Introduction:
This handout is intended to be about both design methodologies and about PIPENET itself. It has the following sections.
2 – Design Tips & Techniques
3 – How to Model a Deluge System
4 – How to Set up the Desktop
5 – Different Phases of Input
5.1. Initialisation
5.2. Libraries (diversion)
5.3. Initialisation (again)
6 – Network Input
7 – The Scenarios
8 & 9 – Additional Examples
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:
One of the most important aspects of successful hydraulic calculations for the design of firewater systems is to perform the calculations for deluge systems and firewater ringmains separately. This is because the objectives of the calculations for the two requirements are somewhat different. So, divide the system into deluge system and firewater ringmain before the calculations are performed.
2.1. Deluge System Calculations Objectives
- Check the velocities and keep them below the specified limits.
Although firewater systems generally do not suffer from erosion problems because they do not work 24 hours a day, it is still important to keep the velocities below a reasonable limit. This would minimise the frictional losses in the pipes. Normally high velocities arise because the pipe diameter is small. For a given flowrate the frictional loss per unit length would be inversely proportional to the fifth power of the diameter. So one must take care while reducing the pipe sizes with the view of reducing costs.
- Achieve good flow balance between the nozzles.
Generally a well designed system will have a low flow imbalance between to nozzles. A high imbalance would generally mean the total flow demanded by the system is higher than necessary. In an extreme case this might even lead to a bigger fire pump.
- Optimise the inlet pressure and flowrate.
It is better to optimise, rather than minimise. Naturally, minimum frictional pressure loss and hence the minimum inlet pressure could be obtained by maximising the pipe sizes. This could increase the cost of the system and its weight. This is why it is important to work close to the maximum velocity but not exceed it.
2.2. Firewater Ringmain Calculation Objectives
- Adequacy of pump performance:
The deluge system calculations would show the system requirements by way of pressure and flowrate. We need to make sure that the fire pump arrangement can meet the requirements under a variety of conditions and a variety of demands. The skill is designing the firewater ringmain system well is in making sure that it would work adequately under a wide variety of circumstances.
- Ability to generate sufficient pressure at inlet to the deluge systems and other users:
It is not just the sizing of the fire pumps which are important. The pipes have to be sized adequately too. They must be small enough to reduce the weight and cost but big enough to offer acceptable pressure drops.
- Longest piping route by blocking pipes:
Firewater ringmains have isolation valves for maintenance, strainers and so on. It is often necessary to ensure that the system would work adequately even if an isolation valve is left closed or a strainer is blocked.
- Integrity of the system if a pipe is broken:
If a firewater ringmain system is being analysed this may be important. For example, if the pipework near the helideck system is broken in a platform with several levels, it is possible that the deluge systems at the low levels might still work properly albeit at a reduced efficiency. This is because the elevation difference might produce enough static head.
- Different fire scenarios:
One of the interesting aspects of the design of a firewater ringmain system is the fact that the same piping system must work under a wide variety of conditions. Clearly this means performing calculations for a wide variety of fire scenarios.
- Different fire pump scenarios:
Usually the system is designed in such a way that one or more pumps would be required to meet the demand. It is necessary to ensure that this is possible.
It is clear that the objectives of the calculations for deluge systems and those for firewater ringmains are somewhat different. This is why best designs are obtained by separating the deluge system calculations from the ringmain calculations.
In this handout we will be considering the range of calculations one might typically perform with deluge systems.
3. How to Model Deluge Systems:
Some of the objectives of modelling deluge systems are the following –
- To keep the velocities below the specified limits
- To minimise flow imbalance
- To select appropriate pipe sizes
- If necessary size the restriction orifice plate required to achieve the required pressure drop
- To assess the impact of low pressure at the inlet of the deluge system
- To assess the impact of high pressure at the inlet of the deluge system
3.1. The First Calculation in Analysing Deluge Systems:
The first calculation is almost always the most remote nozzle option. When the most remote nozzle option is chosen, PIPENET will set the Flowrate through the hydraulically most remote nozzle to the minimum required Flowrate. This is usually the first calculation one would perform with a deluge system because it is the optimum operating condition. This is the optimum condition because if the inlet pressure is less than what is required by this criterion at least one nozzle will supply less than its required Flowrate. If the inlet pressure is higher then too much water will flow through the system. In an extreme case this could lead to the need for a larger fire pump.
In this exercise we will use the most remote nozzle option as the first calculation.
3.2. Subsequent Calculations in Analysing Deluge Systems:
Typically the following calculations will also be performed in this exercise. This is because in the real design environment the supply side may not be exactly what is required.
- Inlet pressure fixed at a low value – This may be necessary if the pressure available from the firewater ringmain is less than what is required by the most remote nozzle option.
- Inlet pressure fixed at a high value – This may be necessary if the pressure available from the firewater ringmain is higher than what is required by the most remote nozzle option.
- Inlet pressure fixed at a high value with an orifice plate – This may be necessary if the pressure available from the firewater ringmain is higher than what is required by the most remote nozzle option but an orifice plate is used to restrict the pressure to the required value.
3.3. The Network Schematic:
The network we will be working with is shown below. This network schematic was developed in PIPENET. It is being shown here for illustration purposes only. Do not input the network at this stage.
The steps involved in inputting the data are shown below.
4. How to Set up the Desktop:
Open PIPENET and set up the desk top to look as shown below. You desktop may not look the same in which case please go through the following steps.
4.1. Desktop Arrangement:
The first step is to make sure that all four windows are open. Then arrange the screen to look as shown below by using the “View” menu and moving the windows on the desktop.
You may also need to follow the steps described in Display options which is shown below.
span style='mso-ignore:vglayout;;z-index:1;margin-left:209px;margin-top:96px;width:47px;height:61px'
4.2. Display Options:
First select the display options in the following dialog box. In order to do this, enter “Options/Display” and select the options as shown below.
4.3. Menu Bar Format:
Those of you, who are experienced users of PIPENET, might prefer to use a menu bar which is similar to that of PIPENET Classic. This may be selected as shown below:
No comments:
Post a Comment