Gas Objects

This section presents an in-depth description of all objects available in SAInt to develop a gas network model. Figure 1 shows the hierarchy and the parent-child relationships between base objects. Table 1 gives a quick and short description of the base objects.

For example, the top level base object in a gas network model is the "Gas Network" object GNET. A gas network contains many other different base objects, such as, for example, gas nodes GNO or gas branches GBR. GNET is the parent, and GNO or GBR is a child. A gas network object has no parents, but only children. A gas component object GCMP cannot be a parent, but only a child. See the schematic below for a visual representation of the complete hierarchical object structure for a gas network model in SAInt.

Relationship between objects in a gas network. A child object is indicated by the head of a pointed arrow, while a parent is by the tail. Please, open the image in another window of your Internet browser to enlarge it.
Figure 1. Relationship between objects in a gas network. A child object is indicated by the head of a pointed arrow, while a parent is by the tail. Please, open the image in another window of your Internet browser to enlarge it.
Table 1. Icons and descriptions of the objects in gas network.
Icon ObjType Display Name Description

gnet

GNET

Gas Network

Models the characteristics and interactions of facilities and/or components of a gas network. Serves as a container for all objects in the gas network

gsub

GSUB

Gas Sub

Models a subset of nodes, branches, and gas externals of an gas network. A gas sub is branch-oriented, i.e., only gas branches can be assigned to a gas sub, and every gas branch belongs to only one gas sub

gzn

GZN

Gas Zone

Models a subset of nodes, branches, and externals of a gas network. A gas zone is node-oriented, i.e., only gas nodes can be assigned to a gas zone, and every gas node belongs to only one gas zone

ggrp

GGRP

Gas Group

Models a subset of different objects in a gas network. Except for the gas network, subs, and zones, any gas object can be added to a gas group. In contrast to gas subs and zones, gas groups do not follow any specific assignment rules. Thus, a gas object can be part of multiple gas groups

gno

GNO

Gas Node

Models a physical or virtual location in the gas network where gas can be injected or extracted through externals (gas demand, supply, storage, etc.)

gpi

GPI

Gas Pipeline

Models the transport of gas between two distant locations

gcs

GCS

Gas Compressor

Models the increase of inlet pressure to a higher outlet pressure to ensure continuous transport and delivery of gas to customers at the contracted nominations and delivery pressures

gcv

GCV

Gas Control Valve

Models the reduction of inlet pressure to lower outlet pressure or the control of gas flow to a downstream network

gva

GVA

Gas Valve

Models a valve station, which is used to route the gas stream and shut down sections of the network for maintenance or in case of a disruption.

gre

GRE

Gas Resistor

Models passive devices that cause a local pressure drop, such as meters inlet piping, scrubbers, coolers, heaters, etc

gsup

GSUP

Gas Supply

Models the injection of gas at a node

gdem

GDEM

Gas Demand

Models the consumption of gas at a node

gstr

GSTR

Gas Storage

Models the withdrawal and injection of gas from/into the storage inventory of an (underground) gas storage facility

lng

LNG

LNG terminal

Models the arrival of LNG-vessels and the discharge, storage, regasification, and injection of liquefied natural gas in an LNG regasification terminal

gqual

GQUAL

Gas Quality

Models the thermo dynamic properties (gross/net calorific value, relative density, etc.) and the mixtures of different gas molecules (gas components) flowing through the network

gcmp

GCMP

Gas Component

Models the thermo dynamic properties (gross/net calorific value, relative density, etc.) of a gas molecule included in the gas mixture

gcus

GCUS

Gas Component Usage

Models the molar percentage of mixture of a gas component included in a gas quality

1. Gas network (GNET)

A gas network object is the top parent object in any model of a gas system in SAInt (Figure 1). A gas network object is modeled as a directed graph consisting of a set of gas nodes, branches, and externals that are connected with one another. A gas network contains a description of all geometric, topological, and relational information, as well as all network child objects and their static properties that do not change during the execution of a simulation (e.g., the length and the diameter of a gas pipeline).

The "nodes" of a gas network are objects that identify either junctions among the network branches or physical points where gas can be supplied to or withdrawn from the system.

The "branches" of a gas network are objects that establish how nodes and externals are connected and can passively (e.g., with gas pipelines) or actively (e.g., with gas compressor stations or control valves) modify the state of the gas that flows through them. Branches of a gas network are "pipelines", "compressor stations", "control valves", "valves", and "resistors".

The "externals" of a gas network represent objects supplying or withdrawing gas from the system. Externals of a gas network are "supplies", "demands", "storages", and "LNG terminals".

