Trans Basic Concepts

PIPENET VISION TRAINING MANUAL TRANSIENT: CHAPTER 1 PAGE 1 OF 13 REVISION 2.3, SEP 2011

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PIPENET VISION TRANSIENT MODULE

CHAPTER 1: BASIC CONCEPTS

1. Introduction

The PIPENET Transient module models flows and forces in pipe networks and how they evolve with time. The results calculated can be displayed as graphs or tables in a report file.

The Transient module has two options, Spray and Standard. The Standard option is the default general use option, whilst the Spray option is for the analysis of fire protection systems in accordance with either NFPA or FOC rules. Other sources of information on using PIPENET Transient are:

The Help menu system

The Training manuals and data files (supplied on the PIPENET CD)

Demonstrations (supplied on the PIPENET CD)

2. Components and Libraries

A PIPENET network model is constructed from Components. Most of these are components through which fluid flows, such as pipes, valves, pumps, or tanks. Additional components allow you to model control systems, such as sensors and PID controllers which change the state of other components when certain conditions occur in the network. We will provide a full summary of the components offered by PIPENET later in this chapter. Many aspects of the components you create will be common to many projects. For example, although the length and position of a specific pipe will be specific to the project in which it is used, the schedules of available pipe materials and diameters can be common to several projects. PIPENET promotes data reuse and consistency via the idea of a Library which contains the reusable data for:

Pipe schedules

Fittings

Valves

Pumps

Other components such as caissons or PID controllers have no reusable aspects and are always defined in-situ. Library data is created via the Libraries option in PIPENETs menu bar. When you save your work, the library information is saved to a file with a .SLF (Sunrise Library File) suffix, and the


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project-specific information is saved to an .SDF (Sunrise Data File). You can reuse an SLF file in another project by opening it as a System Library. When you later re-open your project (SDF) file, any libraries which it references will be opened for you automatically.

3. Creating a Network

3.1 Units

For each measure (length, flow rate, etc), you can choose the units which you want to use and the number of decimal places to display, using the Options | Units menu.

3.2 Drawing in the Schematic Window

Once you have created any library data you need, you can create the components in your network. Components can be placed directly in the Schematic Window using the mouse. Components through which fluid flows have one or more Flow Nodes drawn as solid black dots. Where flow nodes of different components coincide, the components are automatically connected in the network. Similarly, components which are part of a control loop have Information Nodes which are connected where they coincide. Components are automatically assigned a unique label by PIPENET. Prefixing labels with a tag (e.g. RING/27) is recommended, to improve readability and increase the number of different components that can be labelled. Options | Display Options allows you set up an isometric or plan grid to help you input the data, and choose whether or not you want to display node and link labels. If you have an existing drawing in Metafile or Autocad DXF format, you can use View | Import Graphical Underlay to display it. For large networks, you might want to use the overview window (View | Schematic overview) to help you navigate around the network. Schematic drawings are NOT to scale!

3.3 Component properties

The Properties of a component appear in the Properties Window (if it is not visible use ViewProperties) to make the properties of a particular component appear in the property window click on the component in the Schematic window. The properties window allows certain aspects of a component to be edited. For example the properties of a pipe that can be altered are Label, Type, Diameter, Length, Net-height


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change, Roughness and Additional k-factor. If a property cannot be edited by the user, it will appear greyed-out.

3.4 Component specifications

The Specification of a component is a description of how it behaves as time passes. Specifications are also viewed and edited in the properties window. To see the specification of a component, click on the Information Node that is attached perpendicularly to the component in the schematic window (it is drawn as a thick circle).

3.5 More on nodes

We have mentioned two kinds of node above:

A Flow Node - a physical point in the network, through which fluid flows

An Information Node - these are used to attach specifications to components, and to connect together components into a control loop.

If a flow node is a point at which fluid enters or leaves the network, it is also referred to as an Input/Output node.

Input/Output nodes have a specification attached, that describes the boundary value pressures and flow rates with respect to time.

3.6 Fittings

Some examples of fittings are bends, butterfly valves or tee-bends in pipes. They can be added to a selected pipe using the Fittings tab of the Properties window. Alternatively, if the k-factor of the fittings on a pipe is known, then the fittings do not need to be modelled explicitly, and the k-factor can be entered as an additional k-factor in the pipe properties.

