This paper describes how modeling can now help with such analysis, automating the process and making major savings on what is at present a time-consuming and costly task.
Modeling as an aid to hydrant and fire flow analysis
There are two separate types of fire analysis that hydraulic modeling can support: - Fire Hydrant Testing - Fire Incident Analysis
Fire Hydrant Testing
Fire hydrant testing evaluates in detail the ability of a hydrant to supply water at the start of an incident. It does not examine any changes in the flow over a period of time. In essence, fire hydrant testing simulation is a mathematical representation of the real-world fire hydrant test used in most municipalities around the world to assure fire obligations are met.
The details that need to be evaluated include:
- Maximum flow at a hydrant, and whether this meets the flow requirements
- The pressure at maximum flow
- The pressure at required fire flow, and whether this meets the pressure requirements
- Minimum pressure in nodes within the fire zone during the test, and whether this pressure meets zone minimum pressure constraints
- Minimum pressure in nodes within the system outside the fire zone during the test, and whether this pressure meets system minimum pressure constraints
- Hydrant curve (pressure vs. flow curve for this hydrant at this time)
During a fire incident the pressure in the network around the hydrants being used to fight the fires will drop due to the large consumption of water. A city may specify different criteria for minimum pressures during fire fighting, covering:
- Minimum pressure required at the hydrant
- Minimum pressure required in the fire zone
- Minimum pressure required in the system during the fire
Modeling of a hydrant test can replace the real-world hydrant test that has to be performed on hydrants. For example, the procedure in the US is for a Fire Marshal or City Engineer to open the hydrant and measure the maximum flow and the pressure at the hydrant at this flow. Pressure is also evaluated at control points in the system. They will also evaluate pressures at required fire flow for this hydrant. This testing has two negative effects – it is a time consuming task, and it wastes a lot of water. Many municipalities will allow substitution of up to 90% of fire hydrant tests with model results if the model results match test for the remaining 10% of tests, saving time, water, and money.
Fire incident analysis
Fire Incident analysis differs from Hydrant Testing by considering supplying the fire flow Q for a period of time T and assessing the consequence of this on the whole network, such as the emptying of reservoirs and the consequent loss of supply to customers, and water quality issues such as discolored water. Fire Incident Analysis can cover the use of multiple hydrants, and more than one fire.
The results from above two types of analysis allow engineers to identify hydrants that are likely to fail to provide adequate fire flow in the event of a fire, therefore allowing them to look at alternative arrangement such as the provision of fire tanker trucks.
How InfoWorks addresses the analyses
InfoWorks provides two key factors without which modeling fire flow and hydrant testing becomes cumbersome or even impossible. These factors are hydrant objects and fast simulation engine.
InfoWorks WS allows modeling hydrants as a specific element, making it simpler for the user to simulate hydrant testing. All the characteristics of a hydrant, including connecting pipe, valve, and hydrant node are all stored as attributes of the hydrant. This reduces the size of a hydraulic model by the factor of 4 at every hydrant in a system. Considering that an average city can have tens of thousands of hydrants, a resulting model that does not utilize hydrant objects would have hundreds of thousands more elements.
Furthermore, the software can automate the testing of a series of hydrants, or even all hydrants in the system, in one run, taking away the manual intervention of moving the testing from one hydrant to the next. To evaluate hydrant flow at all hydrants in a system, the user needs fast and stable hydraulic solver. The InfoWorks WS simulation engine, recently enhanced, has been proven to clearly outperform other simulation engines in speed and stability.
With an accurately verified and validated hydraulic model, automated fire flow analysis can calculate the available fire flows at hydrants. Water authorities can now rapidly assess the adequacy of their water system to meet target fire flow requirements, identify system weaknesses, and plan for necessary system strengthening and improvements, all from modeling alone.
With the continued and rapid development of hydraulic modeling products and hardware, engineers can rapidly analyze a multitude of scenarios that would take a lifetime to undertake manually. This rapid automated process permits engineers to prepare for unexpected events and have contingency plans available in order to meet statutory obligations and reduce interruption to customers.