A gas network also contains further gas-specific objects like "gas qualities", and "gas components".

Scenario events define a change in the settings of a gas network object during the execution of a scenario. Gas network events can be used to customize the network and simulation settings and to compare the effect of different assumptions across simulation scenarios. The following list describes the scenario events available for a network object.

Summaries for the properties and the events of GNET.

2. Gas network container

The child objects of a network can be grouped into the following subsets, also referred to as "containers": sub, zone, and group. These arrangements can be used to define relevant information for a network, or they can simply be used to conveniently aggregate outputs of a scenario.

2.1. Gas sub (GSUB)

A sub (also referred to as sub-network or sub-system) is a subset of nodes, branches, and externals of a network. A sub is branch-oriented, i.e., only branches can be assigned to a sub, and every branch belongs to only one sub. The FromNode and ToNode of a branch, as well as the externals connected to these two nodes, are implicitly added to the sub. Thus, nodes connecting branches of different subs and the externals connected to these nodes are always included in multiple subs. But all properties of a sub are determined by the branches belonging to it.

Summaries for the properties and the events of GSUB.

2.2. Gas zone (GZN)

A zone is a subset of nodes, branches, and externals of a gas network. Differently from subs, zones are node-oriented, i.e., only nodes can be assigned to a zone, and every node belongs to one zone. Branches with a FromNode and ToNode belonging to the same zone are implicitly added to the corresponding zone. In contrast, branches with a FromNode and ToNode belonging to two different zones do not belong to any zone. Externals are also implicitly added to the zone of the node they are connected to. But all properties of a zone are determined by the nodes belonging to it.

Summaries for the properties and the events of GZN.

2.3. Gas group (GGRP)

A group is a subset of different child object types (e.g., nodes, branches, externals, etc.) of the whole network. In contrast to subs and zones, groups do not follow any specific assignment rules. Thus, a child object can be assigned to multiple groups, and a group can have as many child objects assigned to it as there are child objects in the network.

Summaries for the properties and the events of GGRP.

3. Gas node (GNO)

Nodes represent objects describing a junction among two or more gas branches, as well as a location in the gas network where gas can be injected or extracted through externals (e.g., demand, supply, etc.).

Summaries for the properties and the events of GNO.

4. Gas branch (GBR)

A branch is a general type of object which describes a generic connection between two gas nodes. Different types of branches exist in SAInt and the following sections provides more details on each of them.

4.1. Gas pipeline (GPI)

A gas pipeline is an object which models the transport of gas between two locations and connects two nodes. Gas pipelines (also referred to as pipes) are passive branches, as no active control can be imposed over them. Gas flowing along a gas pipeline is subject to pressure drops due to friction. The magnitude of the pressure drop depends on the flow rate, the composition, the thermodynamic state of the gas (pressure, temperature), and on the properties of the pipelines (diameter, length, internal roughness).

Summaries for the properties and the events of GPI.

Inlet and outlet-derived properties do not depend on the flow direction but on the drawing direction of the pipeline (i.e., from which start node to which end node).

4.2. Gas compressor (GCS)

A gas compressor is an object that increases the inlet gas pressure to a higher outlet pressure to ensure continuous transport and delivery of natural gas to its customers at the contracted nominations and delivery pressures. A gas compressor is an active branch connecting two nodes, and it requires an assigned control mode (scenario event) to operate. Gas compressors typically use part of the fuel that they process to produce compression work. In the current release of SAInt, the fuel consumed at a compressor station is not accounted for any flow balance the compressor station contributes to, but it is set apart.

Summaries for the properties and the events of GCS.

4.3. Gas resistor (GRE)

A gas resistor is an object describing any passive device that reduces the pressure of the gas flowing across it. A gas resistor can be used to model pressure drops across different objects such as gas meters, coolers, heaters, etc.

Summaries for the properties and the events of GRE.

4.4. Gas control valve (GCV)

A gas control valve is an object that reduces the inlet gas pressure to a lower outlet pressure and regulates the quantity of gas flowing through the station. A gas control valve is an active branch, and it requires an assigned control mode (scenario event) to operate.

Summaries for the properties and the events of GCV.

4.5. Gas valve (GVA)

A gas valve is an object that stops or limits the gas flow between two connected nodes. It is an active branch, and it requires an assigned control mode (scenario event) to operate. A gas valve can be modeled as open (bypass) or closed (off). In reality, gas valves can also be partially closed to control flow velocity. In such cases, these objects may exert a non-negligible resistance to the flow.

Summaries for the properties and the events of GVA.