3.7 Using the Data Window

The Data Window is an invaluable tool for viewing and editing your network in tabular form, and/or selecting results that you want PIPENET to calculate. If it is not visible you can enable it with ViewData Window. Within the Data Window, the data tab displays tables showing all objects of the same type that are drawn in the Schematic window. There is a wide range of choices for the object type such as

Any component type (pipe, pump, caisson, etc)

Nodes

Specifications You can sort the items within the data tab to quickly identify those of interest, and clicking on a row in the table will highlight the corresponding item in the schematic. There are copy and


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paste facilities to make the same change to multiple rows, and you can even copy data to and from a Spreadsheet program! We will discuss other tabs of the Data Window later.

3.8 Check Network

We recommend that you check your network for basic errors before attempting a full calculation, using this toolbar button:

4. Forces

PIPENET allows you to define the forces which you wish to measure using the Forces tab of the Data Window. Forces may be Simple or Complex. To define a Complex Force, you need to supply PIPENET with a connected series of components, and two end conditions - one at the inlet and one at the outlet to the component series. Each end condition requires:

The surface normal to the pipe direction, facing outward from the Control volume

containing the selected components

Whether the end condition is rigid (such as an elbow or a section of piping) or

elastic (such as a hose or an open tank)

The resultant force on the component series will be calculated by summing:

The External Body Force, which includes the weight of the components and the fluid

in them.

The static force (due to fluid pressure)

The dynamic force (due the momentum change of the fluid)

Frictional force

A Simple Force is a special case in which all of the components lie in a straight line on the same axis. In this case the surface normal at both the inlet and outlet can be deduced and do not need to be specified by the user. There is no external body force option, as its direction is unlikely to be along the common axis. Simple Force is usually the appropriate choice for third party force analysis programs. Note that the momentum change is measured according to the difference in orientation of the inlet and outlet surface normals, not according to the geometry of any components used. In practice, a force should only use a sequence of components with at most one direction change (defined by the inlet and outlet surface normals), and no internal elastic joints. Sequences which do not meet these criteria should be sub-divided into smaller component sequences.


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5. The Simulation Environment

As well as the network components, there are some additional parameters which we need to specify to determine the outcome of the simulation.

5.1 Fluid properties The fluid properties can be defined in OptionsFluid,

5.2 Ambient conditions The ambient conditions are defined in OptionsModule options. PIPENET needs to know these conditions in case air enters the system.

5.3 Simulation Time and time-step

The Simulation Time is the length of time that the simulation runs for. The Calculation time-step controls the number of calculations that occur during the simulation. Smaller time-steps lead to a higher degree of accuracy but more calculations and a longer run time. The maximum allowed calculation time-step is the time taken for a pressure wave to travel the length of the shortest elastic pipe in the network (you can use short pipes as opposed to elastic pipes to increase the time-step). Both are found in OptionsModule options.

5.4 Graphical/Tabular Times and time-step

Graphical time is the period of time that will be displayed on the graph. The graphical time-step affects the number of data points used to construct the graph. Again a smaller time-step means more data points, which increases accuracy but also the time required to perform the simulation and draw the graph. The tabular time-step, called output time-step is the time-step used in the table of results. Both the graphical and tabular controls are found in CalculationOutput If the graphical or tabular time-steps are smaller than the calculation time-step then they will be used as the calculation time-step.

5.5 Steady state and Run-in Time The simulation requires that the system be in its normal steady state before the perturbations under study are made. PIPENET can usually determine this steady state for you, because is the system is set up with reasonably plausible parameters and left to run, it will naturally converge towards its steady state,


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The Run-in Time is the period of time for which the system is left to run before the simulation proper is started, in order that the simulation begins in the steady state. PIPENET will estimate a suitable value for the time necessary, but you can override it using Calculation Options | Initial State. Where control systems are used, it is more likely that the user will need to use a degree of trial and error to determine an optimal run-in time (this is because the interactions between different parts of the network are more complex and so the system may take longer to settle down into a steady state). In extreme cases, Calculation Options | Initial State also allows you to provide an initial guess file which specifies the steady state of the system, though this is rarely needed. A useful tip to check whether the amount of run-in time chosen is sufficient is to allow the system to remain unchanged for the first couple of seconds of the simulation time (i.e. arrange the perturbations to happen after a few seconds, not at time zero) Your result graphs against time should then show a brief flat line at the start, proving that the system is steady prior to the perturbation.

6. Calculation results When you have everything ready to your satisfaction, use Calculate | Go or the toolbar button to perform the calculation. There are several ways in which you can examine the results.

6.1 Schematic view PIPENET Transient allows you to annotate links and nodes on the schematic view with the results of your calculations. Use the toolbar selectors that look like this:

You may want to change the number of decimal places displayed, which you can do using the Display Precision fields in Options | Units. The red buttons allow you to ask for links and nodes to be coloured according the magnitude of the quantity concerned. PIPENET will automatically divide the range of values into coloured-coded sub-ranges, or you can choose your own ranges using the Colouration menu.

