Introduction:
This chapter builds on what was covered in Chapter 1 and extends the principles to cover firewater ringmain systems. Modelling firewater ringmain systems well requires understanding some of the capabilities of PIPENET which were not covered in Chapter 2. It is intended to be about both design methodologies and about techniques for using PIPENET itself. It has the following sections.
2 – Design Tips & Techniques
3 – Additional Principles in More Detail
4 – How to Model Firewater Ringmain Systems
5 – How to Set up the Desktop
6 – Different Phases of Input
6.1. Initialisation
6.2. Libraries (diversion)
6.3. Initialisation (again)
7 – Network Input
8 – Systems Based on Clack Shut Type Deluge Valves
9 – Systems Based on Elastomeric Type Deluge Valves
10 – Pump Selection Techniques
11 – Some Hints and Tips for Refining the Design
12 – Example of a Real Firewater Ringmain
The material in this document is partly for discussion and partly for actual input. If you have your laptop computer please be sure to use it.
2. Design Tips and Techniques:
In the previous Chapter we looked at the way deluge systems are modelled in some detail. We also saw why it is better to model deluge systems separately from firewater ringmains. In this chapter we will see how to model firewater ringmain systems in detail.
The basic principles of setting up a network are the same as for deluge systems which is covered in Chapter 1. However, we have to learn the following new aspects in order to be able to fully utilise the capabilities of PIPENET for modelling firewater ringmain systems.
- Inputting pump curves – Firewater systems often have pump curves and it is necessary to learn how to input vendor provided pump performance curves.
- How to deal with systems using “clack shut” valves and “elastomeric” valves as deluge valves – The best technique of modelling systems using conventional clack shut valves is quite different from that for modelling elastomeric valves.
- How to deal with multiple inputs and outputs – Firewater ringmain systems often have multiple inputs (for example, multiple pumps) and/or multiple outputs (for example hydrants, hose reels etc.). We need to understand how to apply specifications to systems with multiple inputs/outputs.
- Dealing with monitors, hydrants and hose reels – Firewater ringmain systems have all the above types of item. We need to understand how to model such items.
Usually there will not be one unique way of modelling a firewater ringmain system. This chapter is intended mainly to cover the basic principles. Each individual engineer and each company may wish to develop its own method of applying these principles.
3. Additional Principles in More Detail:
Let us discuss the above items one by one. Please note that the dialog boxes shown are for information only. Do not input data at this stage. The paragraphs below simply discuss the above matters in more detail.
3.1. Inputting Pump Curves:
It is assumed that units have been chosen as required. Go to Libraries/Pumps – coefficients Unknown. As an example, let us take the following tabular data.
Flowrate, lit/min
Pressure, Barg
10000
14.5
20000
12.5
30000
10
Minimum flowrate = 10000 lit/min
Maximum flowrate = 30000 lit/min
The dialog box for entering this data is shown below.
Sometimes you might get the error message “Gradient must be negative over the whole range...” This message will be produced if the Fitted Curve (not necessarily the input data) has a peak between the minimum and maximum flowrates. Effectively this means for some values of pressure there could be two corresponding flowrates. Under these circumstances there may not be a unique solution because two flowrates could give rise to the same pressure.
This problem would generally arise if the steep part of the pump curve is input along with the shallow part. There are perhaps three ways of dealing with this.
- Reduce the range between the minimum and maximum flowrates
- Input the steep part of the pump curve or the shallow part but not both together
- Gently modify the data points so that the peak does not occur
One other point one has to bear in mind is the following, especially in offshore firewater systems. Often the pump curve refers to the flowrate and pressure at the discharge flange of the pump assembly. In other words, it already takes into account the static head loss and frictional loss in the riser pipe. If this were the case, the caisson riser pipe must not be input again.
3.2. How to Deal with Systems using “Clack Shut” Valves and “Elastomeric” Valves as Deluge Valves:
Deluge valves are usually either of the type “clack shut” or elastomeric.
Clack shut deluge valves are characterised by the fact that the flowrate will depend on the inlet pressure. For this reason, if more than one system operates, the deluge systems will interact with each other. It is common to model a system with a clack shut deluge valve by an equivalent nozzle. For example, if the system including the deluge valve requires a flowrate of 5697 lit/min at a pressure of 9 barg, an equivalent nozzle would have a k-factor of 5697/√9 = 1899 (lit/min, bar). As each deluge system in the ringmain would have a different k-factor, they are normally not set up in the nozzle library. Instead they are input as “user defined” nozzles. The minimum and maximum pressures can be set to any reasonable values.
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Elastomeric deluge valves on the other hand work with the principle that they control the downstream pressure. Consequently, they control the flowrate going into the deluge system. As the flowrate will be fixed during commissioning, this will treated as an output with a known flowrate.
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See the section below for a clarification of the meaning of design phase and calculation phase specifications.
3.3. How to Deal with Multiple Inputs and Outputs:
This is mainly a matter of learning how to assign appropriate specifications. The basic mathematical rules are the following:
The number inputs + outputs = Number of pressure specifications +
number of flowrate specifications
Number of pressure specifications ≥ 1
Please note that nozzles are ignored when it comes to the above rules. This is because all the specifications for nozzles are automatically assigned by PIPENET.
When it comes to applying the above rules in PIPENET, the first point we note is the following. PIPENET always performs the calculation twice. They are called the design phase and the calculation phase. In PIPENET the terms Calculation Phase and Analysis Phase mean the same thing. The way in which the above rules are applied is different between the Design Phase and Calculation Phase.
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