Lesson 5 : Calculating the system

In lesson 5 we will look at how you can calculate the hydraulic model which we have entered and at the different ways of selecting heads and revising the head selection together with some of the various calculation options. Lets remind ourselves of some of the basic concepts of hydraulic calculations and the information we need to enter into the FHC a hydraulic model before we can produce a successful calculation. 

 

•Hydraulics the basics 

•Calculating in FHC

•Calculate all heads

•Select a group of heads

•Select Individually

•Add / Amend head selection

 

 

 

Hydraulics the basics

 

The examples that we are going to use in lesson 5 will be based on a fire sprinkler installation where we are required to provide a density discharge over an area of operation which would be typical for a fire sprinkler design, will cover other types of fire protection systems and further lessons.  To enable us to do this we need to provide FHC with a some information, you will need to enter in FHC Project Data the required default design density and you also need to specify the actual head area for each head in the FHC project, you will enter this information when you Add or Edit the pipe.

 

Design density is the density of application of water. To put it another way, the amount of water being discharged over an area so if we know the area the sprinkler is covering and the design density required we can calculate the amount of water we need to flow from the head using the following formula.

 

Q = A x D

 

when

 

Q = flow of water from sprinkler head in L/min

A = the area the sprinkler head is covering in m²

D = the design density required in mm/min

 

Let's take an example we have a sprinkler system which requires 7.5 mm/mim design density and a sprinkler head in the system is covering 8.5 m then:

 

Q = A x D so 8.5 m² x 7.5 mm/min = 63.75 L/min

 

We have now calculated that the minimum flow from this sprinkler head and we will need 63.75 L/min.  As long as we correctly enter the required design density and the area each head is covering in the sprinkler system, FHC will make the necessary adjustments to ensure that the calculation is accurate.  We will now see how this information is entered in to FHC project.

 

 

Project data 

 

When we start an new FHC project we are required to enter some basic information in the FHC Project Data you will see below that the 'Density of coverage mm/mim' is Underlined this is telling you that what you enter into this field will affect the calculation.  In this instance we have entered 7.5 mm/min which will now become the default design density for all the sprinklers heads on this project unless we specified differently in the Add or Edit pipe commands.

 

 

 

 

Add Pipe & Edit Pipe

 

Each time we add a sprinkler head into the FHC hydraulic model we need to specify the actual area the head is covering we enter this number into the text box Area - m2.  We can also enter the head design density but we do not need to do this if we have already specified it in the FHC project data as this will be the default design density for the project and all heads. 

 

  

 

If we have a project where we are required 12.5 mm/min design density and we had only a few sprinklers with 5 mm/min design density then we could set the default design density in the FHC project data 12.5 mm/min and then for the few heads which required the lower design density can be specified when we use the add or edit the pipe command.   

 

 

Calculating in FHC

 

We now know what information we are required to enter in the FHC project data and when we add pipes into the system to enable FHC to make a calculation.  In lesson 5 we are going to practice on a simple tree system shown below

 

 

We will set the project design data to require 7.5 mm/min design density over an area of 240 m² of course when we enter the heads in the system we won't nationally know how many heads we will need to select to get to our 240 m² area of operation.  FHC provides a number of commands which allows you to select a group of heads or individual heads.  You can also use the add and men command to change selection once you've carried out the first calculation. 

 

Below is an image of the FHC toolbar and in the highlighted box 3 icons working from left to right they are as follows.

 

1.Calculate all heads or or calculate selected heads [F12]

2.Select a group of heads

3.Select individual heads

 

You can also select each of these commands from the calculate menu but as you use quite frequently it's worth remembering the location on the toolbar and the associated shortcut keys. 

 

 

Calculate all heads

 

If we open the lesson 5 in FHC and press the shortcut key [F12] this will calculate all the sprinklers heads which we have entered into the project but you should first see the Calculation – options window as below.

 

 

 

We don't really need to worry about all the options at the moment to just go ahead and press the OK button, FHC will now calculate the system and present you with the results window as shown below:

 

 

The calculation summary provides you with detailed information about the calculation but the most important part is in the top line marked (1) above which is telling us that the source pressure for the system is 3485.8 lpm @ 5.647 bar.  Source pressure is the minimum amount of flow and pressure which is required for the system to operate correctly given design parameters which we have entered. 

 

Now that calculation has been performed new information will be attributed to each pipe in the system which we can interrogate by using the pipe properties command [Ctrl+V] we can also use the context menu by clicking the right hand mouse button in the drawer area, we can then select various properties.  In the example below we have selected pipe flow so the flow and direction is being shown on each pipe. You will also notice below one other heads is marked MRH this is the most remote head in the installation so we now know the critical path for the water flow from the water source is to this point in the system.  You will also notice that an additional 3 heads around MRH have also been marked, FHC tries to correctly find the 4 most remote heads in the system which is a requirement of EN 12845

 

 

 

 

Select a group of heads

 

So far we have seen that if we pressed the [F12] key we can calculate all the sprinkler heads which we have input into our hydraulic model of course when we are looking for the most remote and most favourable areas in our design we will have input more heads than are required for our area of operation so we can prove that the heads we have selected are indeed the most remote and most favourable.  FHC allows you to select individual heads or a group of heads and then select section we will show you how you do this.

 

If you open the calculation menu you will see all the commands which were available to us in selecting and calculating the system.

 

 

We have already seen that by pressing the [F12] key will calculate all the sprinkler heads in the FHC hydraulic model but if we wish to calculate a group of heads we can use the menu Calculate -> Select by group command or use [F11] shortcut key or we can select the command from the toolbar see (1) bellow.

 

 

 

If you open lesson 5 in FHC and ensure that you have the plan view selected [F2], then select the 'select by group' [F11] command you now need to draw a marquee tool around the heads you wish to select you do this by pressing the left hand mouse key and holding whilst drawing the area, once you have the head selected released the mouse key, the head you selected will now turn blue as shown below (2).  You can keep using this tool to select and unselect the heads as you require.  You will see on the status bar (3) will be updated to show you the number of heads you have selected and the total area for those heads.  If the area is not shown this is because you are not entered the head area for each the heads and you may need to amend your hydraulic model.

 

Once you have the heads selected which you wish to calculate you can now use the calculate option menu Calculate -> Calculate command or just press [F12]

 

 

 

Select Individually

 

Another option which we have to select heads is to select individual heads we can select the menu Calculate -> Select individual command or press [F9] key or by using the menu command labelled (1) below.  We can now use the mouse and click on each of the heads which would like to include in the calculation if with select the head again it will become unselected.  You will also see that as before the status bar (3) is updated they tell you the number heads which are selected and the area of the selected heads.

 

Once you have the head selected which you wish to calculate you can now use the calculate option Calculate -> Calculate or just press [F12]

 

 

 

 

 

 

 

Add / Amend heads

 

Once we have made a calculation we may wish to amend the number of heads and make a further calculation we can do this by using the commands above to 'Select by the group' or Select individually or alternatively we can use the 'add / amend heads' command on the calculate menu [Ctrl+F12]. Once you selected this command with the mouse you can select and unselect heads as you require when you have made the changes you require press the [F12] key to recalculate.  You can keep using this command until you have correctly found your most remote and most favourable sprinkler heads in your hydraulic model.

 

 

Related Topics

•FHC Project data

•Calculate menu options

 

 

Video tutorial 

 

You can now watch a short video which demonstrates how to select heads & nozzles to calculate using the select individually and select by group commands.