6.2 Output report Calculate | Go will ask you for a location where it will generate a .OUT file, which you can open with Output | Report. This contains:


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Summary of the network and environment

Maximum/minimum pressures, forces and pressure envelope

Additional tables that you can request using the Data Window Tables tab

6.3 Graphs

6.3.1 Creating graph data Graphs are a key feature of PIPENET Transient, clearly communicating how a chosen variable (such as pressure or flow rate) varies over time at a chosen point in the network. The raw graph data is created in a .RES file whose location you specify when you invoke the calculation. Because there is a lot of computation and disk space required for the graph data, PIPENET only generates the data for certain variables and network locations which you can choose. You need to make this choice before you perform the calculation, either by:

Right-clicking on one or more network components, and using the Select Results

menu. This is generally the easiest way for simple networks.

Using the Data Window Result Graphs tab.

Note that pipes, unlike all other components, have a choice of whether the observed quantity is to be measured against time or against distance along the pipe. Also note that Forces, unlike all other observables can only be chosen via using the Results Graph Tab.

6.3.2 Viewing graphs in the Graph Viewer There are two ways to invoke the Graph Viewer:

Right-click a network component, and use the View Results menu

Use Output | Graphs

Once the Graph viewer is running, you have a variety of options at your disposal, such as:

Selecting and overlaying different graphs

Annotating the graphs

Choosing fonts and line styles

Zooming and Printing

Opening a previously created .RES file.

6.4 Forces

When there is pressure surge, pipe systems often become damaged because of transient hydraulic forces. Pressure surges can produce large unbalanced forces, which can be particularly dangerous especially if the system is not well supported. The definitive method


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for determining whether a system can sustain damage is by calculating the transient hydraulic forces and performing pipe stress analysis calculations (using a program such as Caesar II). It is clear, therefore, that hydraulic transient forces are of fundamental importance. In PIPENET VISION, both the change in the pressure and the momentum are used to calculate hydraulic forces. The Transient module offers a choice of calculating either the total forces or just the dynamic forces. The total force is the sum of the steady state and dynamic forces. Information about forces can be viewed in the output report or the Graphic Viewer as described above. In addition, details of each force are output to a .FRC file, whose location you specify when you start the calculation. This file contains the information that is typically required by third party force analysis tools.

7. PIPENET Transient Components Quick Reference

7.1 Pipes

(Elastic) Pipe

These are the normal pipes to use for modelling with PIPENET Transient. They are assumed to have a uniform circular cross sectional area

Short Pipe

The Short Pipe is used to model incompressible flow through a rigid pipe. Pressure transients are assumed to travel across the short pipe instantaneously.

Because PIPENET Transient determines the calculation time step according to the shortest elastic pipes in the network, using a short pipe as a replacement for an elastic pipe can offer great savings in computational time.

Pipe Bundle

Models a bundle of connected pipes that have the same diameters and lengths

Typically used to model heat exchanges and condensers

Breaks and leaks can be modelled.

It is assumed that fluid inside the pipe is a liquid, and outside is a vapour or gas.

Compressible Pipe The compressible pipe is specialised to simulate the first path of a shock wave caused

by a sudden increase in pressure at the inlet of the pipe, such as when a pressure relief valve opens very rapidly.

It is rarely used.


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7.2 Fittings

Fittings For example bends, tee-pieces, and filters, these do not appear explicitly as components, but are added to pipes via the Fittings Tab in the Properties window As noted earlier, you do not need to model fittings explicitly if you know their k-factor.

7.3 Pumps

Simple Pump

Provides a pressure increase which depends on the pump speed and performance (pressure vs flow-rate) curve

A variety of ways are provided to define the performance curve in the library

This model simulates the behaviour of a pump in the positive quadrant only (positive rotor speed and positive flow rate)

Turbo Pump

Like a simple pump but with the added feature that it can spin-down due to a pump failure

Uses Suter Characteristic Curves, which define pressure head and torque as a function of flow rate and pump speed over all possible operating conditions

This model simulates the behaviour of a pump in all four quadrants (any combination of rotor speed and flow directions)

Inertial Pump

Works like a simple pump at the steady state, but can simulate transient behaviour during start-up and stopping

Suter curves are not required for an inertial pump.

This model simulates the behaviour of a pump in the two positive flow quadrants (the rotor speed can be positive or negative)

7.4 Valves

Valves are components that have variable resistances to flow, causing a pressure drop across the valve.

Operating Valve


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General purpose valve whose setting is specified by the user through its information node

Regulator Valve Models a pressure control valve that has a controllable reaction time.

