Understanding Pressure Zones
By Tom DeWitte and Tom Coolidge
Within every pressurized pipe network, whether natural gas, hazardous liquid, water, or district energy, there are one or more pressure zones. Pressure zones are foundational to the engineering and operation of pressurized pipe networks. A formal definition defines a pressure zone as a distinct subset of the pipe network where a minimum and maximum pressure range is maintained by pressure-controlling devices. Yet, knowing this definition of a pressure zone is not the same as understanding a pressure zone.
Understanding a Pressure Zone
Understanding a pressure zone has differing meaning across an organization responsible for managing a pressurized pipe network. System planners require an understanding of the capacity of the pressure zone and the load from customers consuming the commodity traversing the pipe network. Hydraulic engineers need to understand the capacity, load, assets, commodity flow, and the type of customers depending on the provided service. Integrity Engineers require an understanding of the pressure zone assets and their characteristics. System control center operators must understand the standard operating pressure and the current maximum allowable operating pressure (MAOP). For many others across the organization, it means visually seeing the extent of the pressure zone. With this wide range of what it means to understand a pressure zone, how does an organization achieve this understanding?
Defining the Pressure Zone
The need to create an inventory of the assets that comprise a specific pressure zone is foundational to all these various aspects of understanding a pressure zone. From a software perspective, this means utilizing software that can determine a pressure zone. This determination is based on how the pipe assets are connected, knowing the location of the pressure regulating devices, and an understanding of the high side and the low side of those pressure regulating devices. With this understanding of pressure zone components, add some software logic to define commodity flow, and you can now define a pressure zone.
Within ArcGIS, the capability to determine a pressure zone is called the ArcGIS Utility Network. The result of the application of this capability for pressure zones is the creation of a pressure subnetwork.
Defining a pressure subnetwork within ArcGIS generates multiple items of information to aid in understanding the pressure zone. These aids are:
- Creating a persistent graphical representation of the pressure zone.
- Inserting the name of the pressure zone to each asset contained within the pressure zone.
- Tabulating quantities of the identified assets, such as number of meters and number of valves.
- Performing engineering calculations to determine MAOP for the pressure zone.
This inventorying, tabulating, calculating, and visualizing provide pipe organizations with the foundation to understand the pressure zone.
Understanding A Pressure Zones Capacity
A common derivative of the pressure subnetwork representation of the pressure zone is the tabulation of the volume within the pressure zone. Knowing a pressure zone’s pipe volume has historically been the primary challenge in accurately determining the standard operating pressure amount of commodity within this subset of networked pipes. For an individual pipe segment, tabulating volume is an exercise in remembering the middle school geometry equation for a cylinder. That equation leverages the diameter and length of the pipe. Repeating this geometry calculation thousands of times for each distribution main and service line within the pressure zone is hard for humans. It is a simple calculation for a computer.
With the volume automatically tabulated for the pressure zone, the next step is to apply the Ideal Gas Law equation and some commodity specific values to determine the mass and energy stored within the pressure zone.
An arcade script within a pop-up can easily perform this calculation for both the standard operating and the maximum allowable operating pressure. Having the result of this tabulation readily available whenever a user opens a pressure zone pop-up within a web application or mobile map is a significant time saver for engineers and system control operators.
Understanding A Pressure Zone’s Assets
Once the ArcGIS Utility Network capability defines a pressure zone, the capability updates the name of the pressure zone in all traced assets (pipe segments, valves, fittings, customer meters, etc.
This assigning of the pressure zone name to each asset, simplifies follow-on pressure zone asset queries often used in capital planning, such as:
- Average age of pipe
- Length of pipe by material
- Length of pipe by size
Understanding a Pressure Zone’s Pressure Range
Core to managing a safe and reliable pressurized pipe network is knowing the maximum pressure at which the gas utility can safely operate the pressure zone. To most industries this value is known as MAOP (Maximum Allowable Operation Pressure). This value is asset specific and will vary across the thousands of assets comprising a single pressure zone. The pressure zone MAOP is defined by the weakest link within all the assets of the pressure zone. In software terms, this is the application of a minimum function against all the pressure zone asset’s individual MAOP value to find the lowest value. The ArcGIS Utility Network capabilities can automatically tabulate this engineering value as part of defining the pressure zone.
With MAOP now automatically defined and maintained by ArcGIS, whenever a new asset is installed within the pressure zone and the asset’s individual MAOP value is defined, the software will revise the pressure zone MAOP to reflect the installation of the new asset.
Combining the calculation of MAOP with the tabulation of volume for a pressure zone allows for the calculation of maximum line pack. Line packing is a natural gas industry term for increasing the mass of gas within a given pressure zone. Line packing is achieved by increasing the operating pressure. An increase cannot exceed MAOP. Control room operators use this value to understand how much additional mass can be placed within the pressure zone pipe network in anticipation of a forecasted increase in customer demand.
Understanding a Pressure Zone’s Extent
The ability to see a visual representation of the pressure zone is another product of the ArcGIS Utility Network defining a pressure zone.
The ability to see the extent of a pressure zone is key to helping a utility’s staff answer their questions. Typical questions include:
- Emergency Events. Which pressure zones will be impacted by wildfires and floods?
- Planning: Which pressure zone provides service to the address of a potential new customer?
- Engineering: When planning a system modification, what is the extent of the pressure zone?
Understanding a Pressure Zone’s Customer Base
With the pressure zone accurately defined, geospatial analysis methods can be applied to identify and inventory the customers within a given pressure zone. As an example, a buffering of customer meters with the pressure zone name can query the USA Structures Living Atlas feature layer to tabulate valuable information such as:
- Number of hospitals
- Number of schools
- Number of impaired / limited mobility customers
- Quantity of types of customers (residential, commercial, industrial, government
This provides instant clarity of the customer base being served by the pressure zone. This clarity helps emergency event coordinators, engineers, and planners.
Bring it All Together
Historically, the information mentioned in this article has been scattered across an organization’s many data siloes, spreadsheets, and the brains of individual employees. Leveraging ArcGIS and its ArcGIS Utility Network capability to define the pressure zone brings this information together. This aggregation of information and presenting it in a manner that is easy to understand is what turns defining a pressure zone into understanding a pressure zone.
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