5. Gas external (GXT)

A directed connection to a single gas node which allows the extraction and injection of gas at that node. A gas external is used to model demands, supplies, storage facilities and LNG terminals in a gas network. An arbitrary number of gas externals can be assigned to a gas node.

5.1. Gas demand (GDEM)

A gas demand object represents a demand for gas at a particular node. A gas demand withdraws gas from the network, and it can be assigned with a flow rate or pressure control set-point.

Summaries for the properties and the events of GDEM.

5.2. Gas supply (GSUP)

A gas supply object represents a facility supplying gas in correspondence of a particular node. A gas supply injects gas into the network, and it can be assigned with a flow rate or pressure control set-point. A gas supply can be coupled with a gas quality object to specify its gas composition. A gas supply injects gas with a default quality (DefaultQuality property of the gas network) when no quality is specified, or when a turn off quality tracking (QTOFF) event for the gas network is introduced in the simulation scenario.

Summaries for the properties and the events of GSUP.

5.3. Gas storage (GSTR)

A gas storage object represents an underground storage facility located in correspondence with a particular node. A gas storage can supply or absorb gas from the network depending on its control mode and the network conditions. Typically, the rate at which gas can be injected into or withdrawn from the gas storage depends on its inventory. The consequent operating region of a gas storage is reflected by its storage envelope.

Summaries for the properties and the events of GSTR.

5.4. LNG terminal (LNG)

An LNG terminal object represents a liquefied natural gas (LNG) regasification terminal in correspondence with a particular node. An LNG terminal supplies gas to the network, and it can be assigned a flow rate or pressure control set-point. Volumes of gas supplied to the grid are withdrawn from the LNG terminal storage tanks as long as its inventory value is greater than zero. The LNG storage tank can be recharged during a simulation scenario by modeling the arrival of LNG vessels.

Summaries for the properties and the events of LNG.

6. Gas quality (GQUAL)

A gas quality object represents a mixture of gas components. Every gas network has a default gas quality defined. Besides that, other supply qualities can be added from the GUI or by importing them from a file. These can be assigned to gas supplies, storages, and LNG terminals from their input properties.

Input properties of a gas quality object include the molar composition and the thermo-physical characteristics of the gas mixture, like its gross/net calorific values and molar mass. The molar concentrations of the components should sum up to 100%. By default, thermo-physical properties are automatically computed based on the composition. However, custom values can also be used to overwrite calculated properties.

Summaries for the properties and the events of GQUAL.

6.1. Gas component (GCMP)

SAInt is not limited to model transportation and distribution of natural gas. It can effortlessly model a wide variety of gas mixtures (i.e., a gas quality) by creating and managing gas components. SAInt offers a built-in library of common gas components to create and adapt a gas quality to the user’s specific needs. It also provides a straightforward and simple capability for defining a new component when not available. Gas components can be mixed to obtain a certain gas quality, whose properties are estimated by SAInt or derived by the user inputs.

A gas component represents a pure chemical species to be used for the creation of gas quality objects. Every gas network has a list of gas component objects that can be used to define gas qualities. The list of gas components contains, by default, only methane and can be expanded by picking more gas components from the built-in library available in SAInt. New custom gas component objects can also be defined.

The library covers the main hydrocarbons and the most common impurities and can be easily extended. The full list of components, and associated SAInt code in brackets, is:

Hydrocarbons

methane (C1), ethane (C2), propane (C3), iso-butane (IC4), n-butane (NC4), iso-pentane (IC5), n-pentane (NC5), n-hexane (NC6), n-heptane (NC7), n-octane (NC8), n-nonane (NC9), n-decane (NC10);

Non Hydrocarbons

where we can distinguish inert gases like nitrogen (N2), rare gasses like argon (AR) and helium (HE), impurities like hydrogen sulfide (H2S), water (H2O), oxygen (O2), carbon monoxide (CO), or carbon dioxide (CO2), and other gasses like hydrogen (H2).

In brackets is reported the string used for the property ShortName in SAInt for the component and not its chemical notation.

A gas component has a set of physical and energy related properties. As an example, Figure 2 shows the properties for methane. Please, note that the properties of the components in the built-in library cannot be edited. The user can only modify the properties of user-defined components.

create gas component 00
Figure 2. Example of the properties of the gas component methane (C1).

Gas components, as gas qualities, are objects within a network object.

The physical and energy properties (e.g., gross/net calorific value, relative density, molar mass, etc.) of a gas component included in SAInt built-in library cannot be edited. In the case of a custom gas component, specific properties can be described in input.

Summaries for the properties and the events of GCMP.

Refer to the following How-Tos for more details on Add a Gas Component, Create and Edit a Gas Quality in the GUI, or How to Import/Export Gas Qualities.