PIPENET calculates the valve position is calculated so that a specified pressure upstream or downstream of the valve is maintained

7.5 Non-Return Valves Non-return Valves have the characteristic that little to no fluid is allowed to flow in the negative direction.

Non-return Valve Allows unrestricted flow from the input to output node, but prevents all flow in the

reverse direction.

Check Valve

Models a swing-gate type non-return valve

A certain amount of flow is permitted in the reverse direction, according to the flow and pressure near the valve and the valve's physical characteristics.

Fluid Damping Check Valve A kind of check valve with a translating disk and plug

The flow in the forward and reverse directions depends on the hydrodynamic forces acting on a valve plug and damping disk.

Inertial Check Valve Models a check valve with a swing door hinged at the top.

Offers a detailed mechanistic model that takes into account parameters such as damping, the spring constant and the mass of the disc

The valve setting is determined by pressure and flow in the vicinity of the valve.

7.6 Relief Valves Relief valves have the characteristic that the fluid is allowed to flow out if the pressure becomes too high

Liquid Surge Relief Valve Used to relieve pressure surges

This valve allow fluid to flow if the pressure is too high

Bursting Disc


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Protects the system from high pressures

If the defined rupture pressure is reached, the disc will open in a time specified by the user and will not shut again during the simulation

Pressure Surge Release Modelled on a Daniel Model 762 gas loaded relief/back pressure control valve

Regulates and controls maximum pipeline pressures, or maintains a minimum back pressure in a system

Vacuum Breaker

Allows air at atmospheric pressure into the system thereby compensating for cavitation

The air is expelled to the atmosphere when the system pressure begins to rise again 7.7 Tanks

Accumulator Consists of a sealed cylinder or sphere with a single inlet/outlet

Trapped air acts as a cushion against pressure surges

Simulation stops if the accumulator drains completely.

Surge Tank Consists of an open-ended tank in which the fluid can rise/fall as pressure surges

occur near its inlet/outlet

Simulation stops if the surge tank drains or overflows during the simulation

Receiving Vessel Identical to a surge tank, but allows overflow out of the tank

Weir crests can be specified

Simulation stops if the receiving vessel drains completely.

Simple Tank Similar to the Surge tank, but additionally has a flow into the top of the tank,

determined either as a flow rate specification or a connection to the pipe network

7.8 Caissons

One Node Caisson

Pipe that is sealed at one end with an air inlet/outlet valve, with the other end connected to the network

Can be used to models pipes which are part-filled or dry.

If the fluid reaches the critical depth the simulation stops


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Two Node Caisson Type One Same as the one-node caisson except:

It has a built in non-return-valve at the caisson outlet (so the caisson will drain if the flow reverses)

There is also a built in air valve just before the built-in non-return valve which is the only path for an air exchange with the ambient atmosphere

If the caisson fills it behaves as a Short Pipe

Two Node Caisson Type Two Differs from the type one caisson in that:

The valve is opened or closed automatically according to pressure difference and fluid level specifications

When the pipe is full the elastic pipe model is used (instead of the short pipe model)

7.9 General pressure loss

General Pressure Loss Used to model a pressure drop where the resistance factor varies with flow-rate in a

complex relationship (beyond, say, the operating valve models provided).

7.10 Control Systems

Control systems can be introduced into a network to allow components such as valves or pumps to react to changes in pressure at a node or flow rate between two nodes

7.10.1 Sensors

All of the sensors listed below can be analogue or digital. They accept the same input, but the output of digital is discrete, and digital has an additional scan frequency parameter.

Flow Sensor Provides an instantaneous reading for the flow rate

Pressure Sensor Provides an instantaneous reading for the pressure

Pressure Difference Sensor Provides an instantaneous reading for the pressure difference

Transfer Function


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Models the dynamics i.e. response characteristics of a device such as a sensor, a valve, or a pump

PID Controller Takes a signal from a sensor or transfer function and converts it to a control signal for

the device to be controlled

Enables a target value (of flow-rate, pressure or pressure-difference) to be specified for an area/the system to tend to.

There are three types of controller; P proportional, I integral, D differential. These can be used can be used in isolation or conjunction to give a wide range of controller behaviours.

Cascade PID Controller Similar to a PID controller, but the set point (i.e. target value) does not have to be

constant throughout the simulation. It can be time, pressure, flow-rate or pressure-difference dependent or may be the output of another control system.

Switch Used to operate components according to a specific time or a pressure, flow-rate or

pressure difference reading

Signal Selector Used to select the minimum or maximum of two input signals, or to operate according

to the first signal until a designated time and the second signal thereafter.

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