Gas supply systems are made up of interconnected network of pipes, some of which are very large. During production, problems are usually encountered either due to other system failure or debris in the gas pipes. Network analysis is the examination, monitoring and analysing of gas flowing through network of pipes and the interdependence of each pipe on the others in the network. Network analysis model when established helps to solve the problems in the gas network to a greater extent and thereby optimizing production processes of gas.
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1.1 AIMS AND OBJECTIVES OF NETWORK ANALYSIS
The aims and objectives of networks analysis is to determine the flow of gas in all the pipes in the network and the pressure at all junctions of the pipe system. This can be achieved by knowing the fixed pressure at all supply points, the loads or off takes from the system and the minimum pressure at any point in the network.
Unsteady State Flow
Unsteady state flow or transient flow, is a flow where the mass flow rate and pressure are changing with time. When changes occur to a fluid systems such as the starting or stopping of a pump or closing or opening a valve, then transient flow conditions exist; otherwise the system is steady state. Often transient flow conditions persist as oscillating pressure and velocity waves for some time after the initial event that caused it.
Steady State Flow
Steady state flow is defined as that in which the various parameters do not change with time.
1.2 INPUT REQUIREMENTS
The reliability of any network analysis model is dependent on the input data, as such the accuracy of the input parameter/device is important for any network analysis model to be of great value. These input requirements include
This is the fundamental equation to which most gas network analysis model is based on. Although, it is not really an input data but it is the initial step that is considered and taken before any input data or device is selected.
The flow equation must be chosen to suit the network been analysed. For example, considering a low pressure gas supply network, Î”P = K Qn is used where constant k and n will be affected also by the method chosen to calculate the friction factor and other flow dependent parameters.
This is the property of the pipe that determines the cross-sectional area available for gas to flow in the network, most especially the internal diameter. It is directly related to the flow capacity, as such accurate values of the pipe internal diameter (region where the gas passes) of each of the pipes in the network is essential. This is because for each pipe in the network the internal diameter forms part of the constant K, which is proportional to 1/d5. So an error of 1% in the input value of diameter will create an error of 5% in the value of K. This in turn would produce an error of 21/2% in the flow calculation.
The accuracy can be affected by scale of deposit been accumulated around the inside of the pipe. For example, at Carbon Black Petrochemical Plant of (WRPC) Warri Refining and Petrochemical Company (dec.2005), the propane gas in the network was flowing below specification. This was caused by large debris of sulphur accumulated around the corner of the inside of the pipe. The sulphur debris which was as a result of failure of the “Desalted” in the crude distillation unit to remove the sulphur substance associated with the crude, blocks the pipe internal diameter thereby limiting the flow of propane gas in the network. These lines have to be flushed with high pressure steam and then dried with hot air into the gas network.
This is another important input parameter that must be considered in any network analysis. In flow equations, the pipe length is directly proportional to the pressure; as such any error will definitely influence the value of constant K which in turn affects the flow equation. The errors are small and negligible on Distribution Network of reasonable size. However, it is not the same for transmission system analysis because transmission system is made up of fewer pipes with greater length and the therefore accuracy of the pipe length is important in this case.
Source pressure is the fixed gas pressure from an establish source into the gas network. For example, the pressure of natural gas coming into Carbon Black Plant of (WRPC) through the natural gas scrubber is already fixed from the Nigeria Gas Company Liquefied Natural Gas Plant.
This pressure is normally taken to be fixed and their location in relation to the network is therefore important for accurate solution.
This is the amount of gas that is expected to be supplied by the gas network system. It is an area of uncertainty because it is usually estimated at the initial process of the network analysis. This is done by collating meter readings on an annual, quarterly or monthly basis and converted to peak flows or by applying factors that are determined from load research or experience. Also, load forecasts produced by statical analysis for a region, area etc is used to allocate these throughout the network.
One major problem in assessing the peak demand is the rate of growth and the positioning of the different rates of growth accurately within the network. This is because some areas are declining due to depopulation and load growth can be considered negligible, where as some other areas are undergoing rapid development and considerable rates of growth can be forecast; between these extremes is a large area of uncertainty. The solution therefore is by estimating and closely monitors the trends in load growth by updating the network at regular intervals as information on new loads are obtained.
1.3 OUTPUT REQUIREMENTS
The output data is as good as the input data in any network analysis because it depends mainly on the input data. It is important because it is used to fine tune the network analysis model.
Also, the primary requirement of the output from any network analysis program is that it should accurately represent the actual network. This is achieved by:
Ensuring the Input Data is Accurate:
Since the output is strictly dependent on the input data, the accuracy of the input data is essential in the output requirement because if there is error in the input data, the resulting output data must be incorrect.
This is the close monitoring of the load data been estimated at the input requirement stage. This includes the measuring of flow in pipes supplying known number of customers in order to assess the accuracy of the assumptions made in determining the load data. It is done occasionally because it is expensive.
