cost of a new regulator station can be high, but the benefit can also be significant. Regulator station design affects capacity, safety, and reliability of the distribution system that the station serves. The size and type of regulators and the piping size and configuration determines pressure drop
