PDF version of this information: tnhyd001en.pdf

Preface:

Please note: How to calculate water demands for fire fighting is usually described in national standards, see Literature.

1 Task

STANET has integrated hydrant simulation. The following variants exists:

1.1 Basic Conditions

Basic conditions for all simulation variants are:

Diameter: For the simulation the value inserted into the corresponding field of the hydrant will be taken. If no entry exists the value for diameter in dialog File->Network Parameters->Pipe Parameters will be used. If this value is 0.0 it is generally assumed 80 mm.

Roughness: For the simulation the value inserted into the corresponding field of the hydrant will be taken. If no entry exists the value for diameter in dialog File->Network Parameters->Pipe Parameters will be used.

Loss Coefficient (zeta value): Only the field Loss Coefficent of the hydrant will be used.

Height / Altitude / Elevation: The relevant height for the hydrant simulation is written in the field Elevation (interpol.).

  • Either: It will be taken over from the field Elevation, i. e. you can insert the real height.
  • Or: If no entry exists in the field Elevation the value will be interpolated from the node heights of the corresponding pipe. In this case it is assumed that the hydrant outflow is at pipe height.

1.2 Determine outflow pressure of hydrants with given amount

The water demand for fire fighting is inserted into the field Def. Outflow (e. g. 24, 48, 96 or 192 m3/h; corresponding to the working sheet DVGW W405).
Call function Tools->Calculate Hydrant Outflow. With option Calc. pressure for the outflow entered in each hydrant for every hydrant with Def. Outflow > 0 the pressure will be calculated. 		All the time a stationary simulation will be done using the actual settings from dialog File->Start Simulation together with the additional consumption of a single hydrant. The number of hydrants with Def. Outflow > 0 determines the number of simulations to be done.

1.3. Determine the max. outflow to given hydrant pressure

Call function Tools->Calculate Hydrant Outflow and select the option Calculate max. outflow for pressure at hydrant. Afterwards you specify a pressure for all hydrants (global value). The simulation determines for each hydrant the maximum outflow which will be written into the field Calc. Outflow. The number of hydrants in the network determines the number of simulations to be done.

1.4. Determine the max. outflow out of the values 192, 96, 48, 24, 0 m3/h to lowest given pressure in the network

Call function Tools->Calculate Hydrant Outflow and select the option Calculate max. outflow out of (192, 96, 48, 24, 0) m3/h at minimal network pressure. Afterwards you specify a pressure for the whole network. The simulation determines for all hydrants the highest outflow out from the values 192, 96, 48, 24 or 0 m3/h and writes it into the field Calc. Outflow. The number of hydrants in the network determines the number of simulations to be done.

1.5. Simulation for a predefined selection of hydrants

Additional variants for the simulation exists for the cases 1.2 - 1.4:

  • It is possible to simulate fire protection demands while assuming that on more than one hydrant water consumption take place. For that purpose you mark the hydrants with pressed Ctrl key and left mouse clicks. Afterwards you call the dialog Tools->Calculate Hydrant Outflow and activate the checkbox Simulate concurrent usage of all selected hydrants.
  • To simulate the outflow for a single hydrant, mark the hydrant before calling the hydrant calculation. In the dialog activate the checkbox Simulate concurrent usage of all selected hydrants.

In all cases the number of marked hydrants is written behind the checkbox.

1.6. Hints:

  1. For graphical processing hydrant symbols can be colorized using attribute legends.
  1. The simulation splits the pipe for every hydrant. Therefore no average results for pipes, e. g. flow, velocity, etc., could be determined. All results belonging to the whole pipe, e. g. flow, velocity, Δp, heightdifference, etc. are without meaning. To determine the average results for the pipe you must do a normal simulation!

2. Simulation Algorithm

Symbolic representation:
1.) underground hydrant
2.) pillar hydrant

The pressure drop in the hydrant, simulated from point A, the junction between the hydrant and the pipe, to point B, the junction between hydrant and flexible tube of the fire-brigade, will be calculated with the following formula:


Turbulent flow is assumed. If the geodetic height of outflow point B is not inserted in the hydrant table, the geodetic height will be interpolated from the node heights of the corresponding pipe. If roughness or diameter are not inserted, the values will be taken from dialog File->Network Parameters->Pipe Parameters. If hydrant length is not inserted, a value of 1 meter will be used. If the loss-factor (ZETA value) is zero, the simulation uses zero. (Have a look at 1.1 Basic Conditions.)

3. Cases in Practice

In practice, high demands of fire fighting water are taken from neighbouring hydrants and not from a single hydrant. If the total amount is concentrated onto a single node, a much greater pressure drop is achieved than in reality.

To model this case you should use zero for the loss-factor (ZETA value) on the appropriate hydrant. 		(An additional changing of the roughness hasn't great influence.) That means: The deliverability of the pipe will be calculated, while a consumption with small loss is simulated. Because its a simple method it is widly used in practice. Normal loss-factors (ZETA values) for hydrants are between 4 and 5, for free flow hydrants between 0.8 and 1.2.
Links to literature are here.
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