Another output requirement process is by conducting surveys over well defined areas where total loads can be accurately assessed from meter readings in these areas.
Application of Efficiency Factor:
Efficiency factor is applied using data from pressure and flow surveys to the pipes within the area. It is important in the output requirement because it compensates for the difference in the actual gas flow from the ideal gas flow equation and design.
1.4 INDUSTRIAL APPLICATION OF NETWORK ANALYSIS
Network analysis is applied in almost every process industries that require interconnection of large chain of materials for its production and distribution. As such, it is applied in electrical industries where network of circuit are used to supply electricity and more recently in telecommunications, where is been applied in GSM, telephone and computers interconnection. It is also applied in water cooling plants, process pipe work design etc. But the scope of this report is limited to gas network.
The gas distribution companies use network analysis to model their distribution system for low pressure gas distribution. This is done by using the basic steady state equation for low pressure gas.
Î”P = K Qn
Where constant k and n are dependent on the type of gas and the pipe system been analysed.
Î”P is the pressure drop, which is flow of gas in the network depends on
Q is the volume flow rate of the gas in the network.
Figure 1: Diagram of Natural Gas Application
Once a basic gas network model has been established, its uses can be extended to a wide variety of applications. This includes
Identification of Reinforcement Schemes:
This is done by determining the minimum pressure areas within an existing network and hence the requirement for reinforcement of these areas, because if the load data and load growth rates is not accurate, it produces interruptible loads which in turn takes gas during peak demand periods. The optimum scheme for reinforcement will be based on the least cost solution which network analysis program can be used to determine the engineering requirements of alternative options; for example, increasing the sizes of certain mains during a renewal programme could be a cheaper way of providing reinforcement than, say, anew feeder main.
Network analysis is used to optimize pressure in a gas network. Most networks are supplied from a number of sources and the pressure settings of these sources will determine the flow pattern through the network. This pressure optimization process helps to reduce leakage in low pressure networks reinforcement, maximise the use of pipes within the network and minimise the requirement for compression in high pressure systems.
The gas network analysis model helps to plan the sequence of operation during production or distribution, prepare contingency plans if there is any plant failures, pipe breakage or maintenance shut-downs under varying conditions.
Testing the Network:
It helps in testing the network under certain conditions, like the effect of adding a new load. This will determine whether the existing network is capable of taking the new load and, if not, the extent of additional work required to accommodate it.
Designing New Networks:
It is applied in designing of new network to evaluate the kind of materials to use and also providing the least cost solution.
2.0 PRINCIPLES OF NETWORK ANALYSIS
The analysis of network is done by different methods which include Hardy Cross (mesh and nodal) method, Newton Rapson (mesh and nodal) method and the Hybrid methods. All these methods are governed with the same basic rules which are termed principles of network analysis.
Network analysis method
Figure 2.1 Summary of the Network Analysis Methods
However, some important features of network have to been understood in order to appreciate these basic principles and thereby knowing how to apply them in solving network problems.
These features include:
Source: This is any point where gas enters the network at a fixed pressure.
Node: This is a junction of two or more pipes sections in the network and can include the free end of a spur pipe. It is usually a physical feature of the network.
Figure 2.2: A simple node
Tree: This describes a network in which each pipe is connected to a source via one route only. That is, only one flow path exists to supply each pipe or node within the network.
2 3 4
Figure 2.3: A simple Tree
Loop: This describes a condition where two or more paths exist to supply a particular pipe or node within a network. A loop is therefore a closed path which begins and ends at the same point.
2 3 4
Figure 2.4: A simple loop network
2.1 The basic rules that govern the flow of gas through features of the network are known as Kirchhoff’s Law. There are:
Kirchhoff’s First Law:
This states that the volume flow entering a given node is equal to the volume flow leaving that node in a specified period of time.
So therefore, the algebraic sum of all flows entering and leaving a given node is equal to zero, i.e.
Î£Q = 0
Kirchhoff’s Second Law:
This states that, at a given instant in time the pressure difference between any two nodes in a network is fixed and is the same for every flow path between those two nodes.
So therefore, the algebraic sum of the pressure drops around any given loops is equal to zero, i.e.
Î£Î”P = 0
Because only one pressure value can exist at a given node in a network, this law applies to the difference in the pressures squared, i.e.
Î£Î”P2 = 0
Note that a network is not balanced unless both laws are satisfied.
3.0 RULES THAT UNDERPIN ALL NETWORK ANALYSIS METHOD
Before going into the details of the rules, it is important to know the terminologies used in network analysis. They are as follows.
Source- Any point where gas enters the network at a fixed pressure
Node- it is a junction of two or more pipe sections and can also include the free end of s spur pipe. It’s usually a physical feature of the network but you may choose to impose one on the network at some particular point for convenience.
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3.1 Kirchhoff’s First Law
Kirchhoff’s first law states that the volume flow entering a given node is equal to the volume flow leaving that node in a specified period of time. From this it can be stated that the algebraic sum of all flows entering and leaving a given node is equal to zero. That is.
Figure 3.1: Flow diagram of Kirchhoff’s first law
Kirchhoff’s Second Law
Kirchhoff’s second law states that at given instant in time the pressure difference between any two node in a network is fixed and is the same for every flow path between those two nodes. From this it can be stated that the algebraic sum of the pressure drops around any given loop is equal to zero. That is.
âˆ‘âˆ†P â¼ 0
And because only one pressure value can exist at a given node in a network, this law also applies to the difference in the pressure squared. That is.
A network is not balanced unless both laws are satisfied.
Figure 3.2: Kirchhoff’s second law.
Note that the above figure is the 2nd law as applied to electric circuit. The figure is a simple circuit showing the potential differences across the source and the resistor. According to Kirchhoff’s 2nd law the sum of the potential differences will be zero.
4.0 SIMULATION MODELING FOR NETWORK ANALYSIS
4.1 SynerGEE Gas Unsteady-State Modules (USM)
SynerGEE Gas Unsteady-State Module (USM) developed by SynerGEE Stoner Software, performs off-line unsteady flow condition analysis in natural gas networks.
USM can be used to model gas composition, heat content and specific gravity as it varies with time and as system supplies change.
USM allows modelling of simple or complex pipeline systems. Integrated models can include all facilities, including pipes, valves, regulators, compressors, storage fields and other special facilities.
Despite the rate of change in your transient scenario analysis, USM balances the network volumetrically or thermal (which includes option of both heat content tracing and component tracing capabilities).
It is quite expensive to acquire.
4.2 Pipe Flow Compressible
It calculates the flow rates and pressures in the entire piping system by performing a total network analysis.
It Incorporates blowers, compressors, and control valves into the piping system model, showing how the entire piping system operates.
It considers alternate operating conditions for the system and shows how the system operates.
It shows where choked flow conditions exist in a compressed gas system.
It calculates the physical properties of the gas as it flows through the piping system.
It can handle unlimited number of pipes.
It can only handle gases and not liquids.
Stoner Pipeline Simulator
SPS can handle any combination of scenarios including control system analysis, equipment performance analysis or pressure flow capacity analysis with user-defined levels of complexity.
SPS can simulate existing equipment, including pipes, centrifugal and reciprocating compressors and station yard piping, regulators, valves, headers and heat exchangers, exactly as it is configured in the field.
It is highly expensive.
Pipe Flow Professional
Figure 4.1: Pipe flow Professional
It simulates the operation of piping systems transporting liquids and industrial gases under a variety of expected operating conditions.
It has no limits as to the number of pipelines it can handle and provides accurate results for series, branching, looped systems (both open and closed), as well as primary / secondary piping systems.
The program simulates the operation of the complete piping system showing the interaction of pumps, compressors, pipelines, control valves, and components by incorporating the following features into a single, easy to use program.
Pipe Flow Professional cannot be used for long runs of pipelines as those typically found in gas transmission system.
It cannot be used for high pressure drops such as vent and relief headers.
It cannot be used in situations of large changes in pipe diameter.
It cannot be used for low-pressure vacuum systems.
4.5 Simsci Simulator
Figure 4.2: Simsci Simulator
It is very good for complete network system.
Lack of inclusion of compressor, valves and sudden changes in pipe.
4.6 HYSIS Simulator
Figure 4.3: HYSIS Simulator
Hysis simulator allows modeling of simple or complex pipeline systems.
It has no limits as to the number of pipelines it can handle and provides accurate results for series, branching and looped systems.
It is expensive.
It is not possible to model a stream of mixture, say natural gas and crude oil. Each component of the mixture has to be modeled in a separate stream then combine into pipeline.
5.0 COMPUTATION OF LINEPACK VOLUME
The maximum and minimum operating condition for a natural gas transmission pipeline with an internal diameter of 600mm shown below
Description: C:UsersRaphael AkinsanyaDocumentsDownloadsGas Flow Question 6.jpg
Figure 5.1: Operating Condition for a Natural Gas Transmission Pipeline
If the natural gas is a representative of southern North Sea Gas, what is the quantity of line pack released at MSC when the flow rate changes from Qmin to Qmax ?
The gas temperature is 15°C = 273.15 +15 = 288.15°K
Zm at maximum flow rate= 0.875
Zm at minimum flow rate = 0.864
Using the mean pressure (Pm) Equation:-
For at minimum flow rate; P1 is 70 barg = 71.013bar and P2 is 60 barg = 61.013bar
For at minimum flow rate; P1 is 70 barg = 71.013bar and P2 is 50 barg = 51.013bar
Pressure at MSC (Metric Standard Conditions) = 1.01325 bar
Pressure at Qmin = 66.14bar
Pressure at Qmax = 61.56 bar
Length of Pipeline = 100km= 100 000m
Temperature at MSC = 15 °C = 273.15 +15 = 288.15°K
Using the Line pack Equation
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